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An investigation of solvated electrons in hexamethylphosphoramide Flynn, Garry John 1975

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AN INVESTIGATION OF SOLVATED ELECTRONS IN HEXAMETHYLPHOSPHORAMIDE BY GARRY JOHN FLYNN B.Sc, University of British Columbia, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of CHEMISTRY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February, 1975 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Depa rtment The University of British Columbia Vancouver 8, Canada r. - i i i -ABSTRACT T h i s t h e s i s i s concerned w i t h a d e t a i l e d i n v e s t i g a t i o n of the n a t u r e , y i e l d , s t a b i l i t y , and r e a c t i v i t y o f s o l v a t e d e l e c t r o n s i n hexamethy lphosphoramide (HMPA). E v i dence i s p r e sen ted f o r t h e i r f o r m a t i o n by r a d i o l y s i s and f o r t h e i r c o m p a r a t i v e l y l o n g - l i v e d e x i s t e n c e i n t h i s h i g h l y p o l a r a p r o t i c s o l v e n t i n wh ich o r d i n a r y an i on s a r e p a r t i c u l a r l y weak ly s o l v a t e d . By means of m i c r o second p u l s e r a d i o l y s i s methods i t was shown t h a t the s o l v a t e d e l e c t r o n i n HMPA has a i n t e n s e broad s t r u c t u r e l e s s a b s o r p t i o n band w i t h a band maximum at 2200 + 100 nm, a band h a l f - w i d t h of 3600 c m - 1 and maximum molar a b s o r p t i v i t y of (3.2 + 0.5) X 1 0 4 M-1 cm~^. The spectrum i s s i m i l a r to t h a t found i n s o l u t i o n s o f sodium meta l d i s s o l v e d i n HMPA. R a d i o l y s i s s t u d i e s p r o v i d e d a v a l u e of 2.2 + 0.2 f o r the f r e e - i o n y i e l d o f s o l v a t e d e l e c t r o n s . T h i s was e s t a b l i s h e d by both p u l s e and s t e a d y - s t a t e methods and i s a p p r o x i m a t e l y what one would expec t on the b a s i s o f the l i q u i d ' s d i e l e c t r i c c o n s t a n t when compared w i t h o t h e r s o l v e n t s . An i n t e r e s t i n g d e d u c t i o n from the s t e a d y - s t a t e V - r a d i o l y s i s r e s u l t s was t h a t i ^ O does not scavenge e l e c t r o n s on a o n e - t o - o n e b a s i s i n t h i s s o l v e n t , but r a t h e r produced more than one Hr, m o l e c u l e per e l e c t r o n s cavenged . The y i e l d s of f rom ^ 0 s c a v e n g i n g o f e l e c t r o n s i n HMPA/water m i x t u r e s are a l s o r e p o r t e d f o r the f u l l c o m p o s i t i o n range . E l e c t r o n decays were s t u d i e d s p e c t r o -p h o t o m e t r i c a l l y and i t s r a t e of r e a c t i o n w i t h N 9 0 , p y r ene , anthracene and other additives are reported in the thesis. When stable solutions of solvated electrons in HMPA ( a l k a l i metal solutions) were themselves i r r a d i a t e d , a net decrease in electron concentration occurred, indicating that e ^ p ^ reacts not only with i t s concomitant p o s i t i v e ion but also with other ra d i o1y s i s products. Pure HMPA produces hydrogen and methane when i r r a d i a t e d , with yields of 3.3 + 0.3 and 0.29 + 0.03 respectively. The H^  y i e l d was shown to comprise a non-scavengable molecular process (whose y i e l d is 1.4 + 0.1) and a scavengable part (yield 1.9 + 0.2) that c l e a r l y involved solvated electrons. - v -TABLE OF CONTENTS Chapte r 5 Page I. INTRODUCTION 1 A. Hexamethy lphosphoramide 1 B. A l k a l i Me ta l S o l u t i o n s 4 1. L i q u i d Ammonia 4 2. Amines & E t h e r s 10 3. Water - The Hydrated E l e c t r o n 14 C. R a d i a t i o n Chem i s t r y and S o l v a t e d E l e c t r o n s 17 D. The I n t e r a c t i o n of High Energy R a d i a t i o n w i t h M a t t e r 21 1. Gamma Rad i o l y s i s 22 a) The P h o t o e l e c t r i c E f f e c t 22 b) The Compton P roce s s 23 c) P a i r P r o d u c t i o n and P h o t o n u c l e a r R e a c t i o n s 24 2. High Energy E l e c t r o n s 25 E. The Chemica l Consequences of R a d i a t i o n A b s o r p t i o n 32 F. S o l v a t e d E l e c t r o n s 34 1 . Fo rmat i on 34 2. T h e o r e t i c a l Models 40 3. R e a c t i o n s 44 4. Y i e l d 46 G. The P r e s e n t Study - S o l v a t e d E l e c t r o n s i n HMPA . . 54 - v i -Chapter Page I I . EXPERIMENTAL 56 A. S teady S t a t e Expe r iment s 56 1 . M a t e r i a l s 56 2. R a d i a t i o n Source 57 3. Do s imet ry 58 4. E x p e r i m e n t a t i o n 64 a) 6°Co Gamma R a d i o l y s i s 64 b) Sodium Meta l S o l u t i o n s 71 5. A n a l y s i s 74 B. P u l s e R a d i o l y s i s E xpe r iment s 78 1. Genera l O u t l i n e o f the Techn ique s 78 2. Sample P r e p a r a t i o n 78 3. P u l s e R a d i a t i o n Source 83 4. O p t i c a l D e t e c t i o n 85 a) A n a l y s i n g L i g h t Source 85 b) Wavelength S e l e c t i o n 86 c) D e t e c t i o n 87 d) Dos imet ry 38 I I I . RESULTS AND DISCUSSION 93 A. R e s u l t s : S teady S t a t e E xpe r imen t s 93 1. Gamma R a d i o l y s i s S t u d i e s 93 a) Pure HMPA 93 b) Scavenger S t u d i e s 97 i ) N i t r o u s Oxide 97 i i ) N i t r o u s Ox ide P l u s a Second Scavenger 102 - v i i -Chapter Page I I I . c ) HMPA - Water M i x t u r e s 112 2. Na/HMPA S t u d i e s 113 a) Na Meta l S o l u t i o n s 113 b) Sodium Amalgams i n S o l u t i o n 115 B. D i s c u s s i o n : S teady S t a t e E x p e r i m e n t s 120 1. Gamma R a d i o l y s i s S t u d i e s 120 a) P r oce s se s Lead ing to M o l e c u l a r P r oduc t Fo rmat ion 120 b) Free Ion Y i e l d 125 c) Free Ion L i f e t i m e 137 d) N i t r o g e n Y i e l d From the S o l u t i o n s C o n t a i n i n g High C o n c e n t r a t i o n s of N i t r o u s Ox i de : Other Sources 138 i ) D i r e c t R a d i o l y s i s o f i^O 139 i i ) R e a c t i o n of N^C1 W i th Other S p e c i e s Produced E i t h e r D i r e c t l y or I n d i r e c t l y From S o l v e n t R a d i o l y s i s . 139 11.1) E x c i t e d S p e c i e s 140 11.2) R a d i c a l S p e c i e s 142 11.3) I o n i c S p e c i e s 144 i i . 3 . i ) Geminate Ions - A H i s t o r i c a l Background .. 148 i i . 3 . i i ) Geminate Ions i n HMPA . . 154 i i i ) Secondary I o n i c R e a c t i o n s of N i t r o u s Ox ide 161 - v i i i -Chapte r Page I I I . i i i . l ) P o s s i b l e Mechanisms f o r N^Q S caveng i ng 161 i i i . 2) Secondary I o n i c R e a c t i o n s i n HMPA 163 i v ) Geminate Ion S caveng i ng Ver sus Secondary I o n i c R e a c t i o n s 170 i v . l ) In Pure HMPA R a d i o l y s i s . . . . 170 e) R a d i o l y s i s o f L i q u i d M i x t u r e s 174 i ) Background 174 i i ) H M P A / H 2 0 M i x t u r e s 175 f ) The Nature o f the Secondary I o n i c S p e c i e s Lead ing to N 0 i n 1^0 S caveng i ng S t u d i e s . 181 g) S o l u t i o n s of Sodium Me ta l i n HMPA 189 C. P u l s e R a d i o l y s i s S t u d i e s of S o l v a t e d E l e c t r o n s i n HMPA 196 D. D i s c u s s i o n : P u l s e R a d i o l y s i s E xpe r imen t s 201 1. T r a n s i e n t A b s o r p t i o n S p e c t r a 201 2. Mo l a r A b s o r p t i v i t y and 3and O s c i l l a t o r S t r e n g t h 206 3. S o l v a t e d E l e c t r o n Y i e l d i n HMPA 213 4. K i n e t i c S t u d i e s 216 a) Decay of P r ima r y S p e c i e s 216 i ) The S o l v a t e d E l e c t r o n 217 i i ) The UV A b s o r b i n g S p e c i e s 224 i i i ) R e a c t i o n Between T r a n s i e n t S p e c i e s . 228 i v ) E l e c t r o n Decay i n I r r a d i a t e d Sodium Meta l S o l u t i o n 231 - i x -Chapte r Page I I I . b) B e h a v i o r of T r a n s i e n t S p e c i e s i n the P re sence of Added S o l u t e s 233 i ) E l e c t r o n Scavenger s 233 i . l ) Pyrene and A n t h r a c e n e 233 i . 2) Ni t r o u s Ox ide 241 i . 3 ) Oxygen 246 i . 4) Acetone 246 i i ) P o s i t i v e Ion Scavenger s 253 i i . l ) Water and Methano l 253 i i . 2 ) Bromi de Ions 253 i i . 2 . i ) Y i e l d of O x i d i z i n g S p e c i e s 253 i i . 2 . i i ) K i n e t i c C o n s i d e r a t i o n s . 259 i i . 2 . i i i ) Na~ Spect rum 264 - X -LIST OF TARLES Tab le Page I Comb ina t i on t a b l e f o r two t ype s of o b j e c t s t a ken f o u r a t a t ime 53 II S o l u b i l i t i e s of v a r i o u s gases i n HMPA at 23°C as de te rm ined from the s l ope s o f the p l o t s i n F i g u r e 11 - 4 68 I I I T y p i c a l v a l u e s f o r the c h r o m a t o g r a n h i c r e soon se t ime and d e t e c t o r s e n s i t i v i t y of a number of gases f o r the c o n d i t i o n s o u t l i n e d 76 IV Bausch and Lomb G r a t i n g s used i n the p u l s e r a d i o l y s i s e x p e r i m e n t s 86 V E f f e c t of dose on gaseous p r oduc t " y i e l d " i n HMPA - 5 samoles i n i t i a l l y c o n t a i n i n g 7 X 1 0 M n i t r o u s o x i d e 101 VI M o l e c u l a r p r oduc t y i e l d s from the i r r a d i a t i o n o f HMPA samples c o n t a i n i n g known amounts of N^ O and a second s cavenge r 104 VII Gas r e c o v e r e d from the r a d i o l y s i s c e l l as a r e s u l t o f the a d d i t i o n of N^O or to a s o l u t i o n of Na i n HMPA 115 V I I I Y i e l d s o f gaseous p r o d u c t s o b t a i n e d from v a r i o u s s o l v e n t m i x t u r e s uoon the a d d i t i o n o f amalgams c o n t a i n i n g 360 + 20 ymoles o f sodium metal [ N 2 O ^ 6 X 10 2 M 1 1 3 - x i -Tab le Page IX N i t r o g e n y i e l d s f o r low and h igh c o n c e n t r a t i o n s o f n i t r o u s o x i d e i n v a r i o u s l i q u i d s compared to the f r e e i o n y i e l d s de te rm ined i n d e p e n d e n t l y 131 X N i t r o g e n y i e l d s from the gas phase r a d i o l y s i s of v a r i o u s compounds i n the p re sence o f n i t r o u s o x i d e .. 135 XI N i t r o g e n p r e c u r s o r y i e l d and r a t e c o n s t a n t r a t i o s c a l c u l a t e d f o r c o m p e t i t i o n between M^O and second s o l u t e s i n i r r a d i a t e d HMPA 145 XII Comoar i son of absorbances i n a 2n mm c e l l o f e , " l M n n a t HMPA 1000 nm w i t h the r a d i c a l an ion maximum a f t e r a r o m a t i c hyd roca rbon a d d i t i o n . For each p a i r o f r e a d i n g s e q u i v a l e n t r a d i a t i o n pu l s e s are used 210 X I I I R a d i a t i o n y i e l d o f a r o m a t i c an ions f rom An th r a cene and Pyrene at v a r i o u s c o n c e n t r a t i o n s i n HMPA f o r ^ 1 krad p u l s e s 216 XIV K i n e t i c a n a l y s i s o f e l e c t r o n decay da t a i n a samole of HMPA f o r wh ich the dose per p u l s e v a l u e was v a r i e d . The combined f i r s t p l u s second o r d e r t r e a t m e n t d e s c r i b e d i n the t e x t was used to c a l c u l a t e the p a r a m e t e r s , k*^  gg and 2k^ gg 223 XV E f f e c t o f acetone on the e l e c t r o n ab so rbance at 1 550 nm i n i r r a d i a t e d HMPA 248 (VI R e s u l t s o f f i r s t p l u s second o r d e r r e g r e s s i o n a n a l y s e s f o r the e l e c t r o n decay a t 1000 nm i n pure HMPA and the same sample c o n t a i n i n g 0.14M L i B r 260 - x i i -LIST OF FIGURES 29 F i g u r e Page 1-1 Hexamethy lphosphoramide 1 1-2 The ComDton P roce s s 23 1-3 Atomic a b s o r p t i o n c o e f f i c i e n t s f o r w a t e r , a) T o t a l a b s o r p t i o n c o e f f i c i e n t ; b) P h o t o e l e c t r i c a b s o r p -t i o n c o e f f i c i e n t ; c) Compton c o e f f i c i e n t ( w i t h c o h e r e n t s c a t t e r i n g ) ; d) Compton c o e f f i c i e n t ( w i t h o u t c o h e r e n t s c a t t e r i n g ) ; e) P a i r - p r o d u c t i o n - ?4 ? c o e f f i c i e n t . 1 barn = 10 cm 26 1-4 Dose d i s t r i b u t i o n from h igh energy e l e c t r o n s as a f u n c t i o n of depth and energy 1-5 A p o s s i b l e mechanism f o r e l e c t r o n s o l v a t i o n i n p o l a r l i q u i d s . The scheme i s o u t l i n e d i n the t e x t 1-6 D e t e r m i n a t i o n of the f r e e enerqy o f s o l v a t i o n (AG ) f o r the h yd ra ted e l e c t r o n . A l l f r e e energy v a l u e s a re i n k ca l /mo l e 37 1-7 D e f i n i t i o n of c a v i t y d i s t a n c e s i n J o r t n e r ' s s e m i -cont inuum model f o r s o l v a t e d e l e c t r o n s 42 1-8 Free i on y i e l d p l o t t e d as a f u n c t i o n o f s t a t i c d i e l e c t r i c c o n s t a n t f o r a v a r i e t y of s o l v e n t s . . . . 49 1-9 A b s o r p t i o n s p e c t r a of e g i n monoethano lamine ( ) and i n an e q u i m o l a r m i x t u r e o f e t h a n o l and monobuty lamine ( ) r e p o r t e d by Vann ikov and M a r e v t s e v . Arrows i n d i c a t e p o s i t i o n s o f X m , „ f o r max 35 - x i i i -F i g u r e Page I- 9 e~ i n pure e t h a n o l and e t h y l a m i n e 52 I I - l F r i c k e Do s ime t r y . R e s u l t s f rom the 5 0 C o Y - i r r a d i a t i o n o f a p p r o p r i a t e s o l u t i o n s i n the r a d i o l y s i s c e l l 61 11 - 2 Gamma r a d i o l y s i s r e a c t i o n v e s s e l 65 11 - 3 Schemat ic d iagram of vacuum l i n e used f o r the p r e p a r a t i o n and a d d i t i o n o f gaseous s c a venge r s t o HMPA s o l u t i o n s i n the r a d i o l y s i s v e s s e l , G 66 I I -4 S o l u b i l i t i e s of N 2 0 ( O ) , C 0 2 ( • ) , and 0 2 ( A ) at 23 C i n HMPA measured i n the r e a c t i o n v e s s e l . F i l l e d c i r c l e s i n d i c a t e N 2 0 samnles u s i n g Ar as d i l u e n t 69 I I - 5 S o l u b i l i t y at 23 °C of N 2 0 i n H 20/HMPA m i x t u r e s . . . 72 11-6 A. R e a c t i o n v e s s e l f o r sodium meta l s o l u t i o n s B. Sodium/mercury amalgam p r e p a r a t i o n and a d d i t i o n a c c e s s o r y 73 I I - 7 Schemat i c d iagram o f c h r o m a t o g r a p h i c gas s t a n d a r d and sample i n j e c t i o n system 75 11-8 T y p i c a l chromatogram o b t a i n e d from an i r r a d i a t e d sample of HMPA c o n t a i n i n g N 2 0 77 11 - 9 S i m p l i f i e d s chemat i c o f the p u l s e r a d i o l y s i s appa ra tu s at The Ohio S t a t e U n i v e r s i t y 79 11-10 Pu l s e r a d i o l y s i s samnle p r e o a r a t i o n and i r r a d i a t i o n v e s s e l . The o p t i c a l c e l l (20 X 10 X 5 mm) has S p e c t r o s i l windows 81 - X1 v -F i gure Page 11-11 Shape o f l i n a c e l e c t r o n p u l s e 84 11-12 P r o f i l e o f the output from the Xenon -a r c lamp used i n p u l s e mode 85 11-13 T y p i c a l o s c i l l o s c o p e t r a c e showing a t r a n s i e n t a b s o r p t i o n i n HMPA f o l l o w i n g a 400 nsec r a d i a t i o n p u l s e 88 I I - 14 R a d i a t i o n Do s ime t r y . T r a n s i e n t a b s o r p t i o n s r e s u l t i n g from 100 nsec p u l s e s o f 4 MeV e l e c t r o n s obse rved i n a c e l l hav ing an o p t i c a l D a t h l e n g t h o f 20 mm and c o n t a i n i n g A, aqueous KCNS s o l u t i o n and B, t r i p l y d i s t i l l e d water 90 I I I - l T o t a l p r o d u c t i o n o f hydroqen ( O ) and methane ( • ) as a f u n c t i o n of t o t a l absorbed dose from the r a d i o l y s i s of f o u r d i f f e r e n t samples o f HMPA 94 111 - 2 A c t u a l y i e l d s o f hydrogen ( O ) and methane ( • ) as a f u n c t i o n of t o t a l abso rbed dose f o r the da ta o f F i g u r e 111 -1 95 111 - 3 N i t r o g e n y i e l d from i r r a d i a t e d samples o f HMPA as a f u n c t i o n of n i t r o u s o x i d e c o n c e n t r a t i o n 93 111-4 Observed hydrogen ( • ) and n i t r o g e n ( O ) " y i e l d s " f rom i r r a d i a t e d samples of HMPA c o n t a i n i n g v e r y low i n i t i a l N^0 c o n c e n t r a t i o n s 100 111 — 5 S p e c t r a o b t a i n e d from a 1 mm c e l l c o n t a i n i n g : 2.5 X 10 " 3 M Gv i n HMPA ( ); 1.1 X 1 0 " 4 Gv i n HMPA ( ); 1.4 X 1 0 " 5 M Gv i n c y c l o h e x a n e ( ) 108 - XV -F i g u r e Page 111-6 S p e c t r a o b t a i n e d from a 1 mm c e l l c o n t a i n i n g : 7.6 X 10" 4M I 2 i n HMPA ( ); ^ 1 0 " 3 M KI and 12 i n wa te r ( ) 110 111 - 7 P r oduc t y i e l d s f rom v a r i o u s m i x t u r e s o f HMPA and H^O c o n t a i n i n g 0.02M NpO as a f u n c t i o n of mole f r a c t i o n H 2 0 114 I' 11 - 8 P l o t o f the da ta of F i g u r e 111 - 3 u t i l i z i n g e q u a t i o n (xxx ) 128 111 - 9 P l o t o f the data of F i g u r e 111-8 f o r l a r g e c o n c e n t r a t i o n s ( 1 0 ~ 2 to 1 0 " 1 M) o f M 20 129 111-10 P l o t f o r the n i t r o g e n y i e l d da t a u t i l i z i n g e q u a t i o n ( x x x i ) f rom c o m p e t i t i o n s between N 2 0 and C C l ^ ( • ) , CHC1 3 ( O ) , g a l v i n o x y l ( • ) , 1"2 ( • ) , and a ce tone ( A ) 146 I I I - l l P l o t o f t he da ta f o r n i t r o g e n y i e l d i n exces s o f the f r e e i on y i e l d from i r r a d i a t e d N 2 0 s o l u t i o n s of HMPA u t i l i z i n g e q u a t i o n ( x x x v i i ) 156 111-1 2 The e f f e c t of 0.2M Br~ on n i t r o g e n y i e l d f rom NpO i n HMPA. The s o l i d l i n e r e p r e s e n t s the n i t r o g e n y i e l d i n the absence of Br 158 111-13 The e f f e c t o f 0.27M acetone on n i t r o g e n y i e l d from N 2 0 i n HMPA. The s o l i d l i n e r e p r e s e n t s the n i t r o g e n y i e l d i n the absence of a ce tone 160 111-14 P l o t o f the " e x c e s s " n i t r o g e n y i e l d from N 2 0 i n HMPA f o r the da ta o f F i g u r e 111-3 u t i l i z i n g e q u a t i o n ( x l ) 171 - x v i -Page 173 Fi gure 111-15 Theoretical yield curves resulting f rom geminate ion scavenging ( ) and secondary ionic reaction (• ) considerations. The circles represent actual observed data from Figure 111 - 3 111-16 Computer simulation (lines) and observed nitrogen yields ( O ) as a function of solution c o m D o s i t i o n from irradiated mixtures of water and HMPA containing 2 X 1 0 M N^ O. The curves are explained in the text 179 111-17 Transient absorption spectrum in pulse irradiated HMPA. A m a v represents the absorbance at 2200 nm . 199 max 111-18 Absorption spectrum of nure HMPA exhibited in a radiolysis cell having an optical path length of 20 mm 200 111-19 Absorption maxima for e~ in HMPA/EDA mixtures as a function of solution composition expressed as mole ( O ) and volume (•) fraction 203 111-2 0 Combined Gaussion/Lorentzian f i t to the spectrum of Figure 111-17 plotted on an energy scale. Published data from Ma solution in HMPA (•) is included. G and L are explained in the text 205 111-21 Radiation produced transients observed at (i) 470-520 nm for (2.6 ± 0.1) X 10"3M pyrene and ( i i ) -4 710 - 780 nm for (6.3 ± n.5) X 10 M anthracene in HMPA. The species a re attributed to the aromatic negative ions 208 - x v i i -Fi gure Page 1 1 1 - 2 2 Oscillator strength comparison for e~ in several solvents. The figures in brackets _o 1 o represent values of € m *w, X 10 M cm as m a X ^ explained in the text 214 1 1 1 - 2 3 Lifetime of e^ M p^ as a function of radiation dose per pulse. Doses of (a) 3 0 0 , (b) 2 , 0 0 0 , (c) 1 0 , 0 0 0 rads gave electron f i r s t half-lives of 5 0 , 5 , and 2 usee respectively 218 1 1 1 - 2 4 First (a) and second order (b) plots for the data of Figure 1 1 1 - 2 3 a and c. Data for short-lived electrons (O) and long-lived electrons (#) are shown 219 1 1 1 - 2 5 Mixed f i r s t plus second order plot for the data of Figure I I I - 2 4 . The method is explained in the text 222 1 1 1 - 2 6 Simulation of the electron decay at 1 000 nm in HMPA ( ) by means of mechanisms involving bimolecular combination (#) and bimolecular recombination (O) 225 111 - 2 7 Interaction between e ^ p ^ observed at 1 000 nm (a,b,c) and the UV species at 350 nm (d,e,f) as a function of sample purity. The various effects on decays are explained in the text 230 1 1 1 - 2 8 Effect of electron pulses on a solution of Na in HMPA at 750 nm. The second pulse, b, followed 30 sec after the f i r s t and showed that the pulses - x v i i i -Figure Page 111 - 28 gave an i n i t i a l increase in e ^ M p A concentration but a net permanent loss o f the species. Similar results were found at 1 550 nm 232 III-29 Decay of eMpA a t , s o r P t i o n a t 1 0 0 0 (A) and s i m u l t a n e o u s build-up of An" a b s o r D t i o n at 740 nm ( B ) in a solution of (6 ± 2) X 10~5M anthracene in HMPA following a 20 ns radiation pulse 234 111 - 3 0 First order kinetic plots for the e^ MPA decay at 1 000 nm (O) and the "corrected" anthracene anion absorption build-up at 740 nm (#) from an irradiated solution of (6 + 2) X 10"5M An in HMPA. 236 111-31 Decay of aromatic anions produced via electron scavenging in irradiated HMPA. (A) An" at 740 nm from (6 ± 2) X 10"5M An in HMPA. (B) Py" at 495 nm from (3.2 ± 0.3) X 10_4M Py in HMPA 238 111-32 Typical fast time scale o s c i l l o s c o p e traces obtained at 1000 nm following 700 nsec radiation pulses in pure HMPA (A) and HMPA containing (7.9 ± 0.2) X 10~4M N20 (B) 242 111 - 3 3 First order kinetic plot for rapid e^p^ decay in the presence of (7.9 ± 0.2) X 10~4M NgO from Figure III-32(b). Time zero corresponds to the end of the r a d i a t i o n p u l s e 243 111 - 34 Build-up of e ~ M p ^ absorbance at 1 000 nm during 700 nsec "square" radiation pulses of Figure 111- 3 2. In pure HMPA (O) rate of formation was constant, XIX -Figure Page 111-34 dA/dt = (5.2 ± 0.1) X 105 sec" 1. For HMPA containing (7.9 ± 0.2) X 10~4M N20 (#} a steady state absorbance A = 0.044 ± 0.002 was reached 245 111-3 5 Spectra obtained in irradiated HMPA containing (1.6 ± 0.2) X 10_1M acetone immediately ( O ) and 2 usee (•) after the pulse 249 111-36 Build-up of e ^ M p A absorbance at 1 550 nm in irradiated samples of HMPA ( O ) and HMPA containing (3.2 ± 0.5) X 10~2M acetone (•) 251 111 - 3 7 Spectra obtained in irradiated HMPA containing (2.0 + 0.2) X 10-1M LiBr ( O ) , compared to pure HMPA ( ). Also shown are the long-lived transient observed on the addition of (1.3 ± 0.1) X 10"3M N20 (•) and the short-lived e ^ M p A contribution from that solution (•) 254 111-38 Normalized oscilloscope traces showing the relative contributions of e j ^ p A and Br~ at various wavelengths from the irradiation of a solution of HMPA containing (2.0 ± 0.2) X 10_1M LiBr and (1.3 + 0.1 ) X 10"3M N20 257 111- 3 9 Second order kinetic plots for the decay of the Br 2 absorption at 360 nm in irradiated HMPA containing (0.17 ± 0.03) M LiBr. Decay was examined over both short ( O ) and long time scales (•) 261 - X X -ACKNOWLEDGEMENTS The au t ho r would l i k e to exp re s s h i s g r a t i t u d e to Dr. D. C. Walker f o r h i s i n v a l u a b l e gu i dance and c o n t i n u e d encouragement t h r oughou t the cou r se of t h i s s t u d y . S i n c e r e a p p r e c i a t i o n i s ex tended to Dr. L. M. Dorfman and h i s group at The Ohio S t a t e U n i v e r s i t y , Co lumbus, Oh i o . T h e i r c o - o p e r a t i o n i n the use of the p u l s e r a d i o l y s i s f a c i l i t i e s at t h a t i n s t i t u t i o n g r e a t l y enhanced t h i s work. S p e c i a l thanks are due to Or. E. A. Shaede f o r h i s e x p e r t o p e r a t i o n of the l i n e a r a c c e l e r a t o r , many i n v a l u a b l e d i s c u s s i o n s , and i nnumerab le l ong hours of a d d i t i o n a l work. The au tho r i s i n d e b t e d to Dr. F. Y. Jou f o r o b t a i n i n g a b s o r p t i o n da ta p e r t a i n i n g t o HMPA/EDA m i x t u r e s . G r a t i t u d e f o r f i n a n c i a l s uppo r t of t h i s work i s e xp re s s ed to the N a t i o n a l Research C o u n c i l o f Canada f o r p o s t g r a d u a t e s c h o l a r s h i p s and to the U. S. A tomic Energy Commiss ion ( a t The Ohio S t a t e U n i v e r s i t y ) . - x x i -To dear Jennifer - xxii -- 1 -CHAPTER I INTRODUCTION A. HEXAMETHYLPHOSPHQRAMIDE H e x a m e t h y lphosDhoramid e (HMPA), [ (CH 3) _N ] 3 PO , i i s a most r emarkab le p o l a r a p r o t i c s o l v e n t ! I t i s a l i q u i d a t room tempe ra tu re of m o d e r a t e l y h igh d i e l e c t r i c c o n s t a n t (30 a t 2 0°C ) , has a l a r g e d i p o l e moment (5.3D at 2 0 ° C ) , and i s c o m p l e t e l y m i s c i b l e w i t h both p o l a r and - - w i t h the e x c e p t i o n of s a t u r a t e d hydrocarbons - - n o n - p o l a r l i q u i d s . Th i s s o l v e n t d e r i v e s a number of e x t r a o r d i n a r y p r o p e r t i e s f rom i t s p e c u l i a r s t r u c t u r e and charge d i s t r i b u t i o n . F i g u r e 1-1. Hexamethy lphosphoramide. As shown in F i gore J ~ 1, the HMPA molecule has a general pyramidal shape. The phosphoryi group is considered to bo about 50% ionic which accounts for the molecule's large dipole moment. In addition, the resulting high electron density l o c a l i z e d at the oxygiwi eto:!< irp? rts to the molecule an unusually high b a s i c i t y . The positive end of the di pole, on the other hand, is symmetrically dispersed over the phosphorus and three nitrogen atoms and is thereby s t e r i c a l l y screened from the surroundings by six methyl groups. In terms of i t s d i e l e c t r i c arrangement then, the HMPA molecule 'is a sort of "mushroom" shaped dipole having a well defined negative "stock" but an extremely diffuse positive "head". As a r e s u l t , HMPA solvates positive ions extremely strongly but negative ions on i y very weaK ly. i m s , conu>i nea wi tn i t s basicity, renders dissolved anions exceptionally reactive. For example, fluoride and chloride ions are espe c i a l l y strong bases in HMPA. Some reactions that are known to involve the formation of intermediate anions (carbani ons for instance) are greatly accelerated in th1s solv e nt. Equal ly rer.iarkabl e is the innate s t a b i l i t y exhi bited by HMPA tov:a s many types of highly reactive species. Probably because of trie s t e r i c screening of i t s p o s i t i v e region, HMPA is e s s e n t i a l l y i n e rt with r e s pect t e nuc1e o ph i 1 i c a ttack. A l l i t s hydrogen atoms are f; to the polar group and rather v n r e a c t: 1 v e . The s o 1 v c n t d o e s n o t u n d e r 5 0 hydr 01 y sis in th e presence of alkaline mod i a, and only those reagents - 3 -which can a t t a c k the exposed n e g a t i v e r e g i o n o f the m o l e c u l e , i . e . , the oxygen atom, r e a c t t o any g r e a t e x t e n t w i t h HMPA. Thus both B rbn s ted and Lewis a c i d s a c t v i g o r o u s l y on HMPA. Wh i le such r e a c t i o n s u l t i m a t e l y l e a d t o the r e p l a c e m e n t o f one or more d i m e t h y l ami no groups by the an i on (A~ from s ay , r eagen t H + A" ) , i t i s l i k e l y t h a t the i n i t i a l a t t a c k o c c u r s at t he oxygen. Subsequent h e a t i n g of the r e s u l t a n t complex no doubt g i ve s r i s e to the observed s u b s t i t u t i o n p r oduc t p r o b a b l y v i a r e a c t i on ( 1 . 1 ) . ( C H 3 ] 2 N \ ( C H 3 > 2 N \ S> S-( C H 3 ) 2 N — P = = 0 + HA 5 p = ± ( CH3) 2 N - > P _ 0 — H A ( C H 3 } 2 N ^ ( C H 3 ) 2 N ( C H 3 ) 2 N . ± ( C H 3 ) 2 N - ^ P 0 H + A 0 ( C H 3 ) 2 N -( C H 3 ) 2 N . ± ( C H 3 ) 2 N - ^ - p = 0 + A 0 H < C H 3 > 2 N \ © t ( C H 3 ) 2 N - ^ - P = 0 + ( C H 3 ) ? N H + A 0 ( C H 3 ) 2 N . ± ( C H 3 ) 2 N - ^ P = 0 + ( C H 3 ) 2 N H - 4 -Normant 1, i n a comprehens i ve r e v i e w o f the p h y s i c a l and chem ica l p r o p e r t i e s o f HMPA, conc l uded t h a t from a l l p o i n t s o f v iew i t i s t he bes t p o l a r a p r o t i c s o l v e n t . An unde r g r adua te s t u d e n t , J . P. M a t a n o v i c h 2 n o t i c e d t h a t a l k a l i me ta l s d i s s o l v e d i n HMPA to y i e l d s t a b l e b l u e r e d u c i n g s o l u t i o n s . Such s o l u t i o n s have s u b s e q u e n t l y been shown t o c o n t a i n e l e c t r o n s i n s o l u t i o n . T h i s t h e s i s e x p l o r e s the p o s s i b l e o c c u r r e n c e of r a d i a t i o n produced s o l v a t e d e l e c t r o n s i n HMPA. B. ALKALI METAL SOLUTIONS 1. L i q u i d Ammonia More than a c e n t u r y ago, i n 1864, W e y l 3 r e p o r t e d t he p r e p a r a t i o n of intense/y c o l o u r e d s o l u t i o n s f rom the d i s s o l u t i o n o f sodium or po t a s s i um i n l i q u i d ammonia. He found t h a t by b o i l i n g away the ammonia he c o u l d r e c o v e r the pure m e t a l s , so the i n t e n s e c o l o u r was a t t r i b u t e d to o r d i n a r y c h e m i c a l compounds such as NaH^ 'N. Some t h i r t y y e a r s l a t e r , an unde r g r adua te s t uden t named H. P. Cady found t h a t the e l e c t r i c a l c o n d u c t i v i t y o f the meta l -ammonia s o l u t i o n s was h i g h e r than t h a t found f o r s o l u t i o n s of s i m p l e s a l t s i n ammonia. I t was Kraus 1* however who was f i r s t to r e a l i z e the t r u e s i g n i f i c a n c e of t h i s r e s u l t . He m e t i c u l o u s l y s t u d i e d the "me ta l ammoniums" ove r a p e r i o d of more than t w e n t y - f i v e y e a r s and showed c o n c l u s i v e l y t h a t d i l u t e s o l u t i o n s c o n s i s t e d p r i n c i p a l l y of ammoniated meta l i on s and - 5 -and ammoniated e l e c t r o n s (1 .2 ) NH ~ * " + + e am " ~" "am ' c am (1.2) Ogg 5 d e s c r i b e d the ammoniated e l e c t r o n as an e l e c t r o n t r a p p e d i n a s o l v e n t c a v i t y formed as a r e s u l t of the s u r r o u n d i n g s o l v e n t d i p o l e s hav ing a l l t h e i r p ro ton s d i r e c t e d i n w a r d . The r e s u l t i n g net p o s i t i v e charge at the c a v i t y c e n t e r a c t s t o s t a b i l i z e the e l e c t r o n . S o l v a t i o n of the metal c a t i o n o c cu r s s i m i l a r i l y , but i n t h a t case the s o l v e n t d i p o l e s are r e v e r s e d - - the n i t r o g e n atoms be ing d i r e c t e d to the c a v i t y c e n t e r . There i s p r o b a b l y l i t t l e tendency f o r a s imp l e NHg an ion to be f o rmed . One can e a s i l y e n v i s a ge e l e c t r o n c a p t u r e by a r o m a t i c m o l e c u l e s because they have l o w - l y i n g u n f i l l e d m o l e c u l a r ir o r b i t a l s a v a i l a b l e w i t h which to s t a b i l i z e the exces s e l e c t r o n . For the NH~ a n i o n , however, the exces s e l e c t r o n must occupy an a n t i b o n d i n g o r b i t a l - -an e n e r g e t i c a l l y u n f a v o r a b l e a r rangement . Now, the c l o s e o r i e n t a t i o n of s e v e r a l ammonia m o l e c u l e s i n the s o l v e n t c a g e , each hav ing the p o s i t i v e end of i t s d i p o l e d i r e c t e d i n w a r d , would l e a d t o mutual r e p u l s i o n s . Such f o r c e s would r e s u l t i n the expans i on of the c a v i t y wh ich s hou l d be accomoanied by a d e c r e a s e i n the d e n s i t y o f the s o l u t i o n . In p o i n t of f a c t , a s a t u r a t e d s o l u t i o n of l i t h i u m i n l i q u i d ammonia has a d e n s i t y of o n l y 0.477 gm/cm 3 a t room t e m p e r a t u r e 6 and i s the l e a s t dense l i q u i d known at t h a t t e m p e r a t u r e . S i n c e l i t h i u m and ammonia have d e n s i t i e s of 0.534 gm/cm 3 a t 20°C and 0.68 gm/cm3 a t -30°C r e s p e c t i v e l y , c a v i t y e xpan s i on appear s e v i d e n t - 6 -i n metal-ammoniurn s o l u t i o n s . L i p s c o m b 7 made c a l c u l a t i o n s which showed the e l e c t r o n c a v i t y i n l i q u i d ammonia be ing about 3.2 A i n r a d i u s or r o u g h l y t h r e e t imes the volume o f ammonia m o l e c u l e s t h e m s e l v e s . I f one assumes c u b i c - c l o s e - p a c k i n g o f s o l v e n t m o l e c u l e s however , t h i s vacancy c o r r e s p o n d s t o the complete removal of o n l y a s i n g l e NH^ m o l e c u l e . In t h i s r e ga rd t h e n , the s o l v a t e d e l e c t r o n i n ammonia r e semb le s an e l e c t r o n t r apped i n a c r y s t a l d e f e c t ( i . e . an F c e n t r e ) . K r a u s 8 has shown t h a t the m o b i l i t y of the ammoniated e l e c t r o n i s about e i g h t t imes t h a t o f a sodium i o n . T h i s obse rved m o b i l i t y i s much s m a l l e r than would be e xpec ted f o r a f r e e e l e c t r o n , but i s s u f f i c i e n t l y h i gh to r e q u i r e an e l e c t r o n -jump mechanism r a t h e r than m i g r a t i o n o f the e n t i r e c a v i t y . In a more r e c e n t c o n d u c t i v i t y s t u d y , A r n o l d and P a t t e r s o n 9 have shown t h a t e l e c t r o n m o b i l i t y i s c o n c e n t r a t i o n dependent and may p a r t l y i n v o l v e an e l e c t r o n t u n n e l l i n g mechanism. I ndeed , the u n c e r t a i n t y p r i n c i p l e r e q u i r e s t h a t the e l e c t r o n not be l o c a l i z e d i n the t r a p but move f r e e l y i n the r e g i o n o f the c a v i t y . C a l c u l a t i o n s 1 0 i n d i c a t e t h a t the e l e c t r o n i n f a c t spends much of i t s t ime o u t s i d e the c a v i t y . Rega rd l e s s of the a l k a l i meta l d i s s o l v e d , the ammonia s o l u -t i o n s e x h i b i t a broad a symmetr i c a b s o r p t i o n i n the i n f r a - r e d . Th i s band, a s s o c i a t e d w i t h the ammoniated e l e c t r o n , has a maximum at 1500 nm and a w i d t h a t h a l f - h e i g h t of about 650 nm. The q u e s t i o n a r i s e s as to whether the band r e s u l t s f rom t r a n s i t i o n between bound e l e c t r o n i c l e v e l s , or s i m p l y a t r a n s i t i o n from the t r a p to a c o n d u c t i o n band. S t a i r s 1 2 r e f i n e d the c a l c u l a t i o n s of - 7 -Lipscomb and conc luded that on ly a s i n g l e bound s t a t e e x i s t e d . ! i Attempts to d e t e c t p h o t o c o n d u c t i v i t y from these s o l u t i o n s were. u n s u c c e s s f u l ? 3 To d a t e , many attempts have been made to d e s c r i b e these systems. Perhaps the most s u c c e s s f u l t h e o r e t i c a l d e s c r i p t i o n o f the o p t i c a l p r o p e r t i e s of a l k a l i metal/ammonia s o l u t i o n s i s tha t formula ted by Cope l and 3 Ke s tne r , and J o r t n e r J ^ K e s t n e r 1 5 extended the c a l c u l a t i o n s to p r e d i c t the l o c a t i o n of the f i r s t e x c i t e d s t a te of the ammoniated e l e c t r o n . The o v e r a l l consensus of op in ion i s r e f l e c t e d in a r ecen t p u b l i c a t i o n on the s u b j e c t ; 1 6 that the ab so rp t i on band a s s o c i a t e d with the ammoniated e l e c t r o n a r i s e s from a s i n g l e (2p-«—Is) e l e c t r o n i c t r a n s i t i o n between bound s t a t e s . A study of the p h o t o e l e c t r o n emiss ion spectrum from s o l v a t e d e l e c t r o n s in ammonia by Delahey et a l . 1 7 adds p r a c t i c a l support to t h i s c o n j e c t u r e . T h e i r ob se rva t i ons were c o n s i s t e n t with a bound-bound e l e c t r o n i c t r a n s i t i o n ( s i m i l a r to the B band f o r F c e n t r e s ) f o l l owed by a u t o - i o n i z a t i o n . In d i l u t e metal/ammonia s o l u t i o n s , the ions i n t e r a c t on ly very weakly with the r e s u l t tha t t h e i r behav ior resembles spec ie s in the gas phase. More concent ra ted s o l u t i o n s take on a b ronze-l i k e l u s t e r , become even le s s dense, and in some cases separate i n to two l i q u i d phases} 0 Poh ler and Thompson 1, 8 by s imply t r e a t i n g the c o n c e n t r a t i o n s o l u t i o n s as l i q u i d me ta l s , were able to q u i t e a c c u r a t e l y p r e d i c t t h e i r e l e c t r i c a l c o n d u c t i v i t y . In f a c t , s o l u t i o n s c o n t a i n i n g more than 9 mol % metal have s i n c e been shown to behave in most re spec t s as l i q u i d m e t a l s ? 9 C l e a r l y then , a d d i t i o n a l spec ie s are present at the h igher c o n c e n t r a t i o n s . - 8 -I t i s known t h a t the t o t a l s p i n paramagnet i sm o f the s o l u t i o n s , dec rea se s marked l y w i t h i n c r e a s i n g metal c o n c e n t r a t i o n ? 0 H u s t l e r 2 1 p roposed an e q u i l i b r i u m between i o n i c s p e c i e s and d i a m a g n e t i c meta l atom d imer s ( 1 . 3 ) . 2M + 2e~ am am M 2 am (1 .3 ) J o l l y and c o - w o r k e r s 1 1 were f i r s t to a t t empt to d i s t i n g u i s h s p e c t r o s c o p i c a l l y e ~ m , M * m , M and M, a m s p e c i e s or e v a l u a t e a III a III a Ml C a m the e q u i l i b r i u m c o n s t a n t s : '1 .2 am e~ . am_ M ' '1 .3 ( D '2 am M am am_ ( i i ) U n f o r t u n a t e l y , they were on l y a b l e to sugges t t h a t the d e v i a t i o n from B e e r ' s Law at 2100 nm e x h i b i t e d by s o l u t i o n s c o n t a i n i n g g r e a t e r than 0.05 M metal i n ammonia a ro se from d imer or po lymer f o r m a t i o n . B e c k e r , L i n d q u i s t and A l d e r 2 2 p roposed a monomeric s p e c i e s M a m, o r . ( M M eIm) c o n s i s t i n g of an a l k a l i meta l c a t i o n a Hi su r rounded by a p p r o x i m a t e l y s i x o r i e n t e d ammonia m o l e c u l e s w i t h an e l e c t r o n c i r c u l a t i n g around the i o n on the p r o t o n s . The d i s c o v e r y o f n u c l e a r h y p e r f i n e s p l i t t i n g by the meta l f rom EPR s t u d i e s of the s o l u t i o n s 2 3 has demons t ra ted u n e q u i v o c a l l y the p re sence of. a monomeric meta l s p e c i e s . However, t h a t work showed t h a t the s p e c i e s was not r e s p o n s i b l e f o r the v i s i b l e a b s o r p t i o n band and t h a t i t s most l i k e l y s t r u c t u r e was t h a t p roposed by Beeker et a l . , ; -A r n o l d and P a t t e r s o n 2 4 s t u d i e d the proposed models c a r e f u l l y ; and found s e r i o u s d i s c r e p a n c i e s between the t h e o r i e s and expe r iment - - p a r t i c u l a r l y . f o r some e l e c t r i c a l and magnet i c p r o p e r t i e s of t he s o l u t i o n s . . I t became apparent , t h a t a t l e a s t two d i f f e r e n t s p e c i e s c o n t a i n i n g p a i r e d e l e c t r o n s were r e q u i r e d . They proposed the a d d i t i o n of an N" a m c e n t r e i n a d d i t i o n to the n e u t r a l M 0 , m c e n t r e : an M, m c e n t r e be ing an e l e c t r o n t r a p p e d 2 am . am • , i n the f i e l d of the monomeric M a m s p e c i e s as d e p i c t e d by r e a c t i o n ( 1 . 4 ) . - " • • '• M a m + e " < » M a m ( o r M~ ) ( 1 .4 ) am am . . am am ' F i n a l l y , G o l d e n , Guttman and T u t t l e 2 5 assembl ed a l 1 the known data and were a b l e t o q u a n t i t a t i v e l y a ccoun t f o r the p r o p e r t i e s of metal ammonium s o l u t i o n s i n terms of the f o l l o w i n g e q u i l i b r i a M'„m < * M* + e " n o\ am. . am am ( 1 . 2 ; Mam + 2 & " ;.« ' Ma"m ' (1 .5) Mam + Mam « M 2 am (1 .6 ) - 10 -2. Amines & E t h e r s 1 . , . 'V P o s s i b l y f o r h i s t o r i c a l r e a s o n s , the major p o r t i o n of the work w i t h a l k a l i meta l s o l u t i o n s has been conduc ted w i t h l i q u i d ammonia. The s o l v a t e d e l e c t r o n i s an e x t r e m e l y r e a c t i v e s p e c i e s .and seems to r e a c t w i t h many s o l v e n t s and t h e i r t r a c e i m p u r i t i e s . ••' However, a number of amines and e t h e r s have been proven c a p a b l e : of f o rm ing s t a b l e s o l u t i o n s of the a c t i v e m e t a l s 2 6 The p r e -r e q u i s i t e s f o r s o l v e n t s u i t a b i l i t y appear to i n c l u d e an i n h e r e n t permanent d i p o l e and a r e s i s t a n c e to ' r e d u c t i o n . Most of the compounds are i n f a c t " b e t t e r " s o l v e n t s f o r a l k a l i m e t a l s than i s l i q u i d ammonia i n so f a r as. ease of h a n d l i n g and s o l u t i o n s t a b i l i t y i s , c o n c e r n e d . These advantages have had . t h e i r p r i c e however as the amine systems have proven to be the more c o m p l i c a t e d . In the amines , even i n the d i l u t e s o l u t i o n , the a l k a l i meta l ... appears to p l a y a more a c t i v e r o l e than s i m p l y be ing the counter - ion to the s o l v a t e d e l e c t r o n . In c o n t r a s t to the a l k a l i meta1/ammonia systems i n wh ich o n l y a s i n g l e broad a s b o r p t i o n i s o b s e r v e d , when amines are used as a s o l v e n t the s o l u t i o n s g e n e r a l l y e x h i b i t e d a t l e a s t t h r e e d i s t i n c t a b s o r p t i o n b a n d s 2 7 ; - - c a l l e d ,: the IR, R, and V bands. The IR band i n the i n f r a - r e d near 1300 nm was n e a r l y i ndependent of the metal used both i n shape and p o s i t i o n . That band was a t t r i b u t e d to t he s o l v a t e d e l e c t r o n . The R band o c c u r r e d a t h i g h e r energy but the p o s i t i o n of the maximum was dependent upon the meta l d i s s o l v e d . The V band, found around 660 nm, was not dependent upon the a l k a l i meta l u sed . Th i s d i s c o v e r y of a second s t a b l e s p e c i e s a p p a r e n t l y i n v o l v i n g - 11 -o n l y e l e c t r o n s and/or s o l v e n t m o l e c u l e s s t i m u l a t e d a g r e a t dea l of r e s e a r c h . A number of s p e c i e s were proposed to account f o r the V band, among them the d i e l e c t r o n f 8 e 2 • However, t h e r e was a d i s c o n c e r t i n g l a c k of agreement among., v a r i o u s groups p e r t a i n i n g to the r e l a t i v e i n t e n s i t y of t he a b s o r p t i o n 2 9 Then, H u r l e y , T u t t l e and G o l d e n 2 7 o f f e r e d a ve r y r a t i o n a l e x p l a n a t i o n . They proposed t h a t the a b s o r p t i o n was s i m p l y the R band of sodium i n the s o l u t i o n . That i s , i n amine s o l u t i o n s c o n t a i n i n g a l k a l i me ta l s o t h e r than sod ium, the sodium R band appeared as a r e s u l t of sodium i on s be ing l e a c h e d from py rex v e s s e l s . They went on to demons t r a te t h a t p o t a s s i u m / e t h y l a m i n e s o l u t i o n s p repa red and c o n t a i n e d s o l e l y i n . q u a r t z appa ra tu s d i d not e x h i b i t the 660 nm band. However, the a d d i t i o n of sodium s a l t s or c o n t a c t w i t h pyrex g l a s s w a r e r e s u l t e d i n the immediate f o r m a t i o n of the V band, c o n f i r m i n g t h e i r s u s p i c i o n s . The p o s i t i o n of the R band maximum i n amine s o l u t i o n was dependent upon the a l k a l i meta l d i s s o l v e d . The s p e c i e s r e s p o n s i b l e t h e n , c a l l i t Xm, was c l e a r l y a " m e t a l l i c " s p e c i e s and was p r o b a b l y ana logous to one of the s p e c i e s (M* , M M~, or g ) p o s t u l a t e d f o r the a l k a l i m e t a l / 1 i q . ammonia s y s tem. Now, t h e r e was e v i d e n c e t h a t the s p e c i e s were d i a m a g n e t i c 2 3 which r u l e s out the a tomic s p e c i e s M' '. The r e l a t i v e i n t e n s i t y of the R band w i t h r e s p e c t to the IR (e~) band tended to d e c r e a s e upon d i l u t i o n . T h i s , a l ong w i t h the f a c t t h a t p h o t o l y s i s of the R band y i e l d e d an i n c r e a s e i n the IR band i n d i c a t e s a . m e t a l l i c s p e c i e s hav ing " e x c e s s " e l e c t r o n s wh ich was p r o b a b l y i n e q u i l i b r i u m w i t h s o l v a t e d e l e c t r o n s and some o t h e r m e t a l l i c i o n . These f a c t s taken t o g e t h e r - 12 -tend to e l i m i n a t e the p o s s i b i 1 i t y of Xm be i ng the p o s i t i v e i o n , DeBecker and D y e 3 0 managed to measure the o s c i l l a t o r s t r e n g t h of the V band t r a n s i t i o n ( i . e . the R band of s od i um) . They found t h a t ces ium s o l u t i o n s i n e t h y l e n e d i a m i n e showed o n l y the IR (e~) band. The a d d i t i o n o f exces s sodium i on s removed the IR band c o m p l e t e l y , r e p l a c i n g i t w i t h the V band. By u s i n g i n t e r m e d i a t e c o n c e n t r a t i o n s of sodium s a l t s , t hey o b t a i n e d both s p e c t r a and c a l c u l a t e d a v a l u e f o r the V band e x t i n c t i o n c o -4 - 1 - 1 e f f i c i e n t of 8.2 X 10 M cm assuming one sodium i on per a b s o r b i n g s p e c i e s . Th i s v a l u e enab led i n t e g r a t i o n o f the V band s p e c t r a and c a l c u l a t i o n of the o s c i l l a t o r s t r e n g t h f o r the t r a n s i t i o n a v a l u e f = 1.9 + 0.2 be ing o b t a i n e d . N o t i c e t h a t i f the s p e c i e s c o n t a i n e d two sodium n u c l e i t h i s v a l u e would have been d o u b l e d . An o s c i l l a t o r s t r e n g t h of 2 r e q u i r e d t h a t a t l e a s t two e q u i v a l e n t e l e c t r o n s be i n v o l v e d i n the . t r a n s i t i o n . These c a l c u l a t i o n s i n d i c a t e r a t h e r c o n v i n c i n g l y t h a t the s p e c i e s (Xm) r e s p o n s i b l e f o r the R bands of sodium i n amine s o l u t i o n i s the metal a n i o n , . N a " . Dye a l s o showed from f a s t k i n e t i c s t u d i e s o f the r e a c t i o n between N a + and e~ i n s e t h y l e n e d i a m i n e t h a t both the decay o f the e l e c t r o n and the growth of the R band were second o r d e r i n e~ , c o n s i s t e n t w i t h the o t h e r c a l c u l a t i o n s . A c t u a l l y , M a n t a l o n , Golden and O t t o l e n g h i 3 1 had e a r l i e r c onc l uded ( a l t h o u g h f rom much l e s s c o n c l u s i v e work) t h a t meta l an ions were r e s p o n s i b l e f o r the R bands. They had n o t i c e d t h a t the Xm s p e c i e s was e a s i l y p h o t o l y z e d and t h a t the R band s p e c t r a - 13 -i t s e l f e x h i b i t e d a marked b lue s h i f t when the s o l u t i o n s were c o o l e d . S i n c e these p r o p e r t i e s are c h a r a c t e r i s t i c of c h a r g e -t r a n s f e r - t o - s o l vent (CTTS) s p e c t r a , Manta lon et a l . d e c i d e d t o . , examine the amine s p e c t r a i n t h i s l i g h t . In CTTS s p e c t r a the s o l v e n t p l a y s an i n t e g r a l p a r t i n d e t e r m i n i n g the t r a n s i t i o n ene rgy . G e n e r a l l y , v i s i b l e and u l t r a v i o l e t a b s o r p t i o n s of m o l e c u l e s i n v o l v e e l e c t r o n i c e x c i t a t i o n s f rom ground to e x c i t e d m o l e c u l a r s t a t e s - - i . e . , i n t r a m o l e c u l a r t r a n s i t i o n s . The e x c i t e d e l e c t r o n remains bound to the m o l e c u l e and i t s a b s o r p t i o n s p e c t r a o f t e n d e t a i l s the v i b r a t i o n a l p r o p e r t i e s of the m o l e c u l e . CTTS s p e c t r a on the o t h e r hand a r i s e from the t r a n s f e r o f an e l e c t r o n f rom some s o l u t e s p e c i e s , X, to a m o l e c u l e of i t s s o l v a t i o n s h e l l , S ( 1 . 7 ) . X ' - S — — » X-S " (1 .7) The ground s t a t e s p e c i e s i s . r e g e n e r a t e d by t he rma l d e g r a d a t i o n , of the e x c i t e d s t a t e . Because the e l e c t r o n i s o n l y weak l y bound to the s o l u t e , CTTS s p e c i e s r e a d i l y undergo a u t o i o n i z a t i o n . The h a l i d e i on s i n wate r are t y p i c a l examples . CTTS s p e c t r a are smooth and s t r u c t u r e l e s s , u s u a l l y o c c u r r i n g i n the u l t r a -v i o l e t . They u s u a l l y obey B e e r ' s Law. Because o f the d i r e c t i n vo l vemen t of s o l v e n t s t r u c t u r e i n the t r a n s i t i o n s , the a b s o r p t i o n maximum, ^ m a x > i s ve ry s e n s i t i v e t o changes i n s o l v e n t t e m p e r a t u r e , p r e s s u r e , and c o m p o s i t i o n . With these c o n s i d e r a t i o n s i n mind t h e n , M a n t a l o n , Golden and O t t o l e n g h i 3 1 c a r e f u l l y s t u d i e d s o l u t i o n s of sodium metal. i n e thy lamine/ammon ia . They c o n c l u d e d t h a t the s p e c i e s Xm was the meta l an ion (Na~ i n t h a t ca se ) and t h a t i t p roduced a t y p i c a l CTTS s pec t r um. As was the case w i t h metal/ammonia s o l u t i o n s , e x t e n s i v e s p e c t r o s c o p i c 3 2 and e l e c t r o n s p i n s t u d i e s 3 3 ' 3 h ' 3 5 of m e t a l / amine s o l u t i o n s l ed to the d i s c r i m i n a t i o n of a t l e a s t s i x d i s t i n c t s p e c i e s . That i s , i n g e n e r a l s o l u t i o n s o f a l k a l i me ta l s i n v o l v e the s t a b i l i t y , i n t e r a c t i o n and r e a c t i v i t y o f the f o l l o w i n g s o l v a t e d s p e c i e s : S, s o l v e n t m o l e c u l e s M s , n e u t r a l metal monomers or atoms M * , meta l c a t i o n s e~, e l e c t r o n s M~, meta l an i on s M 2 s , n e u t r a l metal d imers Of these s p e c i e s , the s o l v a t e d e l e c t r o n i s p a r t i c u l a r l y i n t e r e s t i n g and i m p o r t a n t . 3. Water - The Hydra ted E l e c t r o n The a d d i t i o n o f a l k a l i meta l s to pure wate r l e a d s not to the f o r m a t i o n of s t a b l e s o l u t i o n s but i n s t e a d to r a t h e r v i o l e n t r e a c t i o n . C l a s s i c a l l y , the p roce s s has been d e s c r i b e d i n terms - 15 -of the f o r m a t i o n of hydrogen atoms (1 .8 ) which combine to produce m o l e c u l a r hydrogen ( 1 . 9 ) . M +-H 20 — > M + + OH" + H (1 .8 ) H + H • H 2 ( 1 .9 ) As the s o l v a t e d e l e c t r o n became more and more, e s t a b l i s h e d as a s p e c i e s of major impo r tance i n metal s o l u t i o n s o f ammonia . and the amines , peop le began to s u s p e c t t h a t i t might a l s o p l a y a r o l e i n the aqueous sy s tem. That i s , h y d r a t e d e l e c t r o n s might be formed i n i t i a l l y (1.10) which then go on to produce hydrogen ( 1 . 1 1 ) . M + H 2 0 H 2 0 6 a q Shaede and W a l k e r 3 6 a t tempted to i n t e r r u p t such a p r oce s s by r e a c t i n g sodium w i t h wate r c o n t a i n i n g s cavenge r s o l u t e s t h a t c o u l d d i s t i n g u i s h between h y d r a t e d e l e c t r o n s and hydrogen atoms as i n t e r m e d i a t e s . I n i t i a l e xpe r imen t s w i t h sodium m i r r o r s and water showed t h a t the p re sence of 0.02 m o l e s / l i t r e s cavenge r had l i t t l e e f f e c t upon hydrogen f o r m a t i o n . Thus , e i t h e r the p roce s s was too f a s t to be i n t e r r u p t e d , or e l s e d i d n ' t i n v o l v e the i n t e r m e d i a t e s p ropo sed . They s u b s e q u e n t l y " d i l u t e d " the -> M + e aq aq (1 .10) -> 3gH-0 + OH' (1 .11) - . 1 6. -sodium by making an amalgam w i t h an exces s o f mercu r y . D i l u t i o n made i t p o s s i b l e t o ex tend the r e a c t i o n t ime from l e s s than a second to s e v e r a l hou r s . Shaede.and Walker were t h e n ; ab l e t o scavenge the hydrogen p r e c u r s o r ; Even i n the p re sence of an exces s o f methanol wh ich i s known to e f f i c i e n t l y scavenge hydrogen atoms ( 1 . 1 2 ) , hydrogen f o r m a t i o n c o u l d be p r e v e n t e d . H + CH 30H R ] , 1 2 • CHgOH + H g ^ where k ] ] 2 = 2 X TO 6 M " 1 s e c " 1 ' ? , . ; , • , . However, at low pH where: h yd r a t ed e l e c t r O n s would q u i c k l y be c o n v e r t e d by p ro ton s t o hydrogen atoms ( 1 . 1 3 ) , hydrogen p r o d u c t i o n was u n a f f e c t e d , by the p re sence o f a d d i t i o n a l s o l u t e s . ' ; e ; q + H + • H; ;! (1 .13) That s tudy proved c o n c l u s i v e l y t h a t hydrogen atoms a re not produced d u r i n g the sodium airi.al gam/water . r e a c t i o n and s t r o n g l y i n d i c a t e d t h a t the h yd ra ted e l e c t r o n i s the r e a c t i v e i n t e r m e d i a t e . S i m i l a r i n v e s t i g a t i o n s ; 3 8 i n wh ich r e a c t i o n r a t e r a t i o s t u d i e s were conduc ted , c o r r o b o r a t e d t h a t c o n c l u s i o n . As can be s e e n , s o l v a t e d e l e c t r o n s were becoming a s p e c i e s of i n c r e a s i n g chemica l i m p o r t a n c e . - 17 -C. RADIATION CHEMISTRY AND SOLVATED ELECTRONS When m a t t e r i s s u b j e c t e d to r a d i a t i o n of s u f f i c i e n t l y h i gh ene r g y , some of i t s .molecu les a re i o n i z e d and e l e c t r o n s e j e c t e d . In a l i q u i d , the exces s energy of such e l e c t r o n s i s q u i c k l y l o s t to the su r round ing . - s o l v e n t m o l e c u l e s . However, b e f o r e i t i s t he rma l i zed . ( reduced t o the rma l e n e r g y , kT) an e l e c t r o n may have t r a v e l l e d q u i t e some d i s t a n c e from the i o n i z e d m o l e c u l e (now a p o s i t i v e i o n ) . T h e r e f o r e , the d i s t i n c t p o s s i b i l i t y e x i s t s t h a t such an e l e c t r o n might become s o l v a t e d and thus s t a b i l i z e d i n the medium r a t h e r than r e t u r n i n g t o i t s p a r e n t , i on and be ing i m m e d i a t e l y . n e u t r a l i z e d . A f t e r a l l , a s i d e from i t s tendency f o r n e u t r a l i z a t i o n , the the rma l e l e c t r o n s h o u l d not appear to the s o l v e n t to be ve ry d i f f e r e n t to an e l e c t r o n from a d i s s o l v e d sodium atom. T h e r e f o r e , i n s o l v e n t s where the s p e c i e s a re a l r e a d y known to be s t a b l e one might w e l l e x p e c t to f i n d a r a d i a t i o n produced s o l v a t e d e l e c t r o n . The i m p l i c a t i o n s of such a s p e c i e s on the r e a c t i o n s a s s o c i a t e d w i t h an i r r a d i a t e d medium ( i . e . i t s r a d i a t i o n c h e m i s t r y ) may be enormous. In a d d i t i o n , such phenomena might f a c i l i t a t e d e t a i l e d s t u d i e s o f the s p e c i e s i t s e l f . W i thou t d o u b t , s t u d i e s of water or aqueous systems have dominated the f i e l d of r a d i a t i o n c h e m i s t r y . T h i s i s not s u p r i s i n g when one c o n s i d e r s t h a t wate r is. the most common s o l v e n t ; can e a s i l y be h i g h l y p u r i f i e d ; and makes up the bu l k of b i o l o g i c a l s y s tems . P r i o r t o 1950, r a d i a t i o n chemica l s t u d i e s of aqueous systems - 13 -were a p p a r e n t l y w e l l e x p l a i n e d i n terms of the p r o d u c t i o n and r e a c t i o n s of hydrogen atoms, H, and h y d r o x y l r a d i c a l s , OH. The f o r m a t i o n of t he se r e d u c i n g and o x i d i z i n g s p e c i e s f o l l o w i n g i r r a d i a t i o n was proposed by W e i s s 3 9 t o o c c u r v i a the f o l l o w i n g mechanism: r a d i a t i o n , H2o A A / W v \ • H 2 0 + +• e " ( 1 u ) H 2 0 + + H 2 0 — • H 3 0 + + 0 H ( K 1 5 ) e- + H 2 0 > OH ' + . H ( 1 > 1 6 ) where e - and H 2 0 + were e x t r e m e l y s h o r t l i v e d s p e c i e s ( T < 1 0 - 1 1 s e c ) . Over the next decade , i t became i n c r e a s i n g l y appa ren t t h a t more than one r e d u c i n g s p e c i e s i s p roduced d u r i n g the r a d i o l y s i s of w a t e r . In 1952, S t e i n " 0 found t h a t C 0 2 i n h i b i t e d the b l e a c h i n g ( r e d u c t i o n ) of aqueous methy lene b l u e d u r i n g i r r a d i a t i o n , y e t C 0 2 i s u n r e a c t i v e towards hydrogen atoms. P la tzman had been making t h e o r e t i c a l c a l c u l a t i o n s on wate r based on knowledge of the ammoniated e l e c t r o n i n a l k a l i meta l s o l u t i o n s . At a. c o n f e r e n c e i n 1 953^ 1 he made the r a t h e r p ro found remark: "I t h i n k i r r a d i a t e d water t u r n s b l ue and we j u s t d o n ' t see i t " . His p o s t u l a t i o n was t h a t a s h o r t - l i v e d s o l v a t e d e l e c t r o n a b s o r b i n g i n the red r e g i o n o f the spect rum shou ld e x i s t i n i r r a d i a t e d w a t e r . The g r e a t r e d u c t i o n i n hydrogen y i e l d from i r r a d i a t e d a c i d i f i e d aqueous methanol s o l u t i o n s ob se rved i n the - 19 -presence o f f e r r i c i on by Baxenda le and Hughe s " 2 i n 1958 c o u l d not be s a t i s f a c t o r i l y e x p l a i n e d i n terms of a c o m p e t i t i o n f o r hydrogen atoms, (1 .12) and ( 1 . 1 7 ) . H + CH 30H — • CH 20H + H £ (1 12) H + F e 3 + • F e 2 + + H + (1 .17) R e a c t i o n (1.17) would not be expec ted to be a b l e to compete w i t h r e a c t i o n ( 1 . 1 2 ) . Bu t , i f a h yd ra ted e l e c t r o n were the p r e c u r s o r of H i n a c i d s o l u t i o n ( 1 . 1 3 ) , then f e r r i c i on s might w e l l be e f f i c i e n t l y reduced by such a s p e c i e s ( 1 . 1 8 ) . e a q + H + — H ( 1 > 1 3 ) e " + F e 3 + • F e 2 + e a q h e h e (1 .18) The f o l l o w i n g y e a r , Hayon and We i s s ' * 3 c o n f i r m e d the s u s p i c i o n s by c h e m i c a l l y d i s t i n g u i s h i n g two r e d u c i n g s p e c i e s . They d i d so by s t u d y i n g a system i n which the two r e d u c i n g s p e c i e s r e a c t e d to g i v e d i f f e r e n t o b s e r v a b l e p r o d u c t s . Aqueous s o l u t i o n s o f m o n o c h l o r o a c e t i c a c i d y i e l d e d hydrogen as the p redominant p roduc t i n a c i d s o l u t i o n but a t pH > 4 , c h l o r i d e i on f o r m a t i o n was f a v o u r e d . These r e s u l t s a re c o n s i s t e n t w i t h the f o l l o w i n g mechanism. - 20 -H ? 0 A A / W — • e " + o t h e r p r o d u c t s (1.19) + H + > H (1 .13) e a q + C1CH 2 C0 2 H • C l + CH 2 C0 2 H (1 .20) H + Cl.C'HgCOgH * H 2 + C1CHC0 2H (1 .21) That i s , r a d i o l y s i s l e a d s t o the f o r m a t i o n o f h yd r a t ed e l e c t r o n s (1.19) which r e a c t a t n e u t r a l o r h igh pH w i t h m o n o c h l o r o a c e t i c a c i d to g i v e c h l o r i d e i on s ( 1 . 2 0 ) . As the pH i s l o w e r e d , h yd ra ted e l e c t r o n s are scavenged by p r o t on s p r o d u c i n g the second r e d u c i n g s p e c i e s , hydrogen atoms ( 1 . 1 3 ) , wh ich upon r e a c t i o n w i t h m o n o c h l o r o a c e t i c a c i d produce a d i f f e r e n t p r o d u c t , m o l e c u l a r hydrogen ( 1 . 2 1 ) . These c o n c l u s i o n s were echoed by B a r r and A l l e n " " who s t u d i e d the prob lem by a t o t a l l y d i f f e r e n t app roach . I n s t ead of t r y i n g t o prove the e x i s t e n c e o f two r e d u c i n g s p e c i e s by l o o k i n g f o r d i f f e r e n t p r o d u c t s f rom a s i n g l e r e a c t a n t under d i f f e r e n t c o n d i t i o n s , t hey s t u d i e d the r e a c t i o n of " hydrogen atoms" produced by d i f f e r e n t methods. They found t h a t hydrogen atoms produced from the f r e e r a d i c a l o x i d a t i o n of m o l e c u l a r hydrogen (1 .22) r e a c t e d much more q u i c k l y w i t h oxygen than w i t h hydrogen p e r o x i d e . H 2 + OH * H + H 2 0 (1 .22) - 21 -However, the r a t e c o n s t a n t s f o r the r e a c t i o n of " hyd rogen atoms1.' produced from aqueous i r r a d i a t i o n w i t h oxygen and p e r o x i d e were e s s e n t i a l l y e q u a l . Th i s a g a i n i m p l i e s the p re sence of a r e d u c i n g s p e c i e s o t h e r than the hydrogen atom. P roo f t h a t the second r e d u c i n g s p e c i e s i s i n f a c t the h y d r a t e d e l e c t r o n was n e a r l y comp le te when i n 1 962 C z a p s k i and Schwarz 1 * 5 de te rm ined f rom k i n e t i c i o n i c s t r e n g t h e f f e c t s t h a t the s p e c i e s c a r r i e s -1 u n i t of c h a r g e . F i n a l l y , t h a t same y e a r , when s u f f i c i e n t l y f a s t d e t e c t i o n equipment had become a v a i l a b l e , Hart and Boag 1 * 6 ob se rved a t r a n s i e n t a b s o r b i n g s p e c i e s i n water i m m e d i a t e l y f o l l o w i n g a s h o r t r a d i a t i o n p u l s e . A l t h o u g h the s p e c i e s l i v e d f o r l e s s than 10 y s e c , Har t and Boag de te rm ined i t s v i s i b l e spect rum and i d e n t i f i e d i t as the h yd ra ted e l e c t r o n , e ~ q . S i n c e t h a t t i m e , the h y d r a t e d e l e c t r o n has been the s u b j e c t of c o u n t l e s s i n v e s t i g a t i o n s . D. THE INTERACTION OF HIGH ENERGY RADIATION WITH MATTER In o r de r to p r o p e r l y i n v e s t i g a t e the f o r m a t i o n , p r o p e r t i e s , r e a c t i v i t y and e xac t n a t u r e of r a d i a t i o n produced s o l v a t e d e l e c t r o n s , i t i s i m p e r a t i v e t o unde r s t and someth ing about the way i n which the h igh energy r a d i a t i o n from the s ou r ce s used i n t e r a c t s w i t h m a t t e r . - 22 -1 . Gamma R a d i o l y s i s The i n t e n s i t y , I, o f e l e c t r o m a g n e t i c r a d i a t i o n p a s s i n g th rough ma t te r i s governed by the r e l a t i o n s h i p : where I i s the i n t e n s i t y of the i n c i d e n t r a d i a t i o n , and u i s the t o t a l l i n e a r a b s o r p t i o n c o e f f i c i e n t of the medium of t h i c k n e s s x. For X or y - r a y s , the o v e r a l l l i n e a r a b s o r p t i o n c o e f f i c i e n t i s the sum o f c o n t r i b u t i o n s f rom the f o u r p r i n c i p l e mechanisms whereby energy i s t r a n s f e r r e d to the m e d i u m 4 7 They are the p h o t o e l e c t r i c e f f e c t , the Compton p r o c e s s , p a i r p r o d u c t i o n , and p h o t o n u c l e a r r e a c t i o n s . The r e l a t i v e impo r t ance o f t he se i n t e r a c t i o n s depends p r i m a r i l y upon the energy o f the i n c i d e n t photons and to a l e s s e r e x t e n t upon the a tom ic number, Z, o f the a b s o r b i n g medium. a) The P h o t o e l e c t r i c E f f e c t When a l l the energy of a photon i s c o m p l e t e l y abso rbed by a s i n g l e a tomic e l e c t r o n , the e l e c t r o n i s e j e c t e d f rom the atom w i t h an ene r g y , E , equa l to the d i f f e r e n c e between the photon ene rgy , E Q , and the b i n d i n g ene r g y , E^, of the e l e c t r o n i n the atom ( i v ) . Th i s i s the p h o t o e l e c t r i c e f f e c t . - 23 -E E - E b ( i v ) e o Momentum c o n s e r v a t i o n i s made p o s s i b l e t h rough atom r e c o i l , so t h a t p h o t o e l e c t r i c i n t e r a c t i o n i s not p o s s i b l e w i t h s i n g l e p a r t i c l e s . The a b s o r p t i o n c r o s s s e c t i o n f o r the p h o t o e l e c t r i c e f f e c t , x , i s governed by e q u a t i o n ( v ) . 4 3 tp a zX (• Thus t h i s p roce s s i s i m p o r t a n t o n l y f o r h igh a tom ic number m a t e r i a l s and low photon e n e r g i e s . b) The Compton P roce s s At e n e r g i e s > 0.1 MeV, photons can undergo e l a s t i c c o l l i s i o n s w i t h f r e e or l o o s e l y bound e l e c t r o n s wh ich r e s u l t i n the t r a n s f e r of o n l y p a r t of the i n c i d e n t photon ene rgy . I n t e r a c t i o n o f t h i s t ype i s c a l l e d the Compton p roce s s and i s d e p i c t e d i n F i g u r e 1-2. E o F i g u r e 1-2. The Compton P r o c e s s . - 24 -Upon c o l l i s i o n , the photon i s s c a t t e r e d a t an ang l e 9, w i t h energy E . The e l e c t r o n r e c o i l s w i t h energy E , a t an a n g l e (J>. As a consequence o f the laws o f c o n s e r v a t i o n o f energy and momentum, the a n g u l a r r e l a t i o n s are dependent upon the e n e r g i e s i n v o l v e d . For example , the energy o f the s c a t t e r e d photon i s g i ven by ( v i ) : E • . _ < s : 1 + ( E 0 / m 0 C 2 ) (1 - cos 9) ( v i ) 2 where mQC i s the r e s t mass energy o f the e l e c t r o n . N e g l e c t i n g i t s b i n d i n g ene r g y , the r e c o i l o f Compton e l e c t r o n has e n e r g y , E e » g i v e n by ( v i i ) . E = F - F e b o h s ( v i i ) I t can be seen from e q u a t i o n s ( v i ) and ( v i i ) t h a t the Compton e l e c t r o n w i l l have maximum energy when the photon i s s c a t t e r e d 180° . c) P a i r P r o d u c t i o n and P h o t o n u c l e a r R e a c t i o n s These are h i gh energy p r o ce s s e s wh ich i n t i m a t e l y i n v o l v e the a tomic n u c l e u s ; P a i r p r o d u c t i o n o c c u r s when a h igh energy photon i s a n n i h i l a t e d near a n u c l e u s , accompanied by the concom i t an t p r o d u c t i o n of an e l e c t r o n / p o s i t i o n p a i r . The r e a c t i o n can o n l y o c cu r w i t h photons hav i ng s u f f i c i e n t energy to produce two e l e c t r o n r e s t masses (2m QC = 1.02 MeV). A r e c o i l n u c l e u s i s r equ i r ed , to con se r ve momentum'; E n e r g i e s i n exces s of 10.MeV are u s u a l l y r e q u i r e d . Even more e n e r g e t i c photons a re r e q u i r e d to b r i n g about p h o t o n u c l e a r r e a c t i o n s wh i ch i n v o l v e the e j e c t i o n of neu t ron s o r p ro ton s from atomic n u c l e i . • The r e l a t i v e impor tance o f t he v a r i o u s i n t e r a c t i o n s of e l e c t r o m a g n e t i c r a d i a t i o n w i t h water , i s e a s i l y seen i n F i g u r e 1-3 which shows the t o t a l a b s o r p t i o n c o e f f i c i e n t as a f u n c t i o n of photon ene rgy . I t can be seen t h a t f o r 6 0 G o r a d i o l y s i s ( y - r a y s at 1.17 and .1.33 MeV)* i n t e r a c t i o n ' w i t h wa te r o c c u r s a lmos t e x c l u s i v e l y v i a the Gompton p roce s s wh ich e s s e n t i a l l y , c o n v e r t s , h i gh energy photons to h i gh energy e l e c t r o n s . I t i s thus ne ce s s a r y to c o n s i d e r the consequences o f i n t e r a c t i o n s between these s p e c i e s and m a t t e r . 2. High Energy E l e c t r o n s High speed p a r t i c l e s , p a r t i c u l a r l y charged s p e c i e s , i n t e r a c t much more s t r o n g l y w i t h m a t t e r than does e l e c t r o m a g n e t i c r a d i a t i o n . As a consequence , such p a r t i c l e s have r a t h e r s h o r t , f i n i t e ranges i n an a b s o r b i n g m a t e r i a l . A charged p a r t i c l e p a s s i n g c l o s e t o an atom i s d e c e l e r a t e d by the e l e c t r i c f i e l d of the n u c l e u s . A c c o r d i n g to c l a s s i c a l p h y s i c s , such i n t e r -a c t i o n must be accompanied by the e m i s s i o n o f e l e c t r o m a g n e t i c r a d i a t i o n ( b r e m s s t r a h l u n g ) . The r a t e of energy l o s s , - dE/dx , 2 2 2 i s p r o p o r t i o n a l to e Z / m where e and Z„ are the e l e c t r o n and n n (D F i g u r e 1-3.. A tomic a b s o r p t i o n , c o e f f i c i e n t s f o r wa te r , a) t o t a l a b s o r p t i o n c o e f f i c i e n t ; •"' b) P h o t o e l e c t r i c a b s o r p t i o n c o e f f i c i e n t ; c) Compton c o e f f i c i e n t ( w i t h c o h e r e n t s c a t t e r i n g ) ; d) Compton c o e f f i c i e n t (w i t hou t , cohe ren t s c a t t e r i n g ) ; e) Pa i r -p r o d u c t i o n c o e f f i c i e n t . 1 barn = 10"** c m 1 . n u c l e a r c h a r g e s . . r e s p e c t i v e l y , and -m the e l e c t r o n mass. , B r ems s t r ah l ung e m i s s i o n i s most e f f i c i e n t f o r m a t e r i a l s o f h i gh a tomic number, thus l e a d and t ung s t en a re g e n e r a l l y used i n X - ray p r o d u c t i o n sy s tems. The p roce s s i s n e g l i g i b l e f o r e l e c t r o n e n e r g i e s below 1 0 0 KeV, becomes s i g n i f i c a n t above 1 MeV, and p redomina te s above 1 0 0 MeV. Note t h a t t h i s e l e c t r o -magnet i c r a d i a t i o n c a n . s u b s e q u e n t l y i n t e r a c t f u r t h e r w i t h the medium as d i s c u s s e d p r e v i o u s l y . E l e c t r o n s p o s s e s s i n g e n e r g i e s below 1 MeV. can undergo, cou l omb i c i n t e r a c t i o n s w i t h the e l e c t r o n s o f the medium. Such i n e l a s t i c c o l l i s i o n s 1ead to e x c i t a t i o n and i o n i z a t i o n o f the mo l e cu l e s o f the medium and are u l t i m a t e l y r e s p o n s i b l e f o r the obse rved chem ica l changes i n t h e . s y s t e m . The l i n e a r r a t e o f energy t r a n s f e r ( L E T ) , -dE/dx , f o r an e l e c t r o n hav i ng v e l o c i t y v i s g i ven by the Be the e q u a t i o n ( y i i i ) . . . -dE •=. 2 T r N e 4 Z 2 dx m v o . .2 i n V E - ( 2 x A N B T - 1 + B 2 ) l n 2 2 I 2 ( l - 3 2 ) + 1 - B 2 + 1/8 (1 - 7.1-B 2) ergs cm",-'. ( v i i i ) where, N i s the number of atoms per c u b i c c e n t i m e t e r , e i s the charge oh the e l e c t r o n , ^ Z i s t he a tomic number of the ab so rbe r , , m- i s the r e s t .ma s s o f the e l e c t r o n , E i s the k i n e t i c energy of the e l e c t r o n i n e r g s , - 28 -3 equa l s v./c where c i s the v e l o c i t y o f l i g h t i n vacuo and I i s the average e x c i t a t i o n p o t e n t i a l o f the a b s o r b e r . The energy d e p o s i t e d (dose) i n a sample i r r a d i a t e d by p a r a l l e l beams o f e l e c t r o n s can be c a l c u l a t e d from t h i s e x p r e s s i o n . F i g u r e 1-4 shows the dose d i s t r i b u t i o n as a f u n c t i o n of depth f o r some moderate e l e c t r o n e n e r g i e s 4 8 N o t i c e t h a t e x cep t f o r very sma l l or t h i n samp le s , i r r a d i a t i o n l e ad s on the macro s c a l e to an inhomogeneous d i s t r i b u t i o n o f s p e c i e s . The r e l a t i v e impo r tance of b r e m s s t r a h l u n g i n t e r a c t i o n s and c o l l i s i o n s i n the energy t r a n s f e r scheme i s app rox ima ted by e x p r e s s i on ( i x ) . ( d E / d x ) b r e m ' EZ ( d E / d x ) c o l l 1600 m 0 c 2 ( . x ) 2 where E, Z, m Q , c are as b e f o r e . For e l e c t r o n s i n the 1 - 3 MeV r ange , b r e m s s t r a h l u n g r a d i a t i o n accoun t s f o r l e s s than 5% of the energy l o s s . The average s p e c i f i c i o n i z a t i o n b rought about by gamma rays i s o n l y one t e n t h to one hundredth t h a t caused by a 8 p a r t i c l e ( h i gh energy e l e c t r o n ) of the same ene rgy . T h e r e f o r e , gamma ray s have a much g r e a t e r range i n a g i v e n medium. However, i t i s s t i l l p o s s i b l e to e s t i m a t e the LET f o r e l e c t r o m a g n e t i c r a d i a t i o n from the Bethe e q u a t i o n p r o v i d e d one sums over the spect rum of Compton e l e c t r o n s p roduced . In at tempts to r e l a t e d i f f e r e n t t ypes o f i r r a d i a t i o n s , an average l i n e a r energy t r a n s f e r (LET) i s o f t e n d i s c u s s e d . LET i s o b t a i n e d s i m p l y by d i v i d i n g F i g u r e 1-4. Dose d i s t r i b u t i o n from h igh energy e l e c t r o n s as a f u n c t i o n of depth and ene rgy . - 30 -the i n i t i a l energy of the p a r t i c l e by i t s r ange . I t s hou l d be p o i n t e d out t h a t the LET v a l u e s f o r 3 MeV e l e c t r o n s and 6 0 C o gamma ray s are comparab le . As e l e c t r o n s i n t e r a c t w i t h m a t t e r by the p r o c e s s e s d i s c u s s e d they are c o n s t a n t l y be ing d e f l e c t e d i n random d i r e c t i o n s . The r e s u l t of t h i s s c a t t e r i n g i s t h a t the t o t a l d i s t a n c e t r a v e l l e d by the e l e c t r o n ( i t s path l e n g t h ) w i l l f a r exceed i t s depth of p e n e t r a t i o n ( range ) i n the medium. The d i s t r i b u t i o n o f e l e c t r o n ranges i n the s ub s t ance as e x p r e s s e d by some number/d i s t ance r e l a t i o n s h i p f a l l s o f f f a i r l y l i n e a r i l y and i s not s i m p l y r e f l e c t e d by the dose d i s t r i b u t i o n wh i ch shows a maximum. I t has been s t a t e d t h a t i t i s t he c o u l o m b i c i n t e r a c t i o n s between h igh energy e l e c t r o n s and the e l e c t r o n s o f the medium which g i v e r i s e t o chemica l e f f e c t s . However, t he se i n e l a s t i c c o l l i s i o n s do not g i v e r i s e t o a homogenous d i s t r i b u t i o n of e x c i t e d o r i o n i z e d s p e c i e s , nor even to c o n t i n u o u s c y l i n d e r s of such s p e c i e s . I n s t e a d , the " t r a c k " of an i n c i d e n t p a r t i c l e (be i t a p r i m a r y or Compton e l e c t r o n ) c o n s i s t s o f a s e r i e s of c o l l i s i o n s or " p r i m a r y e v e n t s " o c c u r r i n g at i r r e g u l a r i n t e r v a l s . P r imary i o n i z a t i o n event s l e a d i n g to the p r o d u c t i o n of s econda ry e l e c t r o n s hav ing s u f f i c i e n t energy to escape immediate r e -c o m b i n a t i o n w i t h t h e i r p o s i t i v e p a r en t i o n produce one o f s e v e r a l types o f e n e r g i z e d r e g i o n s . Secondary e l e c t r o n s w i t h e n e r g i e s l e s s than 100 eV have a h igh LET v a l u e and c o n s e q u e n t l y q u i c k l y i n i t i a t e a r a p i d s u c c e s s i o n of t e r t i a r y e x c i t a t i o n s and i o n i z a t i o n s . T h i s l e ad s - 31 -to the f o r m a t i o n o f r o u g h l y s p e r i c a l " s p u r s " c o n t a i n i n g a r e l a t i v e l y sma l l number o f low energy e l e c t r o n s , p o s i t i v e i on s and e x c i t e d " mol e c u l es'. More e n e r g e t i c seconda ry e l e c t r o n s (>100 eV) have s u f f i c i e n t range t o produce t h e i r own s h o r t t r a c k s , c a l l e d 5 - r a y s . F u r t h e r i o n i z a t i o n s and spur f o r m a t i o n can o c cu r and i f the seconda ry e l e c t r o n i s o n l y m o d e r a t e l y e n e r g e t i c (100 - 500 eV) the spur s w i l l o v e r l a p to produce e l o n g a t e d " b l o b s " . H i ghe r energy secondary or 6 - e l e c t r o n s (500 eV - 5 keV) form a t r u e t r a c k of t h e i r own - - a " s h o r t t r a c k " . In wate r f o r example spurs of perhaps 20A d i a m e t e r a re s e p a r a t e d by thousands o f Angstrom u n i t s when produced by a 1 MeV e l e c t r o n . A l s o , as the p r i m a r y e l e c t r o n l o s e s i t s energy i t s LET i n c r e a s e s and p r i m a r y event s beg in t o c o n v e r g e . The p roce s s i s not u n l i k e a t h r e e d i m e n s i o n a l ana logue of the d i s t u r b a n c e p a t t e r n produced when a f l a t s tone i s s k i pped a c r o s s a ca lm wa te r s u r f a c e . 6 0 C o gamma r ay s g i v e r i s e t o s econda r y e l e c t r o n s hav ing a mean energy ' o f about 440 k e V 4 9 T h i s energy i s m a i n l y d i s s i p a t e d i n the form o f spurs (^64%) w i t h t h e ' r ema i n i n g energy d e p o s i t e d as s h o r t t r a c k s (^25%) and b l ob s (^11%). F u r t h e r , i t may be s t a t e d t h a t , i n g e n e r a l , energy d e p o s i t e d by i o n i z i n g r a d i a t i o n seems to be about e q u a l l y d i v i d e d between i o n i c and e x c i t e d s p e c i e s . Thus i t can be seen t h a t the p h y s i c a l e f f e c t of r a d i a t i o n on m a t t e r i s to produce a g r o s s l y inhomogenous d i s t r i b u t i o n of s p e c i e s on both the m i c ro and macro s c a l e s . The subsequent r e a c t i o n s and i n t e r a c t i o n s o f these s p e c i e s must u l t i m a t e l y govern the obse rved chemica l changes. E. THE CHEMICAL CONSEQUENCES OF RADIATION ABSORPTION Upon s u b j e c t i o n to i o n i z i n g r a d i a t i o n , m a t t e r undergoes changes which' a re be s t d e s c r i b e d i n terms o f t h r e e s t a g e s . The P h y s i c a l s t age encompasses t h a t p e r i o d of t ime ( u s u a l l y l e s s than 1 0 " ^ sec ) d u r i n g which r a d i a n t energy i s d i s s i p a t e d i n the s y s tem. The next s t a g e , l a s t i n g f o r perhaps 1 0 - ^ s e c , i s c a l l e d the P h y s i c o c h e m i c a l s t a g e . E x c i t e d s t a t e and p h o t o -p h y s i c a l r e a c t i o n s , geminate i o n r e c o m b i n a t i o n s and p r o ton t r a n s f e r or f r a g m e n t a t i o n r e a c t i o n s o c cu r d u r i n g t h i s s t age as the system a t t a i n s therma l e q u i l i b r i u m . F i n a l l y , s u r v i v i n g . ... r e a c t i v e i n t e r m e d i a t e s p e c i e s beg in to d i f f u s e i n t o the bu l k of the medium where the decay or r e a c t t h rough normal chem ica l r e a c t i o n s . T h i s , the Chemica l s t a g e , o c c u r s d u r i n g a p e r i o d o f 10" sec or l o n g e r u n t i l such t ime as c hem i ca l e q u i l i b r i u m i s r e - e s t a b l i shed. When the r a d i o l y s i s of a sub s tance r e s u l t s i n the f o r m a t i o n of some measurab le p r o d u c t , the r a d i a t i o n , c h e m i s t d i s cu s se s , the p roce s s i n terms of a r a d i o l y t i c y i e l d . For c o n v e n i e n c e , t he y i e l d or "G v a l u e " of such a p roduc t i s d e f i n e d as the number of mo l e cu l e s changed or produced f o r each 100 "eV of r a d i a n t energy absorbed by the medium. T h i s v a l u e was a r b i t r a r i l y chosen so as to g i ve t y p i c a l G v a l u e s near u n i t y . R e c a l l however, t h a t t h i s energy i s about t h a t g e n e r a l l y a s s o c i a t e d w i t h the average - 33 -spu r . Du r i ng the e a r l y days of r a d i a t i o n c h e m i s t r y , i o n n e u t r a l i z a t i o n i n condensed media was c o n s i d e r e d so r a p i d t h a t o n l y r a d i c a l s p e c i e s were c o n s i d e r e d l ong enough l i v e d to be c apab l e of i n i t i a t i n g c hem i ca l r e a c t i o n . In a t h e o r e t i c a l t r e a t m e n t of aqueous systems f o r example , Samuel and M a g e e 5 0 accounted f o r the obse rved p r oduc t y i e l d s s o l e l y i n terms o f the d i f f u s i o n k i n e t i c s of H and OH r a d i c a l s . M o l e c u l a r p r oduc t y i e l d s , i n g e n e r a l , were: found to be h i g h l y dependent upon the LET p r o p e r t i e s of the i o n i z i n g r a d i a t i o n . Th i s was c o n s i d e r e d to be a consequence of the inhomogenous d i s t r i b u t i o n o f the r e a c t i v e s p e c i e s . For example , m o l e c u l a r hydrogen i s thought t o be formed i n i r r a d i a t e d water when d i f f u s i o n a l l o w s spur s t o o v e r l a p e n a b l i n g i n t e r s p u r c o m b i n a t i o n o f hydrogen atoms. As d i s c u s s e d e a r l i e r , 6 0 C o y - i r r a d i a t i o n produces a s e r i e s of w e l l i s o l a t e d spurs i n w a t e r . Heavy he l i um n u c l e i (a p a r t i c l e s ) on the o t h e r hand i n t e r a c t s t r o n g l y w i t h m a t t e r - - i . e . have a h i gh LET v a l ue and produce a s h o r t s e r i e s o f o v e r l a p p i n g spur s e q u i v a l e n t to a c y l i n d e r of e x c i t e d and i o n i z e d s p e c i e s . Thus , as one might e x p e c t , i n w a t e r , the m o l e c u l a r hydrogen y i e l d from y - r a d i o l y s i s GCHg) = 0 . 4 2 i s much l e s s than t h a t f rom a - r a d i o l y s i s where G ( H j = 1 . 57 ? 1  v 2 a - 34 -F. SOLVATED ELECTRONS 1. Fo rmat i on As more da ta became a v a i l a b l e i t was becoming i n c r e a s i n g l y obv ious to the t h e o r e t i c i a n s t h a t r a d i c a l d i f f u s i o n k i n e t i c c o n s i d e r a t i o n s a l one were not s u f f i c i e n t to d e s c r i b e the o b s e r v a t i o n s . The p re sence o f i o n i c and e l e c t r o n s cavenge r s had s i g n i f i c a n t e f f e c t s on p r o d u c t y i e l d s so t he f a t e of t he e l e c t r o n s on t h e r m a l i z a t i o n had to be r e c o n s i d e r e d . F u r t h e r -more, s cavenge r da ta p o i n t e d to e l e c t r o n s p e c i e s hav ing l i f e -t imes c o n s i d e r a b l y l o n g e r than one would a s s o c i a t e w i t h f r e e e l e c t r o n s . I n d i c a t i o n s were t h a t e l e c t r o n s were be ing s t a b i l i z e d or " s o l v a t e d " i n l i q u i d med i a . Normal an ions become s o l v a t e d i n p o l a r l i q u i d s by a t t r a c t i n g the p o s i t i v e ends of nearby s o l v e n t m o l e c u l e s t h e r e b y , f o rm ing a s o l v e n t s h e a t h . G e n e r a l l y , t h i s f i r s t l a y e r o f s o l v e n t m o l e c u l e s i s t i g h t l y bound to the i on and moves w i t h i t t h rough the s o l u t i o n t h e r e b y d e c r e a s i n g the i o n m o b i l i t y . On the o t h e r hand, an e l e c t r o n can move much more r a p i d l y than b u l k y s o l v e n t m o l e c u l e s . Du r i ng the t ime r e q u i r e d f o r the s o l v e n t d i p o l e s t o r o t a t e and p o i n t towards the e l e c t r o n ( ^ 1 0 - 1 1 s e c , the d i p o l a r r e l a x a t i o n t ime ) the e l e c t r o n moves.on. As a r e s u l t , the s o l v e n t d i p o l e s can o n l y o r i e n t t hemse l ve s towards the average p o s i t i o n o f the e l e c t r o n . T h i s however g i v e s us a c l u e as to a p o s s i b l e mechanism which c o u l d l e a d to e l e c t r o n s t a b i l i z a t i o n . F i g u r e 1-5 i l l u s t r a t e s the p r o c e s s . In (a) F i g u r e 1-5. A p o s s i b l e mechanism f o r e l e c t r o n s o l v a t i o n , i n p o l a r l i q u i d s . The scheme i s o u t l i n e d i n the t e x t . - 36 -random therma l mot ion o f p o l a r s o l v e n t m o l e c u l e s has c r e a t e d a r e g i o n of a c c i d e n t a l p o l a r i z a t i o n . A nearby the rma l e l e c t r o n i s a t t r a c t e d to t h i s r e g i o n and thus spends a g r e a t e r p r o -p o r t i o n of i t s t ime i n i t s v i c i n i t y . In (b) the p o s i t i v e ends of s o l v e n t d i p o l e s i n the r e g i o n are now a t t r a c t e d by the e l e c t r o n and the p o l a r i z a t i o n i s enhanced. Through t h i s f e e d -back p r oce s s the mutual a t t r a c t i o n i n c r e a s e s u n t i l a f u l l y formed t r a p (c ) i s c a p a b l e o f s t a b i l i z i n g the e l e c t r o n . T h i s p roce s s of p r o d u c i n g a s o l v a t e d e l e c t r o n c o u l d o c c u r i n about 1 0 " ^ s e c . N o t i c e t h a t as the s o l v e n t d i p o l e s beg in to a l i g n themse l ve s w i t h t h e i r p o s i t i v e ends a l l p o i n t i n g i n w a r d s , they w i l l m u t u a l l y r e p e l one a n o t h e r t o c r e a t e an e n l a r g e d c a v i t y devo i d o f s o l v e n t m o l e c u l e s . A l s o , i t i s i n t e r e s t i n g t o t h i n k about the s o l v e n t c a v i t y t h a t would remain i f the e l e c t r o n were sudden l y c o m p l e t e l y removed. The r e m a i n i n g p o s i t i v e " h o l e " might i n f a c t not be ve r y d i f f e r e n t e n e r g e t i c a l l y f rom the Is o r b i t a l o f a p r o t o n . From t h i s v i e w , i t i s perhaps u n d e r s t a n d a b l e t h a t s o l v a t e d e l e c t r o n s behave ve r y much l i k e , and w e r e . 1 o n g ' m i s t a k e n f o r , hydrogen atoms. I ndeed , as Ha r t commented; ' 2 "Our u n d e r s t a n d i n g o f the chem ica l r e a c t i o n s o f the s o l v a t e d e l e c t r o n w i l l be much improved i f we t h i n k o f i t as an e lement hav ing n u c l e a r mass 0, a tom ic number 0 and e l e c t r i c charge - 1 " . Ano the r way i n which the p roce s s o f s o l v a t i n g a t he rma l e l e c t r o n c o u l d be i n i t i a t e d i s as f o l l o w s . The p re sence o f an e l e c t r o n c o u l d q u i c k l y i nduce e l e c t r o n i c p o l a r i z a t i o n i n nearby s o l v e n t m o l e c u l e s . The s h a l l o w t r a p thus produced might w e l l - 37 -se rve to s t a b i l i z e the e l e c t r o n long enough to a l l o w d i p o l a r r e l a x a t i o n ( i . e . r o t a t i o n o f the permanent d i p o l e s ) o f the media . 1ead ing to t r u e s o l v a t i o n . The e l e c t r o n , r e t a r d e d as a r e s u l t o f the p o l a r i z a b i 1 i t y of the medium, (we might c a l l i t a " d r y " e l e c t r o n ) may w e l l i t s e l f i n i t i a t e the s o l v a t i o n p roce s s or " d i g i t s own g r a v e " . In o r d e r t o demons t r a te t h a t e l e c t r o n s o l v a t i o n was indeed e n e r g e t i c a l l y f a v o u r a b l e , B a x e n d a l e 5 3 c a l c u l a t e d i t s h y d r a t i o n energy from the c y c l e g i v e n i n F i g u r e 1-6. U s i ng a l l energy v a l u e s f o r the r e a c t i o n s g i v e n i n k c a l / m o l e he o b t a i n e d a f r e e energy of -38 .2 k c a l / m o l e (1.66 eV) f o r the p r o c e s s . 6 ( a q ) + H ( a q ) -AG 260. 5 e ( g ) + H ( g ) -63.9 •314.0 -> H (g) •48.6 0.0 • F i g u r e 1-6. D e t e r m i n a t i o n o f the f r e e energy o f s o l v a t i o n ( A G s ) f o r the h yd ra ted e l e c t r o n . A l l f r e e energy v a l u e s are i n k c a l / m o l e . S o l v a t e d e l e c t r o n s e x h i b i t i n t e n s e a b s o r p t i o n s p e c t r a . The a b s o r p t i o n u s u a l l y t ake s the form o f a s i n g l e broad a s y m e t r i c and s t r u c t u r e l e s s band i n the v i s i b l e o r near i n f r a - r e d r e g i o n - ' - 38 - • of t he s pec t r um. The p o s i t i o n o f the band maximum i s i dependent on the medium (env i r onment ) but a p p a r e n t l y i ndependent of the phase of a g i v en medium. S o l u t i o n s o f s o l v a t e d e l e c t r o n s a re h i g h l y c o n d u c t i v e , i n d i c a t i n g the p re sence of i o n i c s p e c i e s . S t u d i e s i n d i c a t e an e l e c t r o n m o b i l i t y h i g h e r than t h a t of s imp l e an i on s but much l ower than t h a t g e n e r a l l y . a s s o c i a t e d w i t h s o - c a l l e d " f r e e " e l e c t r o n s . Minday and c o - w o r k e r s 5 4 c onc l uded t h a t even i n n o n - p o l a r hyd roca rbon s , e l e c t r o n t r a n s p o r t appeared to i n v o l v e s h o r t - l i v e d t r a p s i n ' f l u i d . F u r t h e r , they sugges t t h a t the t r a p s must be a c o l l e c t i v e p r o p e r t y of the f l u i d r a t h e r than be i ng a s s o c i a t e d w i t h i n d i v i d u a ! m o l e c u l e s . The pa ramagnet i c na tu re o f s t a b i l i z e d e l e c t r o n s : tends to c o r r o b o r a t e t h e s e c o n c l u s i o n s . S o l v a t e d e l e c t r o n s e x h i b i t e l e c t r o n s p i n re sonance s p e c t r a c h a r a c t e r i z e d by a s i n g l e narrow l i n e w i t h a g y romagnet i c f a c t o r g = 2 . 0 0 2 2 , v e r y near t h a t of the f r e e e l e c t r o n , = 2 .0023. The absence of any p r o t on h y p e r f i n e s p l i t t i n g and the l i n e sha rpnes s i n d i c a t e t h a t t h e r e i s no s t r o n g magnet i c i n t e r a c t i o n between the e l e c t r o n and the s o l v e n t m o l e c u l e s of an o r de red s o l v a t i o n s h e a t h . That i s , the s o l v e n t m o l e c u l e f o rm ing the c a v i t y must be ve r y l o o s e l y bound, and the o v e r l a p o f e l e c t r o n d e n s i t y w i t h s p e c i f i c p r o t o n s ve ry weak. Wh i l e r a d i o l y s i s t e c h n i q u e s o f f e r a d i r e c t method f o r g e n e r a t i n g s o l v a t e d e l e c t r o n s , the p r o d u c t i o n o f t h i s s p e c i e s i n many o t h e r ways has been c o n f i r m e d . S o l v a t e d e l e c t r o n f o r m a t i o n upon the d i s s o l u t i o n of a l k a l i me ta l s has a l r e a d y been d i s c u s s e d i n d e t a i l . W a l k e r 5 5 has p r e s en ted e v i d e n c e to -. 39 -suggest t h a t the s p e c i e s i s a l s o an i n t e r m e d i a t e i n the 3+ r e d u c t i o n o f wate r by U s a l t s ( 1 . 2 3 ) . u 3 + — + U 4 + + P -U U a q . e a q (1 .23) P h o t o i o n i z a t i o n of a number o f aqueous i on s has been a c h i e v e d by c o n v e n t i o n a l f l a s h p h o t o l y s i s t e c h n i q u e s . Such r e a c t i o n o f t e n l e ad s t o the p r o d u c t i o n of the h yd r a ted e l e c t r o n . Those i on s wh ich e x h i b i t c h a r g e - t r a n s f e r - t o - s o l vent s p e c t r a ( the i o d i d e i on f o r example) seem p a r t i c u l a r l y s u i t a b l e f o r such r e a c t i on (1 . 24) . '••:-Ki. " hv ! a q ~ ~ ~ * 1 + e a q . (1 .24) A number o f s t u d i e s have shown t h a t s o l v a t e d e l e c t r o n s can be p repa red e l e c t r o l y t i c a l l y . In an e a r l y s t u d y , W a l k e r 5 6 l o o k e d f o r and found e v i d e n c e f o r . e ~ . f r o m the e l e c t r o l y s i s of w a t e r . A l s o , i t may w e l l be t h a t the h yd ra ted e l e c t r o n i s an i m p o r t a n t i n t e r m e d i a t e of many pho tochem i ca l r e a c t i o n s ( p h o t o s y n t h e s i s f o r example) i n,:whi'ch.- i t has not been c o n s i d e r e d . Rega rd l e s s of the method whereby they were p r o d u c e d , the s o l u t i o n s c o n t a i n i n g s o l v a t e d e l e c t r o n s e x h i b i t e d i d e n t i c a l s p e c t r a . F u r t h e r , many of the r e a c t i o n s of the s p e c i e s were., a l s o i ndependent o f i t s o r i g i n . These f a c t s sugges t t h a t the s o l v a t i o n mechanism does not i n v o l v e the c o u n t e r - i o n to any s i g n i f i c a n t e x t e n t . T h i s i s a ve r y i m p o r t a n t f i n d i n g , f o r i t - 40 -a l l o w s one to s tudy s o l v a t e d e l e c t r o n s by w h i c h e v e r t e c h n i q u e i s c o n v e n i e n t and s t i l l be a b l e t o . r e l a t e the r e s u l t s w i t h r e s e a r c h per fo rmed by o t h e r means/ .-..•'':'.''''.;•;.'.»'•.. > 2. T h e o r e t i c a l Models ; . ' ' '' " ] ' :"'' ' Many a t tempt s have been made to q u a n t i t a t i v e l y account ' f o r the obse rved chem i ca l and p h y s i c a l p r o p e r t i e s of s o l v a t e d e l e c t r o n s . P a r t i c u l a r a t t e n t i o n has been pa i d . to. t h e o r e t i c a l models which c o u l d a c c o u n t , f o r the shape and p o s i t i o n o f t h e i r a b s o r p t i o n s p e c t r a . Ogg57•••'.proposed t h a t t he ammoniated e l e c t r o n be c o n s i d e r e d t r a p p e d i n a s p h e r i c a l c a v i t y , t he e l e c t r o n was taken to be c o n f i n e d by an i n f i n i t e l y s t eep p o t e n t i a l w e l l p roduced by the e l e c t r i c p o l a r i z a t i o n of the s u r r o u n d i n g s o l v e n t m o l e c u l e s . A s i m p l e e l e c t r o n - i n - a - b o x c a l c u l a t i o n gave v a l u e s r o u g h l y a p p r o x i m a t i n g e x p e r i m e n t e r d a t a . J o r t n e r ! 8 3 v b a p p l i e d the p o l a r o n i d e a o f L a n d a u 5 8 0 and P e k a r 5 e d to e~ and c a l c u l a t e d the energy by means o f a d i e l e c t r i c -cont inuum model i n wh ich l o n g - r a n g e i n t e r a c t i o n s were c o n s i d e r e d That i s , the p o l a r i z a t i o n p o t e n t i a l on the e l e c t r o n was t r e a t e d as c o n s t a n t w i t h i n the c a v i t y . .(zero as the f i r s t a p p r o x i m a t i o n ) , and as cou l omb i c o u t s i d e . As a consequence o f the energy l e v e l s o f t h a t model be i ng de te rm ined by l o h g - r a n g e i n t e r a c t i o n s the l o w e s t o b s e r v a b l e t r a n s i t i o n was r e l a t e d to the q u a n t i t y r ( 1 /D Q p - 1/D s) - - where D Qp and D $ are the h i gh f r e q u e n c y and s t a t i c d i e l e c t r i c c o n s t a n t s r e s p e c t i v e l y . The p o t e n t i a l energy f u n c t i o n as a f u n c t i o n o f d i s t a n c e r , P ( r ) , f o r a s o l v a t e d - 41 -e l e c t r o n i n a c a v i t y o f r a d i u s R was taken a s : P ( r ) = - 6e 2 /R f o r ' r < R = - B e 2 / r f o r r > R (x) where B = ( 1 / D ' - 1 / D S ) and e i s the u n i t e l e c t r i c charge. C o n s i d e r i n g an e l e c t r o n i c t r a n s i t i o n i n v o l v i n g h y d r o g e n i c -l i k e Is and 2p s t a t e s , J o r t n e r was ab l e to match e x p e r i m e n t a l v a l ue s of the a b s o r p t i o n band maxima f o r many s o l v e n t s . However, s i n c e the c a v i t y r a d i u s , R, was an a d j u s t a b l e pa ramete r i n h i s c a l c u l a t i o n s , J o r t n e r 1 s. model was s t i l l s e m i - e m p i r i c a l a t b e s t . A major f l a w o f the e a r l y t h e o r e t i c a l models i s t h a t they p r o v i d e d no i n f o r m a t i o n about - - i n f a c t i g n o r e d — s h o r t - r a n g e i n t e r a c t i o n s . C l e a r l y , the e l e c t r o s t a t i c p o t e n t i a l g r a d i e n t i n the immediate v i c i n i t y of the s o l v a t e d e l e c t r o n would be l a r g e . F u e k i 5 9 a t tempted to account f o r the s h o r t - r a n g e i n t e r a c t i o n s i n terms o f d i e l e c t r i c c o n s t a n t , D, th rough the use of e x p r e s s i o n ( x i ) i n h i s c a l c u l a t i o n s . D D op f o r 0 < r < r = ( r / r 2 ) D s f o r r, < r < r f o r r 0 < r ( x i ) - 42 -where r-j = ( D 0 D / D s ) r 2 a n c ' r 2 w a s t a k e n *P b e 9 A -J o r t n e r , Cope land and K e s t n e r 6 0 p r e s e n t e d a s em i - c on t i nuum d i e l e c t r i c model u t i l i z i n g an even more comple te p o t e n t i a l e x p r e s s i o n . E l e c t r o n s t a b a l i z a t i o n was s u b d i v i d e d i n t o s h o r t -range a t t r a c t i o n s , l o n g - r a n g e p o l a r i z a t i o n s and s h o r t - r a n g e r e p u l s i o n s . Those i n t e r a c t i o n s were e x p r e s s e d i n terms o f v a r i o u s s p e c i f i c d i s t a n c e s a s s o c i a t e d w i t h the c a v i t y t h a t can be seen i n F i g u r e 1-7 which d e p i c t s the p h y s i c a l i m p l i c a t i o n s of the mode l . F i g u r e 1-7. D e f i n i t i o n o f c a v i t y d i s t a n c e s i n J o r t n e r 1 s s e m i -cont inuum model f o r s o l v a t e d e l e c t r o n s . The d i s t a n c e s u t i l i z e d i n the model are namely: - 43 -r , the a c t u a l v o i d r a d i u s o f the c a v i t y r ,' the e f f e c t i v e s o l v e n t m o l e c u l e r a d i u s a , the e f f e c t i v e hard co re o f the m o l e c u l e s r^, the d i s t a n c e o f the s o l v e n t m o l e c u l e s f rom the c a v i t y c e n t e r r , the d i s t a n c e from the c a v i t y c e n t r e to the b e g i n n i n g o f the cont inuum and R, the normal or mean c a v i t y r a d i u s J o r t n e r et a l . showed t h a t the e l e c t r o n / m e d i u m i n t e r a c t i o n p o t e n t i a l , P ( r ) , f o r t h i s model t ake s the form of e q u a t i o n (x i i ) . P ( r ) = - N y e / r 2 -'3e 2 / r f o r 0 < r < R = - Nue/rjj - B e 2 / r + V Q f o r R < r . < T d 2 - Be / r + V Q f o r r^ < r ( x i i ) where e and 3 are as d e f i n e d f o r e q u a t i o n ( x ) . N i s number o f n e a r e s t m o l e c u l e s of the e l e c t r o n , V Q i s t he energy of the q u a s i f r e e ( c o n d u c t i o n ) e l e c t r o n s t a t e and y i s the e f f e c t i v e d i p o l e moment o f the n e a r e s t m o l e c u l e s . The e f f e c t i v e d i p o l e term y t ake s i n t o account the s p e c i f i c o r i e n t a t i o n of the a c t u a l d i p o l e moment v e c t o r , ' u , w i t h r e s p e c t to the r a d i u s v e c t o r , - 44 -u = u,<cose> / • • • \ o ( x m ) The important feature of this detailed inclusion of short-range interactions and orientations in the model is that the. configuration stability of the energy states can be established so that a unique cavity radius corresponding to a minimum in the total energy of the system can be calculated. Thus, the semi-continuum model is not plagued by the necessity of the earlier models to include an empirical cavity radius parameter and is thereby inherently more useful for predicting physical properties. For example, Jortner showed that for the ammoniated electron system, the model correctly predicts volume expansion, co-ordination number, heat of solution and many spectroscopic properties. To date, additional improvements and approaches to solvation models have been made61 which when taken in.total enable one to more readily think of solvated electrons in terms of ordinary, discrete chemical species. 3. Reacti ons In early steady-state 6 0Co y-radiolysis studies, measurements of product yields from solute mixtures enabled rate constant ratios for electron reactions to be determined. With the advent in the early '60's of pulse radiolysis and fast kinetic analysis equipment i t became possible to monitor the electrons (and other reactive intermediates) to determine reaction rates - 45 -d i r e c t l y . For example , i n 1967, Anbar and N e t a 3 7 c o m p i l e d a l i s t c o n t a i n i n g p u b l i s h e d r a t e c o n s t a n t s f o r r e a c t i o n between hyd ra ted e l e c t r o n s and more than 600 s o l u t e s . S e v e r a l e x p e r i m e n t e r s n o t i c e d t h a t the p re sence of l a r g e c o n c e n t r a t i o n s o f e l e c t r o n s cavenge r s i n wate r r e s u l t e d i n a dec rea sed m o l e c u l a r hydrogen y i e l d upon r a d i o l y s i s . The D r agan i c s f o r e x a m p l e 6 2 obse rved t h i s e f f e c t whether t hey 2 + used a c a t i o n (Cu ). an a n i o n . (NOg) o r n e u t r a l s p e c i e s ( H 2 0 2 , a ce tone ) as e l e c t r o n s c a venge r . Th i s e f f e c t has been a t t r i b u t e d to the i n t e r r u p t i o n of a n e u t r a l i z a t i o n r e a c t i o n which o r d i n a r i l y l e ad s u l t i m a t e l y to hydrogen atom f o r m a t i o n (1 .25 ) . ea"q + H2°aq ( o r H 3 ° a V — > H + 0 H ( o r ^ O ) . . " ( 1 . 2 5 ) I ndeed, t h e r e has even been e v i dence to suggest t h a t the i n t e r m e d i a t e i o n i c s p e c i e s c o u l d be scavenged p r i o r to s o l v a t i o n H a m i l l 6 3 c l a i m e d t h a t the " d r y h o l e " i n water ( H 2 0 + ) c o u l d be scavenged by ha logen an i on s at h igh c o n c e n t r a t i o n ( 1 . 2 6 ) . . H 2 0 + • X - — • H 2 0 • X ( 1 . Now, s i n c e the " d r y h o l e " i s thought to r e a c t r a p i d l y w i t h a nearby s o l v e n t m o l e c u l e t o produce a h y d r d x y l r a d i c a l (1 .18) i t i s p o s s i b l e t h a t r e a c t i o n of the an i on w i t h t h a t r a d i c a l (1 .27) c o u l d e a s i l y be m i s t a ken f o r ho le s c a v e n g i n g . - 46 -OH + X" • 0H"+ X (1 .27 ) However, c h l o r i d e i on cannot be o x i d i z e d by OH i n n e u t r a l s o l u t i o n so a d i s t i n c t i o n was. e a s i l y made and t h a t p o s s i b i l i t y e x c l u d e d . S ugge s t i on s have a l s o been made t h a t " d r y " e l e c t r o n s can a l s o be scavenged. I t c o u l d not be shown c o n c l u s i v e l y however whether the e f f e c t s were not s i m p l y a consequence o f the t ime dependent r a t e c o n s t a n t s ( inhomogenous k i n e t i c s ) p o s s i b l e at l a r g e s o l ute c o n c e n t r a t i o n s 6 "* In any event i t i s appa ren t t h a t charge n e u t r a l i z a t i o n must be a major s ou rce of e x c i t e d and f ragmented s p e c i e s d u r i n g r a d i o l y s i s . S c h u l e r e t a l 6 5 f o r i n s t a n c e have e s t i m a t e d t h a t about 701 o f the m o l e c u l a r hydrogen from c y c l o h e x a n e r a d i o l y s i s a r i s e s i n t h a t way. 4. Y i e l d I t became r e a d i l y appa ren t t h a t the y i e l d s o f f r e e i on s ( i . e . those i on s e s c a p i n g geminate r e c o m b i n a t i o n ) were much l a r g e r f rom tho se s o l v e n t s p o s s e s s i n g a h i gh d i e l e c t r i c c o n s t a n t or permanent d i p o l e moment. Wh i l e h i g h l y p o l a r m o l e c u l e s might be expec ted to s o l v a t e an e l e c t r o n more s t r o n g l y t h a n ' weak ly p o l a r m o l e c u l e s , t h e r e i s no a s p e c t of the s o l v a t i o n p roce s s i t s e l f as d i s c u s s e d t h a t c o u l d account f o r a h i g h e r y i e l d of s o l v a t e d e l e c t r o n s . A l s o , r e c a l l t h a t the i n i t i a l i o n i z a t i o n y i e l d depends o n l y on the e l e c t r o n d e n s i t y o f the medium as a w h o l e , not on the p r o p e r t i e s o f i n d i v i d u a l m o l e c u l e s . C l e a r l y t h e n , the e f f e c t o f m o l e c u l a r p o l a r i z a t i o n (o r - 47 -p o l a r i z a b i 1 i t y ) must be t o i n c r e a s e t h e p r o b a b i 1 i t y f o r an i o n i z e d e l e c t r o n t o e s c a p e g e m i n a t e r e c o m b i n a t i o n a n d t h e r e b y s u r v i v e l o n g e n o u g h t o be s o l v a t e d . The e j e c t e d e l e c t r o n i s , on t h e r m a l i z a t i o n , s u b j e c t t o m o t i o n o f two t y p e s . . F i r s t l y , due t o c o u l o m b i c a t t r a c t i o n , t h e . e l e c t r o n moves t o w a r d s i t s c o u n t e r - i o n w i t h v e l o c i t y , v ~ . The m a g n i t u d e o f t h i s v e l o c i t y b e i n g i n v e r s l y p r o p o r t i o n a l t o t h e i o n - e l e c t r o n d i s t a n c e . S e c o n d l y , s i m p l y as a c o n s e q u e n c e o f i t s s m a l l mass and t h e r m a l e n e r g y , t h e e l e c t r o n i s s u b j e c t t o d i f f u s i v e m o t i o n , T h i s d i f f u s i o n , b e i n g a r a n d o m p r o c e s s a k i n t o B r o w n i a n m o t i o n , may w e l l h a v e , a n o n - z e r o c o m p o n e n t d i r e c t e d away f r o m t h e c o u n t e r - i o n ( v + ) . A t some i o n - e l e c t r o n d i s t a n c e , r , t h e s e two o p p o s i t e v i r t u a l v e l o c i t i e s w o u l d be e q u a l . I f t h e a c t u a l d i s t a n c e i s g r e a t e r t h a n r c t h e e l e c t r o n e s c a p e s ; i f i t i s l e s s t h e e l e c t r o n r e t u r n s t o t h e i o n . T h i s c r i t i c a l d i s t a n c e , r , c a l l e d t h e e s c a p e r a d i u s , has b e e n s h o w n 6 6 t o be g i v e n by e q u a t i o n ( x i v ) . , r c = e 2 /3ekT ( x i v ) w h e r e e i s t h e e l e m e n t a r y c h a r g e , e i s t h e d i e l e c t r i c c o n s t a n t o f t h e s o l v e n t , k i s t h e B o l t z m a n n c o n s t a n t a n d T t h e a b s o l u t e t e m p e r a t u r e . O n s a g e r 6 7 i n a s t a t i s t i c a l t r e a t m e n t s h o w e d t h a t f o r an e l e c t r o n t h e r m a l i z e d a t a d i s t a n c e R f r o m i t s g e m i n a t e i o n t h e p r o b a b i l i t y t h a t i t w o u l d e s c a p e r e c o m b i n a t i o n p ( R ) Q C r > i s g i v e n by ( x v ) : - 4 8 -P ( R ) e s c = ' e x p ( - e 2 / e R k T ) . ( x v ) Thus i n media of h igh d i e l e c t r i c c o n s t a n t the s o l v a t e d e l e c t r o n y i e l d would be expec ted to be higher because polarized solvent . m o l e c u l e s a c t to s c reen the e l e c t r o n from i t s p a r en t i o n , t h e r e b y i n c r e a s i n g i t s chance t o e scape geminate r e c o m b i n a t i o n . The r e l a t i o n s h i p ' ' b e t w e e n d i e l e c t r i c c o n s t a n t and f r e e i on y i e l d s f rom the r a d i o l y s i s of v a r i o u s l i q u i d s has been examined by a number, of i n v e s t i g a t o r s 6 8 F i g u r e 1-8 shows t h a t w h i l e a g e n e r a l t r e n d e x i s t s , t h e r e a re a number o f e x c e p t i o n a l cases and i t i s c l e a r t h a t o t h e r f a c t o r s are i m p o r t a n t . I t has been s u g g e s t e d 6 9 t h a t e l e c t r o n s a re l e s s r e a d i l y l o c a l i z e d when the s o l v e n t m o l e c u l e s are more n e a r l y s p h e r i c a l . Thus a secondary e l e c t r o n would p r o b a b l y t r a v e l f u r t h e r f rom i t s pa ren t i o n b e f o r e becoming l o c a l i z e d i n , s ay , neopentane than n-pentane a l t h o u g h both compounds possess i n d e n t i c a l d i e l e c t r i c c o n s t a n t s . The f r e e i on y i e l d s of 0.9 and 0.1 r e s p e c t i v e l y tend to s uppo r t such c o n s i d e r a t i o n s . Even when c a r r i e d to the extreme case o f l i q u i d a r g o n , wh ich does not form a l o c a l i z e d e l e c t r o n s t a t e , the f r e e i on y i e l d i s about 5 i n d i c a t i n g t h a t the e l e c t r o n s are t h e r m a l i z e d a t g r e a t d i s t a n c e from t h e i r p a r en t i o n s . In a d d i t i o n to t h e i r y i e l d , the p o s i t i o n of the a b s o r p t i o n maximum o f s o l v a t e d e l e c t r o n s appears to be dependent upon the d i e l e c t r i c p r o p e r t i e s o f the med ia . Sauer et a l 7 0 have shown f o r example t h a t the a b s o r p t i o n maximum f o r the a l c o h o l s e x h i b i t s a red s h i f t w i t h a d e c r e a s i n g d i e l e c t r i c c o n s t a n t . I t Figure 1-8. Free ion y i e l d p l o t t e d as a f u n c t i o n of s t a t i c d i e l e c t r i c constant f o r a v a r i e of s o l v e n t s . shou ld be p o s s i b l e t o t e s t the t h e o r e t i c a l mode l s , e s p e c i a l l y r e g a r d i n g the i n vo l v emen t o f bu l k p r o p e r t i e s , t h rough s t u d i e s o f mixed s o l v e n t s . One c o u l d va r y d i e l e c t r i c c o n s t a n t f o r example and obse rve i t s e f f e c t on r e s u l t s . I f , as p o s t u l a t e d , the e l e c t r o n s are t r u e ! y d e l o c a l i z e d and thus sample t h e i r average e n v i r o n m e n t t h e i r y i e l d and o p t i c a l p r o p e r t i e s w i l l be dependent upon such 1 mac r o s cop i c p r o p e r t i e s of m i x t u r e s . B i n a r y m i x t u r e s o f a l c o h o l s and water were s t u d i e d by A r a i and S a u e r 7 1 They found t h a t , i n d e e d , the e l e c t r o n a b s o r p t i o n e x h i b i t e d a s i n g l e band i n t e r m e d i a t e between t ho se f o r the two pure s u b s t a n c e s . The p o s i t i o n of the maximum.was dependent upon the a c t u a l m i x t u r e c o m p o s i t i o n . S i m i l a r r e s u l t s have been found w i t h m i x t u r e o f water w i t h ammonia and e t h y l enedi ami n e 7 : 2 , However , a t t empt s were made to s t udy a m i x t u r e c o n t a i n i n g m o l e c u l e s o f v a s t l y d i f f e r e n t p o l a r i t y 7 3 (.ethanol , D g .= 25.7 and n-hexane D g = 1 . 8 ) , the band maximum remained a t t h e / p o s i t i o n f o r pure e thano l t h r o u g h o u t . In t h a t c a s e , i t s magnitude and the f r e e i on y i e l d were both d i r e c t l y p r o p o r t i o n a l to the e t hano l c o n c e n t r a t i o n . However, s i n c e the l i f e t i m e , o f the e l e c t r o n s p e c i e s was i ndependent o f m i x t u r e c o m p o s i t i o n i t seems more l i k e l y t h a t i t formed o n l y i n c a v i t i e s formed by c l u s t e r s of e t hano l m o l e c u l e s . S i m i l a r r e s u l t s have been o b t a i n e d for m i x t u r e s o f methanol i n t e t r a h y d r o f u r a n and c y c l o h e x a n e 7 3 as w e i l as f o r 3 -methy lhexane i n e t h a n o l and methanol ik ; ; • These i n v e s t i g a t i o n s tend to i n d i c a t e t h a t w h i l e s o l v a t e d e l e c t r o n s a re c e r t a i n l y not s t r o n g l y a s s o c i a t e d w i t h i n d i v i d u a l - 51" -s o l v e n t m o l e c u l e s i t i s the f i r s t s h e l l o r perhaps the f i r s t few l a y e r s of s o l vent ..that have the most i n f l u e n c e on the . p r o p e r t i e s o f t h e . s p e c i es . T h i s has been p a r t i c u l a r l y w e l l demons t ra ted i n a r a t h e r e l e g a n t i n v e s t i g a t i o n by Vann i kov and M a r e v t s e v 7 5 These a u t h o r s measured the a b s o r p t i o n s p e c t r a of s o l v a t e d e l e c t r o n s i n monoethanol ami ne (MEA), HOCH^CH^NH,,, and an e q u i m o l a r m i x t u r e of e t hano l , HOCH^H^, and mono-b u t y l a m i n e , C H 3 C H 2 C H 2 C H 2 N H 2 , (ETOH-MBA). These two systems have s i m i l a r , mac r o s cop i c c hem i ca l c o m p o s i t i o n but v a s t l y d i f f e r e n t d i e l e c t r i c p r o p e r t i e s . The d i e l e c t r i c c o n s t a n t o f ''•< MEA i s 57.7 and t h a t of the ETOH-MBA m i x t u r e , 1 0 . 5 . As i s shown i n F i g u r e 1-9,. d e s p i t e t h e i r f i v e - f o l d d i f f e r e n c e i n d i e l e c t r i c c o n s t a n t , the systems were found to e x h i b i t n e a r l y i d e n t i c a l s o l v a t e d e l e c t r o n s p e c t r a , each w i t h a b s o r p t i o n maxima . i n t e rmed i a t e between t h a t of pure e thano l and e t h y l a m i n e . Vann ikov and Marev t sev a t t r i b u t e d no p a r t i c u l a r impo r t ance to the f a c t t h a t t h e - s o l v a t e d e l e c t r o n band i n the ETOH-MBA m i x t u r e i s much b roader than t h a t i n MEA. To the c o n t r a r y , t h i s r e s u l t i s most s i g n i f i c a n t f o r i t a f f o r d s f u r t h e r e v i d e n c e o f t he i n f l u e n c e o f the e x p l i c i t s o l v e n t c a v i t y s t r u c t u r e . C o n s i d e r . . f o r " . s imp! i c i t y t h a t , t he se c a v i t i e s c o n s i s t e d of a t e t r a h e d r a l arrangement o f f o u r s o l v e n t m o l e c u l e s . ' In MEA, the m o l e c u l e s f o rm ing the t r a p s are e q u i v a l e n t so t h a t the p o t e n t i a l e x p e r i e n c e d by s t a b i l i z e d e l e c t r o n s i n d i f f e r e n t c a v i t i e s w i l l be s i m i l a r . As a r e s u l t , the a b s o r p t i o n band f o r e l e c t r o n i c t r a n s i t i o n s shou ld be r e l a t i v e l y nar row. On the o t h e r hand, i n the ETOH-MBA m i x t u r e , s o l v e n t cages would c o n t a i n m o l e c u l e s of two t y p e s . W A V E L E N G T H ( n m ) Figure 1-9. Absorption spectra of ef in monoethanolamine ( ) and in an equimolar mixture of ethanol and monobutyl amine (.......) reported by Vannikov and Marevtsev. Arrows indicate positions of X for e~ in pure ethanol and ethyl amine. • V \ . v ' ^ - V - V : £ : - 53 - • \ '• S imp le s t a t i s t i c a l c o n s i d e r a t i ons d i c t a t e t h a t randomly formed t e t r a h e d r a l t r a p s e x h i b i t a p r e d i c t a b l e d i s t r i b u t i o n o f s t r u c t u r e c o m p o s i t i o n s . Tab l e I l i s t s the number o f p e r -mu ta t i o n s p o s s i b l e f o r each o f the f i v e c o m b i n a t i o n s o f two o b j e c t s taken f o u r a t a t i m e . TABLE I Comb ina t i on Number o f P e r m u t a t i ons AAAA ' : . :" AAAB 4 A ' A B B 6 . ABBB 4 • BBBB 1 E l e c t r o n i c t r a n s i t i o n s i n v o l v i n g such a d i s t r i b u t i o n o f c a v i t i e s would be e xpec ted to produce a much b roade r a b s o r p t i o n band compared t o a sys tem i n which the t r a p s a re a l l s i m i l a r . T h i s w a s ' p r e c i s e l y the obse rved r e s u l t . C l e a r l y , t he se e x p e r i m e n t s c o n f i r m t h a t the m i c r o s c o p i c p r o p e r t i e s o f media a re v e r y . i m p o r t a n t i n d e t e r m i n i n g many o f the p h y s i c a l p r o p e r t i e s o f s o l v a t e d e l e c t r o n s i Kenney -Wa l l ace and H e n t z 6 s C had c o n c l u d e d f o r example t h a t l o c a l s o l v e n t s t r u c t u r e and p a c k i n g were much more i m p o r t a n t than bu lk d i e l e c t r i c p r o p e r t i e s . G. THE PRESENT STUDY - SOLVATED ELECTRONS IN HMPA Thus, M a t a n o v i c h ' s 2 d i s c o v e r y t h a t a l k a l i me ta l s d i s s o l v e i n HMPA to produce s t a b l e b l ue r e d u c i n g s o l u t i o n s becomes v e r y s i g n i f i c a n t . That i s , d e s p i t e i t s v e r y poor a n i o n s b l v a t i n g power, HMPA appears c a p a b l e o f accomodat i ng s o l v a t e d e l e c t r o n s . A;number o f v i n v e s t i g a t i o n s have been c a r r i e d out on s o l u t i o n s of a l k a l i me ta l s i n HMPA. A b r o a d , a s y s m e t r i c , i n f r a -red a b s o r p t i o n b a n d ! 6 a s i n g l e , narrow e . s . r . a b s o r p t i o n 7 7 and e l e c t r i c a l c o n d u c t i v i t y 7 8 8 a l l c h a r a c t e r i s t i c o f s o l v a t e d e l e c t r o n s , have been r e p o r t e d f o r such s o l u t i o n s . In a d d i t i o n , S t e r n b e r g e t a l ' 7 8 - -have obse rved the f o r m a t i o n o f dark b l u e g l o b u l e s a t a p l a t i n u m cathode when a s o l u t i o n of l i t h i u m c h l o r i d e i n HMPA was e l e c t r o l y z e d . A l p a t r o v a and G r i s h i n a 7 9 u s i n g e . s . r . s p e c t r o s c o p i c t e c h n i q u e s , a t t r i b u t e d t h i s c o l o u r -a t i o n t o s o l v a t e d . e l e c t r o n s . ATI of the HMPA s t u d i e s f o r wh ich s t a b l e s o l u t i o n s of s o l v a t e d e l e c t r o n s have been proposed c o n t a i n l a r g e c o n c e n t r a t i o n s of f o r e i g n c a t i o n s ( u s u a l l y one o f the a l k a l i m e t a l s ) . Thus i t i s not c l e a r whether o r not i t i s the p re sence of such c a t i o n s which i s i n f a c t r e s p o n s i b l e f o r the e l e c t r o n s t a b i l i z -a t i o n i n those sy s tems. I t was proposed to s tudy by means of. r a d i o l y s i s t e c h n i q u e s the a b i l i t y o f HMPA to accommodate and s t a b i l i z e s o l v a t e d e l e c t r o n s . P r e l i m i n a r y s t e a d y - s t a t e r a d i o l y s i s s t u d i e s 8 0 u t i 1 i z i ng. c o m p e t i t i v e ' s cavenger t e c h n i q u e s demons t ra ted the f e a s i b i l i t y o f the p r o j e c t . That work was deve l oped f u r t h e r - 55 - • ' and i n c o n j u n c t i o n w i t h s t u d i e s o f Na s o l u t i o n s i n HMPA the y i e l d , s t a b i 1 i t y and r e a c t i v i t y of e " M p A were f a i r l y w e l l e s t a b l i s h e d . F i n a l l y , the work was ex tended to p u l s e r a d i o l y s i s t e c h n i q u e s which f a c i 1 i t a t e d d i r e c t o b s e r v a t i o n of the s p e c i e s , i t s s pec t r um, and r e a c t i o n s a l ong w i t h an i ndependent measurement of i t s y i e l d . • ; - 56 -•-/;•<: CHAPTER II • EXPERIMENTAL A. STEADY STATE EXPERIMENTS 1. . M a t e r i a l si -Hexamethy lphosphoramide was p u r i f i e d i n the f o l l o w i n g manner. F r e s h l y c u t p i e c e s o f sodium meta l were added to s e v e r a l l i t e r s of F i s h e r S c i e n t i f i c t e c h n i c a l grade HMPA i n a vacuum d i s t i l l a t i o n a p p a r a t u s . The samples were a g i t a t e d a t a tmo sphe r i c p r e s s u r e by means of a he l i um gas purge u n t i l a s t a b l e b l ue s o l u t i o n o f sodium i n HMPA was a t t a i n e d . T h i s s o l u t i o n was f r a c t i o n a l l y d i s t i l l e d under vacuum. The m i d d l e t w o - t h i r d s ( t h a t which b o i l e d at 87 °C f o r about one t o r r p r e s s u r e ) was r e t a i n e d . The . p u r i f i c a t i o n t e c h n i q u e was t hen r epea ted w i t h t h a t sample as s t a r t i n g m a t e r i a l , the m i d d l e f r a c t i o n be ing c o l l e c t e d and s t o r e d u n t i l r e q u i r e d . When sodium metal was added t o the s i n g l y d i s t i l 1ed sample o f HMPA p r i o r to the second d i s t i l l a t i o n , the s o l u t i o n became s t a b l e b l ue a lmos t i m m e d i a t e l y i n d i c a t i n g t h a t few o x i d i z i n g i m p u r i t i e s r ema ined . Water used as s cavenger and f o r HMPA/water m i x t u r e s t u d i e s was t r i p l y d i s t i l l e d . S i n g l y d i s t i l l e d water was r e f l u x e d from an a c i d i f i e d d i c h r oma te s o l u t i o n , gamma i r r a d i a t e d o v e r n i g h t (0.5 Mrad) to remove t r a c e o r g a n i c i m p u r i t i e s s then r e f l u x e d - 57 -from an a l k a l i n e permaganate s o l u t i o n . N i t r o u s o x i d e , ca rbon d i o x i d e , oxygen and argon were p u r i f i e d i n the vacuum system by c o n v e n t i o n a l " t r a p - t o - t r a p " d i s t i l l a t i o n s i n v o l v i n g m u l t i p l e f r eeze -pump- thaw c y c l e s . These gases were s t o r e d on the vacuum l i n e i n s topped b u l b s u n t i l r e q u i r e d . Sodium meta l was p u r i f i e d and p repa red f o r a d d i t i o n to l i q u i d samples as amalgams w i t h mercu r y . Smal l weighed s l u g s of f r e s h l y c u t sodium meta l were s e a l e d i n t o a l o ng py rex tube which was s u b s e q u e n t l y e v a c u a t e d . The meta l was d i s t i l l e d th rough a number o f c o n s t r i c t i o n s i n the t u b e . The r e s i d u e chamber f rom each d i s t i l l a t i o n was s e a l e d - o f f and removed w i t h the r e s u l t t h a t the l a s t chamber c o n t a i n e d a b r i g h t sodium m i r r o r . " C l e a n " mercury was d i s t i l l e d onto t h i s m i r r o r and the r e s u l t i n g amalgam s t o r e d under vacuum. Other c h e m i c a l s used as s cavenge r s were a n a l y t i c a l grade or b e t t e r and were not s u b j e c t e d to f u r t h e r p u r i f i c a t i o n . S p e c i a l c a r e was t aken to ensure t h a t g l a s swa re was s c r u p u l o u s l y c l e a n . V e s s e l s were washed w i t h permangan ic a c i d ; r i n s e d w i t h d i s t i l l e d w a t e r ; washed w i t h a s o l u t i o n o f hydrogen p e r o x i d e i n n i t r i c a c i d ; and f i n a l l y r i n s e d w i t h s i n g l y and t r i p l y d i s t i l l e d w a t e r . G l a s sware was u s u a l l y d r i e d i n a 120 °C oven and on o c c a s i o n annea led o v e r n i g h t i n a gl a s s b l ower's oven. 2. R a d i a t i o n Source An Atomic Energy of Canada Gammacell 220 was used as a - 58 -source of gamma r a y s . T h i s mach ine , wh ich had a nomina l a c t i v i t y of 3800 c u r i e s , uses 6 0 C o as i t s s o u r c e . 6 0 C o has a h a l f - l i f e of 5.26 y e a r s and decays w i t h the e m i s s i o n o f gamma r a y s o f 1.173 MeV and 1.333 MeV. 6 0 C o a l s o em i t s 0.32 MeV beta p a r t i c l e . s ( e l e c t r o n s ) but i n the Gammacell 220 tho se a re absorbed b e f o r e t hey reach the sample i r r a d i a t i o n chamber. 3. Dos imetry, ; In o r de r t o be a b l e t o d i s c u s s the e x p e r i m e n t a l e f f e c t s of r a d i a t i o n q u a n t i t a t i v e l y i n terms of G v a l u e s , i t i s n ece s s a r y to de te rm ine the r a d i a t i o n energy absorbed by a g i ven sample. Absorbed dose i s u s u a l l y e xp re s s ed i n rads --1 13 one rad be ing the a b s o r p t i o n of 100 e rgs gm or 6.24-X 10 eV g m " 1 . S i n c e the dose r a t e i n the r a d i a t i o n chamber o f the Gammacell i s not s p a t i a l l y u n i f o r m , c a r e was t aken to ensure r e p r o d u c i b l e p lacement of the i r r a d i a t i o n c e l l s . In a d d i t i o n , d o s i m e t r y measurements were made u t i l i z i n g a p s e u d o - c e l l wh ich a c c u r a t e l y r ep roduced sample p o s i t i o n i n g . Chemica l d o s i m e t r y was per fo rmed wh ich mon i t o r ed the o x i d a t i o n o f f e r r o u s i o n s . Known as F r i c k e d o s i m e t r y , the t e c h n i q u e i s a u n i v e r s a l l y a c cep ted s t anda rd t e c h n i q u e . 1 An a i r s a t u r a t e d s o l u t i o n of t r i p l y d i s t i l l e d - 3 - 3 wate r c o n t a i n i n g 1 X 1 0 M f e r r o u s ammonium s u l p h a t e , 1 X 1 0 M sodium c h l o r i d e , and 4 X 10~ 1 M s u l p h u r i c a c i d was p r e p a r e d . I r r i d a t i o n of such a s o l u t i o n i s thought to l e a d to the f o l l o w i n g p r o c e s s e s : - 59 -n H 2 O A A W y V - ^ e a q + H + OH + H 2 0 2 + H 2 (2 1) e a q + H + ~ * H ( 1 .13 ) > H ° 2 (2 .2 ) H o 2 + F e 2 + + H + • F e 3 + + H ^ ( 2 > 3 ) H 2 0 2 + F e 2 + + H + — • F e 3 + + OH + H 2 0 (2 .4 ) OH + F e 2 + + H + • F e 3 + + H 2 0 (2 .5 ) From the se r e a c t i o n s , i t can be seen t h a t : G ( F e 3 + ) = 3 [ 'G(e~ q.) + G(H)] + G(0H) + 2 G ( H 2 0 2 ) ( x v i ) Based on d i r e c t c o l o r i m e t r i c measurements the f e r r i c i o n y i e l d 3+ -1 G(Fe ) = 1 5 . 5 molec (100 eV) has been measured w i t h g r e a t p r e c i s i o n 8 . 1 C l e a r l y , t he f e r r i c i o n y i e l d i s v e r y s e n s i t i v e to a number of i n t e r m e d i a t e s p e c i e s i n the r a d i o l y s i s o f w a t e r . In p a r t i c u l a r , the o x i d a t i o n o f o r g a n i c i m p u r i t i e s (RH) by hyd roxy ! r a d i c a l s (2 .6) and the subsequent r e a c t i o n w i t h oxygen (2.7) can produce o r g a n i c p e r o x i d e s . 0H.+ RH • R + H 2 0 (2 .6 ) 60 -R + °2 • R ° 2 (2.7) Such p e r o x i d e s c o u l d then r e a c t w i t h f e r r o u s i on s i n a r e a c t i o n scheme ana lagous to ( 2 .3 , 2.4, 2 .5) . The r e s u l t would be to i n c r e a s e the f e r r i c i o n y i e l d . However, the p r e s ence o f c h l o r i d e i on s i n the F r i c k e s o l u t i o n s e r ve s to s upp re s s such a p r oce s s by s caveng i ng the h y d r o x y l r a d i c a l s ( 2 . 8 ) . OH + Cl ~ • 0H~ + Cl (2.8) The c h l o r i n e atom then o x i d i z e s a f e r r o u s i on (2.9) so t h a t the f e r r i c i on y i e l d i s p r e s e r v e d . Cl + F e 2 + • F e 3 + + C l " (2.9) F r i c k e s o l u t i o n s were i r r a d i a t e d f o r v a r i o u s p e r i o d s o f t i m e . The r e s u l t i n g f e r r i c i on c o n c e n t r a t i o n i n each sample was measured s p e c t r o p h o t o m e t r i c a l l y u s i n g a Beckman DU s p e c t r o p h o t o m e t e r which had been c a l i b r a t e d w i t h a known po ta s s i um chromate s o l u t i o n . F i g u r e I I - l shows t h a t the 3 + observed o p t i c a l d e n s i t y at 304 nm ( f rom Fe i o n ) was a l i n e a r f u n c t i o n of the Gammacell t i m e r s e t t i n g . The p o s i t i v e i n t e r c e p t to be seen i n t h a t f i g u r e r e f l e c t s a p e c u l i a r i t y of the Gammacell o p e r a t i o n . That i s , because the t i m e r mechanism i s o n l y a c t i v a t e d when the c a v i t y drawer i s at i t s l owermost p o s i t i o n , samples are s u b j e c t e d to an unt imed but c o n s t a n t r e s i d u a l dose o f r a d i a t i o n d u r i n g the p e r i o d s of de s cen t f rom - 61 -I R R A D I A T I O N P E R I O D ( T I M E R M I N ) F i gure I I - l . F r i c k e Do s ime t r y . R e s u l t s f rom of a p p r o p r i a t e s o l u t i o n s i n the the 6 0 C o y -r a d i o l y s i s i r r a d i a t i o n c e l l . - 62 -and a s cen t to the upper l o a d i n g p o s i t i o n . As can be s e e n , the r e s i d u a l dose ; was e q u i v a l e n t to an i r r a d i a t i o n of about 5 sec as measured by the t i m e r . The absorbed dose r a t e , R Q , f o r the F r i c k e d o s i m e t e r s o l u t i o n i s g i v en by e q u a t i o n ( x v i i ) : D _ 9.65 X 1 0 8 AO.D./At rad m i n - 1 e 3 F ^ 4 3 + . l . p . G ( F e 3 + ) ( x v 1 i ) F e 3 + 3+ where e 3 Q 4 i s the molar a b s o r p t i v i t y o f Fe a t 304 nm (2174 M 1 c m - 1 ) , 1 i s t he o p t i c a l pa th l e n g t h o f t h e c e l l (1 cm), P i s the d e n s i t y of the F r i c k e s o l u t i o n (1 .024 ± 0.001 a t 20 "C) and G ( F e 3 + ) = 15.5 molec (100 e V ) " 1 . For the da ta o f F i g u r e I I - l a dose r a t e , RQ = 6580 rad m i n - ^ was c a l c u l a t e d . . Now, the major energy t r a n s f e r mechanism — the Compton p roce s s - - depends on the e l e c t r o n d e n s i t y o f the a b s o r b i n g medium. T h e r e f o r ej • the c o r r e s p o n d i n g r a d i a t i o n dose r a t e f o r HMPA samples i n the same p h y s i c a l p o s i t i o n i n r a d i a t i o n chamber i s g i ven by ( x v i i i ) . . n _ n ( Z / A ) H M P A HMPA % ^ Z / A ) F R I C K E ( x v i i i ) - 63 -where Z and A are the a tom ic numbers and w e i g h t s of the two med ia . In a d d i t i o n to the p h y s i c a l i n f l u e n c e s on dose r a t e , 6 0 C o i t s e l f i s s u b j e c t to an i n h e r e n t n a t u r a l decay . The a c t i v i t y o f r a d i o a c t i v e m a t e r i a l a t t ime t , A^, i s r e l a t e d to some i n i t i a l a c t i l i t y , A Q , by the e x p r e s s i o n ( x i x ) . A t = A o e A t ( x i x ) where A i s the decay c o n s t a n t (xx ) 0.693 X = ( xx ) ' a and T j i s the r a d i o a c t i v e h a l f - l i f e (5.27 y e a r s f o r 6 0 C o ) . The dose r a t e , R^, f o r a sample i r r a d i a t e d t days a f t e r d o s i m e t r y was per formed would be g i v e n by the e x p r e s s i o n ( x x i ) : R = R e - ( 0 . 6 9 3 t / 1 9 2 5 ) t o ( x x i ) A s i m p l e computer program was w r i t t e n t h a t took i n t o accoun t sample e l e c t r o n d e n s i t y , c e l l p o s i t i o n , i r r a d i a t i o n t i m e , r e s i d u a l dose , t ime s i n c e d o s i m e t r y measurements , d o s i m e t r y r e s u l t s , and G 0 C o decay and r e t u r n e d a v a l u e r e p r e s e n t i n g the - 6 4 -a c t u a l absorbed dose f o r a g i v en e x p e r i m e n t . 4. . E x p e r i m e n t a l on a) . 6 0 C o Gamma R a d i o l y s i s F i g u r e 11 - 2: d e p i c t s the r e a c t i o n c e l l used i n the s t e a d y -s t a t e r a d i o l y s i s s t u d i e s . That pyrex v e s s e l c o u l d be r e p r o d u c i b l y p l a c e d i n the Gammacell r a d i a t i o n chamber. I t s c o n s t r u c t i o n f a c i l i t a t e d the r a p i d i n t r o d u c t i o n , homogeneous s o l u t i o n f o r m a t i o n , and q u a n t i t a t i v e r e t r i e v a l o f gaseous s u b s t a n c e s . The f ou r -way s t opcock was removed and known amounts o f l i q u i d s o l u t i o n s i n t r o d u c e d i n t o the c e l l t h r ough the v e n t . The s t opcock was s p a r i n g l y l u b r i c a t e d w i t h i n e r t Ap i e zon N and h e l d f i r m l y i n p l a c e w i t h a a luminum r e t a i n e r which c o u l d w i t h s t a n d the p r e s s u r e s r e q u i r e d f o r a n a l y s i s . Samples were then deoxygenated by f l u s h i n g w i t h argon f o r 15 m i n u t e s . T u r n i n g the s t opcock 45° to a f u l l y c l o s e d p o s i t i o n r e t a i n e d the l i q u i d above the f r i t t e d g l a s s d i s k . A t t h i s p o i n t the sample ( g e n e r a l l y HMPA w i t h or w i t h o u t s c a venge r s ) was e i t h e r i r r a d i a t e d or a t t a c h e d t o a vacuum l i n e f o r the a d d i t i o n o f gaseous s c a v e n g e r s . The vacuum appa ra tu s enab led one to i n t r o d u c e measured amounts o f gas or a m i x t u r e o f gases d i r e c t l y i n t o the r e a c t i o n v e s s e l . F i g u r e 11-3 g i v e s s chemat i c d e t a i l s o f the equ ipment . V a r i o u s pure gases ( N 2 0 , CO,,, 0,,, Ar ) were s t o r e d i n l a r g e f l a s k s A, B, or C. F l a s k A was g e n e r a l l y r e s e r v e d f o r the - 65 -F i g u r e " 1 1 - 3 . S chemat i c d iagram o f vacuum l i n e used f o r the p r e p a r a t i o n and a d d i t i o n o f gaseous s cavenge r s to HMPA s o l u t i o n s i n the r a d i o l y s i s v e s s e l , G. - 67 -\ major gaseous s c a v e n g e r , N 2 0 . Through the use of a l i q u i d n i t r o g e n bath and bu lb D, N^O samples c o u l d be c o n v e n i e n t l y r e c o v e r e d from o t h e r r e g i o n s o f the l i n e , p u r i f i e d , and r e t u r n e d to s t o r a g e bu lb A. The r e a c t i o n c e l l G, c o n t a i n i n g the HMPA s o l u t i o n be ing s t u d i e d , w a s a t t a c h e d to the vacuum l i n e by means of two b a l l and s o c ke t j o i n t s . S i n c e a t room tempe ra tu re the vapour p r e s s u r e of HMPA i s n e g l i g i b l e , samples of the l i q u i d c o u l d be r e a d i l y degassed by e v a c u a t i n g the r e a c t i o n c e l l . T h i s was a c comp l i s hed by open ing s t o p c o c k s S-| , S 2 , Sg, Sg, S jQ , S.|i (between Sg and S^ 3 ) and s l o w l y , S ^ . When e q u i 1 i b r i u r n was e s t a b l i s h e d S- j 3 was c l o s e d i s o l a t i n g the c e l l . was r o t a t e d so t h a t add i t i on bu lb E would be e v a c u a t e d . I f two gaseous s cavenge r s were to be added to the r e a c t i o n v e s s e l S 1 2 would be opened so as to a l l o w e v a c u a t i o n o f bu l b F as w e l l . These bu l b s were a t t a c h e d by means of cone and s o c k e t j o i n t s and c o u l d be r e p l a c e d w i t h a number o f bu lb s of v a r i o u s known c a p a c i t i e s . Gaseous s cavenge r s to be added to the r e a c t i o n c e l l were t r a n s f e r r e d to bu lb E and/or F a t a known p r e s s u r e and t e m p e r a t u r e . Two s cavenge r s c o u l d be i n d e p e n d e n t l y m e t e r e d , the f i r s t go ing i n t o b u l b F and t he second i n t o bu lb E. S topcock S ^ was opened between Sg and S ^ and the l i n e evacuated as b e f o r e . At the same t ime s t opcock S-j 2 would be opened to a l l o w tho rough m i x i n g o f s cavenge r s when two were be ing used. The s cavenger or m i x t u r e was i n t r o d u c e d i n t o the r e a c t i o n c e l l and m o n i t o r e d by c l o s i n g S-j (as w e l l as S 3 , S 4 , Sg, S 7 ) and open ing S 2 , Sg, S g , S 1 0 , S 1 3 , S - j 2 and s l o w l y S-ji (between S ^ and S 1 3 ) . When b u b b l i n g had ceased - 68 -and e q u i l b r i u m a t t a i n e d , the t empe ra t u r e and p r e s s u r e o f r e s i d u a l s cavenge r gases measured. S i n c e a l l r e l e v e n t volumes i n the system were a c c u r a t e l y known and assuming i d e a l gas b e h a v i o r i t was p o s s i b l e to c a l c u l a t e the c o n c e n t r a t i o n of gaseous s cavenge r (o r s c a venge r s ) d i s s o l v e d i n the l i q u i d . Measurements!were made to de te rm ine the s o l u b i l i t y o f each o f the gaseous s cavenge r s i n HMPA. F i g u r e 11-4 shows the v a r i a t i o n o f c o n c e n t r a t i o n w i t h the ob se r ved e q u i l i b r i u m p a r t i a l p r e s s u r e s of N 2 0 , 0 2 and C 0 2 - I t can be seen t h a t i n each of those cases the s cavenger c o n c e n t r a t i o n was a l i n e a r f u n c t i o n o f i t s e q u i l i b r i u m p a r t i a l p r e s s u r e . T a b l e I I g i v e s s o l u b i l i t i e s f o r the gases i n HMPA at 23 9C as de te rm ined from the s l o p e s o f the p l o t s i n F i g u r e 11-4. TABLE II S o l u b i l i t y i n HMPA at 23 X as a f u n c t i o n Gas o f e q u i l i b r i u m p a r t i a l p r e s s u r e (M/mm Hg) N 2 0 1.85 ± 0.05 X 1 0 " 4 °2 4.2 ± 0.4 X 1 0 " 5 co2 2.4 ± 0.2 X 1 0 " 4 I t s hou ld be noted t h a t e x t r a p o l a t i o n o f the p l o t s of F i g u r e 11-4 l e a d to c o n s t a n t but non - ze ro i n t e r c e p t s . T h i s appear s to a r i s e as consequence of the p a r t i c u l a r e x p e r i m e n t a l s e t up. That i s , E Q U I L I B R I U M P A R T I A L P R E S S U R E ( t o r r ) F i g u r e 11-4. S o l u b i l i t i e s o f Hz0 « > ) . C 0 x ( O ) . . and 0* ( A ) a t 23 X i n HMPA measured i n the r e a c t i o n v e s s e l . F i l l e d c i r c l e s i n d i c a t e Hz0 samples u s i ng Ar as d i l u e n t . - 70 -because of a l a r g e p r e s s u r e drop a c ro s s the s i n t e r e d d i s k i n the r e a c t i o n c e l l , a gas p r e s s u r e of a t l e a s t 100 t o r r was r e q u i r e d to i n i t i a t e b u b b l i n g . P resumably t h i s p r e s s u r e was i e s s e n t i a l to f o r c e l i q u i d out of the d i s k so t h a t the gas i t s e l f c o u l d pass t h r o u g h . U n f o r t u n a t e l y , the i n i t i a l p r e s s u r e r e q u i r e m e n t o f 100 t o r r tended to p l a c e a l ower l i m i t on the c o n c e n t r a t i o n o f s cavenge r t h a t one c o u l d o b t a i n . T h i s was e s p e c i a l l y a p rob lem w i t h N 2 0 because i t was h i g h l y d e s i r a b l e t o produce s o l u t i o n s o f ve r y low but known c o n c e n t r a t i o n s of t h a t s c a v e n g e r . The prob lem was somewhat overcome by l i m i t i n g the t o t a l amount o f s cavenger th rough the use o f a sma l l bu lb a t E. To a c h i e v e ve r y low c o n c e n t r a t i o n s o f N 2 0 , argon gas was mixed w i t h the s cavenge r to p r o v i d e the r e q u i r e d i n i t i a l p r e s s u r e . That i s , v e r y s m a l l amounts o f N 2 0 were t r a n s f e r r e d to bu lb F, then s u f f i c i e n t argon t r a n s f e r r e d t o bu lb E t o ensure the m i x t u r e o f gases i n E and F c o u l d i n i t i a t e b u b b l i n g . Argon i t s e l f was found to be e s s e n t i a l l y i n s o l u b l e i n HMPA. The N 2 0 p r e s s u r e s were s t i l l o f s u f f i c i e n t magnitude so as to a l l o w i d e a l gas c a l c u l a t i o n s . The f i l l e d c i r c l e s and the dashed l i n e i n the f i g u r e show the s o l u b i l i t y r e l a t i o n s h i p o b t a i n e d f o r N 2 0 i n HMPA at 23 °C when used i n c o n j u n c t i o n w i t h c o n s t a n t amount of argon " c a r r i e r " gas. In t h a t c a s e , the s l o pe g i v e s a v a l u e f o r the N 2 0 s o l u b i l i t y S N Q / A r = 1.47 ± 0.05 X 1 0 " 4 M/mm f o r low N 2 0 p r e s s u r e s which s hou ld be compared to the v a l u e f o r N 2 0 -4 a long at h i g h e r p r e s s u r e of S M n = 1.85 ± 0.05 X 10 M/mm. - 7 1 -I t must be emphas ized t h a t t h i s i s a p u r e l y e m p i r i c a l r e l a t i o n -s h i p a p p l y i n g t o a p a r t i c u l a r e x p e r i m e n t a l a r rangement . Where - 3 e x t r e m e l y Tow c o n c e n t r a t i o n s (<10 M) o f ^ 0 were d e s i r e d , i d e a l gas c a l c u l a t i o n s were i m p r a c t i c a l , so the ^ 0 c o m p o s i t i o n was o b t a i n e d from an e x t r a p o l a t i o n of the da ta of F i g u r e 11 - 4. A d m i t t e d l y t h i s was pu sh i ng t he sys tem beyond r e a s o n a b l e l i m i t s and undoub ted l y l e d to l a r g e u n c e r t a i n t i e s i n the a c t u a l N 2 0 c o n c e n t r a t i o n s . However, the method appeared t o g i ve s e n s i b l e and c o n s i s t e n t r e s u l t s and c e r t a i n l y one c o u l d e xpec t the p r o p o r t i o n a l i t y between NgO a d d i t i o n s to be p r e s e r v e d . I t was d e s i r e d to compare e xpe r imen t s i n wh ich equa l c o n c e n t r a t i o n s o f NgO were added to v a r i o u s m i x t u r e s o f wa te r and HMPA. S o l u b i l i t y da ta was o b t a i n e d f o r a number o f m i x t u r e s and t he r e s u l t s a re shown i n F i g u r e 1 1 - 5 . When samples c o n t a i n i n g the d e s i r e d c o n c e n t r a t i o n s o f s cavenge r s were p r e p a r e d , S -j ^  w a s c l o s e d , the c e l l removed from the vacuum 1 i ne and the sample i r r a d i a t e d w i t h an a p p r o p r i a t e dose o f gamma r a d i a t i o n i n the Gammace l l . b) Sodium Meta l S o l u t i o n s The r e a c t i o n c e l l used i n the s tudy of the e f f e c t s o f t he d i s s o l u t i o n of sodium meta l i n HMPA s o l u t i o n s i s d e p i c t e d i n F i g u r e 11-6. Though somewhat d i f f e r e n t i n d e s i g n f rom the r a d i o l y s i s v e s s e l , t h i s c e l l pos ses sed s i m i l a r c a p a b i l i t i e s . HMPA c o n t a i n i n g v a r i o u s m a t e r i a l s i n t r o d u c e d d i r e c t l y i n t o the c e l l ( i n c l u d i n g sodium m e t a l ) c o u l d be degassed w i t h an gure 11- 5. A. R e a c t i o n v e s s e l f o r sodium meta l s o l u t i o n s . B. Sodium/mercury amalgam p r e p a r a t i o n and a d d i t i o n , .'-accessory. - 74 -i n e r t gas purge v i a the s i n t e r e d b u b b l e r . The o u t l e t s were c o m p a t i b l e w i t h the vacuum system l ead s so t h a t the v e s s e l c o u l d be evacua ted and gaseous s cavenge r s added. A s i d e arm p r o v i d e d acce s s f o r i n t r o d u c t i o n o f measured a l i q u o t s o f p repa red sod ium/mercury amalgams. T r a n s f e r c o u l d be pe r fo rmed i n vacuo o r , i f the r e a c t i o n v e s s e l c o n t a i n e d v o l a t i l e m a t e r i a l s , the amaglam q u i c k l y f o r c e d i n t o the c e l l by means o f an exces s p r e s s u r e of a r gon . A s m a l l T e f l o n s t i r r i n g magnet was used i n the r e a c t i o n v e s s e l to a g i t a t e the amalgam t h e r e b y i n c r e a s i n g i t s r e a c t i v e s u r f a c e a r e a . Upon c o m p l e t i o n o f r e a c t i o n s , the b u b b l e r f a c i l i t a t e d q u a n t i t a t i v e r e c o v e r y o f gaseous m a t e r i a l s f o r a n a l y t i c a l pu rpo se s . 5. A n a l y s i s Sample a n a l y s i s was pe r fo rmed u s i n g a V a r i a n Ae rog raph S e r i e s 1700 Chromatograph w i t h dua l 20 f o o t by % i n c h s t a i n l e s s s t e e l co lumns. The co lumns , c o n t a i n i n g 13X m o l e c u l a r s i e v e m a t e r i a l , were m a i n t a i n e d a t 50°C. Argon c a r r i e r gas w i t h a f l o w r a t e o f 30 ml m i n - ^ was e l u t e d th rough WX the rma l c o n d u c t i v i t y d e t e c t o r s . These d e t e c t o r s , m a i n t a i n e d a t 110 *0 , were o p e r a t e d a t a f i l a m e n t c u r r e n t o f 100 mA. Output was mon i t o r ed v i a a W e s t r o n i c s c h a r t r e c o r d e r . F i g u r e 11-7 g i v e s a s chemat i c d iagram o f the e x t e r n a l sample and s t a n d a r d i n l e t a r rangement s . F o l l o w i n g r a d i o l y s i s o r r e a c t i o n w i t h sodium meta l or amalgam the c e l l i n q u e s t i o n was a t t a c h e d to the chromatograph F i g u r e I I - 7 . S chemat i c d iag ram o f c h r o m a t o g r a p h i c gas s t anda rd and sample i n j e c t i o n s y s tem. - 76 -as shown i n F i g u r e 11-7 - The v a r i o u s s t o p c o c k s were t u r n e d so as to f l u s h the a i r from the e x t e r n a l l i n e and c e l l l e a d s . When the a i r s l u g had e l u t e d , the c e l l s t opcock S-j^ was r o t a t e d 90° which f l u s h e d gases f rom the c e l l i n t o the ch romatog r aph . Va r i ou s s t anda rd gases were i n t r o d u c e d a t a t m o s p h e r i c p r e s s u r e v i a S-j 5 i n t o the c a l i b r a t e d s t a n d a r d l oop L. When i t was c o n v e n i e n t , such s t anda rd s were i n j e c t e d th rough S-^ i n t o the ch romatog raph . I n t e r c h a n g i n g l oop L and the r e a c t i o n c e l l (and p re - co l umn) a l l o w e d one to i n j e c t known amounts o f v a r i o u s gases i n t o the r e a c t i o n c e l l i m m e d i a t e l y p r i o r to a n a l y s i s . P r o d u c t s were i d e n t i f i e d by compar ing t h e i r r e t e n t i o n t ime w i t h known s t a n d a r d s . Over the range s t u d i e d , the d e t e c t o r r e sponse was l i n e a r f o r the gases mon i t o red thus q u a n t i t a t i v e r e s u l t s were o b t a i n e d by t r i a n g u l a r i n t e g r a t i o n of the r e c o r d e r ou tpu t peaks . Tab le I I I l i s t s t y p i c a l v a l u e s f o r the re sponse t ime and d e t e c t o r s e n s i t i v i t y o f a number of gases f o r the c o n d i t i o n s o u t l i n e d . A t y p i c a l chromatogram i s shown i n F i g u r e 11 -8 . TABLE I I I GAS R e t e n t i o n Time (min) D e t e c t o r S e n s i t i v i t y 2 (cm /ymole) H 2 ; 58.1 °2 13 6.6 N 2 23 5.2 C H 4 35 15.2 C 2^ 2 >60 12.7 •r-,,-HMPA - 3-61 0.023 M..N 20 : A 7' c; R QJF. D - T ; : 27/U73--CM s CM CO X CM X A. - • 30 ml/min.. . . : _std.: A r - ; A . A A A r _JQ0 JTlA CM " \ CM X CM CM \ CM X CJ 32 CM u CD Q. E in T IME p i c a l chromatogram o b t a i n e d from an i r r a d i a t e d sample o f HMPA c o n t a i n i n g N 20 - 78 -B. PULSE RADIOLYSIS 1. Genera l O u t l i n e of the Techn ique P u l s e r a d i o l y s i s s t u d i e s of HMPA were conduc ted d u r i n g b r i e f v i s i t s to the Ohio S t a t e U n i v e r s i t y . The r a d i a t i o n f a c i l i t i e s a t the e s t a b l i s h m e n t were p a r t i c u l a r i l y w e l l s u i t e d to the t ype s of e xpe r imen t s t h a t might remove many of the a m b i g u i t i e s i n h e r e n t w i t h i n d i r e c t s t e a d y - s t a t e 6 0 C o Y - r a d i o l y s i s s t u d i e s . The p u l s e r a d i o l y s i s equipment a l l o w s one to d i r e c t l y obse rve t r a n s i e n t a b s o r p t i o n s p roduced by s h o r t b u r s t s o f i o n i z i n g r a d i a t i o n . F i g u r e 11 - 9 i s a s i m p l i f i e d s chemat i c d iagram showing the l a y o u t of the v a r i o u s components. A p o r t i o n o f the e l e c t r o n beam from the l i n e a r a c c e l e r a t o r was absorbed i n an i r r a d i a t i o n c e l l c o n t a i n i n g the sample under s t u d y . Any t r a n s i e n t s p e c i e s formed as a r e s u l t of such i n t e r a c t i o n which absorbed between 300 and 2300 nm were mon i t o r ed by means o f a beam o f a n a l y z i n g l i g h t p a s s i n g th rough the sample c e l l a t r i g h t ang le s to the e l e c t r o n beam. Through the j u d i c i o u s s e l e c t i o n o f the samp le , f i l t e r s , d e t e c t o r and s e t t i n g s o f the monochromator and o s c i l l o s c o p e , s p e c t r a l and k i n e t i c i n f o r m a t i o n was o b t a i n e d f o r a number of t r a n s i e n t s p e c i e s . 2. Sample P r e p a r a t i o n Dow Chemica l Company " D o r c o l " brand HMPA was p u r i f i e d by - 79 -f i. Xe lamp F i g u r e 11-9. S i m p l i f i e d s chemat i c of the p u l s e r a d i o l y s i s a o c a r a t u s at The Ohio S t a t e u n i v e r s i t y . - 80 -s t i r r i n g w i t h m e t a l l i c sodium under vacuum u n t i l a s t a b l e dark b lue s o l u t i o n o f s o l v a t e d e l e c t r o n s was f o rmed. T h i s sodium s o l u t i o n was then f r a c t i o n a l l y d i s t i l l e d under vacuum, the m idd l e p o r t i o n be ing c o l l e c t e d i n a s c r u p u l o u s l y c l e a n evacua ted f l a s k equ ipped w i t h a T e f l o n s t o p c o c k . Samples f o r i r r a d i a t i o n were p repa red by r e - d i s t i l l i n g HMPA f rom t h i s s t o r a ge bu lb d i r e c t l y i n t o r a d i o l y s i s c e l l s . A t y p i c a l c e l l i s shown i n F i g u r e 11-10. A g a i n , o n l y T e f l o n s t o p c o c k s were employed, and a t tachment of the c e l l to the vacuum system was a c h i e v e d th rough the use of S o l v - S e a l g l a s s and T e f l o n j o i n t s : Each c e l l i n c l u d e d a l a r g e h e a r t - s h a p e d chamber wh ich f a c i l i t a t e d the removal o f t r a c e s o f d i s s o l v e d gases th rough a m u l t i p l e f r eeze -pump- thaw p r o c e d u r e . A l o ng s i d e arm t e r m i n a t e d by a p r e c i s i o n o p t i c a l c e l l p r o v i d e d a means whereby s m a l l p o r t i o n s o f t he bu lb sample c o u l d be i r r a d i a t e d and o b s e r v e d . O p t i c a l c e l l s were e i t h e r S u p r a s i l q u a r t z w i t h an o p t i c a l path o f 20 mm or t h i n V i t r o s i l (3 mm path l e n g t h ) . The path o f r a d i a t i o n through the c e l l was a t r i g h t ang l e s to the o p t i c a l path and was 10 mm i n l e n g t h . The c e l l shown i n the d iagram had a sma l l chamber between two T e f l o n s t o p c o c k s wh ich c o u l d be used to i n t r o d u c e under vacuum, s m a l l known amounts o f s o l u t e s . Chemica l s used as s cavenger s were a n a l y t i c a l r e agen t grade or b e t t e r . An th racene and p y r ene , used i n the s o l v a t e d e l e c t r o n y i e l d d e t e r m i n a t i o n , were A l d r i c h " G o l d L a b e l " z one -r e f i n e d (99.9+% p u r i t y ) g r ade . Gaseous s cavenge r s were p u r i f i e d by m u l t i p l e t r a p - t o - t r a p d i s t i l l a t i o n on the vacuum l i n e . - 81 Solv-Seal Teflon joint Optical Cell F i g u r e 11-10. Pu l s e r a d i o l y s i s sample p r e p a r a t i o n and i r r a d i a t i o n v e s s e l . The o p t i c a l c e l l (20 X 10 X 5 mm) has S p e c t r o s i l windows. - 82 -Samples of HMPA c o n t a i n i n g ve r y s m a l l amounts of n i t r o u s o x i de were p repa red by q u a n t i t a t i v e l y f r e e z i n g measured amounts of the gas d i r e c t l y i n t o the r a d i o l y s i s c e l l . The r e s u l t i n g s cavenge r c o n c e n t r a t i o n c o u l d then be c a l c u l a t e d from known p a r a m e t e r s . That i s , assuming i d e a l gas b e h a v i o r , the number of m o l e s , n, of n i t r o u s o x i d e t r a n s f e r r e d from a sma l l bu lb on the vacuum l i n e i s g i v e n by ( x x i i ) . P I V I R T I ( x x i i ) where Pj- = N',,0 p r e s s u r e i n bu lb (mm Hg) V j. = bul b v o l ume (1) T j = bu lb t empe ra tu re (°K) • R ' = 62,3 mm Hg ° K _ 1 mole _ 1 A f t e r q u a n t a t i v e t r a n s f e r to a r a d i o l y s i s c e l l c o n t a i n i n g a known amount o f HMPA, the n i t r o u s o x i d e has components i n both the l i q u i d and gas phases ( x x i i i ) . P F v r  r b + s p p v L RT C r >--\ F ( x x i l 1 ) where Pp = p r e s s u r e of n i t r o u s o x i d e (mm Hg) above HMPA Vg = volume o f gas space i n r a d i o l y s i s c e l l (1) V L = volume o f HMPA l i q u i d (1) S = s o l u b i l i t y o f N 2 0 i n HMPA ( 1 . 8 5 x l 0 ~ 4 M/mm Hg) - 83 -Tp = r a d i o l y s i s c e l l t empe ra t u r e ( "K) Assuming the t empe ra tu re of the two bu lb s were equa l ( i . e . a l l o w e d to reach ambient room t e m p . ) , one can combine e q u a t i o n s ( x x i i ) and ( x x i i r ) • o b t a i n i n g the f i n a l e q u i l i b r i u m p r e s s u r e of NgO i n the r a d i o l y s i s c e l l ( x x i v ) , V P P F -( V R + V,SRT) / . \ v G L ' ( x x i v ) and t h e r e f o r e the c o n c e n t r a t i o n o f NgO i n s o l u t i o n , jj^ oj M^PA' by e q u a t i o n ( x x v ) . N 2 0 J HMPA P F S ( xxv ) For 100 mm N,,0 i n a 13.37 ml bu lb t r a n s f e r r e d to a c e l l c o n t a i n i n g 86.0 ml HMPA and hav ing a gas space o f 20.1 ml the f i n a l c o n c e n t r a t i o n o f N 2 0 i n HMPA was c a l c u l a t e d to be (7.9 ± 0.2) X 1 0 " 4 M . 3. Pu l s e R a d i a t i o n Source A V a r i a n V-7715A microwave l i n e a r a c c e l e r a t o r ( l i n a c ) was used as a sou rce o f h i gh energy e l e c t r o n s . T h i s machine produced e l e c t r o n p u l s e s i n the energy range 2 - 6 MeV w i t h pu l s e w i d t h c o n t i n u o u s l y v a r i a b l e from 0.005 to 1.6 y s e c . Most of the p r e s e n t e xpe r imen t s u t i l i z e d s i n g l e p u l s e s of 4 MeV . ; , ; , v . ' - 84 - , e l e c t r o n s at a p u l s e c u r r e n t o f 300 mA, c o r r e s p o n d i n g to a dose r a t e of about l O 1 ^ rad s e c - 1 . P u l s e to p u l s e r e p r o d u c -i b i l i t y of the a c c e l e r a t o r was g e n e r a l l y b e t t e r than ±5% o f the mean. Through the use o f a f e r r o e l e c t r i c d e v i c e , i t was p o s s i b l e to examine i n d e t a i l the shape, i n t e n s i t y , and d u r a t i o n of r a d i a t i o n p u l s e s . That d e v i c e made from a ce r am i c m a t e r i a l c o n t a i n i n g l ead z i r c o n a t e t i t a n a t e (PZT) has a l a r g e p y r o -e l e c t r i c c o e f f i c i e n t . I n c i d e n t r a d i a t i o n produces an i n s t a n t a n -eous sma l l t empe ra tu re i n c r e a s e i n the d e v i c e which d e l i v e r s an e l e c t r i c a l impu l se c o r r e s p o n d i n g to the t ime r a t e o f t empera tu re change. The d e v i c e was p a r t i c u l a r l y u s e f u l f o r m o n i t o r i n g the l i n a c p u l s e s because i t had a r i s e t ime of l e s s than two nsec and responded l i n e a r i l y to even ve r y i n t e n s e energy f l u x e s . F i g u r e 11-11 d e p i c t s the re sponse o f the PZT d e v i c e to a 600 nsec e l e c t r o n p u l s e . As can be s e e n , t h e l i n a c produces r emarkab l y n e a r - r e c t a n g u l a r p u l s e s o f e l e c t r on s/ . Hrel) — » rsec •4-if" 1 F i g u r e 11 -11 . Shape of l i n a c e l e c t r o n p u l s e . - 85 -4. O p t i c a l D e t e c t i o n a) A n a l y s i n g L i g h t Source T r a n s i e n t s p e c i e s produced i n the sample were s t u d i e d by o b s e r v i n g t h e i r o p t i c a l a b s o r p t i o n s p e c t r o p h o t o m e t r i c a l l y . A 500 wat t Osram xenon -a r c lamp, type XBO, was used as w h i t e l i g h t s o u r c e . The lamp c o u l d be ope r a t ed i n e i t h e r c o n s t a n t i n t e n s i t y or f l a s h e d modes. When a p u l s e d c u r r e n t of 150A was added to the c o n t i n u o u s o p e r a t i o n c u r r e n t o f 20A, the lamp i n t e n s i t y was i n c r e a s e d by a f a c t o r of 25 to 40 depend ing on w a v e l e n g t h . The r e s u l t i n g l i g h t p u l s e l a s t e d s e v e r a l m i l l i s e c o n d s and had a u s ab l e c o n s t a n t i n t e n s i t y p o r t i o n of about 100 ysec which c o u l d be s y c h r o n i z e d w i t h the a c c e l e r a t o r e l e c t r o n beam. F i g u r e 11-12 shows the p r o f i l e o f a t y p i c a l f l a s h . 0 — 0-2, Krel) F i g u r e IT -12 . P r o f i l e of the ou tpu t from the Xenon -a rc lamp used i n pu l s e mode. - 86 -The ou tpu t from the lamp was f o cu s sed onto the r e a c t i o n c e l l and aga in on the monochromator e n t r a n c e s l i t . U n t i l j u s t p r i o r to each r a d i a t i o n p u l s e , the s h u t t e r was c l o s e d to p r e ven t h e a t i n g and p h o t o l y s i s o f samples by the lamp - -o p e r a t i n g i n i t s c o n s t a n t i n t e n s i t y mode. i b) Wavelength S e l e c t i o n A Bausch and Lomb f /3 . 5 g r a t i n g monochromator, t ype 3 3 -86-26 was used i n c o n j u n c t i o n w i t h a number o f g r a t i n g s . T a b l e l i s t s p e r t i n e n t i n f o r m a t i o n f o r the g r a t i n g s . TABLE IV Bausch and LOmb G r a t i n g s used i n the p u l s e r a d i o l y s i s e x p e r i m e n t s . G r a t i n g Type . S p e c t r a l Reg ion (nm) D i s p e r s i o n (nm/mm) 33-86-02 33-86-03 33-86-04 350 - 800 700 - 1600 1400 - 3200 6.4 12.8 25.6 Each g r a t i n g was c a l i b r a t e d i n the monochromator w i t h r e f e r e n c e to l i n e e m i s s i o n s from a lamp c o n t a i n i n g Hg, A r , Xe, and He. A p p r o p r i a t e Co rn i n g g l a s s f i l t e r s were p l a c e d i n the o p t i c a l path - - b e f o r e the r a d i o l y s i s c e l l to p r e v e n t sample p h o t o l y s i s - - and b e f o r e the monochromator to e l i m i n a t e s e cond -o r d e r components ( e s p e c i a l l y from Cerenkov r a d i a t i o n ) and the - 87 -e f f e c t s of s c a t t e r e d l i g h t , c) Detec t i on T r a n s i e n t a b s o r p t i o n s below 1200 nm were measured th rough the use of one of s e v e r a l p h o t o m u l t i p i i e r tubes (RCA 7200, 7102, 1P28 o r HTV 196 ) . For wave leng th s between 800 and 2300 nm an i n d i u m - a n t i m o n i d e pho tod i ode was employed. T h i s d e v i c e was mounted beh ind a s a p p h i r e window on a c o l d f i n g e r of a l i q u i d n i t r o g e n dewar. I t had a s e n s i t i v e a rea o f about 0.04 cm and was connec ted d i r e c t l y to the i n p u t of a h i g h -speed s o l i d s t a t e o p e r a t i o n a l a m p l i f i e r ! 2 The 10 - 90% r i s e -t ime of t h i s i n f r a - r e d d e t e c t o r was about 100 n s e c . E xpe r imen t s g e n e r a l l y u t i l i z e d a T e k t r o n i x 545 dua l beam o s c i l l o s c o p e . For some s t u d i e s , where f a s t e r t ime r e s o l u t i o n was p a r t i c u l a r l y d e s i r a b l e , a p h o t o m u l t i p i i e r system w i t h a 2 nsec r i s e - t i m e (RCA 1P28) was used w i t h a F a i r c h i l d 777V o s c i l l o s c o p e . A r e s o l u t i o n t ime of 5 nsec was a t t a i n e d w i t h t h i s a r rangement . Permanent r e c o r d s o f the t r a n s i e n t s i g n a l s were c a p t u r e d on P o l a r o i d speed 3000 or 10,000 f i l m . In a d d i t i o n , the v a r i o u s s h u t t e r s and pu l s e s were t imed such t h a t t r a c e s from s i g n a l s r e p r e s e n t i n g ze ro and 100 p e r c e n t t r a n s m i s s i o n of the a n a l y z i n g l i g h t th rough the r e a c t i o n m i x t u r e were a l s o r e c o r d e d on each pho tog raph . F i g u r e 11-13 i s a t r a c i n g o f a t y p i c a l pho tog raph i n which these f e a t u r e s can be seen . F i g u r e 11-13. T y p i c a l o s c i l l o s c o p e ' t r a c e showing a t r a n s i e n t a b s o r p t i o n i n HMPA f o l l o w i n g a 400 nsec r a d i a t i p u l s e . I t can be seen t h a t th rough the j u d i c i o u s s e l e c t i o n o f e x p e r i m e n t a l pa rameter s one c o u l d o b t a i n a d e t a i l e d h i s t o r y of the f o r m a t i o n and decay o f t r a n s i e n t s p e c i e s f o l l o w i n g i r r a d i a t i o n . d) Do s imet r y Measurement o f the energy absorbed by the samples per r a d i a t i o n pu l s e was per fo rmed th rough the use of aqueous po ta s s i um t h i o c y a n a t e as a chemica l d o s i m e t e r . Do s imet r y was c a r r i e d out i n c e l l s o f the same type and d imens i on s as those c o n t a i n i n g the samp les . Measurements were made j u s t p r i o r to i n v e s t i g a t i o n s on v a r i o u s s amp le s . A f t e r c o m p l e t i o n of a s e r i e s o f e xpe r imen t s (o r p e r i o d i c a l l y d u r i n g l o n g 89 -e x p e r i m e n t s ) dose measurements were aga i n made to ensu re t h a t the a c c e l e r a t o r c o n d i t i o n s had not changed. R a d i o l y s i s o f aqueous KCNS s o l u t i o n s g i v e s r i s e to a l ong l i v e d s p e c i e s hav ing an a b s o r p t i o n maximum at 475 nm. Curve A i n F i g u r e 11-14 shows the spect rum o b t a i n e d from a s o l u t i o n o f 0.1 M KCNS s a t u r a t e d w i t h n i t r o u s o x i d e . The a b s o r p t i o n had been a s s i g n e d 8 3 to the t h i o c y a n a t e r a d i c a l formed from r e a c t i o n of h y d r o x y l r a d i c a l s ( 2 . 1 0 ) . OH + CMS" • CNS + OH" (2 .10) L a t e r , Adams et a l ? 1 * r e p o r t e d a v a l u e f o r the mo la r a b s o r p t i v i t y 3 -1 -1 e 500 = 7 ' 1 x 1 0 M c m a n d conceeded t h a t the da t a was a l s o c o n s i s t e n t w i t h the t r a n s i e n t be ing a d i m e r i c s p e c i e s , (CNS) " ( 2 . 1 1 ) . CNS + CNS • (CNS) " ( 2 n ) Baxenda le et a l ? 5 c o n f i r m e d the f o r m a t i o n of t h i s d i m e r i c s p e c i e s . Behar and c o - w o r k e r s 8 6 ob se r ved two t r a n s i e n t i n t e r m e d i a t e p r e c u r s o r s of (CNS)! - w i t h maxima a t 330 nm and 390 nm. The f o rmer was a t t r i b u t e d to the t h i o c y a n a t e r a d i c a l , CNS and the l a t e r was a s s i g n e d to a new i n t e r m e d i a t e s p e c i e s , CNSOH" ( 2 . 12 , 2 . 13 ) , OH + CNS * CNSOH (2.12) 7 3 5 7 7 5 F i g u r e 11-14 W A V E L E N G T H ( n m ) R a d i a t i o n D o s i m e t r y . T r a n s i e n t a b s o r p t i o n s r e s u l t i n g from 100 nsec p u l s e s o f 4 MeV e l e c t r o n s ob se r ved i n a c e l l hav ing an o p t i c a l path l e n g t h of 20 mm and c o n t a i n i n g A, aqueous KCNS s o l u t i o n and B, t r i n l y d i s t i l l e d w a t e r . - 91 -CNSOH" • CNS + OH" (2.13.) In n e u t r a l w a t e r , 6(OH) = 2.9 and f o r s o l u t i o n s c o n t a i n i n g -3 g r e a t e r than about 5 X 1 0 M CNS , a l l r e a c t i o n s o f h yd r o x y ! r a d i c a l s save (1 .10) are s u p p r e s s e d . For such s o l u t i o n s G ^ C N S ) ' ) = G(0H) = 2.9. In the p re sence of a s a t u r a t e d s o l u t i o n of n i t r o u s o x i d e , h yd ra ted e l e c t r o n s are c o n v e r t e d to hyd roxy ! r a d i c a l s ( 2 . 1 4 ) . aq 2 H2° N 2 ° ] * 0 H + 0 H ~ + N 2 (2 .14) Under those c o n d i t i o n s , the (CNS)^ y i e l d i s g i v e n by ( x x v i ) . G ( ( C N S ) - ) = G(0H) + G ( , ; q ) - 5.7 ( x x y 1 ) where the h yd ra ted e l e c t r o n y i e l d i s t aken as 2 .8. For a s p e c i e s of known G v a l u e and mo la r a b s o r p t i v i t y , absorbed dose , D, i s g i v e n by ( x x v i i ) . D ( e V 1 ) = 0 elG ( x x v i i ) where A = absorbance of d o s i m e t e r s p e c i e s N Q = Avagadro's number (molec m o l e - ^ ) e = mo la r a b s o r p t i v i t y of d o s i m e t e r s p e c i e s (mol e"^ 1 cm"^ ) - 92 -1 = o p i t i c a l path l e n g t h (cm) G = G v a l ue of d o s i m e t e r s p e c i e s (molec (100 e V ) - 1 ) The dose absorbed by the HMPA samples was o b t a i n e d by c o r r e c t i n g f o r the r e l a t i v e e l e c t r o n d e n s i t i e s o f HMPA and w a t e r . The 20 absorbed dose from a 400 nsec p u l s e was t y p i c a l l y 4 X 10 eV/1. Pu l s e to p u l s e r e p r o d u c i b i l i t y of the a c c e l e r a t o r was u s u a l l y b e t t e r than ±5% o f the mean. As a check , a second d o s i m e t e r - - pure wate r - - was employed. Curve B i n F i g u r e 11-14 shows the s p e c t r a o b t a i n e d i n the r e g i o n near 715 nm from the r a d i o l y s i s o f t r i p l y d i s t i l l e d w a t e r . A v a l ue o f G e e a q = 5.0 X 1 0 4 M " 1 c m " 1 molec (100 e V ) " 1 71 5 was taken f o r the h yd ra ted e l e c t r o n a t 715 nm! 7 Because the h yd ra ted e l e c t r o n i s r e l a t i v e l y s h o r t - l i v e d , the r e s u l t s were c o r r e c t e d to account f o r decay d u r i n g the p u l s e . The two d o s i m e t r y t e c h n i q u e s gave r e s u l t s w i t h i n about 10% of each o t h e r . The r e s u l t s of the t h i o c y a n a t e d o s i m e t e r , by v i r t u e of the s t a b i l i t y of i t s a b s o r b i n g s p e c i e s , were averaged and used i n the c a l c u l a t i o n s . - 93 -, CHAPTER I I I RESULTS AND DISCUSSION A. RESULTS: STEADY STATE EXPERIMENTS 1. Gamma R a d i o l y s i s S t u d i e s i I a) Pure HMPA Gamma r a d i o l y s i s o f samples of HMPA at 23 + 2 t y i e l d e d hydrogen and methane as gaseous p r o d u c t s . F i g u r e I I1-1 shows the y i e l d s o f these p r o d u c t s o b t a i n e d from f o u r d i f f e r e n t samples of HMPA as a f u n c t i o n o f accumu la ted dose . The methane y i e l d was i ndependent of dose up to at l e a s t 1.5 X 10^ rads (9.4 X 1 0 1 9 eV g " 1 ) . From the s l o p e of the l i n e the y i e l d of methane, G ( C H 4 ) , e xp re s sed as m o l e c u l e s per 100 eV of energy ab s o r bed , was c a l c u l a t e d to be: G(CH 4 ) = 0.29 ± 0 . 0 3 As can be seen i n the f i g u r e , the hydrogen y i e l d depen -dence was more complex. The curves were d i f f e r e n t f o r the samp le s , but each e x h i b i t e d a f i n a l l i n e a r p o r t i o n c o r r e s p o n d i n g to G (H 2 ) = 3.3 ± 0 . 3 . The e f f e c t can be seen more c l e a r l y i n F i g u r e I I I - 2 . The re , hydrogen and methane y i e l d s ( c a l c u l a t e d from the s l o p e d( product}/d(Dose)) f o r each i r r a d i a t i o n a re - 94 F i c u r e 111-1 A C C U M U L A T E D D O S E ( M r a d ) Tot ? ] p r o d u c t i o n o f hydrogen ( O ) and methane ( • ) as a f u n c t i o n of t o t a l absorbed dose from the r a d i o l y s i s of f o u r d i f f e r e n t samples o f HMPA. 1 0. 0 0 .5 1.0 1.5 A C C U M U L A T E D D O S E ( M r a d ) F i g u r e 111 - 2. A c t u a l y i e l d s of hydrogen ( O ) and methane ( • ) as a f u n c t i o n of t o t a l absorbed dose f o r the da ta of Ffg-jre I I M . - 96 -p l o t t e d as a f u n c t i o n o f t o t a l absorbed dose. The r e s u l t s suggest t h a t the samples i n i t i a l l y c o n t a i n e d low c o n c e n t r a -t i o n s of i m p u r i t y which r e a c t e d w i t h a p r e c u r s o r of m o l e c u l a r hydrogen and was consumed i n the p r o c e s s . The samples were p repared from d i f f e r e n t batches of t e c h n i c a l grade HMPA. S p e c i a l ca re was taken i n the p r e p a r a t i o n o f sample f o u r . That sample was s u b j e c t e d to s e v e r a l a d d i t i o n a l s t age s o f d i s t i l l a t i o n from sod ium. In a d d i t i o n , i t was s t o r e d and t r a n s f e r r e d under vacuum. The hydrogen y i e l d da t a shown i n F i g u r e 111 - 2 s uppo r t s the " i m p u r i t y " t h e o r y as t h a t sample q u i c k l y a t t a i n e d the p l a t e a u hydrogen y i e l d v a l u e . R a d i o l y s i s o f the t e c h n i c a l grade HMPA gave a c o n s t a n t low hydrogen y i e l d G(H 2 ) = 1.3 ± 0.2 f o r a t o t a l dose up to 2 Mrad. One c o u l d not e xpec t to s u b j e c t samples of HMPA to u n l i m i t e d doses o f r a d i a t i o n w i t h o u t a f f e c t i n g the p r i m a r y p r o c e s s e s . I ndeed, the p r o d u c t i o n of hydrogen and methane gave e v i d e n c e t h a t m o l e c u l a r d e g r a d a t i o n o c c u r s . O b v i o u s l y o t h e r l i q u i d p r o d u c t s were a l s o produced i n c o n j u n c t i o n w i t h those m o n i t o r e d . T h e r e f o r e , when i t was c l e a r t h a t a " p u r e " sample c o n d i t i o n had been o b t a i n e d , v a r i o u s s cavenge r s were added p r i o r to subsequent i r r a d i a t i o n so as to f a c i l i t a t e much more i n f o r m a t i v e s t u d i e s o f the system and i t s r e a c t i o n s . A f t e r a number o f e xpe r imen t s i n which s cavenge r s were u s e d , the samples were once aga in i r r a d i a t e d w i t h no added s o l u t e s . Return of the " p l a t e a u " hydrogen y i e l d i n d i c a t e d t h a t i m p u r i t y b u i l d - u p was p r o b a b l y u n i m p o r t a n t . - 97 -b.) Scavenger S t u d i e s i ) N i t r o u s Ox ide ' N i t r o u s o x i d e was used e x t e n s i v e l y i n t h i s s tudy f o r the purpose o f d e t e r m i n i n g the y i e l d and n a t u r e o f the r e d u c i n g s p e c i e s formed d u r i n g the r a d o l y s i s o f HMPA. F o l l o w i n g i r r a d i a t i o n ^ samples o f HMPA c o n t a i n i n g d i s s o l v e d N^O were found to c o n t a i n s i g n i f i c a n t amounts o f m o l e c u l a r n i t r o g e n . For a g i ven c o n c e n t r a t i o n o f N 2 0 , the n i t r o g e n y i e l d was i ndependent o f dose and t o t a l absorbed dose . Methane gas p r o d u c t i o n from HMPA r a d i o l y s i s was u n a f f e c t e d by the p re sence of N 2 0 . The hydrogen y i e l d on the o t h e r hand was reduced from an i n i t i a l v a l u e of G (H 2 ) = 3.3 ± 0.3 t o a c o n s t a n t v a l u e o f G (H 2 ) = 1.4 + 0.1 f o r any s o l u t i o n c o n t a i n i n g -3 g r e a t e r than about 10 M N,,0. E v i d e n t l y N 2 0 r e a c t s e f f i c i e n t l y w i t h a p r e c u r s o r o f some o f the hydrogen. F i g u r e 111-3 shows a p l o t of the obse rved n i t r o g e n y i e l d as a f u n c t i o n o f N 2 0 c o n c e n t r a t i o n . The f i l l e d c i r c l e s r e p r e s e n t those e xpe r imen t s i n wh ich argon " c a r r i e r gas " was used to ensure thorough m i x i n g of sma l l amounts o f N 2 0 . The n i t r o g e n y i e l d i n c r e a s e d r a p i d l y to G(N 2 )>3 f o r N 2 0 up to _ p about 1 X 10 M and then i n c r e a s e d more s l o w l y t o G (N 2 ) > 4.5 at 1.3 X 1 0 - 1 M N' 20. For ve r y low c o n c e n t r a t i o n s o f N 2 0 i t i s somewhat m i s l e a d i n g to r e f e r to " y i e l d s " o f m o l e c u l a r p r oduc t s t h a t are a f f e c t e d by the p resence of t h a t s c a venge r . For the HMPA sample s i z e and F i g u r e 111-3. N i t r o g e n y i e l d c o n c e n t r a t i o n . f rom i r r a d i a t e d samples o f HMPA as a f u n c t i o n of n i t r o u s o x i d e - 99 -j r a d i a t i o n doses employed i n these e x p e r i m e n t s , a p p r o x i m a t e l y -4 1 5 X 10 M r e d u c i n g s p e c i e s ( w i t h a y i e l d o f 2) would be formed d u r i n g the r a d i o l y s i s . A s c a veng i n g r e a c t i o n w i t h N^O c o u l d r e s u l t i n a s u b s t a n t i a l dec r ea se - - p o s s i b l e d e p l e t i o n - - o f s cavenge r from samples i n i t i a l l y c o n t a i n i n g sma l l amounts o f N , ,0. P r o d u c t " y i e l d s " f r om such s o l u t i o n s a re denoted as p s e u d o - y i e l d s , G ' ( X ) , so as to d i f f e r e n t i a t e them from y i e l d s o b t a i n e d under c o n s t a n t s cavenger c o n c e n t r a t i o n . F i g u r e 111 -4 d e t a i l s the p s e u d o - y i e l d s o f n i t r o g e n and hydrogen o b t a i n e d f rom the r a d i o l y s i s o f s o l u t i o n s i n i t i a l l y c o n t a i n i n g l e s s than 1 X 10~ 3 M N 2 0 . Between 0 and 5 X 1 0 - 4 M N 2 0 , the hydrogen p s e u d o - y i e l d dec rea sed from G ' ( H 2 ) = 3.4 ± 0.3 to G ' ( H ) = 1.4 ± 0 . 1 . Over t h i s same r ange , n i t r o g e n f o r m a t i o n i n c r e a s e d from 0 to G ' (N,>) = 2.2 ± 0 .2 . I t i s wor thy o f a t t e n t i o n t h a t the sum of the two " y i e l d s " remained f a i r l y c o n s t a n t t h r o u g h o u t , w i t h G ' ( H g + N 2 ) = 3.4 ± 0.3 to 3.7 ± 0 . 3 . These r e s u l t s were more s u g g e s t i v e o f the e xpec ted " d e p l e t i o n " e f f e c t . To c o n f i r m t h i s s u s p i c i o n , samples of ve r y pure HMPA c o n t a i n i n g a c o n s t a n t low i n i t i a l c o n c e n t r a t i o n of N 2 0 (^ 7 X 10 " 5 M) were i r r a d i a t e d f o r d i f f e r e n t l e n g t h s of. t i m e . That c o n c e n t r a t i o n co r re sponded to o n l y one or two m i c romo le s of N 2 0 i n the samp les . The obse rved e f f e c t o f dose on p r o d u c t " y i e 1 d s" i s g i v e n i n T a b 1 e V. As one would e x p e c t , the a b s o l u t e amounts o f methane, hydrogen and n i t r o g e n formed d u r i n g r a d i o l y s i s i n c r e a s e d w i t h dose. N i t r o g e n , however, r eached a l i m i t i n g v a l u e o f 1.2 F i g u r e 111 - 4. Observed hydrogen ( • ) and n i t r o g e n ( O ) " y i e l d s " from i r r a d i a t e d samples o f HMPA c o n t a i n i n g v e r y low i n i t i a l N,0 c o n c e n t r a t i o n s . TABLE V E f f e c t of dose on gaseous p roduc t " y i e l d " i n HMPA samples i n i t i a l T y -5 c o n t a i n i n g 7 X 10 M n i t r o u s o x i d e . Dose ( rad s X 104) P roduc t Fo rmat i on ( ymo les ) CH4 N2 H2 G'(CH4) Product G'(N2) " Y i e l d " G'(H2) G'(N2 + H2) 1 .74 0.1 0.7 0.9 0.3 • 1.5 1 .9 3 4 3.45 0.2 0.9 2.0 0.3 1.0 2.2 3 2 5.16 0.4 1 .2 3.4 0.3 0.9 2.5 3 4 6.87 0.5 1 .2 4.8 0.3 0.6 2.7 3 3 - 102 -micromo le s f o r . t h e l a r g e r doses - - a v a l u e w i t h i n e x p e r i m e n t a l e r r o r of the t o t a l e s t i m a t e d i n i t i a l N,>0 c o n t e n t . P r o d u c t y i e l d s ( p r o d u c t formed per u n i t dose) f rom s t u d i e s u t i l i z i n g h igh c o n c e n t r a t i o n s o f s cavenge r were i ndependent o f dose and t o t a l absorbed dose. Here , o n l y the methane " y i e l d " was i ndependent of dose. The n i t r o g e n " y i e l d " dec rea sed w i t h dose w h i l e t h a t of H 2 i n c r e a s e d . A g a i n , the sum o f the H 2 and N 2 p s e u d o - y i e l d s remained c o n s t a n t a t G 7 ( H 2 + N 2 ) = 3.3 ± 0.1. The f a c t s are c o n s i s t e n t w i t h a scheme i n wh ich n i t r o u s ox i de scavenges a p r e c u r s o r of some o f the hyd rogen , and t h a t as n i t r o u s o x i d e i s d e p l e t e d the hydrogen f o r m a t i o n r e a c t i o n p r edom ina te s . Note t h a t hydrogen f o r m a t i o n i s not p r e c l u d e d by the H^O s c a veng i n g r e a c t i o n but r a t h e r a s i m p l e c o m p e t i t i o n f o r a p r e c u r s o r e x i s t s . Two f a c t s f rom the t a b l e p o i n t to t h i s c o n c l u s i o n . F i r s t l y , i n the l ower dose e x p e r i m e n t s (where N 20 was not d e p l e t e d ) the hydrogen p s e u d o - y i e l d s t i l l i n c r e a s e d w i t h dose. S e c o n d l y , even f o r the s m a l l e s t dose s t u d i e d ( 1 . 7 X 1 0 4 r ad s ) the hydrogen y i e l d G ' ( H 2 ) = 1.9 was h i g h e r than the p l a t e a u G ( H 2 ) = 1.4 ± 0.1 o b t a i n e d from samples c o n t a i n i n g > 1 0 " 3 M N 20. i i ) N i t r o u s Oxide P l u s a Second Scavenger In o r d e r to e l u c i d a t e the na tu re of the N 2 p r e c u r s o r the e f f e c t o f the a d d i t i o n of a second s cavenge r on gaseous p r o d u c t y i e l d s i n HMPA and s o l u t i o n s of N 20 i n HMPA was i n v e s t i g a t e d . S e v e r a l known r a d i c a l and e l e c t r o n s cavenger s were employed. - 103 -Some of the da ta are summarized i n Tab l e V I . S i l v e r n i t r a t e , and s u l p h u r i c a c i d were u n s u i t a b l e as second s c a v e n g e r s , as the a d d i t i o n o f these s ub s t ance s l e d to d e c o m p o s i t i o n o f HMPA. Smal l amounts of g a l v i n o x y l (Gv ) , a s t a b l e f r e e r a d i c a l , or i o d i n e i n HMPA y i e l d e d s o l u t i o n s hav ing a b s o r p t i o n s p e c t r a c h a r a c t e r i s t i c o f k n o w n i o n i c s p e c i e s , s u g g e s t i v e o f i n t e r a c t i o n i • • w i t h s o l v e n t or i m p u r i t i e s . The v i s i b l e a b s o r p t i o n s p e c t r a o f g a l v i n o x y l i n HMPA e x h i b i t e d i n t e n s e bands at 430 nm and 580 nm as shown i n F i g u r e 111 - 5. The. bands were s i m i l a r i n p o s i t i o n and shape to those a t t r i b u t e d to the g a l v i n o x y l f r e e r a d i c a l (Gv) i n i s o - o c t a n o l and the g a l v i n o x y l r a d i c a l an i on (Gv " ) i n a l k a l i n e e t h a n o l 8 8 The s p e c i e s have r e p o r t e d e x t i n c t i o n c o e f f i c i e n t s of 1.8 X 1 0 " 5 M " 1 cm " 1 and 2.2 X 1 0 5 M " 1 c m " 1 r e s p e c t i v e l y i n • those s o l u t i o n s . A l s o shown i n the f i g u r e i s the s p e c t r a o b t a i n e d f o r a s o l u t i o n o f g a l v i n o x y l i n c y c l o h e x a n e , where o n l y the f r e e r a d i c a l band was o b s e r v e d . I t would appear t h a t i n HMPA, g a l v i n o x y l r e a c t e d w i t h the s o l v e n t (3 .1 ) o r some i m p u r i t y , Im, (3 .2 ) to form the g a l v i n o x y l a n i o n . Gv + HMPA • HMPA + + Gv" ( 3 .1 ) Gv + Im — • Im + + Gv" (3 .2 ) E xam ina t i on of the a b s o r p t i o n s p e c t r a o f HMPA samples c o n t a i n i n g d i f f e r e n t amounts of g a l v i n o x y l would tend to f a v o u r r e a c t i o n w i t h i m p u r i t y ( 3 . 2 ) . From the obse rved a b s o r b a n c e s , - 104 -TABLE VI M o l e c u l a r p r oduc t y i e l d s from the i r r a d i a t i o n o f HMPA samples c o n t a i n i n g known amounts o f N,,0 and a second s c a venge r . Second Scavenger [s2](M) [N 2(](M) G ( N 2 ) ' G ( H 2 ) G ( C H 4 ) N I L ( o n l y N ? 0 ) 3 . 2 A 0.29 - : 0.0005 2.2 1.4 0.28 0.001 2.5 1.4 0.27 0.01 3.4 1.4 0.29 0.10 4.4 1.4 0.26 Water 0.22 - - 1.4 0.27 0.22 0.016 3.3 1.4 0.27 0.22 0.030 3.6 1.4 0.27 0.22 0.045 3.8 1.4 0.25 1.0 0.045 3.8 1.4 0.24 Methanol 0.10 - - 1.4 0.27 0.10 0.015 3.3 1.4 0.28 0.10 0.030 3.7 1.4 0.27 0.10 0.044 3.9 1.4 0.26 1.0 0.030 3.4 1.4 0.27 Acetone 0.03 - - 1.3 0.31 0.03 0.015 3.5 1.3 0.26 0.03 0.030 4.2 1.4 0.28 0.03 0.044 4.2 1.3 0.29 - 105 -TABLE VI ( c o n t i n u e d ) • i Second i Scavenger [ s 2 ] ( » ) [N 2O](M) G ( N 2 ) G ( H 2 ) G ( C H 4 ) Acetone 0.27 0.013 3.8 . 1 .3 0.28 ( c o n ' t ) 0.27 0. 05 4.6 1.4 0.26 0.27 0.11 5.1 1 .4 0.28 CHC1 3 0.015 0.015 1 .5 1.4 0.25 0.015 0.029 2.2 1 .4 0.26 0. 015 0.043 2.6 1 .4 0.26 0.045 0. 01 5 0.8 - 1 . 3 0.22 0.045 0.029 1.2 1 .3 0.24 0.045 0.044 1 .6 1 .3 0.24 c c i 4 0.01 5 0.015 1 .4 1.3 0.25 0.015 0.029 2.2 1.3 0.25 0.01 5 0.043 2.6 1 .4 0.25 0.030 0.015 0.9 1 .4 0.24 0.030 0.020 1.2 1.3 0.25 0. 030 0. 029 1 .4 1.3 0.26 0.030 0.044 1.8 1.3 0.26 0.045 0.016 0.6 1.3 0.22 0. 045 0.029 1.1. 1 .3 0.22 0. 045 0.044 1 .5 1.3 0.24 0.10 • 0.011 0.5 1 .1 0.12 0.10 0.018 0.7 1 .2 0.11 - 106 -TABLE VI ( c o n t i n u e d ) Second ! Scavenger [S2](M) |N20|(M) G (N 2 ) G (H 2 ) G(CH 4 ) I 2 ( c o n ' t ) 0.1 0 0.10 0.10 0.028 0. 051 0.133 1 .0 1 .5 2.6 1 .2 1 .1 1 .1 0.13 0.12 0.10 G a l v i noxy l 0.0001 1 0.014 3.2 1 .4 0.25 0.0025 - - 1 .4 0.26 0.0025 0.008 2.5 1 .4 0.26 0.0025 0.014 2.8 1 .4 0.26 0.0025 0.024 3.1 1 .4 0.25 0.002.5 0.053 3.6 1 .4 0.26 0. 046 0.011 0.7 1 .2 0.12 0.046 0.014 0.8 T .2 0.13 0.046 0.025 1.2 1 .2 0.11 0.046 0.050 1 . 9 1 .2 0.12 0.046 0.124 2.7 1 .2 0.11 0.002 0.002 0.002 0.002 0.011 0.014 0.026 0.042 0.059 0.014 3.3 3.4 3.9 4.3 3.1 1 .4 1 .4 1 .5 1 .5 1.4 0.24 0.24 0.25 0.24 0.21 to, 0.001 0.01 5 0.014 0.014 3.2 3.3 1 .4 1 .4 0.27 0.25 - 107 -TABLE VI ( c o n t i n u e d ) Second Scavenger [s2]w [N2O](M) G(N2) G(H2) G(CH4) C02 ( c o n ' t ) 0.034 0.014 3.3 1 .5 0.26 0.011 - - 1.3 0.24 L i Br 0.20 - -_ 1.3 0.28 0.20 .0.014 3.6 1 .4 0.25 0.20 0. 028 3.8 1 .4 0.23 0.20 0.044 4.1 1 .4 0.24 0.20 0.060 4.3 1 .4 0.23 0.20 0.12 4.5 1 .4 0.25 a da ta a c c u r a c y e s t i m a t e d to be ± (5 to 10)%. W A V E L E N G T H ( n m ) Figure III-5. Spectra obtained from a 1mm cell containing: •"2.5 -X-10" M Gv in HMPA (—) ; 1.1 X 10"*Gv in HMPA . (——) ; . 1.4 X 10 M Gv in cvclohexane ( •). - 109 -and assuming t h a t ^ t h e e x t i n c t i o n c o e f f i c i e n t s f o r the r a d i c a l and i o n i c g a l v i n o x y l s p e c i e s i n HMPA are i d e n t i c a l t o t ho se r e p o r t e d f o r the o t h e r s o l v e n t s , one c a l c u l a t e s t h a t 1.1 X 10 5 M g a l v i n o x y l i n HMPA produces a s o l u t i o n c o n t a i n i n g . 5 X 10" M r a d i c a l s p e c i e s (Gv) and 5 X 10 M i o n i c s p e c i e s ( G _ ) 1 3 For a more c o n c e n t r a t e d s o l u t i o n ( c o n t a i n i n g 2.5 X 10" M g a l v i n o x y l i n HMPA) the an ion band i n t e n s i t y i n c r e a s e ^ o n l y two f o l d , r e p r e s e n t i n g the p re sence o f o n l y 1.0 X 10 " M g a l v i n o x y l a n i o n . The r a d i c a l band i n t e n s i t y on the o t h e r hand i n c r e a s e d many t imes as would be e xpec t ed i f most o f the g a l v i n o x y l were i n i t s r a d i c a l form i . e . , 2.4 X 10 M g a l v i n o x y l r a d i c a l i n the 1 mm s p e c t r o p h o t o m e t e r c e l l u s e d , would have an absorbance a t 430 nm g r e a t e r than f i f t y . These r e s u l t s sugges t t h a t g a l v i n o x y l i n HMPA r e a c t s w i t h i m p u r i t y t o y i e l d sma l l amount o f g a l v i n o x y l an ion but t h a t f o r c o n c e n t r a t i o n s g r e a t e r than 10 " M the bu l k of t he g a l v i n o x y l i n HMPA i s i n i t s f r e e r a d i c a l f o rm. I od i ne a l s o showed unusua l b e h a v i o r when d i s s o l v e d i n HMPA. F i g u r e 111- 6 shows the spect rum o b t a i n e d f o r a s o l u t i o n -4 of 7.6 X 10 M i o d i n e i n HMPA. Th i s s p e c t r a , w i t h peaks at 295 nm and 360 nm, i s compared i n the f i g u r e t o t h a t a t t r i b u t e d to I-, ob se rved i n a s o l u t i o n o f i o d i n e and i o d i d e i o n i n w a t e r . Buck l e s e t a l . r e p o r t e d the s p e c t r a o f \^ l~ and I", o b t a i n e d from s o l u t i o n s of i o d i n e , t e t r a m e t h y l ammonium mono iod ide and t e t r a m e t h y l ammonium t r i i o d i d e i n e t h y l e n e c h l o r i d e ! 9 They found the i o d i n e a b s o r p t i o n to be c e n t e r e d at 500 nm, the i o d i d e i on a b s o r p t i o n below 280 nm and the t r i i o d i d e a b s o r p t i o n w i t h peaks at 295 nm and 365 nm s i m i l a r t o F i g u r e 111- 6. From i t s a b s o r p t i o n s p e c t r a , 7.6 X 10 M I" i n HMPA would appear t o be p r e s e n t e n t i r e l y as the t r i i o d i d e a n i o n , i m p l y i n g r e a c t i o n w i t h s o l v e n t o r i m p u r i t y , Im. P o s s i b l e r e a c t i o n s m i g h t b e : - 1 1 0 -5 5 0 - I l l -I + im « (3 .3 ) 2 — - Im + I 2 I •+ Im + * — ± - Iml + I ( 3 .4 ) 2 I ? + HMPA 4 — * HMPA + I ? ( 3 .5 ) 12 + HMPA + 4 * HMPA + I + I ( 3 .6 ) I 2 < * I + I ( 3 .7 ) l2 + l2 < = * I 3 + I ( 3 .8 ) : l2 + 1 < — h (3 .9) I + 1 4 — l2 (3 .10 ) I + I 2 « =* h (3 .11 ) S i m i l a r f o r m a t i o n of p o l y i o d i d e an ions has been ob se r ved i n p y r i d i n e by Andrev/s e t a l . 0 They a t t r i b u t e d the c o n d u c t i v i t y of s o l u t i o n s o f i o d i n e i n p y r i d i n e to IPy , I", and i Z i o n s . S o l u t i o n s of i o d i n e i n HMPA up t o 0.1M showed s t r o n g a b s o r p t i o n below 500 nm (p resumab ly due to i Z ) but l i t t l e a b s o r p t i o n above 500 nm where I « would be e xpec t ed t o a b s o r b . Thus, u n l i k e f o r g a l v i n o x y l , i m p u r i t y r e a c t i o n w i t h K i s i n s u f f i c i e n t t o a ccoun t f o r the ob se r ved f o r m a t i o n of the p o l y i o d i d e i o n s . The s u i t a b i l i t y o f i o d i n e and g a l v i n o x y l as s cavenge r s i n the HMPA system was v i g o r o u s l y pu r s ued . T h i s was because these s cavenger s o f f e r e d a hope o f o b t a i n i n g i ndependen t y i e l d da ta from s p e c t r o p h o t o m e t r i c means. U n f o r t u n a t e l y , as can be s e e n , both systems were too c o m p l i c a t e d f o r p r a c t i c a l p u rpo se s . Both i o d i n e and g a l v i n o x y l possess l a r g e e l e c t r o n a f f i n i t i e s . Perhaps t h i s combined w i t h the h i gh b a s i c i t y and unusua l s o l v a t i o n p r o p e r t i e s o f HMPA account f o r the obse rved r e s u l t s . The methane y i e l d from i r r a d i a t e d HMPA was l i t t l e a f f e c t e d - 112 -by the p re sence of most s c a v e n g e r s . I o d i ne at 0.10M and g a l v i n o x y l at 0.046M, both e f f i c i e n t e l e c t r o n and r a d i c a l s c a v e n g e r s , reduced the methane y i e l d by about 60% from G(CH 4 ) = 0.29 ± 0.03 to 0.12 ± 0 .02. The hydrogen y i e l d was reduced from G(H 2 ) = 3.3 ± 0.3 to a c o n s t a n t v a l u e o f G (H 2 ) = 1.4 ± 0.1 i n the p re sence o f most o f the s c a venge r s over a l a r g e c o n c e n t r a t i o n range . E x c e p t i o n to t h i s were i o d i n e a t 0.10M and g a l v i n o x y l a t 0.046M which reduced the hydrogen f u r t h e r t o v a l u e s o f G (H 2 ) = 1.1 ± 0.1 and 1.2 ± 0.1 r e s p e c t i v e l y . The y i e l d o f n i t r o g e n from s o l u t i o n s c o n t a i n i n g n i t r o u s ox i de and a second s cavenge r v a r i e d c o n s i d e r a b l y . Wate r , methanol and carbon d i o x i d e had l i t t l e e f f e c t on G ( N 2 ) . Oxygen lowered the n i t r o g e n y i e l d s l i g h t l y when p r e s e n t a t a c o n c e n t r a -t i o n comparab le to n i t r o u s o x i d e . I o d i n e , g a l v i n o x y l , ca rbon t e t r a c h l o r i d e and c h l o r o f o r m a l l reduced the n i t r o g e n y i e l d s u b s t a n t i a l l y . F i n a l l y the p re sence o f l i t h i u m bromide o r acetone r e s u l t e d i n an i n c r e a s e i n the n i t r o g e n y i e l d . c) HMPA - Water M i x t u r e s E x t e n s i v e s t u d i e s o f the r a d i o l y s i s o f wate r have f a i r l y w e l l e s t a b l i s h e d the p r o ce s s e s wh ich o c c u r i n t h a t s o l v e n t . HMPA and water be ing t o t a l l y m i s c i b l e i n a l l p r o p o r t i o n s , i t was proposed t h a t a s t udy o f HMPA/HgO m i x t u r e s might he l p e l u c i d a t e the mechH'rii sms h l '6f t : the ° p re se'rit^ s tudy ' ; 1 Of p a r t i c u l a r i n t e r e s t was the s c a veng i n g mechanism o f N 2 0 i n i r r a d i a t e d HMPA. In o r d e r to a s se s s the s i t u a t i o n p r o p e r l y , - 1 1 3 -m i x t u r e s of v a r i o u s HMPA/H 20 c o m p o s i t i o n s were a l l made 0.02M i n N2O. T h i s was a c h i e v e d by d e t e r m i n i n g the s o l u b i l i t y o f N,,0 f o r each m i x t u r e and then i n t r o d u c i n g the gas a t the a p p r o p r i a t e p a r t i a l p r e s s u r e . The y i e l d s o f n i t r o g e n , hydrogen and methane o b t a i n e d from the r a d i o l y s i s of such s o l u t i o n s as a f u n c t i o n o f m i x t u r e c o m p o s i t i o n i s d e t a i l e d i n F i g u r e 111-7 . 2. Na/HMPA S t u d i e s a) Na Meta l S o l u t i o n s Smal l amounts of sodium meta l were added to degassed samples o f HMPA.. The meta l d i s s o l v e d s l o w l y a t room t e m p e r a t u r e to g i ve r e l a t i v e l y s t a b l e i n t e n s e l y b lue c o l o u r e d s o l u t i o n s . I f l e f t s t a n d i n g f o r s e v e r a l h o u r s , the s o l u t i o n s s l o w l y decayed to g i v e s t a b l e y e l l o w - o r a n g e s o l u t i o n s . The d e c o m p o s i t i o n was m o n i t o r e d by gas chromatography and was found to p roceed i n p a r t th rough the c o n s t a n t p r o d u c t i o n o f s m a l l amounts o f hydrogen and methane. The p r o d u c t i o n of t he se gases ceased as the l a s t o f the b lue c o l o u r d i s s a p p e a r e d — the t o t a l y i e l d of each r e p r e s e n t i n g o n l y about 3% of the number of moles of Na metal used to p repa re the s o l u t i o n . The a d d i t i o n o f exces s N 2 0 to a sodium s o l u t i o n i n HMPA r e s u l t e d i n the immediate removal of i t s b l ue c o l o u r a t i o n . The r e a c t i o n was s t u d i e d i n more d e t a i l by p a s s i n g equa l amounts (27.0 umoles) o f N 2 0 th rough both b lue and orange sodium meta l s o l u t i o n s i n HMPA. For c a l i b r a t i o n p u r p o s e s , e q u i v a l e n t amounts I— o Q o 01 Q_ M O L E F R A C T I O N H 2 0 F i g u r e 111- 7. P roduc t y i e l d s from v a r i o u s m i x t u r e s o f HMPA and HZG c o n t a i n i n g 0.Q2M N 2 0 as a f u n c t i o n o f mole f r a c t i o n H o0• - 115 -of n i t r o g e n gas were a l s o passed th rough the two s o l u t i o n s . Tab le V I I l i s t s the amount o f n i t r o g e n gas r e c o v e r e d from each of the gas a d d i t i o n s . The a d d i t i o n of 27 ymoles N 2 0 i n t o the b l ue s o l u t i o n a l s o r e s u l t e d i n the f o r m a t i o n o f 0.3 ± 0.1 ymoles o f hydrogen. None was e v o l v e d when N 2 0 was i n j e c t e d i n t o the orange sodium s o l u t i o n . No methane was produced i n the Na s o l u t i o n s upon t r e a t m e n t w i t h N o 0 . TABLE VI I Sample N 2 ob se rved (ymoles ) f o l l o w i n g the i n j e c t i o n o f 27.0 ymole samples o f : N 2 N 2 0 B lue s o l u t i o n of Na i n HMPA 2 7 . 3 + 0 . 5 27.2 ± 0.5 Orange (decayed) s o l u t i o n o f Na i n HMPA 27.7 ± 0.5 0.0 ± 0 . 5 b) Sodium Amalgams i n S o l u t i o n Because of t h e i r i n h e r e n t r e a c t i v i t y , i t i s e x t r e m e l y d i f f i c u l t to p repa re s t a b l e s o l u t i o n s o f sodium meta l i n amines which w i l l . c o n t a i n known, r e p r o d u c i b l e c o n c e n t r a t i o n s of s o l v a t e d e l e c t r o n s . F u r t h e r m o r e , i n v e s t i g a t i o n s have - 116 -sugges ted t h a t N^O m o l e c u l e s a re i n v o l v e d i n more than one s tep of the s caveng i ng r e a c t i o n . T h e r e f o r e , even i f "known" metal s o l u t i o n s c o u l d be p r e p a r e d , t h i s p o s s i b i l i t y r e q u i r e s t h a t homogenous NgO s o l u t i o n s o f known c o m p o s i t i o n e x i s t i n i t i a l l y . That i s , Na must be added to a s o l u t i o n a l r e a d y c o n t a i n i n g n i t r o u s o x i d e . In o r d e r to i n t r o d u c e a known amount of pure sodium and to r e t a r d i t s r a t e of d i s s o l u t i o n (and hence i t s r e a c t i o n ) , l i q u i d amalgams o f sodium i n mercury were employed. Equal a l i q u o t s of a g i ven amalgam were t r a n s f e r r e d from an e vacua ted r e s e r v o i r to an i n t e r m e d i a t e c e l l chamber. They were then f o r c e d q u i c k l y i n t o the r e a c t i o n s o l u t i o n by means of a h i gh b a c k p r e s s u r e o f i n e r t gas. T h i s method was employed f o r two r e a s o n s . F i r s t l y , the t e c h n i q u e p r e v e n t e d the l o s s o f v o l a t i l e s cavenge r s f rom the r e a c t i o n chamber. S e c o n d l y , i t l i m i t e d p o s s i b l e r e a c t i o n w i t h amalgam p r i o r to i t s e n t r y i n t o the s o l u t i o n be i ng s t u d i e d . Expe r iment s were conducted u s i ng HMPA, w a t e r , or m i x t u r e s of the two - - both w i t h and w i t h o u t N^O. No b l u e c o l o u r a t i o n was ever obse rved f o l l o w i n g the a d d i t i o n o f amalgam to any o f the samp les . Subsequent to the c o m p l e t i o n of r e a c t i o n i n s o l u t i o n , the samples were a n a l y z e d by means of a tomic a b s o r p t i o n s p e c t r o s c o p y f o r sodium i on c o n t e n t . These a n a l y s e s c o n f i r m e d t h a t a l l a l i q u o t s d i d indeed c o n t a i n the same expec ted amounts o f sodium m e t a l . For example , a p i e c e of sodium meta l w e i g h i n g 0.14 ± 0.01 gm was s ub l imed i n vacuo s e v e r a l , t imes then mixed w i t h - 117 -34.76 ± 0.01 gm of j p u r i f i e d mercu r y . A l i q u o t s o f t h i s amalgam produced s o l u t i o n s c o n t a i n i n g a mercury r e s i d u e w e i g h i n g 2.23 ± 0.02 gm. Based s i m p l y on the f r a c t i o n o f the t o t a l amalgam used (assuming t h a t the sodium meta l s l u g was 100% pure and t h a t no Ijoss o c c u r r e d d u r i n g i t s p u r i f i c a t i o n ) the sodium c o n t e n t o f each a l i q u o t would be g i v e n by e q u a t i o n ( x x v i i i ) . T h e o r e t i c a l a l i q u o t Na c o n t e n t 2.24 + 0.02 gm Hg r e s i d u e X 0.14 ± 0.01 gm Na 34.76 + 0.01 gm Hg t o t a l X 2.30 X 1 0 " 6 gm Na/ymole = 393 ± 32 ymoles Na ( x x v i i i ) However, s i n c e the sodium metal was c u t i n a i r i t would be cove red w i t h an o x i d e l a y e r i n i t i a l l y and each s u b l i m a t i o n p r i o r to the amalgam f o r m a t i o n l e f t a sma l l r e s i d u e . T h e r e f o r e , the a c t u a l sodium c o n t e n t o f the a l i q u o t s would be e xpec t ed to be somewhat l e s s than t h i s e s t i m a t e d v a l u e . I ndeed , a n a l y s i s o f the spent s o l u t i o n s by a tom ic a b s o r p t i o n t e c h n i q u e s r e v e a l e d the p re sence o f 360 ± 20 ymoles o f N a + i on s i n each case - - a v a l ue i n d i c a t i n g e x c e l l e n t a c co rd w i t h e x p e c t a t i o n s . The gaseous p r oduc t s r e s u l t i n g from the a d d i t i o n o f sodium amalgam t o the v a r i o u s s o l u t i o n s were measured by gas chroma-t og r aphy . T y p i c a l r e s u l t s are g i ven i n T a b l e V I I I . In a d d i t i o n , one amalgam a l i q u o t c o n t a i n i n g 360 ± 20 ymoles Na was i n t r o d u c e d - 118 -T A B L E V I I I Y i e l d s o f g a s e o u s p r o d u c t s o b t a i n e d f r o m v a r i o u s s o l v e n t m i x t u r e s u p o n t h e a d d i t i o n o f a m a l g a m s c o n t a i n i n g 360 + 20 y m o l e s o f s o d i u m m e t a l . [N2O] ^ 6 X 10~2M. M o l e F r a c t i o n / ' G a s e o u s P r o d u c t s N20 ( y m o l e s ) HMPA H 2 0 P r e s e n t ? V 1 . 0 0 0 . 0 0 X 4 + 2 a 0 / 2 ± 1 165 ± 15 0 . 6 7 0 . 3 3 b X 13 ± 3 a 0 / 12 + 3 100 ± .10 0 . 2 9 0 . 7 1 X 8 + 2 0 • 1 9 + 4 45 ± 5 0 . 0 3 0 . 97 X 92 ± 10 0 0 . 00 1 . 00 X 195 ± 15 0 / 2 ± 1 200 ± 15 a a m a l g a m e s s e n t i a l l y s t a b l e i n t h i s s o l u t i o n . A d d i t i o n o f e x c e s s w a t e r r e s u l t e d i n r e a c t i o n . b 1 ml H o 0 i n 20 ml HMPA. - 119 -i n t o the empty and evacua ted r e a c t i o n c e l l . N i t r o u s o x i d e a t one atmosphere p r e s s u r e was s u b s e q u e n t l y added to the c e l l . Immed ia te l y a f t e r the NgO a d d i t i o n , the s u r f a c e p r o p e r t i e s of the amalgam cha'nged and i t tended to s t i c k to the c e l l w a l l s . A n a l y s i s o f any gases produced showed t h a t some n i t r o g e n (a- 25 ymoles ) had been formed - - p re sumab ly by a d i r e c t s u r f a c e r e a c t i o n between sodium i n the amalgam and gaseous n i t r o u s o x i d e . S e v e r a l i m p o r t a n t r e s u l t s are i m m e d i a t e l y obv i ou s f rom the da ta of T a b l e V I I I . F i r s t l y , the amalgams were s t a b l e i n pure HMPA or HMPA c o n t a i n i n g moderate amounts o f w a t e r . Even i n a s o l u t i o n hav ing 0.33 mole f r a c t i o n o f wate r (1 ml HgO i n 20 ml HMPA or > 5 X 1 0 4 ymoles HgO), the amalgam c o n t a i n i n g 360 ymoles o f Na atoms was u n r e a c t i v e . The amalgam r e a c t e d -2 w i t h pure wate r to g i ve 195 ± 15 ymoles o f Hg- When 6 X 1 0 M NgO was p r e s e n t i n w a t e r , the hydrogen r e a c t i o n was s u r p r e s s e d and 200 ± 15 ymoles of Ng produced i n s t e a d . That i s , one mole of Hg was produced from the r e a c t i o n o f two moles of Na but a l s o o n l y one mole of Ng was produced from 2 moles o f Na. From s o l u t i o n s o f HMPA c o n t a i n i n g 6 X 10 " 2 M NgO, s l i g h t l y l e s s (165 ± 15 ymoles ) n i t r o g e n was p r oduced , but l i t t l e hydrogen was produced i n any p r e d o m i n a t e l y HMPA s o l u t i o n . Aga in the p r o d u c t ! of one mole o f " n i t r o g e n from NgO r e a c t i o n r e q u i r e d a t l e a s t two moles o f Na un l e s s a l t e r n a t e r e a c t i o n s o c c u r r e d which d i d not l ead to gaseous p roduc t f o r m a t i o n . - 120 -B. DISCUSSION: i STEADY STATE EXPERIMENTS HMPA was s t u d i e d w i t h the o b j e c t i v e of i n v e s t i g a t i n g the r o l e of s o l v a t e d e l e c t r o n s i n t h a t s o l v e n t . In s o - c a l l e d s teady s t a t e r a d i o l y s i s e xpe r imen t s one cannot i n g e n e r a l obse rve r e a c t i v e i n t e r m e d i a t e s d i r e c t l y . T h e r e f o r e , one must make i n d i r e c t i n v e s t i g a t i o n s by o b s e r v i n g s t a b l e r e a c t i o n p r o d u c t s and s t u d y i n g the e f f e c t s of v a r i o u s s o l u t e s on t h e i r y i e l d s . In t h i s way one can o f t e n deduce v a l u a b l e i n f o r m a t i o n about i n t e r m e d i a t e s p e c i e s . I t was w i t h t he se c o n s i d e r a t i o n s i n mind t h a t r e s u l t s o f HMPA expe r imen t s were i n t e r p r e t e d . 1. Gamma R a d i o l y s i s S t u d i e s a) P r o ce s s e s Lead i ng t o M o l e c u l a r P r o d u c t s F o r m a t i o n The r a d i o l y s i s o f pure HMPA y i e l d e d hydrogen and methane as gaseous m o l e c u l a r p r o d u c t s . The p re sence o f known e l e c t r o n s cavenger s such as a c e t o n e , carbon t e t r a c h l o r i d e , c h l o r o f o r m , and n i t r o u s o x i d e had l i t t l e e f f e c t on the methane y i e l d . T h i s would tend to r u l e out i o n i c p r o ce s s e s i n the f o r m a t i o n o f t h a t p r o d u c t . G a l v i n o x y l which i s a s t a b l e f r e e r a d i c a l , would be expec ted to scavenge r a d i c a l s p e c i e s and has been shown to do so i n 2 , 2 , 4 - t r i m e t h y l p e n t a n e ? 1 I o d i ne i s a l s o known to e f f i c i e n t l y scavenge r a d i c a l s i n many s y s tems . In the p re sence of e i t h e r of these s u b s t a n c e s , the methane y i e l d was reduced i n d i c a t i n g t h a t a p r e c u r s o r i s a r a d i c a l s p e c i e s - - p r o b a b l y - 121 -the methy l r a d i c a l . A l t h o u g h , as d i s c u s s e d , i o d i n e i n HMPA p r o b a b l y e x i s t s as the t r i i o d i d e i on t h a t s p e c i e s would a l s o be e xpec t ed to a c t as a r a d i c a l s c a venge r . S i n c e 0.10M i o d i n e orfj.046M g a l v i n o x y l i n HMPA reduced the methane y i e l d by o n l y 60% i t i s appa ren t t h a t methane i s a l s o formed th rough un -s c a vengab l e r a d i a t i o n - i n d u c e d m o l e c u l a r p r o c e s s e s . Such p roce s se s c o u l d i n c l u d e spontaneous d i s s o c i a t i o n of e l e c t r o n -i c a l l y e x c i t e d s o l v e n t m o l e c u l e s , i n t r a s p u r r e a c t i o n s or " h o t atom" ( r a d i c a l ) r e a c t i o n s . S i n c e the p re sence o f any of s e v e r a l e l e c t r o n s cavenge r s over a l a r g e c o n c e n t r a t i o n range r educed the hydrogen y i e l d from G ( H 2 ) = 3.3 + 0.3 t o a c o n s t a n t v a l u e o f G ( H 2 ) = 1.4 ± 0 . 1 , hydrogen must be formed v i a a t l e a s t two p r o c e s s e s , one o f wh ich i n v o l v e d a c o m p a r a t i v e l y l o n g - l i v e d r e d u c i n g s p e c i e s s c a vengab l e by N 2 0 . That t h i s s p e c i e s was u n l i k e l y t o be the hydrogen atom was shown from da ta f o r samples c o n t a i n i n g methanol o r oxygen i n a d d i t i o n to N 2 0 . Methanol i s known to r e a c t r a p i d l y w i t h hydrogen atoms i n many s o l v e n t s v i a r e a c t i o n (1 .12) to produce m o l e c u l a r hydrogen ? 2 H + CH 30H T h e r e f o r e , i f the r e d u c i n g s p e c i e s were hydrogen atoms, the a d d i t i o n o f methanol to HMPA s hou l d have had no e f f e c t on or i n c r e a s e d hydrogen y i e l d upon r a d i o l y s i s . As can be seen i f rom the da ta o f Tab le V I I , the hydrogen y i e l d from a s o l u t i o n H 2 + CH 20H (1 .12) 1 22 -c o n t a i n i n g 0.1M methanol was reduced to G ( H 2 ) = 1.4 + 0.1. Fur the rmore i t has been r e p o r t e d t h a t n i t r o u s o x i d e does not i r e a c t w i t h hydrogen atoms d u r i n g the r a d i o l y s i s o f l i q u i d m e t h a n o l 9 3 or c y c l o p e n t a n e ? k In g e n e r a l , the s c a veng i n g o f hydrogen atoms by NgO i s a c c e p t e d as o c c u r i n g w i t h a r e l a t i v e l y low r a t e c o n s t a n t ? 5 ' 9 6 Fo r examp le , i n aqueous s o l u t i o n 5 methanol and oxygen r e a c t w i t h hydrogen atoms some 20 and 10 t imes f a s t e r r e s p e c t i v e l y than does N g O 3 7 From s o l u t i o n s o f - 2 -1 HMPA c o n t a i n i n g 10 to 10 M N,,0, the obse rved n i t r o g e n y i e l d was l i t t l e a f f e c t e d by the p re sence o f even 0.1M methanol or 0.01 M oxygen. Now, w h i l e t h i s d a t a cannot r u l e out t h e i n vo l v emen t of hydrogen atoms i n a l a t e r s t age o f the p r o ce s s l e a d i n g to m o l e c u l a r hydrogen f o r m a t i o n , i t does s t r o n g l y p r e c l u d e the p o s s i b i l i t y of t he i n i t i a l r e d u c i n g s p e c i e s be ing the hydrogen atom. Th i s sugges t s t h a t the l o n g - l i v e d r e d u c i n g s p e c i e s i s p r o b a b l y a n e g a t i v e i o n , X~, e i t h e r a s o l v a t e d e l e c t r o n ( e - ) or a m o l e c u l a r an i on ( A - ) . I f the r e d u c i n g s p e c i e s , X - , were a n i o n i c i n n a t u r e then i t s hou l d be r e a d i l y scavenged by p r o t o n s . F u r t h e r m o r e , such r e a c t i o n might d i s t i n g u i s h between m o l e c u l a r an i on s and s o l v a t e d e l e c t r o n s by p r o d u c i n g d i f f e r e n t p r o d u c t s , a c c o r d i n g to r e a c t i o n s ( 3 . 1 2 ) and ( 3 . 1 3 ) . + HMPA H + e » H • H s H 2 (3..12) H + + A: * MH • p r o d u c t s ( 3 . 1 3 ) - 1 2 3 -U n f o r t u n a t e l y , HMPA i s a t t a c k e d by e l e c t r o p h i 1 e s so t h a t p ro ton s are u n s t a b l e i n t h i s s o l v e n t } The i n t r o d u c t i o n o f i n a second hydrogen f o r m i n g p roce s s s i n c e h i gh c o n c e n t r a t i o n s o f e f f i c i e n t r a d i c a l s cavenge r s reduced the hydrogen y i e l d below the p l a t e a u v a l u e of G (H 2 ) = 1 . 4 ± 0 . 1 . G a l v i n o x y l at 0.046M and " i o d i n e " a t 0.1M reduced G (H 2 ) to 1.2 ± 0 . 1 and 1.1 + 0.1 r e s p e c t i v e l y . At t he se c o n c e n t r a t i o n s the r a d i c a l s cavenger s would have been e xpec t ed to p i c k up a l l the r a d i c a l s p e c i e s p roduced d u r i n g the r a d i o l y s i s . Thus , as was the case f o r methane f o r m a t i o n , m o l e c u l a r hydrogen a l s o appears t o a r i s e t h rough un scavengab le m o l e c u l a r p r o c e s s e s . From the ob se rved m o l e c u l a r p r o d u c t y i e l d s and the e f f e c t s o f s cavenge r s on tho se y i e l d s , one can deduce p o s s i b l e mechanisms f o r the p r o ce s s e s o c c u r i n g d u r i n g r a d i o l y s i s . The f o l l o w i n g scheme would accoun t f o r the obse rved r e s u l t s i n pure HMPA. For c o n v e n i e n c e , the f o r m u l a f o r HMPA, ( ( C H g ^ N ^ P O , i s e xp re s sed as CHAMPA). The wavy ar row denotes r a d i a t i o n induced p r o c e s s e s , square b r a c k e t s i n d i c a t e p r o c e s s e s o c c u r r i n g w i t h i n the spurs and an a s t e r i s k s i g n i f i e s e l e c t r o n -i c a l l y e x c i t e d s p e c i e s . 10 M H 2 S 0 4 i n t o a sample of HMPA produced a c l o u d y s u s p e n s i o n i n d i c a t i n g t h a t d e c o m p o s i t i o n had o c c u r r e d . Hydrogen atoms do appear to be i n v o l v e d to some e x t e n t (3 .15) (3 .14 ) 1 24 •CH 3 + CH 3 , (MPA)] (MPA)] [e~ + CH3(|MPA)] [e " + CH 3 (MPA ) + ] [•(MPA), -CH 2 (MPA)] •H + CH. CH 3] [•«} [•-]• [ C H 3 ( M P A ) + ] [ CH 3 (MPA ) _ ] CH 3 + CH.3(MPA) •H + CH 3 (MPA) -+ CH 4 + -CH 2 (MPA) [H2 + -CH2(MPA)] [cH 3 ( M P A ) ~ ] [cH 3 (MPAJ* ] — • P r o d u c t s -> -CH. -> -H -> e. -> CHg(MPA)* •> A' CH 4 + -CH 2 ( FPA ) * H 2 + -CH 2 (MPA) ( 3 .16 ) (3 .17) (3 .18) (3 .19) (3 .20 ) (3 .21) (3 .22) (3 .23) (3 .24) (3 .25) (3 .26) (3 .27) e~ + CHAMPA)* • CHAMPA)* » H + -CHAMPA) ( 3 2 Q ) A~ + CHg(MPA)* • • H + -CH2(MPA) + CHAMPA) ( 3 2 g ) CH 2(MPA) P r oduc t s (3 .30) I t was not supposed t h a t t h i s mechanism r e p r e s e n t e d the a c t u a l p r o c e s s e s , but i t s i m p l y se r ved as a b a s i s wh ich sugges ted - 125 -r e a s o n a b l e i n v e s t i g a t i v e l i n e s f o r the l a t e r e x p e r i m e n t s . The observed o n e - t o - o n e co r r e spondence between AG ( N g ) and AG ( H g ) i n the n i t r o u s o x i d e d e p l e t i o n e x p e r i m e n t s and the f a c t t h a t no ethane was ob se r ved i m p l i e s t h a t r a d i c a l - r a d i c a l r e a c t i o n s such as ( 1 . 9 ) , (3 .31) and (3 .32) a re not i m p o r t a n t . •H + • H • H £ • H + ; C H 3 • C H 4 •CH- + -CH, • C 0 H C 3 3 2 6 Th i s i s p r o b a b l y a r e f l e c t i o n of the e f f i c i e n c y o f t he m o l e c u l a r p r o c e s s e s ( 3 . 1 6 ) , (3 .17 ) and the hydrogen a b s t r a c t i o n p r o c e s s e s ( 3 . 2 6 ) , ( 3 . 2 7 ) . b) Free Ion Y i e l d Of a l l the s p e c i e s t h a t a r i s e as a r e s u l t of the r a d i o l y s i s o f HMPA, the h i g h l y r e a c t i v e r e d u c i n g s p e c i e s X~, i s most i m p o r t a n t . Because of the s t r o n g p o s s i b i l i t y t h a t X" i s a s o l v a t e d e l e c t r o n , a n d t h e r e f o r e a f r e e i o n , d e t e r m i n a t i o n o f i t s y i e l d i s o f paramount impo r t ance to an u n d e r s t a n d i n g o f HMPA r a d i o l y s i s . He re , the term f r e e i o n r e f e r s to any compara-t i v e l y l o n g - l i v e d i o n i c s p e c i e s t h a t has escaped geminate r e -c o m b i n a t i o n and as such i s f r e e to d i f f u s e t h rough the medium and r e a c t i n the normal c hem i ca l s en se . (1,9) (3 .31) (3 .32) - 126 -The a n i o n i c r e d u c i n g s p e c i e s X~ i s r e a d i l y scavenged by n i t r o u s o x i d e . Based on the n i t r o g e n y i e l d from s o l u t i o n s of up to 0.13M n i t r o u s o x i d e i n HMPA, as shown i n F i g u r e I I I - 3 , X" i s produced w i t h a. y i e l d o f G(X~) > 4.5 ± 0 . 3 . T h i s assumes of cou r se t h a t n i t r o u s o x i d e scavenges o n l y X ~ and t h a t each such r e a c t i o n u l t i m a t e l y l e a d s to t he f o r m a t i o n o f a s i n g l e n i t r o g e n m o l e c u l e a c c o r d i n g to r e a c t i o n ( 3 . 3 3 ) . k N 2 0 • + p r o d u c t s X s + N 2 0 (3 .33) In the absense of n i t r o u s o x i d e , X~ must r e a c t w i t h i t s c o u n t e r p o s i t i v e i o n , i m p u r i t y , the s o l v e n t , or i t s e l f . I f such a l t e r n a t e f a t e s can be r e p r e s e n t e d by a s i n g l e r e a c t i o n (3 .34) where S r e p r e s e n t s a s p e c i e s -other than N,,0, then the da ta of F i g u r e 111- 3 can be a n a l y z e d on the b a s i s o f a s i m p l e c o m p e t i t i o n between r e a c t i o n s (3 .33) and ( 3 . 3 4 ) . k xs + S -* p r o d u c t s (3 .34 ) That i s , f o r t h i s c o m p e t i t i o n , G (N 2 ) = G(X~) 'N 2 0 [ N 2°] fc] : N 2 0 [ N2°] M + k s [SJ\ ( x x i x) or r e a r r a n g i n g , 127 G ( N 2 ) , G ( X ~ ) 1 + , 5 KN20 [ 0 ;: N 2 0 ( xxx ) A p l o t o f 1/G(N2) ve r su s 1/|N20IJ s hou l d be a s t r a i g h t l i n e hav ing a s l o p e equa l to k S J / k ^ Q G ( X ~ ) and an i n t e r c e p t equa l to 1 / G ( X ~ ) i f t h i s !mechanism h o l d s . F i g u r e I I I - 8 shows such a p l o t f o r the da t a o f F i g u r e 111-3 f o r 5 X 1 0 _ 4 M t o 1.3 X 1 0 _ 1 M n i t r o u s o x i d e i n HMPA. The f i l l e d c i r c l e s r e p r e s e n t those expe r imen t s f o r which the argon d i l u t i o n t e c h n i q u e was employed i n o r d e r to a t t a i n Tow c o n c e n t r a t i o n s o f n i t r o u s o x i d e . The data f o r samples i n i t i a l l y c o n t a i n i n g l e s s than about 5 X 10~ 4 M n i t r o u s o x i d e were not i n c l u d e d because o f p r o b a b l e n i t r o u s o x i d e d e p l e t i o n d u r i n g r a d i o l y s i s . Wh i l e the u n c e r t a i n t y i n . the da ta f o r low n i t r o u s o x i d e c o n c e n t r a t i o n s may be l a r g e , the p l o t appears to be n o n - l i n e a r . However, f o r the h i gh c o n c e n t r a -- 2 -1 t i o n r ange , 10 to 10 M n i t r o u s o x i d e , a near l i n e a r r e l a t i o n -s h i p was o b t a i n e d as shown i n F i g u r e 111-9 and f rom the i n t e r c e p t a v a l ue o f G ( X ~ ) = 4.3 ± 0.3 was c a l c u l a t e d . I t c o u l d be s noted t h a t t h i s v a l u e i s o b t a i n e d at n i t r o u s o x i d e c o n c e n t r a t i o n s ve ry much h i g h e r than would be r e q u i r e d t o scavenge f r e e i on s w i t h l i f e t i m e s of at l e a s t p s e c . F u r t h e r , the y i e l d i s much l a r g e r than the f r e e i o n y i e l d t h a t one would e x p e c t f rom a l i q u i d hav ing a d i e l e c t r i c c o n s t a n t of 3 0 6 8 ' 9 7 ' 9 8 Thus the s i m p l e c o m p e t i t i o n mechanism r e p r e s e n t e d by r e a c t i o n s (3.33) and (3.34) p r o b a b l y does not a d e q u a t e l y d e s c r i b e the HMPA system and i t i s c onc l uded t h a t the h igh n i t r o g e n y i e l d o b t a i n e d a t h i gh N20 does not r e p r e s e n t the " f r e e - i o n " y i e l d . - 130 -N i t r o g e n y i e l d s f rom i r r a d i a t e d NgO s o l u t i o n s have been found to be much l a r g e r than " f r e e - i o n " y i e l d s o b t a i n e d by o t h e r methods f o r s e v e r a l s y s tems . Tab l e IX shows t y p i c a l r e s u l t s which compare.G(N 2 ) f rom NgO w i t h G ( f r e e i o n ) measured i n d e p e n d e n t l y . S i m i l a r l y , T a b l e X shows data from the vapour phase r a d i o l y s i s o f some n i t r o u s o x i d e / h y d r o c a r b o n m i x t u r e s . For low c o n c e n t r a t i o n s o f n i t r o u s o x i d e i n s e v e r a l s y s t ems , the n i t r o g e n y i e l d i s comparab le to the f r e e i o n y i e l d . I ndeed , i n some cases such as w a t e r , methane, and neopentane , a p l o t o f G(Ng) ve r su s low n i t r o u s o x i d e c o n c e n t r a t i o n e x h i b i t s a p l a t e a u r e g i o n where G (N 2 ) ^ G ( f r e e i o n ) . T h i s b e h a v i o r would be e xpec ted i f the r e d u c i n g s p e c i e s were i n f a c t a c o m p a r a t i v e l y l ong l i v e d f r e e i o n . In the absence of s c a v e n g e r s , the f a t e o f the r e d u c i n g a n i o n , X~, must u l t i m a t e l y be n e u t r a l i z a t i o n r e c o m b i n a t i o n w i t h a c a t i o n . In HMPA,.such a p roce s s appears to l e a d a t l e a s t i n p a r t to the f o r m a t i o n o f m o l e c u l a r hydrogen - - p o s s i b l y v i a r e a c t i o n s - (3.35) ' and ( 3 . 2 7 ) . X^ + C H A M P A ) * — • CH 3 (MPA) * • -H + -CHAMPA) •H + CH 3 (MPA) • H 2 + -CH 2 (MPA) That X" i s i ndeed a f r e e i on i s e v i d e n c e d by the f a c t t h a t ve r y low c o n c e n t r a t i o n s o f e l e c t r o n s cavenge r s (even t r a c e i m p u r i t i e s ) e f f i c i e n t l y s u r p r e s s the above mechanism to g i v e o n l y the low p l a t e a u hydrogen y i e l d of G (H 0 ) = 1.4 ± 0 . 1 . (3 .35) (3 .27) N i t r o g e n y i e l d s f o r low and h i gh c o n c e n t r a t i o n s of n i t r o u s o x i de i n v a r i o u s l i q u i d s compared to the f r e e i on y i e l d s de te rmined i n d e p e n d e n t l y . S o l v e n t Free Ion Y i e l d G ( f . i . ) N 2 0 (M) G (N 2 ) P l a t e a u E x h i b i t e d Re fe r ence s Ethane 0.13 0.0004 0.20 0.6 5.4 ? 99 Propane 0.08 0.0004 1.5 0.3 5.4 1 99 Neopentane 0.9 0.0001 1.7 0.95 4.7 Yes 100 2,2,4 T r i m e t h y l p e n t a n e 0 .3 -0 .4 0.001 0-7 0.8 5.5 ? 101 , 91 Cyc l op ropane 0.04 0.0004 1.8 ^0.1 5.0 Yes 100 TABLE IX ( c o n t i n u e d ) F ree Ion Yi e l d P l a t e a u S o l v e n t G ( f . i . ) N 2 0 (M) G (N 2 ) Exh i b i ted Re fe r ence s C y c l o p e n t a n e 0.16 0.001 0.9 No 94, 102 1.1 5.5 Cyc lohexane 0.17 0.0009 0.5 No 103, 98, 91 , 0.4 5.4 104 E t h y l e n e 0.02 0.0004 0.2 Yes 99 2.0 4.2 P r o p y l e n e 0.04 0.0003 <0.1 Yes 99 1 .6 4.2 Benzene 0.06 0.04 0.7 No 104, 91 4.6 6.7 Methanol 1.1,1.9 0.003 2.0 Yes 93 , 105 1.0 4.5 TABLE IX ( c o n t i n u e d ) Free Ion Y i e l d P I a teau S o l v e n t G ( f . i . ) N 2 0 (M) G (N 2 ) Exhi b i ted - Re fe rence s E t hano l 1.0,1.7 0.00001 0.2 .106, 98 0.0001 1.3 Yes? 0.4 4.5 2 - P r opano l 1 .0,1.2 0.00001 0.3 107 0.001 1 .1 No 1.0 3.5 D i e t h y l E t he r 0 . 2 - 0 . 4 0.0003 0.6 No 108, 102 0.29 4.0 1,4 D ioxane 0 . 1 , 2 . 2 <0.001 <1 No 109, 110, 98 0.21 3.1 Water 2.7 0.00004 2.5 Yes 111 , 112, 113 0.20 3.9 TABLE IX ( c o n t i n u e d ) F ree Ion Y i e l d P l a t e a u S o l v e n t G ( f . i . ) N 2 0 (M) G(N 2 ) E x h i b i t e d " R e f e r e n c e s L i q . Xenon 4.6,7.1 0.001 •^4 No 114, 115 1 .0 ^12 Formami de >0. 7 0.01 1.9 No 116, 98 0.05 3.2 D imethy l 1.3,1.6 ^0.005 0.4 ? 117, 98 S u l p h o x i d e 0.02 1 .0 1 .8 P r o p y l e n e 2.3 0.001 0.2 No 118, 98 Carbonate 0.10 2.0 - 135 -TABLE X N i t r o g e n y i e l d s from the gas phase r a d i o l y s i s o f v a r i o u s compounds i n the p re sence o f n i t r o u s o x i d e . Ion Y i e l d N 2 0 Compound G (e " ) G (N 2 ) Mole % R e f e r e n c e ethane 4.2 5.6 %4 119, 120 propane 4.3 6.3 0/4 119, 120 n-butane 4.4 7.0 "u4 119, 120 i s o b u t a n e 4.4 7.1 0,4 119, 120 1-butene 4.1 8.0 %4 119, 120 c i s - 2 - bu tene 4.1 8.2 0,4 119, 120 t r a n s - 2 - b u t e n e 4.2 8.1 ^4 119, 120 p r o p y l e n e 4.0 4.2 ^4 119, 120 i s o b u t e n e • 4.1 4.1 ^4 119, 120 methyl c y c l ohexane 4.4 22.0 2-10 121 a c e t y l e n e 3.9 6 .0 -18.2 1-20 122 i s o p r o p a n o l 3.5 12.5 2 123 - 136 -Based on the se c o n s i d e r a t i o n s , the n i t r o g e n y i e l d f rom i r r a d i a t e d HMPA c o n t a i n i n g v e r y low c o n c e n t r a t i o n s o f NgO s hou l d more a c c u r a t e l y r e f l e c t the f r e e i o n y i e l d . As e x p l a i n e d however, s u i t a b l e s o l u t i o n s were most d i f f i c u l t t o p r epa re - -and those o b t a i n e d s u b j e c t to s cavenger d e p l e t i o n . F o r t u n a t e l y , the a l t e r n a t e f a t e of the r e d u c i n g s p e c i e s a l s o r e s u l t e d i n the f o r m a t i o n of an o b s e r v a b l e p r o d u c t , namely m o l e c u l a r hydrogen. Thus, t ho se e x p e r i m e n t s i n wh ich N 2 0 d e p l e t i o n was s i g n i f i c a n t were e a s i l y d i s t i n g u i s h a b l e . That i s , the n i t r o g e n y i e l d f rom the sample c o n t a i n i n g the l o w e s t i n i t i a l NgO c o n c e n t r a t i o n wh ich gave no e v i d e n c e f o r s cavenge r d e p l e t i o n ( i . e . i n c r e a s e d Hg y i e l d ) s hou ld r e p r e s e n t the f r e e i o n y i e l d . As d e t a i l e d i n F i g u r e 111 —4 , the hydrogen y i e l d i n c r e a s e s f rom samples o f HMPA i n i t i a l l y c o n t a i n i n g l e s s than about 3 X 10 M NgO. From s o l u t i o n s c o n t a i n i n g > 3 X 10 M NgO, the hydrogen y i e l d e x h i b i t e d a p l a t e a u v a l u e 1.4 ± 0 . 1 . Over -4 the c o n c e n t r a t i o n range 3 to 10 X 10 M NgO, the n i t r o g e n y i e l d was G (N 2 ) = 2.2 ± 0 .2 . The Hg and Ng y i e l d da ta over the "NgO d e p l e t i o n r e g i o n " f rom 0 to ^ 3 X 1 0 " 4 M NgO p r o v i d e d a d d i t i o n a l c o n s i s t a n t e v i d e n c e . As shown i n F i g u r e 111 -4, the sum of Hg and Ng y i e l d s was c o n s t a n t , G (Ng + Hg) = 3.5 ± 0 .3 . T h i s i m p l i e s a s i m p l e c o m p e t i t i o n f o r the r e d u c i n g s p e c i e s . Over t h a t c o n c e n t r a t i o n range Ng p r o d u c t i o n must r e f l e c t the s c a veng i n g of a s i n g l e s p e c i e s by NgO. A l s o , the f a c t t h a t above 3 X 1 0 _ 4 M NgO, a t h r e e f o l d i n c r e a s e i n NgO c o n c e n t r a t i o n r e s u l t e d i n o n l y a s l i g h t i n c r e a s e i n N ? y i e l d i m p l i e s t h a t n e u t r a l i z a t i o n r e c o m b i n a t i o n - 1 3 7 -r e p r e s e n t s the o n l y a l t e r n a t e f a t e o f the f r e e i o n s under t ho se cond i t i o n s . In a d d i t i o n , the o n e - t o - o n e r e l a t i o n s h i p between H 2 o r N 2 p r o d u c t i o n means t h a t the y i e l d of r e d u c i n g s p e c i e s would a l s o be g i v en by the d i f f e r e n c e i n H 2 y i e l d s f rom pure HMPA i n the p re sence and absence o f N 20, AG(H 2 ) = (3.4 ± 0.3 -1.4 ± 0.1) = 2.0 + 0 .4 . On the b a s i s of these e xpe r imen t s the f r e e i o n y i e l d i n i r r a d i a t e d HMPA was thus de te rm ined to be G ( f r e e i o n ) = 2.2 ± 0.2. S i m i l a r mechanisms have been proposed to a ccoun t f o r the dependence o f hydrogen y i e l d on n i t r o u s o x i d e c o n c e n t r a t i o n i n s e v e r a l o t h e r s y s t e r n s ! 3 ' 9 " ' 1 2 h ' 1 2 5 c) Free Ion L i f e t i m e I f one makes c e r t a i n a s sumpt ions about the r a t e c o n s t a n t s i n v o l v e d , the e xpe r imen t s conduc ted a t low n i t r o u s o x i d e c o n c e n t r a t i o n a l l o w an e s t i m a t i o n of the f r e e i on l i f e t i m e to be c a l c u l a t e d . I f N 20 d e p l e t i o n i s i n s i g n i f i c a n t , r e a c t i o n (3.33) becomes pseudo - f i r s t o r d e r . Assuming the r e a c t i o n to be d i f f u s i o n c o n t r o l l e d ( i . e . k 3 33 ^ 2 X 1 0 1 0 M _ 1 s e c " 1 ) , one c a l c u l a t e s From T a b l e VI i t can be seen from the H 2 and N 2 da ta f o r s o l u t i o n s i n i t i a l l y c o n t a i n i n g 7 X 10~ 5 M N 2 0 t h a t n e u t r a l i z a t i o n (3.35) and f r e e i on s caveng ing (3.33) o c c u r r e d a t comparab le the mean l i f e t i m e T ( g i v e n by (k 3. 33 nsec - 138 -r a t e s . T h e r e f o r e , ^ t h e i on l i f e t i m e , T, as c a l c u l a t e d above shou ld r e p r e s e n t the f r e e i on l i f e t i m e i n the absence o f s c avenge r . I ndeed , s i n c e from the t a b l e i t i s e v i d e n t t h a t s cavenger d e p l e t i o n was o c c u r r i n g i n those s o l u t i o n s the v a l u e i s at be s t a l ower l i m i t . The r e s u l t does i m p l y t h a t the r e d u c i n g s p e c i e s l i ' v e s a t l e a s t m i c r o second s and i s t h e r e f o r e j u s t i f i a b l y l a b e l l e d a f r e e i o n , s i n c e i n g e n e r a l , s p e c i e s I 8 undergo ing spur r e a c t i o n s have l i f e t i m e s <10 s e c . I ndeed , as w i l l be d e t a i l e d l a t e r , t h e r e i s c o n s i d e r a b l e e v i d e n c e f rom the p r e s e n t s tudy to i d e n t i f y the p r i m a r y r e d u c i n g s p e c i e s i n HMPA as a s o l v a t e d e l e c t r o n , e ^ p ^ -d) N i t r o g e n Y i e l d From the S o l u t i o n s C o n t a i n i n g H igh  C o n c e n t r a t i o n s o f N i t r o u s O x i d e : Other Sources I t can be seen f rom F i g u r e 111-3 t h a t the r a d i o l y s i s o f HMPA c o n t a i n i n g > 1 0 - 1 M n i t r o u s o x i d e produced n i t r o g e n w i t h a y i e l d G (N 2 ) > 4.5 ± 0 . 2 , much h i g h e r than the f r e e i o n y i e l d G ( f r e e i on ) = 2.2 ± 0.2 e s t i m a t e d i n the p r e v i o u s s e c t i o n . There are t h r e e p o s s i b l e p r o c e s s e s b e s i d e s f r e e i on s caveng i ng which c o u l d l e ad to n i t r o g e n f o r m a t i o n from n i t r o u s o x i d e s o l u t i o n s . They a r e : i ) D i r e c t r a d i o l y s i s o f n i t r o u s o x i d e , i i ) R e a c t i o n o f n i t r o u s o x i d e w i t h o t h e r s p e c i e s produced e i t h e r d i r e c t l y o r i n d i r e c t l y f rom s o l v e n t r a d i o l y s i s . i i i ) Secondary i o n i c r e a c t i o n s of n i t r o u s o x i d e . - 139 -i ) D i r e c t R a d i o l y s i s o f N ? 0 D i r e c t r a d i o l y s i s o f NgO l e ad s to the f o r m a t i o n o f n i t r o g e n a c c o r d i n g to the gene ra l r e a c t i o n ( 3 . 3 6 ) . , NgO / V V V V W — • Ng + p r o d u c t s ^ 3 3 i I The n i t r o g e n y i e l d s from gaseous NgO a t room t empe ra tu re and l i q u i d NgO at 185 °K a re r e p o r t e d to be 10 and 13 r e s p e c t i v e l y } 2 6 ' 1 2 7 ' 1 2 8 However, d e s p i t e those l a r g e y i e l d s , c a l c u l a t i o n s r e v e a l t h a t d i r e c t r a d i o l y s i s o f NgO i n the HMPA s t u d i e s c o u l d account f o r at the most two p e r c e n t of the n i t r o g e n o b s e r v e d . Th i s i s because the r a d i a t i o n energy abso rbed d i r e c t l y by any component o f a system i s p r o p o r t i o n a l to t h a t component ' s e l e c t r o n ( d e n s i t y ) f r a c t i o n . By f a r , the s o l v e n t r e p r e s e n t e d the major bu lk of the a b s o r b i n g medium i n the p r e s e n t s t u d y . i i ) R e a c t i o n o f NgO With Other S p e c i e s Produced E i t h e r  D i r e c t l y or I n d i r e c t l y From S o l v e n t R a d i o l y s i s The r a d i o l y s i s of HMPA can be e xpec ted to produce a v a r i e t y of e x c i t e d , r a d i c a l and i o n i c s p e c i e s . The i n t e r a c t i o n of a l l o f the se w i t h NgO must be c o n s i d e r e d as p o s s i b l e p r e c u r s o r s of m o l e c u l a r n i t r o g e n . - 140 -i i . 1 ) E x c i t e d S p e c i e s The i n t e r a c t i o n o f HMPA w i t h i o n i z i n g r a d i a t i o n u n d o u b t e d l y l ead s t o the f o r m a t i o n o f h i g h l y e x c i t e d s o l v e n t m o l e c u l e s i n wh ich bond d i s s o c i a t i o n o c c u r s and the m o l e c u l e s a re f r agmented ( p r e d i s s o c i a t i o n ) . E v i dence f o r t h i s comes f rom the ob se r ved unscavengab le y i e l d o f m o l e c u l a r hydrogen and methane p roduced by s o - c a l l e d " m o l e c u l a r p r o c e s s e s " . In t h i s way a number o f r a d i c a l and p o s s i b l e i o n i c s p e c i e s c o u l d be f o rmed . Of c once rn h e r e , however, i s the f a t e o f e x c i t e d s o l v e n t m o l e c u l e s t h a t do not t hemse l ve s d i s s o c i a t e . Such s p e c i e s are p r o b a b l y s h o r t -l i v e d but might undergo energy t r a n s f e r p r o c e s s e s w i t h s o l u t e s . However, t h e r e seems to be l i t t l e e v i d e n c e to sugge s t t h a t n i t r o u s o x i d e i s c a p a b l e o f s c aveng i ng such e x c i t e d n e u t r a l s o l v e n t m o l e c u l e s . Da i n t on and L o g a n 1 1 3 o b se r ved the n i t r o g e n y i e l d f rom the r a d i o l y s i s o f aqueous s o l u t i o n s o f n i t r o u s o x i d e and n i t r i t e i o n . They sugges ted t h a t some o f the n i t r o g e n o b t a i n e d f rom s o l u t i o n s o f h i gh n i t r o u s o x i d e c o n c e n t r a t i o n (up to 0.2M) was a . r e s u l t of the s c a v e n g i n g o f e x c i t e d w a t e r m o l e c u l e s . R u s s e l l and F r e e m a n 1 2 9 o b t a i n e d s i m i l a r r e s u l t s f rom aqueous n i t r o u s o x i d e s o l u t i o n s c o n t a i n i n g e t h a n o l . However, they a t t r i b u t e d the i n c r e a s e d n i t r o g e n y i e l d from c o n c e n t r a t e d s o l u t i o n s o f n i t r o u s o x i d e t o nonhomogenous s caveng i ng o f i o n i c s p e c i e s w i t h i n the spur s (geminate i o n s c a v e n g i n g ) . H o l r o y d 1 3 0 s t u d i e d and compared the s caveng i ng o f s p e c i e s produced by the p h o t o l y s i s and r a d i o l y s i s o f a l k a n e s . He -2 c onc l uded t h a t a t c o n c e n t r a t i o n s g r e a t e r than 10 M, n i t r o u s - 141 -o x i de scavenged e x c i t e d s o l v e n t m o l e c u l e s i n c y c l o h e x a n e , but not i n 2 , 2 , 4 - t r i m e t h y l p e n t a n e . However, as has been p o i n t e d o u t i 3 1 H o l r o y d ' s o b s e r v a t i o n s may have been an a r t i f a c t o f h i s e x p e r i m e n t a l ar rangement wh ich c o u l d have a l l o w e d d i r e c t p h o t o l y s i s o f NgO near the o p t i c a l windows. Salmon and c o - w o r k e r s 1 3 2 examined d i r e c t l y the f l u o r e s c e n c e from e x c i t e d s t a t e s of a r o m a t i c s o l u t e s i n c y c l o h e x a n e and benzene. They found f o r benzene s o l u t i o n s t h a t the t r i p l e t y i e l d s o f naph tha l ene and 2 , 5 - d i p h e n y l o x a z o l e were l i t t l e a f f e c t e d by the p re sence of l a r g e amounts of n i t r o u s o x i d e (•> 0.1M). In c y c l o h e x a n e s o l u t i o n s , the a r o m a t i c s o l u t e t r i p l e t y i e l d s were r e d u c e d , but t h e i r f l u o r e s c e n c e l i f e t i m e s were u n a f f e c t e d . Salmon e t a l . c onc l uded t h a t a r o m a t i c t r i p l e t s were formed d i r e c t l y by energy t r a n s f e r f rom e x c i t e d s o l v e n t mo l e cu l e s i n benzene but from i on n e u t r a l i z a t i o n p r o c e s s e s i n c y c l ohexane where e x c i t e d s o l v e n t s t a t e s would be e xpec t ed to be s h o r t l i v e d . In the l a t t e r c a s e , n i t r o u s o x i d e competed f o r the i o n i c p r e c u r s o r s and thus reduced the ob se r ved t r i p l e t y i e l d . In any e v e n t , the o b s e r v a t i o n s c l e a r l y i n d i c a t e t h a t n i t r o u s o x i de d i d not r e a c t w i t h s o l v e n t or s o l u t e e x c i t e d s t a t e s i n those sy s tems. The o n l y system i n which t h e r e may be some e v i d e n c e of r e a c t i o n between, e x c i t e d s o l v e n t m o l e c u l e s and NgO i s d i o x a n e i 3 3 However, o t h e r r a d i o l y s i s s t u d i e s 1 0 3 ' 1 1 0 ' 1 3 1 * have shown t h a t system to be p a r t i c u l a r l y "mes s y " . In o r d e r to e x p l a i n o b s e r v a t i o n s i t was u s u a l l y n e c e s s a r y to i n voke mechamisms i n wh ich a l a r g e number o f e x c i t e d and i o n i z e d s p e c i e s underwent - 142 -r a t h e r s p e c i a l i z e d i r e a c t i o n s . Mayer and Baxe'nda 1 e 1 3 5 ob se r ved the e m i s s i o n from benzene and t o l u e n e i n the. p u l s e r a d i o l y s i s o f c y c l o h e x a n e . They conc l uded t h a t the upper l i m i t f o r the y i e l d s of c y c l o h e x a n e e x c i t e d s t a t e s was o n l y G(RH ) < 0 . 3 , wh ich i s p r o b a b l y t y p i c a l f o r the h y d r o c a r b o n s . HMPA, w i t h i t s many methy l g r o u p s , i s p r o b a b l y not ve r y d i f f e r e n t from the hyd roca rbons i n t h i s r e s p e c t . Thus , the e v i d e n c e f rom most systems s t u d i e d seems to i n d i c a t e t h a t r e a c t i o n o f n i t r o u s o x i d e w i t h e x c i t e d s p e c i e s i n HMPA i s u n l i k e l y . Even i f such a p r oce s s d i d o c cu r the r e s u l t s o f Mayer and Baxenda le sugges t t h a t o n l y a s m a l l y i e l d of n i t r o g e n would be p roduced . i i . 2 ) R a d i c a l S p e c i e s R a d i c a l s p e c i e s most l i k e l y to be produced d u r i n g the r a d i o l y s i s o f HMPA are hydrogen atoms, H, and methy l r a d i c a l s , CHg. The obse rved y i e l d s of m o l e c u l a r hydrogen and methane from pure HMPA and the e f f e c t s of v a r i o u s e l e c t r o n and r a d i c a l s cavenger s on the se y i e l d s c o n f i r m e d the p re sence of these r a d i c a l s . As no ethane was ob se r ved among the r a d i o l y s i s p r o d u c t s , e t h y l r a d i c a l s were p r o b a b l y not produced d u r i n g the s o l v e n t r a d i o l y s i s . The e x i s t e n c e o f o t h e r r a d i c a l s p e c i e s , R, wh ich would not l e a d t o the f o r m a t i o n of v o l a t i l e m o l e c u l a r p r oduc t s was not i n v e s t i g a t e d . The r e a c t i o n of n i t r o u s o x i d e w i t h these r a d i c a l s p e c i e s - 143 -might l e a d to n i t r o g e n f o r m a t i o n a c c o r d i n g t o r e a c t i o n s ( 3 . 3 7 ) , (3 .38) and ( 3 . 3 9 ) . N 2 0 + H N 2 0 ' ; + C H 3 N 2 0 + R As d i s c u s s e d e a r l i e r , n i t r o g e n f o r m a t i o n th rough hydrogen atom s caveng i n g (3^37) can be r u l e d out i n HMPA from the da ta o b t a i n e d from samples c o n t a i n i n g oxygen or methanol as second s cavenge r . Sek i and Imamura 9 3 showed t h a t r e a c t i o n (3 .37) was un impo r t an t i n l i q u i d m e t h a n o l . They found t h a t as the pH o f a s o l u t i o n of n i t r o u s o x i de i n methanol was l o w e r e d , the n i t r o g e n y i e l d d e c r e a s e d . At pH y 1 where r e a c t i o n w i t h p r o t on s would c o n v e r t a l l the s o l v a t e d e l e c t r o n s t o hydrogen atoms ( 1 . 1 3 ) , l i t t l e or no n i t r o g e n was o b s e r v e d . S i n c e a c i d i c s o l u t i o n s o f HMPA are u n s t a b l e , the ana logous r e a c t i o n s i n HMPA c o u l d not be i n v e s t i g a t e d d i r e c t l y . Scaveng ing o f methyl r a d i c a l s by n i t r o u s o x i d e (3 .38) i n HMPA can a l s o be e l i m i n a t e d as an a d d i t i o n a l s ou r ce o f n i t r o g e n . The p re sence o f 0.1M n i t r o u s o x i d e had l i t t l e e f f e c t upon the y i e l d o f methane gas , w h i l e known r a d i c a l s cavenge r s ( i o d i n e and g a l v i n o x y l ) reduced the methane y i e l d s u b s t a n t i a l l y . I ndeed , the data f o r the c o m p e t i t i o n s between n i t r o u s o x i d e and i o d i n e or g a l v i n o x y l would tend to r u l e out the i n v o l v e m e n t o f any r a d i c a l + N 2 + OH •* N 2 + CH 3 0 -* N 2 + OR (3 .37 ) (3 .38) (3 .39) - 144 -s caveng i ng p roce s s (3.39) i n the n i t r o g e n f o r m a t i o n mechanism, i i . 3 ) I o n i c S p e c i e s In o r d e r to i n v e s t i g a t e the p o s s i b l e i n v o l v e m e n t o f more than one i o n i c s p e c i e s i n the n i t r o g e n f o r m a t i o n mechanism, a d d i t i o n a l s cavenger s were i n t r o d u c e d i n t o n i t r o u s o x i d e s o l u t i o n s to compete f o r the p r e c u r s o r s of n i t r o g e n . To -2 -1 10 M to 10 M n i t r o u s o x i d e s o l u t i o n s , comparab le c o n c e n t r a t i o n s of the known e l e c t r o n s cavenge r s carbon t e t r a c h l o r i d e , c h l o r o f o r m , g a l v i n o x y l , i o d i n e and ace tone were added. With the e x c e p t i o n of a c e t o n e , the n i t r o g e n y i e l d was c o n s i d e r a b l y reduced i n the p re sence of these second s cavenge r s as shown i n T a b l e V I I . The data were a n a l y z e d i n terms o f a s i m p l e c o m p e t i t i o n between n i t r o u s o x i d e and a second s c a v e n g e r , S, f o r an u n d e f i n e d n i t r o g e n p r e c u r s o r , P, a c c o r d i n g to r e a c t i o n s (3 .40) and ( 3 . 4 1 ) . iv ? 4D M 20 + P • N 2 + p r o d u c t s (3 .40) S + p 3 , 4 1 • p r o d u c t s (3 .41) A s t e a d y s t a t e t r e a t m e n t o f t h i s mechanism ( s i m i l a r to t h a t done f o r r e a c t i o n (3 .33) and (3 .34) l e ad s to the k i n e t i c e x p r e s s i o n ( x x x i ) . 1 . 1 ( 1 + k 3 . 4 l [ S ] ) G(N 2 ) S ( P ) k 3 i 4 0 [N 20] ( x x x 1 ) - 145 -F i g u r e 111-10 shows the r e s u l t s of p l o t s o f 1/G(N 2 ) v e r s u s [s]/[N2'°] f o r n i t r o u s o x i d e i n c o m p e t i t i o n w i t h carbon t e t r a c h l o r i d e , c h l o r o f o r m , g a l v i n o x y l , i o d i n e and a c e t o n e . The f a c t t h a t l i n e a r r e l a t i o n s h i p s were found i n d i c a t e s t h a t the second s cavenge r s were compet ing f o r the n i t r o g e n p r e c u r s o r s . A l s o , the l i n e a r i t y i n d i c a t e s t h a t a l l the n i t r o g e n comes from the same s o u r c e . . Tab le XI l i s t s the y i e l d and r a t e c o n s t a n t r a t i o , k3 41/ k3 40' f o r the n i t r o g e n p r e c u r s o r s i n HMPA o b t a i n e d from the s l o p e s and i n t e r c e p t s of the c o m p e t i t i o n p l o t s o f F i g u r e 111 -1 0 . TABLE XI N i t r o g e n p r e c u r s o r y i e l d and r a t e c o n s t a n t r a t i o s c a l c u l a t e d f o r c o m p e t i t i o n between N 2 0 and second s o l u t e s i n i r r a d i a t e d HMPA. k 3 . 4 1 / k 3 .40 Second k 3 . 4 1 / ( Fo r 6 a q ' From Scavenger^ G(P) k 3 . 4 0 Data of Ref . 37) Carbon T e t r a c h l o r i d e 4 . 2 ± 0 . 4 1 . 8±0 . 3 5.4 C h l o r o f o r m 3 .9±0. 4 1 .4±0 . 3 3.6 G a l v i n o x y l 3 .9±0 . 4 1.1+0. 2 -I od i ne 4 . 5 ±0 . 4 1 .0±0 . 2 9.1 Acetone 4 . 2 ±0 . 4 < 0. 01 1 .1 The c a l c u l a t e d r e d u c i n g s p e c i e s y i e l d o f G(P) ^ 4 i s much h i g h e r than the proposed f r e e i on y i e l d , i m p l y i n g a mechanism i n v o l v i n g - 146 -ecu C H C U • Gv A Acetong A 0 5 1 0 [ S ] / [ N 2 0 ] 111 -1 0. P lot, f o r the n i t r o g e n y i e l d da ta u t i l i z i n g e q u a t i o n ( x x a i ) f rom c o m p e t i t i o n s between N^O and C C l r ( • ) , CHC1, ( O ) , g a l v i n o x y l ( • ) , I, { • ) , and ace tone / A \ - 147 -a d d i t i o n a l s p e c i e s . The r e l a t i v e s caveng i ng r a t e s of NgO compared to the o t h e r e l e c t r o n s cavenge r s (as de te rm ined from the r a t e c o n s t a n t r a t i o s ) were s m a l l e r than have been r e p o r t e d f o r e~ i n the r a d i o l y s i s o f aqueous s o l u t i o n s 3 8 T h i s a g a i n aq c o u l d i n d i c a t e t h a t s p e c i e s o t h e r than s o l v a t e d e l e c t r o n s a re i n v o l v e d . More l i k e l y , the o b s e r v a t i o n s sugges t t h a t s i n c e e l e c t r o n s are not e xpec ted to be s t r o n g l y s o l v a t e d i n HMPA, t h e i r s caveng i ng r e a c t i o n s may w e l l be d i f f u s i o n c o n t r o l l e d . As s u ch , the r a t e s would tend to be i ndependent o f s o l u t e . The r e s u l t s f rom acetone/NgO m i x t u r e s i n i r r a d i a t e d HMPA samples were un i que . In HMPA, acetone (known to be an e f f i c i e n t e l e c t r o n s cavenge r i n o t h e r s y s t e m s ) 1 3 6 d i d not appear to compete w i t h NgO f o r n i t r o g e n p r e c u r s o r s . I ndeed , the n i t r o g e n y i e l d was s u b s t a n t i a l l y i n c r e a s e d from samples c o n t a i n i n g NgO and h igh c o n c e n t r a t i o n s o f a c e t o n e . A p o s s i b l e e x p l a n a t i o n f o r t he se r e s u l t s i s t h a t n i t r o u s o x i de undergoes a charge t r a n s f e r r e a c t i o n w i t h the ace tone an i on formed from the i n i t i a l c o m p e t i t i o n f o r the n i t r o g e n p r e c u r s o r , P, a c c o r d i n g to the f o l l o w i n g sequence: * N 9 + p r o d u c t s (3 .40) CH 3 C0CH 3 + P + (CH 3 C0CH 3 ) (3 .42 ) ( C H 3 C 0 C H 3 ) " + NgO > N 9 + p r o d u c t s (3 .43) I f t h i s were the c a s e , the i n c r e a s e i n the n i t r o g e n y i e l d i s - 148 -s i m p l y a r e f l e c t i o n of the i n c r e a s e i n the t o t a l s cavenge r c o n c e n t r a t i o n . V i | d y a r t h i 1 3 7 found t h a t ace tone q u i c k l y scavenges hydratedj e l e c t r o n s (k ^ 10^ M _ 1 s e c - 1 ) . More i m p o r t a n t , he showed t h a t the r e s u l t i n g ace tone an ion c o u l d , i s u b s e q u e n t l y be p h o t o l y z e d , r e g e n e r a t i n g h y d r a t e d e l e c t r o n s w i t h a y i e l d of'v u n i t y . Th i s r e s u l t s ugges t s t h a t the ace tone an ion i s f a i r l y l o n g - l i v e d ( i n water a t l e a s t ) and t h a t i t r e a d i l y g i v e s up i t s exces s e l e c t r o n . I ndeed , Chaudhr i and A s m u s 1 3 8 found t h a t the acetone an ion c o u l d a c t as a r e d u c i n g s p e c i e s by t r a n s f e r r i n g an e l e c t r o n to o t h e r s o l u t e s . And, more p e r t i n e n t to the p r e s e n t c o n s i d e r a t i o n , B u r c h i l l and W o l l n e r 1 3 9 p r e s e n t e d s t r o n g e v i d e n c e to sugges t j u s t such a t r a n s f e r r e a c t i o n to n i t r o u s o x i d e from s t u d i e s o f the r a d i a t i o n -i nduced o x i d a t i o n of 2 - p ropano l by NgO i n a l k a l i n e aqueous s o l u t i o n . In HMPA t h e n , the ana logous e l e c t r o n t r a n s f e r r e a c t i o n (3.43) might e a s i l y be e n e r g e t i c a l l y f a v o u r a b l e , a c c o u n t i n g f o r the obse rved r e s u l t s . A s i m i l a r mechanism has been proposed to e x p l a i n the r e s u l t s o f a n i t r o u s o x i d e / a c e t o n e c o m p e t i t i o n f o r r a d i o l y s i s p r oduc t s i n d i m e t h y l s u l p h o x i d e } k 0 i i . 3 1 ) Geminate Ions - A H i s t o r i c a l Background Gamma i r r a d i a t i o n of media i n the gaseous phase produces a u n i f o r m d i s t r i b u t i o n o f f r e e e l e c t r o n s and p o s i t i v e i on s wh ich can be c o l l e c t e d upon the a p p l i c a t i o n o f an e l e c t r i c f i e l d . For h y d r o c a r b o n s , the y i e l d o f these f r e e i on s i s found to be G ( f . i . ) a a ( , ^ 4 . 0 . The r a d i a t i o n - i n d u c e d i o n i z a t i o n p r o ce s s e s - 149 -are not cons ide red to depend upon the d e n s i t y of the medium, but , c o n d u c t i v i t y measurements on l i q u i d hydrocarbons i n d i c a t e a much lower f r e e ion y i e l d G ( f . i . ) , . ^ 0 .1 . E l e c t r o n s i n j e c t e d i n to l i q u i d media would undergo i n e l a s t i c c o l l i s i o n s with nearby molecules and q u i c k l y become t h e r m a l i z e d . A p p a r e n t l y , in the hydrocarbons , most of these e l e c t r o n s are not s u f f i -c i e n t l y removed from t h e i r concomitant p o s i t i v e ion to escape i t s coulombic a t t r a c t i o n and r a p i d geminate recombinat ion o c c u r s . Thus, in l i q u i d s , only a few e l e c t r o n s escape recap tu re and u l t i m a t e l y become un i f o rm ly d i s t r i b u t e d as f r e e i o n s . C a l c u l a t i o n s ,based on a model suggested by Samuel and Magee 1 1* 1 i n d i c a t e a r e a c t i o n time f o r geminate recombinat ion -13 of MO sec . Such s h o r t - l i v e d geminate. i ons would not be expected to r eac t with added s o l u t e s . However, f o r many systems and scavengers , i t has been found that the y i e l d of products a t t r i b u t a b l e to e l e c t r o n scavenging was o f ten much g r e a t e r than the f r e e ion y i e l d . That i s , many s o l u t e s , S, appear ab le to i n t e r c e p t the ion recombinat ion process (3.44) by scavenging the geminate el ec t rons (3 .45 ) . R * (3.44) e + S s ,(3.45) A l s o , Khorana and H a m i l l 1 1 * 2 showed that h a l i d e i o n s , X " , cou ld do l i k e w i s e by scavenging the o x i d i z i n g geminate s p e c i e s . They repor ted that from aqueous s o l u t i o n s of high h a l i d e c o n c e n t r a t i o n s 150 -the s o l v a t e d e l e c t r o n y i e l d was i n c r e a s e d and s p e c i e s i d e n t i f i e d as d i h a l i d e i o n s , X^, o b s e r v e d . S i m i l a r r e s u l t s have been r e p o r t e d f o r a l c o h o l i c s o l u t i o n s of h a l i d e s a l t s } 4 3 - These groups a t t r i b u t e d t h e i r o b s e r v a t i o n s to geminate i on r e a c t i o n s . They c o n c l u d e d t h a t h a l i d e i o n s , wh ich have low e l e c t r o n d e t a c h -ment e n e r g i e s 5 eV f o r I" ), r e a d i l y underwent e l e c t r o n t r a n s f e r aq r e a c t i o n w i t h the o x i d i z i n g geminate s p e c i e s ( 3 . 4 6 ) , i . e . , e i t h e r the p o s i t i v e geminate i on or i t s o x i d i z i n g d e c o m p o s i t i o n p r o d u c t . The r e s u l t i n g h a l i d e atom then r e a c t e d w i t h a second h a l i d e i on (3.47) to produce the observed d i h a l i d e i o n s . X + X • X 2 (3 .47) Scaveng ing of p o s i t i v e i on s by h a l i d e would produce n e u t r a l s p e c i e s , end ing the cou l omb i c a t t r a c t i o n f o r the c o n c o m i t a n t geminate e l e c t r o n s and t h e r e b y l e s s e n i n g t h e i r l i k e l i h o o d o f geminate r e a c t i o n . R e a c t i o n between the e l e c t r o n s and n e u t r a l h a l i d e atoms ( 3 . 4 8 ) . + X * X + S (3 .46) + X * X (3 .48 ) would not be expec ted to compete w i t h d i h a l i d e i on f o r m a t i o n (3.47) because of the huge exces s o f h a l i d e i o n s a t the - 151 c o n c e n t r a t i o n s employed and the r e l a t i v e s t a b i l i t y o f the d i h a l i d e i o n . T h e r e f o r e , i n the p re sence of h i gh c o n c e n t r a t i o n s of h a l i d e i o n s , e l e c t r o n s t h a t would have undergone geminate r e c o m b i n a t i o n are spa red r a p i d n e u t r a l i z a t i o n and become f r e e i o n s , r e s u l t i n g i n the ob se rved i n c r e a s e d s o l v a t e d e l e c t r o n y i e l d s . I The p o s s i b i l i t y o f geminate i on s c a veng i n g means t h a t the r e a c t i o n t ime f o r geminate r e c o m b i n a t i o n must be l o n g e r than was o r i g i n a l l y t h o u g h t . Freeman and F a y a d h 6 8 s t a t e d t h a t s i n c e t r apped or s o l v a t e d e l e c t r o n s have been found to have m o b i l i t i e s comparab le t o o r d i n a r y i o n s , most i on p a i r s t h a t undergo geminate r e c o m b i n a t i o n would do so i n about s e c . Thus i n o r d e r f o r a s c a v e n g e r , S, to a p p r e c i a b l y i n t e r c e p t n e u t r a l i z a t i o n , the f o l l o w i n g r e l a t i o n ( x x x i i ) must be s a t i s f i e d : Tf geminate e l e c t r o n s c a veng i n g were c o r r e c t l y d e s c r i b e d i n terms of a s i m p l e c o m p e t i t i o n between i on r e c o m b i n a t i o n and e l e c t r o n c a p t u r e by s cavenger then the y i e l d o f a p r o d u c t , P, would be g i v e n by ( x x x i i i ) . - ( 1 0 ± 1 ) sec -1 ( x x x i i ) G(P) = G ( g . i . ) 1 + T k 3.45 ST ( x x x i i i ) - 152 -where G(g . i .), i s the geminate i o n y i e l d and x = l / k 3 i s the c h a r a c t e r i s t i c geminate i o n l i f e t i m e . However, the n e u t r a l i z a t i o n r e a c t i o n does not take p l a c e randomly between u n i f o r m l y d i s t r i b u t e d s p e c i e s but between i s o l a t e d p a i r s of i o n s wh ich a re c o n s t r a i n e d by t h e i r mutua l cou lomb i c p o t e n t i a l . The l i f e t i m e f o r r e c o m b i n a t i o n o f such ion p a i r s t h e r e f o r e depends upon the i n d i v i d u a l charge s e p a r a t i o n d i s t a n c e s . E l e c t r o n t h e r m a l i z a t i o n would be e xpec t ed to produce a whole spect rum o f i on p a i r s e p a r a t i o n s r e f l e c t i n g the i n i t i a l e l e c t r o n energy s pec t r um. T h e r e f o r e , no s i n g l e c h a r a c t e r i s t i c l i f e t i m e c o u l d be a s s i g n e d to the r e c o m b i n a t i o n p r oce s s (3 .44) and the dependence of p r o d u c t y i e l d on s cavenge r would not i n f a c t be e xpec ted to obey the r e l a t i o n s h i p of e q u a t i o n ( x x x i i i ) . At low s cavenge r c o n c e n t r a t i o n s i n h y d r o c a r b o n s , a p r oduc t y i e l d G(P) ^ 0.1 i s o b t a i n e d c o r r e s p o n d i n g n i c e l y w i t h the f r e e i on y i e l d de te rm ined from c o n d u c t i v i t y measurements. S c h u l e r e t a l J 1 * 1 * showed t h a t as the s cavenge r c o n c e n t r a t i o n was i n c r e a s e d , the e x p e r i m e n t a l l y obse rved dependence of p r o d u c t y i e l d s a r i s i n g s p e c i f i c a l l y from e l e c t r o n s caveng i ng r e a c t i o n s d e v i a t e d c o n s i d e r a b l y from e q u a t i o n ( x x x ) . S e v e r a l a t t empt s have been made t o d e r i v e an e x p r e s s i o n c apab l e of d e s c r i b i n g the s o l u t e c o n c e n t r a t i o n dependence on i on s caveng i ng } 4 5 " 1 "*8 U n f o r t u n a t e l y , due to u n c e r t a i n t i e s i n the i n i t i a l r e l a t i v e s p a t i a l d i s t r i b u t i o n o f i o n s , t h e i r m o t i o n s , encoun te r r a d i i and e f f i c i e n c i e s f o r r e a c t i o n , a l l o f t he se models f a i l e d to p r o v i d e an e x p l i c i t e x p r e s s i o n d e s c r i b i n g the p r o d u c t - 153 y i e l d o f s caveng i ng as a f u n c t i o n o f s o l u t e c o n c e n t r a t i o n . S chu l e r 1 1 * 1 * r e p o r t e d a p u r e l y em p i r i c a l r e l a t i o n s h i p ( x x x i v ) t h a t would c o r r e c t l y d e s c r i b e e x p e r i m e n t a l r e s u l t s f o r a wide range o f s o l u t e c o n c e n t r a t i o n s . G ( P ) . = G ( f . i . ) + G ( g - i - ) 1 + 1 ( x x x i v) The q u a n t i t y , a c a l l e d the r e a c t i v i t y , i s p r o p o r t i o n a l to the r a t e c o n s t a n t f o r the r e a c t i o n o f s o l u t e , S, w i t h the geminate e l e c t r o n s . a g was found e x p e r i m e n t a l l y to have a v a l u e from 10 - 15 M " 1 f o r good e l e c t r o n s c a v e n g e r s . At low s o l u t e concen-t r a t i o n s e q u a t i o n ( x x x i v ) reduces to ( xxxv ) wh ich p r e d i c t s a s imp l e square r o o t r e l a t i o n s h i p between s o l u t e c o n c e n t r a t i o n and p r oduc t y i e l d . G(P) > G ( f . i . ) + G ( g . i . ) (a [s])h ( xxxv ) Th i s e x p r e s s i o n i s s i m i l a r i n form to t h a t o b t a i n e d by Hummel 1 1 * 7 u s i n g a ma thema t i c a l model t o d e s c r i b e i o n s c a v e n g i n g a t low s o l u t e c o n c e n t r a t i o n s ( x x x v i ) , G(P) .= , . G ( f . i . ) + G ( g . i . ) - K . ( k 3 4 5 [s])*- ( x x x v i ) where k 3 A ^ was the r a t e c o n s t a n t f o r r e a c t i o n o f i on s w i t h s o l u t e and K was a c o n s t a n t i n v o l v i n g the a p p r o p r i a t e - 154 -i n t e g r a l s over the s p a t i a l p a r a m e t e r s . These e x p r e s s i o n s are both i n a c c o r d w i t h a square r o o t dependence obse rved e a r l i e r by W i l l i a m s ? 4 9 S c h u l e r has been a b l e to d e s c r i b e o t h e r a s p e c t s o f geminate i o n r e a c t i o n s by s i m p l y e x t e n d i n g the e m p i r i c a l r e l a t i o n s h i p of e q u a t i o n ( x x x i v ) . Through the i n c l u s i o n o f s u i t a b l e terms i n t o the e q u a t i o n , the r e a c t i v i t y o f s c avenge r s t h a t produce no measurab le p r o d u c t upon c a p t u r e o f geminate e l e c t r o n s have been de te rm ined from o b s e r v a t i o n s o f the e f f e c t o f such s o l u t e s on the hydrogen y i e l d . A l s o , e q u a t i o n s to d e s c r i b e c o m p e t i t i v e s caveng i ng have been p r e s e n t e d . The c o n s i s t e n c y w i t h which e q u a t i o n s based on the s i m p l e e m p i r i c a l r e l a t i o n s h i p d e s c r i b e and c o r r e c t l y p r e d i c t e x p e r i m e n t a l o b s e r v a t i o n s g r e a t l y enhances the model's v e r a c i t y . R e c e n t l y , i n a r i g o r o u s ma themat i c a l t r e a t m e n t , A b e l ! and F u n a b a s h i 1 5 0 f o r m u l a t e d a " d i f f u s i o n " model f o r geminate i on r e a c t i o n s . I t i s s i g n i f i c a n t t h a t t h e i r c a l c u l a t i o n s p r e d i c t t h a t S c h u l e r ' s e m p i r i c a l e x p r e s s i o n ( x x x i v ) s hou ld q u i t e a c c u r a t e l y d e s c r i b e the p r o c e s s . i i . 3 1 i ) Geminate Ions i n HMPA The r e a c t i o n between e l e c t r o n s and n i t r o u s o x i d e i n aqueous s o l u t i o n i s c l o s e to the d i f f u s i o n c o n t r o l l e d l i m i t , i . e . , the r a t e c o n s t a n t f o r e l e c t r o n c a p t u r e i s k ^ l O 1 ^ M _ 1 s e c - 1 . I f t h i s a p p l i e s f o r the N ?0/HMPA system as w e l l , c o n d i t i o n ( x x x i i ) - 155 -would be s a t i s f i e d f o r l a r g e n i t r o u s o x i d e c o n c e n t r a t i o n s i n HMPA and geminate e l e c t r o n s caveng i ng c o u l d be s i g n i f i c a n t . A c c o r d i n g to S c h u l e r ' s e m p i r i c a l phen'omenol o g i c a l mode l , the n i t r o g e n y i e l d i n exces s o f the f r e e i on y i e l d i n HMPA s h o u l d then f o l l o w a square r o o t dependence on s o l u t e c o n c e n t r a t i o n . R e a r r a n g i n g e q u a t i o n ( x x i v ) to a l i n e a r form g i v e s the f o l l o w i n g e q u a t i o n ( x x x v i i ) to d e s c r i b e the HMPA/NgO system f o r N,,0 c o n c e n t r a t i o n s where G (N 9 ) > G ( f . i . ) . s t r a i g h t l i n e w i t h s l o p e 1/°^ Q G ( g . i . ) and i n t e r c e p t 1 / G ( g . i . ) . F i g u r e I I I - l l shows such a p l o t f o r the da ta o f c o n c e n t r a t i o n s of n i t r o u s o x i d e where the argon d i l u t i o n t e c h n i q u e was employed ( f i l l e d c i r c l e s ) , i t i s d i f f i c u l t to t e l l whether t h i s d e v i a t i o n i s r e a l or s i m p l y a r e f l e c t i o n of s y s t e m a t i c e r r o r s . In any e v e n t , the n o n - l i n e a r i t y sugges t s t h a t a mechanism i n v o l v i n g geminate i on s caveng i ng by N,,0 does not a p p l y to the HMPA sys tem. From the s l o p e and i n t e r c e p t o f the " b e s t " l i n e th rough the p o i n t s , one c a l c u l a t e s a geminate i o n y i e l d G ( g . i . ) = 4.2+0.6 and a r e a c t i v i t y of N 2 0 towards geminate e l e c t r o n , o<N Q •=' 1 1 + 3 M " 1 . 2 _ - i The l a t t e r v a l u e i s comparab le to Q = 8 - 10 M r e p o r t e d f o r c y c l ohexane s o l u t i o n s . However, when combined w i t h the f r e e i on y i e l d of G ( f . i . ) = 2.2 ± 0 . 2 , a geminate i o n y i e l d G C g . i . ) = 4.2 ± 0.6 would i n d i c a t e a t o t a l i on y i e l d o f G ( t . i . ) = 6.4 ± 0 .8 . (1 + (G (N 2 ) - G ( f . i . ) ) G ( g . i .) ( x x x v i i ) 2 F i g u r e I I I - l l . P l o t o f the da ta f o r n i t r o g e n y i e l d i n exces s o f the f r e e ion y i e l d from i r r a d i a t e d N 20 s o l u t i o n s of HMPA u t i l i z i n g e q u a t i o n ( x x x v i i ) . - 157 -In terms o f the i n i t i a l i o n i z a t i o n p r o c e s s , HMPA s h o u l d not be much d i f f e r e n t f rom the hyd roca rbons f o r wh ich t o t a l i o n i z a t i o n y i e l d s of about f o u r a re g e n e r a l l y acknowledged . T h i s f a c t a l ong w i t h the g e n e r a l n o n - l i n e a r i t y of F i g u r e I I I - l l l e a d s t h i s a u t ho r to r e j e c t the mechanism. R e s u l t s f rom j s e ve r a l e x p e r i m e n t s i n wh i ch m u l t i p l e s o l u t e s were employed tend to c o r r o b o r a t e t h i s c o n c l u s i o n . F i g u r e 111 -12 shows the e f f e c t of t he p re sence o f 0.2 M L i B r on the n i t r o g e n y i e l d from NgO/HMPA s o l u t i o n s . The s o l i d l i n e i n the f i g u r e r e p r e s e n t s the y i e l d of n i t r o g e n i n the absence o f bromide i o n . As can be s een , the n i t r o g e n y i e l d was l i t t l e a f f e c t e d . , At 0.2M, B r - would be e xpec ted to scavenge p o s i t i v e geminate ions i n HMPA and t h e r e b y i n c r e a s e the f r e e an i on y i e l d and l i f e -t i m e . I f the e x t r a y i e l d o f n i t r o g e n above the f r e e i o n y i e l d ( G ( f . i . ) = 2.2) a ro se f rom geminate e l e c t r o n s caveng i ng one would have e x p e c t e d a marked i n c r e a s e i n the n i t r o g e n y i e l d i n those s o l u t i o n s - - e s p e c i a l l y f o r the l owe r c o n c e n t r a t i o n s o f NgO. T h i s was c l e a r l y not the c a s e . C o n s e q u e n t l y t h e r e i s the i m p l i c a t i o n t h a t geminate s c a veng i n g i s no t r e s p o n s i b l e f o r n i t r o g e n y i e l d s up to two t imes the f r e e i o n y i e l d i n HMPA. Expe r imen t s i n wh ich ace tone was used as a second s o l u t e a l s o l e a d to t h i s c o n c l u s i o n . As has been d e s c r i b e d , a ce tone appears to scavenge e l e c t r o n s i n HMPA to produce an i n t e r m e d i a t e s p e c i e s ( p r o b a b l y the ace tone an i on ) wh ich can go on to reduce NgO t o n i t r o g e n . Now, a l a r g e c o n c e n t r a t i o n o f a ce tone would be e xpec ted t o r e a c t w i t h any " s c a v e n g e a b l e " geminate e l e c t r o n s i n N , 0 ] . x 1 0 M F i g u r e 111 -1 2 The e f f e c t o f 0.2M B r " on n i t r o g e n y i e l d from N^O i n HMPA. The s o l i d l i n e r e p r e s e n t s the n i t r o g e n y i e l d i n the absence o f B r " . - 159 -HMPA. H u n t 1 5 1 has shown t h i s to be the case i n wa te r f o r i n s t a n c e . Thus , a l a r g e acetone c o n c e n t r a t i o n would s e r ve to c o n v e r t s h o r t - l i v e d geminate e l e c t r o n s i n t o r e l a t i v e l y l o n g -l i v e d acetone an ions wh ich c o u l d i n t u r n be scavenged by a much l ower c o n c e n t r a t i o n o f n i t r o u s o x i d e . Tn t h i s way, the c o n t r i b u t i o n of geminate e l e c t r o n s to the t o t a l y i e l d o f s cavengeab le r e d u c i n g s p e c i e s would be r e v e a l e d . F i g u r e 111-13 r e v e a l s t h a t the p re sence of 0.27M ace tone i n NgO/HMPA s o l u t i o n s r e s u l t e d i n a near u n i f o r m i n c r e a s e i n n i t r o g e n y i e l d o f about AG(Ng) ^ 0 .7 . C l e a r l y the r e s u l t s are not c o n s i s t e n t w i t h geminate s c a veng i n g p r o c e s s e s b e i n g r e s p o n s i b l e f o r the h igh n i t r o g e n y i e l d s obse rved from l a r g e c o n c e n t r a t i o n s o f NgO a l o n e . The h igh i o n i z a t i o n p o t e n t i a l o f n i t r o u s o x i d e (12.9 eV i n the gas p h a s e ) 1 5 2 makes r e a c t i o n w i t h p o s i t i v e i on s (3 .49) to produce n i t r o g e n u n l i k e l y . R + + NgO • Ng + p r o d u c t s ^ 3 4 9 ^ The p re sence o f known p o s i t i v e i on s cavenge r s i n HMPA s u p p o r t s t h i s s u p p o s i t i o n . Water or bromide i on s a t >0.2M c o n c e n t r a t i o n s had l i t t l e o r no e f f e c t on G(Ng) . Rzad e t a l l 5 3 found t h a t i n c y c l o h e x a n e , the a d d i t i o n o f NgO (o r o t h e r e l e c t r o n s cavenge r s ) r e s u l t e d i n an i n c r e a s e of p o s i t i v e i on s caveng ing by c y c l o p r o p a n e . CM z CD o N , O ] x 1 0 2 M F i g u r e 111-13, The e f f e c t of 0.27M acetone on n i t r o g e n y i e l d from N z 0 i n HMPA, l i n e r e p r e s e n t s the n i t r o g e n y i e l d i n the absence of a ce tone . The s o l i d - 161 -i i i ) Secondary I o n i c R e a c t i o n s o f N i t r o u s Oxide i i i • 1) P o s s i b l e Mechanisms f o r NgO Scaveng ing In s p i t e o f the l a r g e number o f i n v e s t i g a t i o n s u t i l i z i n g n i t r o u s o x i de as ain e l e c t r o n s cavenge r t h e r e remains c o n s i d e r a b l e u n c e r t a i n t y as to the na t u r e of the i n t e r m e d i a t e s i n v o l v e d . Wentworth e t a l l 5 4 have shown t h a t the a t t a chment o f t he rma l e l e c t r o n s t o n i t r o u s o x i d e i n the gas phase t a k e s p l a c e d i s s o c i a t i v e l y v i a a two body p r oce s s ( 3 . 5 0 ) . e g + N 2 0 • N 2 • 0 - ( However, they p r e d i c t e d from t h e i r o b s e r v a t i o n s t h a t the s p e c i e s NgO - s hou ld a l s o be s t a b l e . Other groups s t u d i e d e l e c t r o n a t tachment r e a c t i o n s i n n i t r o u s o x i d e a t much h i g h e r p r e s s u r e . By m o n i t o r i n g the e l e c t r o n decay they found t h a t a t h r e e - b o d y p r o c e s s , p r o b a b l y (3 .51) ,was i n d i c a t e d 1 5 5 » 1 5 6 e " + N 2 Q + N 2 0 N 2 0 " + NgO ( 3 5 1 ) P a u l s o n J 5 7 u s i n g a doub le mass s p e c t r o m e t e r s y s t em, has ob se rved NgO - formed v i a e l e c t r o n t r a n s f e r from NO - (3 .52) t h e r e b y c o n f i r m i n g t h a t i t has a p p r e c i a b l e s t a b i l i t y . NO" + N 2 0 • N 2 0 - + N0 ( 3 ^ 5 2 ) - 162 -1 2 5 N i t r o u s o x i d e used as an e l e c t r o n s cavenge r i n the r a d i o l y s i s o f gaseous systems would t h e r e f o r e be e x p e c t e d t o produce 0~ or N20~ i on s as i n t e r m e d i a t e s . Johnson and Warman i n some e a r l y work on gaseous propane r a d i o l y s i s p roposed a mechanism i n c l u d i n g a secondary r e a c t i o n between n i t r o u s o x i d e and the o x i d e i o n . ( 3 . 5 3 ) to accoun t f o r a n i t r o g e n y i e l d t w i c e the e l e c t r o n y i e l d . e +• N O • N + o" d 2 (3 .50) 0 - + N 2 0 N £ + 0- { 3 5 3 ) W a r m a n 1 1 9 l a t e r showed t h a t the a d d i t i o n o f C 0 2 t o N 2 0/propane m i x t u r e s r e s u l t e d i n a dec rea se i n the n i t r o g e n y i e l d to a l i m i t i n g value, near the e l e c t r o n y i e l d . C 0 2 does not appear to scavenge the rma l e l e c t r o n s i n the gas phase. P a u l s o n 1 5 8 has i n f a c t c a l c u l a t e d t h a t the e l e c t r o n a f f i n i t y of gaseous C 0 2 i s n e g a t i v e . Nor i s C0 2 known to r e a c t w i t h o t h e r r a d i c a l o r i o n i c s p e c i e s produced i n gaseous hyd roca rbon r a d i o l y s i s . However, a r a p i d r e a c t i o n w i t h the o x i d e i o n (3 .54 ) has been obse rved J 5 9 0" + C 0 ? • co: 2 3 ( 3 .54 ) Thus, Warman's C0 2 /N 2 0/p ropane gas pha se .da ta c o u l d be e x p l a i n e d i n terms o f a s i m p l e c o m p e t i t i o n between C 0 2 and N 2 0 f o r - 163 -i n t e r m e d i a t e oxide, i o n s . Warman s t u d i e d a number o f a l k ane and a l k e n e gases and obse rved n i t r o g e n y i e l d s between one and two t imes the e l e c t r o n y i e l d . He sugges ted t h a t o x i d e i on s might r e a c t w i t h the v a r i o u s h y d r o c a r b o n s , R, to produce i o n i c s p e c i e s (3 .55) which c o u l d s u b s e q u e n t l y r e a c t w i t h NgO to produce s econda r y n i t r o g e n ( 3 . 5 6 ) , • 0 " + R : • RO" (3 ,55) RO - + NgO • Ng + p r o d u c t s ( 3 .56 ) Invo lvement of such r e a c t i o n s to v a r i o u s e x t e n t s c o u l d a ccoun t f o r h i s f i n d i n g s , and c o m p e t i t i o n w i t h COg tended to s u p p o r t t h i s c o n j e c t u r e . Warman f u r t h e r sugges ted t h a t an a l t e r n a t e path f o r the secondary r e a c t i o n between 0 " and NgO (3.57) c o u l d a l s o r e s u l t i n i n t e r m e d i a t e n i t r o g e n y i e l d s . 0 - + N 2 0 • NO" + NO ( 3 i 5 7 ) I ndeed, mass s p e c t r o m e t r i e s 6 0 d r i f t t u b e 1 , 6 1 and i on c y c l o t r o n r e s o n a n c e 1 6 2 e xpe r imen t s have shown t h a t r e a c t i o n (3 .57) i s the dominant p roce s s when 0 - i s r e a c t e d w i t h NgO i n the gas phase. F e r g u s o n 1 6 3 p o i n t e d out t h a t s i n c e the e l e c t r o n a f f i n i t y of NO was ve ry sma l l (< 0.1 eV) the N0~ i o n would r e a d i l y undergo c o l l i s i o n a l detachment ( 3 . 5 8 ) . - 164 -•NO", + M • NO + M + e " (3 .58) R e g e n e r a t i o n o f e l e c t r o n s i n t h i s way would u n d o u b t e d l y l e a d to a c h a i n r e a c t i o n f o r m a t i o n of n i t r o g e n and thus such a mechanism c o u l d not account f o r the somewhat dep re s sed n i t r o g e n y i e l d s o b s e r v e d . More r e c e n t l y , J o h n s o n 1 6 1 * examined the r a d i o l y s i s o f p ropane, n i t r o u s o x i d e and hydrogen c h l o r i d e m i x t u r e s . I t was expec ted t h a t HC1 would r e a c t w i t h 0 _ by r e a c t i o n ( 3 . 5 9 ) , 0" + HC1 • OH + C l " ( 3 .59 ) and t h e r e b y reduce the n i t r o g e n y i e l d to the e l e c t r o n y i e l d . Such was indeed the case as the a d d i t i o n of HC1 dec r ea s ed G(Ng) r a p i d l y to a v a l u e near the e l e c t r o n y i e l d w h i l e the hydrogen y i e l d remained u n a f f e c t e d . However, as the c o n c e n t r a t i o n of HC1 was i n c r e a s e d , the n i t r o g e n y i e l d s l o w l y de c r ea s ed f u r t h e r w h i l e the hydrogen y i e l d i n c r e a s e d . Gaseous HC1 i s known t o r e a c t w i t h e l e c t r o n s v i a r e a c t i o n ( 3 . 6 0 ) . e " + 2HC1 • H 2 + p r o d u c t s ^ g 0 ^ These r e s u l t s s uppo r ted the e a r l i e r c o n j e c t u r e t h a t the exces s n i t r o g e n a rose f rom seconda ry r e a c t i o n w i t h an i o n i c i n t e r m e d i a t e . That i s , at low c o n c e n t r a t i o n , HC1 i n t e r f e r s w i t h the seconda ry i o n i c r e a c t i o n and a t h i g h e r c o n c e n t r a t i o n competes w i t h NgO f o r e l e c t r o n s d i r e c t l y . However, Johnson found t h a t a t low c o n c e n -- 165 -t r a t i ons o f HCl , the n i t r o g e n y i e l d d e p r e s s i o n was i ndependent o f N 2 0 c o n c e n t r a t i o n . Upon r e - i n v e s t i g a t i n g the C0 2 -N , ,0 -propane s y s tem, he found a s i m i l a r i ndependence . Under t he se c i r c u m s t a n c e s , the data were not c o n s i s t e n t w i t h 0 " be i ng the r e a c t i v e i n t e r m e d i a t e . T h e r e f o r e , i t was sugges ted t h a t e l e c t r o n c a p t u r e by N 2 0 proceeded v i a the t h r e e body a s s o c i a t i v e mechanism (3.51) and r e s u l t e d i n the f o r m a t i o n o f the N,>02 i o n . e" + N 2 0 + N 2 0 • N 2 0 - • N 20 ( 3 5 1 ) N 2 ° " + N 2 ° » N 2 ° 2 + N2 (3.61) Johnson proposed t h a t i t was the subsequent seconda ry decay of the N 2 0~ i on (3 .62) ( p o s s i b l y by n e u t r a l i z a t i o n ) t h a t a ccoun ted f o r the exces s n i t r o g e n . N 2 0~ ( R H ^ • N ? + p r o d u c t s ^ 3 S2y The e f f e c t s of HCl and C 0 2 would then be e x p l a i n e d i n terms of a c o m p e t i t i o n f o r the N 2 0 2 i on ( 3 . 6 3 ) , ( 3 . 6 4 ) , and r e a c t i o n ( 3 . 6 2 ) . N 20" + ' HCl • N 2 0 + OH + C l " ( 3 > 6 3 ) N 2 0 " + C0 2 > N 2 0 + CO" ( 3 > 6 4 ) - 1 6 6 ' The NgOg i on has i n f a c t been i d e n t i f i e d as a s t a b l e s p e c i e s both i n the gas p h a s e 1 6 1 5 1 6 2 and i n aqueous s o l u t i o n } 6 5 ' 1 6 6 - The o x i d e i o n , 0~, has been p o s t u l a t e d as an i m p o r t a n t i n t e r m e d i a t e formed from e l e c t r o n s c a veng i n g by n i t r o u s , o x i d e i n - the . ' . r ad io l y s i s . ' o f gaseous methyl c y c l o h e x a n e \ 2 1 a c e t y l e n e 1 , 2 2 and b e n z e n e } 6 7 In t he se s y s t ems , n i t r o g e n y i e l d s o f g r e a t e r than 18 have been a t t r i b u t e d to c h a i n r e a c t i o n s i n v o l v i n g r e a c t i o n between 0 _ and the h y d r o c a r b o n , f o l l o w e d by e l e c t r o n t r a n s f e r to NgO. In the a c e t y l e n e s t u d y , the a d d i t i o n o f COg to NgO/acety lene m i x t u r e s r e s u l t e d i n a r e d u c t i o n i n the n i t r o g e n y i e l d , t e n d i n g to a l ower l i m i t around the e l e c t r o n y i e l d . T h i s was. taken as e v i d e n c e f o r the i n v o l v e m e n t o f 0 " i on s i n the c h a i n mechanism. However, s i n c e t h e s e e x p e r i m e n t s were conducted at r e l a t i v e l y h igh p r e s s u r e s (near a t m o s p h e r i c p r e s s u r e i n some ca se s ) , e l e c t r o n c a p t u r e would be e xpec ted to produce the NgO - i on v i a the t h r e e body p r o ce s s ( 3 . 5 1 ) . . T h e r e f o r e , the i n v o l v e m e n t of 0 " i ons would depend upon the l i f e t i m e o f NgO" w i t h r e s p e c t to d i s s o c i a t i o n ( 3 . 6 5 ) . N 2 ° " N 2 + ° " (3 .65) From a k i n e t i c s tudy of the c o m p e t i t i o n between d i s s o c i a t i o n and i t s r e a c t i o n w i t h d e u t e r i u m i o d i d e , a l i f e t i m e of > 1 0 " 4 sec has been e s t i m a t e d f o r the NgO" i on i n gaseous h y d r o c a r b o n s } 6 8 Thus i t seems l i k e l y t h a t NgO" (o r NgO") i o n s were the i n t e r m e d i a t e r e a c t i v e s p e c i e s . B a i l e y and D i x o n 1 2 3 sugges ted t h a t t he se s p e c i e s were - 167 -r e s p o n s i b l e f o r , t h e i r r e s u l t s from the vapour phase r a d i o l y s i s o f n i t r o u s o x i d e i n w a t e r / i s o p r o p a n o l m i x t u r e s . They found the n i t r o g e n y i e l d frOm NgO about equa l to the e l e c t r o n y i e l d i n wate r vapour but g r e a t e r than t h r e e t imes the e l e c t r o n y i e l d i n i s o p r o p a n o l vapour . For w a t e r / a l c o h o l m i x t u r e s the r e s u l t s i n d i c a t e d a c o m p e t i t i o n f o r some i n t e r m e d i a t e s p e c i e s . Complete r e a c t i o n o f e i t h e r NgO - o r NgOg w i t h wa te r ( 3 . 6 6 , 3.67) would 1 ead to an o v e r a l T, n i t r o gen y i e l d ' e q u a l to the e l e c t r o n y i e l d . e " + NgO + M • NgO" + M ^ g ^ NgO; : + NgO . N g + NgO" ( 3 > g 0 ) NgO" + .HgO . N g + OH + OH" ( 3 . 6 6 ) NgO": + HgO . — .NgO + OH + OH ( 3 g y j In l i q u i d phase r a d i o l y s i s s t u d i e s , the mechanisms whereby n i t r o g e n i s formed from e l e c t r o n s caveng i ng by n i t r o u s o x i d e appear even l e s s w e l l u n d e r s t o o d . The ve r y n a t u r e of l i q u i d r a d i o l y s i s l e ad s t o inhomogenous d i s t r i b u t i o n s of r a d i o l y t i c p r oduc t s and the s o l v a t i o n p r o p e r t i e s o f l i q u i d s undoub ted l y a f f e c t the s t a b i l i t y o f the v a r i o u s i o n i c i n t e r m e d i a t e s . For example , the n i t r o g e n y i e l d o b t a i n e d from the r a d i o l y s i s o f n i t r o u s o x i d e i n l i q u i d benzene was o n l y about one f i f t h t h a t o b t a i n e d from a gas phase s t u d y } 6 7 Sato e t a l } 0 1 * were f i r s t to - 168 -propose seconda ry i o n i c r e a c t i o n s o f n i t r o u s o x i d e i n l i q u i d phase r a d i o l y s i s from s t u d i e s o f c y c l o h e x a n e . By a n a l o g y w i t h gas phase work, the o x i d e i o n was proposed as the r e a c t i v e i n t e r m e d i a t e . S i n c e t h a t t ime a number o f a u tho r s have proposed secondary n i t r o g e n f o r m a t i o n i n many l i q u i d s y s t e m s 9 3 ' 1 0 1 ' 1 0 8 ' 1 1 1 * ' 1 1 5 ' 1 6 9 ' 1 7 0 ' 1 7 1 I t i s p a r t i c u l a r l y i n t e r e s t i n g t h a t S c h u l e r ' s g r o u p 1 7 0 a r r i v e d a t such a c o n c l u s i o n . U n l i k e most s o l u t e s , d a t a f rom NgO s t u d i e s i n c y c l o h e x a n e c o u l d not be p r e d i c t e d from t h e i r e m p i r i c a l phenomeno log i ca l model (based on e q u a t i o n . ' ( x x x i v ) ) . Fo rced to conc l ude t h a t secondary i o n i c r e a c t i o n must o c c u r i n N 2 0 s o l u t i o n s they m o d i f i e d t h e i r mode l . The changes r e s u l t e d i n a much more complex (but l o g i c a l ) e x p r e s s i o n wh ich then a l l o w e d c o r r e c t p r e d i c t i o n s t o be made f o r the c y c l o h e x a n e s o l u t i ons--. i i i - 2) Secondary I o n i c R e a c t i o n s i n HMPA I t seems q u i t e p o s s i b l e then t h a t i n the p r e s e n t i n v e s -t i g a t i o n of HMPA r a d i o l y s i s , the h igh n i t r o g e n y i e l d s from Re-s o l u t i o n s might a r i s e from secondary i o n i c r e a c t i o n s i n v o l v i n g Ng0. E l e c t r o n c a p t u r e by NgO must l e a d , at l e a s t i n i t i a l l y , to the f o r m a t i o n of N 9 0 _ (3. 6 8 ) . e " + / N 2 0 (3.68) 169 The i n t e r m e d i a t e might more g e n e r a l l y be denoted as (NgO) -which r e p r e s e n t s the NgO - i o n i t s e l f . , o r some daugh te r i o n formed from decay or d i s s o c i a t i o n . Such an, i on might then undergo any o f s e v e r a l p r o ce s s e s l e a d i n g to the f o r m a t i o n of a s i n g l e m o l e c u l e o f n i t r o g e n as r e p r e s e n t e d by the pseudo-f i r s t , o r d e r p roce s s ( 3 . 6 9 ) . ( N 2 0 ) " — k 3 ' 6 9 • Ng + p r o d u c t s (3.69) On the o t h e r h a n d r e a c t i o n o f (NgO) - w i t h a second NgO m o l e c u l e (3 .70) c o u l d l e a d to the e v e n t u a l f o r m a t i o n of two mo l e cu l e s of n i t r o g e n ( 3 . 7 1 ) . ( N 2 ° ) " + N 2 ° k 3 , 7 0 » N 2 + ° ( N 2 ° > ~ (3 .70) 0 ( N 2 0 ) " — k 3 , 7 1 » N 2 + p r o d u c t s (3 .71) For such a scheme, the f r a c t i o n o f (NgO) - i on s unde rgo i ng secondary r e a c t i o n w i t h NgO, f s e c » would be g i v e n by ( x x x v i i i ) f s e c > k 3 - 7 0 [N 2O] k 3 . 70 NgO + k' 3.69 ( x x x v i i i ) The n i t r o g e n . y i e l d , f r o m a s o l u t i o n c o n t a i n i n g NgO and r e a c t i n g v i a t h i s mechanism would t h e r e f o r e be g i v en by ( x x x i x ) . 170 g ( N 2 ) . G . C f . i . ) (1 + 3.70 [N 2O] •3. 70 N 2 0 + k 3.69 ( x x x i x ) T h i s e x p r e s s i o n c a n be r e a r r a n g e d t o g i v e ( x l ) . 3.69 ( G ( N 2 ) - G ( f . i . ) ) G ( f . i . ) G ( f . i . ) k 3.70 N 2 0 ( x l ) G ( N 2 ) - G ( f . i . ) i s s i m p l y t h e n i t r o g e n y i e l d i n e x c e s s o f t h e f r e e i o n y i e l d . A . p l o t o f 1/(G(N2) - G ( f . i . J v e r s u s l / i ^o] s h o u l d be a s t r a i g h t l i n e w i t h i n t e r c e p t 1 / G ( f . i . ) a n d s l o p e k 3 g g / G ( f . i . ) kg F i g u r e 111-14 shows, s u c h a p l o t f o r t h e n i t r o g e n y i e l d s f r o m N 2 0 / H M P A s o l u t i o n s o f F i g u r e 111-3, t a k i ng G ( f . i . ) = ,2.2. As c a n be s e e n , a f a i r l y l i n e a r r e l a t i o n s h i p was o b t a i n e d . F r o m t h e s i ope a n d , i n t e r c e p t o f a l e a s t s q u a r e s f i t o f t h e d a t a , a v a l u e o f k 3 6 5 / k 3 7 Q = 1.2 ± 0.1 X 10 M was c a l c u l a t e d . What t h i s means i s t h a t i f t h i s i s i n f a c t t h e o p e r a t i v e m e c h a n i s m , t h e n f o r an HMPA s o l u t i o n c o n t a i n i n g 1.2 X 1 0 - 2 M 1^0, f u l l y h a l f t h e ( N 2 0 ) ~ i o n s u n d e r g o s e c o n d a r y i o n i c r e a c t i o n t o g i v e a s e c o n d m o l e c u l e o f n i t r o g e n . i v ) G e m i n a t e I o n S c a v e n g i n g V e r s u s S e c o n d a r y I o n i c R e a c t i o n s i v . l ) In P u r e HMPA R a d i o l y s i s It. i s evident; tha t of the many processes which might lead, to a n i t r ogen y i e l d from i r r a d i a t e d s o l u t i o n s of NgO in HMPA in excess of the f r e e ion y i e l d on ly two mechanisms are reasonab ly compat ib le with the o b s e r v a t i o n s . These processes, are the r e a c t i o n of.NgO w i t h : - Geminate ions - S e c o n d a r y .NgO,- ions or one of t h e i r decay products Based on the mechanisms d i s cu s sed e a r l i e r f o r each o f these schemes, t h e . n i t r o g e n y i e l d s observed as a f u n c t i o n of n i t r o u s ox ide c o n c e n t r a t i o n would be g iven by: - Geminate ion . scaveng ing . G ( Vg/i.s. = G ^ - 1 ' - ) + G(o . i . ) (1 + ( a NgO NgO |) ( x i i ) Secondary ion r e a c t i o n with (NgO) ^^ s ' i . r : . = G( f- i-) ^ + ki 1 • + , 3.69 3.70 NgO ( x i i i ) T h e o r e t i c a l .'curves,; f o r these equat ions are shown in F i gu re 111 -1 5 which i n c l u d e s the observed n i t r ogen y i e l d s taken from F i gu re 111-3. The dashed l.ine was obta ined from the geminate scavenging F i g u r e 111 -1 5. T h e o r e t i c a l Ht y i e l d c u r v e s r e s u l t i n g from geminate ion s c aveng i ng (———) and seconda ry i o n i c r e a c t i o n (......) c o n s i d e r a t i o n s . The c i r c l e s ' r e p r e s e n t a c t u a l ob se r ved da t a f rom F i g u r e I I I - 3 . - 174 -e q u a t i o n ( x l i ) t a k i n g G ( f . i . ) = 2 . 2 , G ( g . i . ) = 5 . 7 and g -4.7 M . The d o t t e d l i n e was o b t a i n e d from the s e conda r y i o n i c r e a c t i o n e q u a t i o n ( x l i i ) t a k i n g G ( f . i .) = 2 . 2 and k'3 gg/•<3 7 0 = 1 . 2 X 1 0 " 2 M " 1 . Both f u n c t i o n s f i t the da ta f a i r l y w e l l . The seconda ry i o n i c cu r ve tends towards a p l a t e a u , w h i l e the R ob se rved y i e l d c o n t i n u e s to! i n c r e a s e w i t h R O c o n c e n t r a t i o n . At ve r y h i gh s cavenge r c o n c e n t r a t i o n s a small, c o n t r i b u t i o n f rom geminate i on seaveng ing wou1d be a n t i c i p a t e d which would account f o r the d e v i a t i o n . . " . The da ta from R O a l one i n HMPA thus does not s u f f i c e to e l u c i d a t e t h e R O s c a veng i n g mechanism. As p o i n t e d o u t , the a d d i t i o n o f o t h e r s o l u t e s tends to r u l e out geminate i o n s c a v e n g i n g . , : e) R a d i o l y s i s o f L i q u i d M i x t u r e s i ) Background. It. might be: expec ted t h a t r a d i o l y s i s p r oduc t y i e l d s f rom a m i x t u r e o f two l i q u i d s would be i n t e r m e d i a t e between the y i e l d s of the pure s o l v e n t s . Schmidt and Al 1 en i 0 2 u s i n g the c l e a r i n g f i e l d t e c h n i q u e , , s t u d i e d a v a r i e t y of hyd roca rbon m i x t u r e s and found t h a t i n d e e d , i n each c a s e , i n t e r m e d i a t e f r e e i on y i e l d s were o b t a i n e d . The i on y i e l d from a m i x t u r e w i l l depend m a i n l y upon the e l e c t r o n f r a c t i o n of the components and perhaps t h e i r s o l v a t i o n p r o p e r t i e s . While: i t i s t r u e t h a t i n a m i x t u r e a d d i t i o n a l t ypes of s o l v e n t - - s o l v e n t i n t e r a c t i o n s c o u l d i n f l u e n c e the o v e r a l l s o l v a t i n g p r o p e r t i e s , i n g e n e r a l , an o v e r a l l a v e r a g i n g o f those p r o p e r t i e s would t end towards the i n t e r m e d i a t e y i e l d s o b t a i n e d . Scaveng ing o f the r e s u l t a n t i on s on the o t h e r hand shou ld be much more dependent on. m i c r o s c o p i c p r o p e r t i e s o f the m i x t u r e s , e s p e c i a l l y i f the two s o l v e n t s produce "common" p r i m a r y s p e c i e s ( e ~ , H, C H 3 , e t c ) . I t i s proposed t h a t i n HMPA, exces s N 2 a r i s e s as a r e s u l t o f s econda ry i o n i c s c aveng i ng r a t h e r than f rom geminate i o n s c a v e n g i n g . I f t h i s i s the c a s e , then i t was a n t i c i p a t e d t h a t a s tudy o f the n i t r o g e n y i e l d from m i x t u r e s of HMPA and a s o l v e n t i n wh ich secondary i o n i c r e a c t i o n s are known not to o ccu r might s e r ve to e l u c i d a t e the am"on . scaveng ing mechanism of n i t r o u s o x i d e i n HMPA. The y i e l d of s cavengeab le e l e c t r o n s i n wa te r has been measured from s t u d i e s u t i l i z i n g many s o l u t e s and t e c h n i q u e s } 7 2 The data suggest t h a t ' n i t r o g e n f o r m a t i o n from i o n i c r e a c t i o n s of n i t r o u s o x i de i n water does not i n v o l v e seconda ry i o n i c r e a c t i o n s . ; That i s , t h e n i t r o g e n y i e l d s ob se rved were cpmparabl to the s cavengeab le e l e c t r o n y i e l d de te rm ined i n d e p e n d e n t l y . i i ) HMPA/H20 M i x t u r e s C o n s i d e r t h e . r a d i o l y s i s o f HMPA/HgO m i x t u r e s . I f , as i s i n d i c a t e d from the r e s u l t s d i s c u s s e d e a r l i e r , the f r e e i o n y i e l d i n HMPA i s G ( f . i . ) = 2.2 and the h i g h e r y i e l d s o f n i t r o g e n a r i s e from secondary i o n i c r e a c t i o n s , t h e f o l l o w i n g model can be - 176 -p r opo sed . -For a s o l u t i o n , of 0.02M NgO i n w a t e r , the n i t r o g e n y i e l d i s G(Ng)^ Q = 3.2]72 wh ich i s a c c e p t e d as the s c a vengeab l e e l e c t r o n y i e l d f o r a s o l u t e a t t h a t c o n c e n t r a t i o n . S i n c e the e l e c t r o n d e n s i t i e s of wate r and HMPA are n e a r l y e q u a l , the y i e l d of s o l v a t e d e l e c t r o n s produced by i o n i z i n g r a d i a t i o n i n HMPA-water m i x t u r e s would be g i ven by ( x l i i i ) : G ( e s > m i x . = G.KV'V + G ( e s } H M P A P . - W ' / i •'• ^ 2 2 2 ( x l m ) where V H Q i s the volume f r a c t i o n o f wate r i n the m i x t u r e ; G ( e s ) H > 0 and G ( e ~ ) H M p A are the s o l v a t e d e l e c t r o n y i e l d s i n the pure s o l v e n t s . In s o l u t i o n s c o n t a i n i n g n i t r o u s o x i d e , the s o l u t e scavenges these i o n i c s p e c i e s to produce n i t r o g e n . The a s sumpt ion i s made t h a t i f the s caveng i ng mechanism i n v o l v e s HMPA m o l e c u l e s , more than a s i n g l e n i t r o g e n m o l e c u l e c o u l d u l t i m a t e l y be p roduced . I f wate r m o l e c u l e s are i n v o l v e d , i t i s assumed t h a t on l y a s i n g l e Ng r e s u l t s . These mechanisms are r e p r e s e n t e d by r e a c t i o n s (3.72) and ( 3 . 7 3 ) . e . mi x k 3. 7 2 + N 2 ° H^0 \ h + P r o d u c t s : (3 .72) k e m i x + H2° 3 ' 7 3 > 0 + " ) N 2 + p r o d u c t s { 3 J 3 ) HMPA - 177 -w h e r e n > 0 . 0 and d e p e n d s u p o n t h e R O c o n c e n t r a t i o n , e v e n i n p u r e HMPA. The f r a c t i o n , f , o f e l e c t r o n s t h a t c o u l d g i v e r i s e t o more t h a n a s i n g l e m o l e c u l e o f n i t r o g e n i s t h e n g i v e n by ( x l i v ) : 1 1 + k 3 . 7 2 3 . 7 3 [HMPA] ( x l i v ) The n i t r o g e n y i e l d f r o m a m i x t u r e o f HMPA a n d w a t e r c o n t a i n i n g a f i x e d R O c o n c e n t r a t i o n w o u l d t h e r e f o r e be g i v e n by ( x l v ) : G ( N 2 > m i x = G ( e - ) m , v . ( l + n f ) m i x ( x i v ) C o m b i n i n g e q u a t i o n s ( x l i i i ) t o ( x i v ) one o b t a i n s ( x l v i ) , G ( N 2 > m i x G ( e s " > H 2 0 - V H 2 0 + G< es~>HMPA ^ H ^ 1 + + K [H 20] [HMPA] ( x v i i ) w h e r e K = kg^/kg^g F o r G ( e ~ ) H Q = 3 . 2 and G ( e J ) U M D f l = 2 . 2 , e q u a t i o n ( x l v i ) r e d u c e s s HMPA 178 -to ( x l v i i ) : G ( N 2 > m i x = <V + 2 - 2 ) ^ + H T T ^ 1 + K L H2°-[HMPA] ( x l v i i ) For pure HMPA c o n t a i n i n g 0.02M RO a v a l u e o f n ^ 0.6 i s i n d i c a t e d f rom F i g u r e 111 - 3. That i s a t t h i s c o n c e n t r a t i o n G(R) ^ 1.6 G ( f . i . ) H M p A (a s suming G ( f . i . ) H M P A = 2 . 2 ) . F i g u r e 111 -16 shows p l o t s of e q u a t i o n ( x l v i i ) f o r n = 0.64 and K = - 3 -1 2 10 , 1 0 and 10 . I t can be seen t h a t the g e n e r a l shape o f the c a l c u l a t e d p l o t s i s d e t e rm ined by K, the r a t i o o f the ' r a t e c o n s t a n t s ' f o r the s c a veng i n g mechamism. T h i s i s l o g i c a l because K d i s c l o s e s which s o l v e n t dominates the o v e r a l l s c a veng i ng scheme t h e r e b y d e t e r m i n i n g the e x t e n t of p o s s i b l e secondary p r o c e s s e s . A l s o i n c l u d e d i n the f i g u r e a re the n i t r o g e n y i e l d s of. F i g u r e 111 - 7 obse rved e x p e r i m e n t a l l y f rom HMPA/H 20 m i x t u r e s c o n t a i n i n g 0.02M R0 i n the p r e s e n t s t u d y . Equa t i on ( x l v i i ) i n d i c a t e s a maximum p o s s i b l e v a l u e f o r G ( N 2 ) m i x = (1 + 2.2) (1 + 0.6) = 5 .1 . T h i s c o u l d be a c h i e v e d i f a t r a c e of HMPA i n wa te r made r e a c t i o n (3 .73) dominate over (3.72) so t h a t 1.6 R were formed per e~. The f a c t t h a t a maximum i s reached at a l l means t h a t K << 1 and i m p l i e s t h a t HMPA has t h a t e f f e c t on w a t e r . T h i s then i m p l i e s t h a t i n n e a r l y pure wate r the p roce s s ( 3 . 6 5 ) , N 2 ° " • N 2 + ° " ( 3 . 6 5 ) - 179 -F i g u r e 111-16 M O L E F R A C T I O N H 2 0 Computer s i m u l a t i o n ( l i n e s ) and ob se r ved n i t r o g e n y i e l d s ( O ) as a f u n c t i o n o f s o l u t i o n c o m p o s i t i o n from i r r a d i a t e d m i x t u r e s o f wate r and HMPA c o n t a i n i n g 2 X 10"2M N ? 0 . The cu r ve s are e x p l a i n e d i n the t e x t . - 180 -i s not ve ry f a s t or does not s i m p l y end t h e r e . The t h e o r e t i c a l cu rve f o r K = 10 and n = 0.64 r e p r e s e n t s the be s t f i t to the da ta from a computer s i m u l a t i o n u s i n g e q u a t i o n ( x l v i i ) . C l e a r l y , the s i m p l e o v e r a l l scheme proposed does not a d e q u a t e l y d e s c r i b e the o p e r a t i v e mechanism, but f u r t h e r a t tempt s to r e f i n e the model would be pure s p e c u l a t i o n and o f l i t t l e v a l u e . S u f f i c e i t to say t h a t the Ng y i e l d d a t a f rom NgO i n HMPA/HgO m i x t u r e s i t c o n s i s t e n t w i t h a r e a c t i o n scheme i n v o l v i n g s e c o n d a r y , i o n i c r e a c t i o n s . S i n c e a c o n s t a n t c o n c e n t r a t i o n (0.2M) o f NgO was used i n a l l m i x t u r e s , the r e s u l t s p o i n t to a second i m p o r t a n t c o n c l u s i o n . That i s , u n l e s s the i n i t i a l r a d i o l y t i c p r o ce s s e s are much d i f f e r e n t i n the m i x t u r e s , the obse rved Ng y i e l d s c o u l d not have a r i s e n th rough geminate i o n s c a v e n g i n g p r o c e s s e s . There have been r e l a t i v e l y few s t u d i e s conduc ted on t h e r a d i o l y s i s o f l i q u i d m i x t u r e s , and fewer s t i l l i n wh ich NgO was used as s c a venge r . The p r e s e n t work appears to be a un ique a t tempt to use m i x t u r e s to t r y to e l u c i d a t e s c a veng i n g mechanisms. Hentz and S c h e r m a n 1 0 9 s t u d i e d the r a d i o l y s i s o f d i o xane / wate r m i x t u r e s c o n t a i n i n g NgO. They c onc l uded t h a t l i k e w a t e r , the n i t r o g e n y i e l d co r re sponded t o the s o l v a t e d e l e c t r o n y i e l d i n d i oxane. Now, w h i l e t hey d i d not examine t h e i r Ng da ta from d i o x a n e / HgO m i x t u r e s i n terms o f a f u n c t i o n o f c o m p o s i t i o n , such c o n s i d e r a t i o n s are p o s s i b l e from t h e i r p u b l i s h e d d a t a . A p l o t of dioxane/HgO data s i m i l a r to F i g u r e 111-16 f o r the HMPA/HgO shows Ng v a l ue s as a s i m p l e l i n e a r f u n c t i o n o f c o m p o s i t i o n - -- 181 -e x a c t l y what one would p r e d i c t from a m i x t u r e of s o l v e n t s i n n e i t h e r o f which seconda ry i o n i c r e a c t i o n s a re t hough t t o o c c u r . f ) The Natu re o f the Secondary I o n i c S p e c i e s L e a d i n g  to Ng In NgO Scaveng ing S t u d i e s S i n c e the e x p e r i m e n t s had f a i r l y w e l l e s t a b l i s h e d t h a t secondary i o n i c r e a c t i o n s o c c u r r e d i n HMPA c o n t a i n i n g NgO,an a t tempt was made to i d e n t i f y the i n t e r m e d i a t e s p e c i e s i n v o l v e d As d i s c u s s e d e a r l i e r , e l e c t r o n s c a veng i n g by NgO i n l i q u i d s a lmos t c e r t a i n l y l e a d s i n i t i a l l y t o the f o r m a t i o n o f NgO" ( 3 . 6 8 ) . Now, the f a t e o f t h i s NgO" i o n must be paramount i n d e t e r m i n i n g the e x t e n t to wh ich i o n i c r e a c t i o n s l e a d to exce s s n i t r o g e n f o r m a t i o n . C e r t a i n l y a l i f e t i m e l e s s than the 10 - 3 to 10 sec a t t r i b u t e d to t h i s s p e c i e s i n the gas p h a s e 1 0 3 ' 1 2 1 would be e x p e c t e d . In aqueous systems f o r e x a m p l e 1 7 3 ' 1 7 h ' 1 7 5 t h e r e i s e v i d e n c e t h a t NgO~ i s r a p i d l y c o n v e r t e d t o a h y d r o x y l r a d i c a l th rough p ro ton t r a n s f e r f rom s o l v e n t ( 3 . 6 6 ) . N 2 0 - • H 2 0 • 0 H + 0H- + N 2 - 6 A h a l f - l i f e l e s s than 3 X 10 sec was i n d i c a t e d f o r t h i s p r o c e s s and A d a m s 1 7 6 e s t i m a t e d a v a l u e as low as 1 n s ec . C z a p s k i 1 7 7 p o i n t e d out t h a t 0 " ^ , r a t h e r than NgO" ( 3 . 7 4 ) , m ight i n f a c t be the r e a c t i v e i n t e r m e d i a t e s p e c i e s i n the aqueous system and t h a t - 9 -8 as such might l i v e f o r perhaps 10 to 10 sec b e f o r e be i n g - 182 -c o n v e r t e d to a hydroxy ! r a d i c a l C3.75):. * N 2 + 0 s (3 .74) * OH + OH ( 3 .75 ) Zehav i and R a b a n i 1 7 8 conduc ted a tho rough m u l t i - s o l u t e s t udy o f RO s a t u r a t e d aqueous s o l u t i o n s . They p r e s e n t e d r a t h e r c o n v i n c i n g e v i d e n c e t h a t N20~, 0- and OH were formed i n t u r n from aqueous e l e c t r o n s v i a r e a c t i o n s ( 3 . 6 8 ) , (3 .74 ) and (3 .75) w i t h h a l f - l i v e s of 10?< 100, and 7 nsec r e s p e c t i v e l y . Salmon e t a l l 7 9 however, c onc l uded from s t u d i e s o f RO and g a l v i n o x y l , Gv, i n c y c l o h e x a n e t h a t R0 - has a l i f e t i m e > 20 y sec i n t h a t s o l v e n t . They mon i t o red the b u i l d - u p o f the s t r o n g o p t i c a l a b s o r p t i o n o f the Gv" a n i o n . From a s o l u t i o n - 5 c o n t a i n i n g 5 X 10 M g a l v i n o x y l i n c y c l o h e x a n e the p re sence o f 0.1M RO had no e f f e c t upon the y i e l d of G v - , a l t h o u g h the k i n e t i c s o f i t s f o r m a t i o n changed. Now, N20 i s known to be a e f f i c i e n t e l e c t r o n s c a v e n g e r , t h e r e f o r e , f rom the se s o l u t i o n s where [RO] / [GV] = 5 X 1 0 3 the e l e c t r o n s would be scavenged by N 2 0 . Salmon e t a l . c onc l uded t h a t Gv" was formed v i a e l e c t r o n t r a n s f e r from N o 0 ( 3 . 7 6 ) . H 0~ + Gv +> N o 0 + Gv (3 .76) The f a c t t h a t G v - f o r m a t i o n c o n t i n u e d f o r more than 20 y sec was - 183 -taken as an i n d i c a t i o n o f the NgO" l i f e t i m e . On the o t h e r hand, the p re sence o f .1 M i o d i n e , ano the r e f f i c i e n t e l e c t r o n s c a v e n g e r , c o m p l e t e l y s u r p r e s s e d Gv~ f o r m a t i o n . However, s i n c e Salmon e t a l . d i d not measure the n i t r o g e n y i e l d s from t h e i r s amp le s , t h i s a u t ho r f e e l s t h a t t h e r e i s no j u s t i f i c a t i o n f o r the a s s i gnment o f NgO" as the l o n g - l i v e d i n t e r m e d i a t e t r a n s f e r r i n g change w i t h Gv. That i s , t h e i r mechanism i m p l i e s t h e r e s h o u l d be comp le te s u r p r e s s i o n o f n i t r o g e n f o r m a t i o n from NgO" c o n t a i n i n g s o l u t i o n s i n the p re sence o f even sma l l amounts o f g a l v i n o x y l . An a l t e r n a t i v e (and a t l e a s t e q u a l l y p l a u s i b l e ) e x p l a n a t i o n f o r t h e i r o b s e r v a t i o n s i s t h a t g a l v i n o x y l r e a c t e d not w i t h NgO", but w i t h some daugh te r an ion a r i s i n g from the NgO" decay mechanism — p o s s i b l y 0 " , NgOg, 0g or some o t h e r s p e c i e s - - a f t e r Ng f o r m a t i o n . I ndeed , S c h u l e r ' s r e c e n t s t u d i e s on c y c l o h e x a n e 1 7 1 i n d i c a t e t h a t i f s econdary i o n i c r e a c t i o n s of NgO o c cu r i n t h a t s y s t e m , then they must be e x t r e m e l y r a p i d . I t was thought t h a t g a l v i n o x y l might prove to be an e x c e l l e n t probe o f the NgO s caveng i ng mechanism i n HMPA. The proposed mechanism c o u l d be r e p r e s e n t e d by the f o l l o w i n g s i m p l i f i e d scheme ( 3 . 7 7 ) , ( 3 . 7 8 ) , ( 3 . 7 9 ) . e HMPA (3 .77 ) HMPA/N o0 NgO s • *• N„ + A (3.78) - 184 -HMPA/N ?0 — * * N 2 + B " ( 3 .79 ) That i s , i n HMPA s o l u t i o n , n i t r o u s o x i d e scavenges the e l e c t r o n s to produce NgO" wh ich then goes on to produce one or more mo l e cu l e s of n i t r o g e n . Now, c o n s i d e r the e f f e c t upon the obse rved n i t r o g e n y i e l d o f the a d d i t i o n o f a s m a l l amount o f g a l v i n o x y l . I f g a l v i n o x y l e f f e c t i v e l y scavenged NgO", n i t r o g e n f o r m a t i o n would be t o t a l l y s u r p r e s s e d . I f the A " an i on o f the proposed scheme were s cavenged , then the n i t r o g e n y i e l d would be reduced to a c o n s t a n t v a l u e equa l to the e l e c t r o n y i e l d . F i n a l l y , i f B~ (or some daugh te r an i on ) were scavenged by g a l v i n o x y l , the n i t r o g e n y i e l d would be u n a f f e c t e d ( o r p a r t i a l l y r e d u c e d ) . As was shown i n T a b l e V I I , the a d d i t i o n o f 10~ 4 M g a l v i n o x y l to a s o l u t i o n o f 1.4 X 1 0 " 2 M NgO i n HMPA had l i t t l e e f f e c t upon the n i t r o u s y i e l d . Only when the c o n c e n t r a t i o n of g a l v i n o x y l was i n c r e a s e d to a l e v e l where d i r e c t c o m p e t i t i o n between Gv and NgO f o r the e l e c t r o n s t hemse l ve s would be p r o b a b l e d i d the n i t r o g e n y i e l d drop a p p r e c i a b l y . Even t h e n , a p l a t e a u v a l ue was not o b s e r v e d ; the n i t r o g e n y i e l d was s i m p l y a f u n c t i o n o f the s o l u t e c o m p o s i t i o n as shown i n F i g u r e 111-10. I t i s s i g n i f i c a n t t h a t even i n the p re sence o f 0.05M g a l v i n o x y l the n i t r o g e n y i e l d from a s o l u t i o n o f 0.12M NgO i n HMPA was G(Ng) = 2 .7 , a v a l u e h i g h e r than the proposed f r e e i o n y i e l d G ( f . i . ) = 2 .2 . Thus , even at a c o n c e n t r a t i o n where g a l v i n o x y l would be compet ing f o r e l e c t r o n s d i r e c t l y , t h e r e i s e v i d e n c e s u g g e s t i v e o f a secondary i o n i c mechanism o f n i t r o g e n f o r m a t i o n . Now, s i n c e the g a l v i n o x y l a n i o n s , G v " , would be n e u t r a l i z e d - 185 -e v e n t u a l l y by the c o n c o m i t a n t p o s i t i v e i o n s produced d u r i n g the r a d i o l y s i s , the y i e l d of t he se i on s ( o r i n f a c t t h e i r p re sence ) c o u l d not be measured i n the p r e s e n t e x p e r i m e n t s . C a p e l l o s and A l l e n 9 1 found t h a t the g a l v i n o x y l an i on decays on. the m i c r o second t i m e s c a l e i n h yd roca rbon s o l v e n t s . However, the expe r imen t s w i t h HMPA i n d i c a t e t h a t e i t h e r NgO" does no t t r a n s f e r an e l e c t r o n to g a l v i n o x y l or e l s e i t i s too s h o r t l i v e d i n HMPA to do so. A s i m i l a r argument would a p p l y to s p e c i e s A" o f r e a c t i o n (91) wh ich most l i k e l y would be 0 " o r NgOg i n the HMPA sy s tem. The e v i d e n c e , a l t h o u g h not s t r o n g , sugges t s t h a t g a l v i n o x y l c o u l d i n t e r r u p t o n l y a l a t e r s t age o f the NgO s c a v e n g i n g mechanism. A t tempt s were made to f u r t h e r d e f i n e the n a t u r e o f p o s s i b l e s p e c i e s A " . The NgO" i on might be s h o r t l i v e d i n HMPA because o f decay to an o x i d e r a d i c a l an i on ( 3 . 8 0 ) . HMPA N 2 ° ~ — > 0 " + N 2 ( 3 > 8 0 ) In t h i s c a s e , seconda ry n i t r o g e n f o r m a t i o n would have a r i s e n from the subsequent r e a c t i o n s o f the o x i d e i o n . Now, m o l e c u l a r oxygen i s known to r e a c t r e a d i l y w i t h t h i s i on (3 .81) to - 9 produce the o z o n i d e i o n , 0 3 - - a v a l u e o f k 3 g-j = 2.6 X 10 M"^ s e c " ^ be ing r e p o r t e d f o r the r a t e c o n s t a n t s o f aqueous s o l u t i o n s } 8 0 o + o5 • o: • , 2 3 ( 3 .81 ) - 186 -Oxygen was added to samples of N o 0 i n HMPA. I f the o x i d e i on were i m p o r t a n t i n the seconda ry IRO s c a v e n g i n g mechanism, one would expec t the n i t r o g e n y i e l d to be reduced to a c o n s t a n t v a l ue i n d i c a t i v e o f the e l e c t r o n y i e l d . However, the p re sence of oxygen, even i n l a r g e amounts ( comparab le w i t h the R O c o n c e n t r a t i o n ) had l i t t l e e f f e c t upon the n i t r o g e n y i e l d . T h i s r e s u l t i s somewhat ambiguous, however, because the 0 " sys tem may not be q u i t e so s t r a i g h t f o r w a r d . Beher and C z a p s k i 1 8 1 have sugges ted t h a t the Og i on can d i s s o c i a t e u n i m o l e c u ! a r i l y to r e g e n e r a t e the 0 _ i on ( 3 . 8 1 ) . °3 * °2 + ° ~ (3 .81) Th i s mechanism was s uppo r ted by the work o f G a l l and D o r f m a n 1 8 2 3 -1 who r e p o r t e d an a b s o l u t e r a t e c o n s t a n t kg g-j = 3.3 X 10 sec f o r t h i s r e a c t i o n . T h e r e f o r e , i f the o zon i de i o n had no a l t e r n a t i v e f a t e i n the HMPA s y s t em, r e g e n e r a t i o n o f o x i d e c o u l d produce the observed r e s u l t s . Now, the o x i d e i o n , 0 " , a l s o r e a c t s w i t h methanol (3 .82) (kg 8 2 = 5.8 X 1 0 8 M " 1 s e c - 1 ) } 8 3 k 3 . 8 2 0 " + CHgOH • CH 20H + OH" ( 3 g 2 ^ In the p r e s e n t work, T a b l e VI I shows t h a t the p re sence o f 1 M methanol i n N 2 0 s o l u t i o n s o f HMPA had no o b s e r v a b l e e f f e c t upon . the n i t r o g e n y i e l d . T h i s r e s u l t taken i n c o n j u n c t i o n w i t h the oxygen a d d i t i o n da ta tends to p r e c l u d e the i n v o l v e m e n t o f o x i d e i on s ( r e a c t i o n 3.80) i n the N 2 0 secondary i o n i c r e a c t i o n mechanism - 187 -i n HMPA r a d i o l y s i s . The R0" i on c o u l d a l s o be s h o r t l i v e d i n HMPA because o f subsequent r e a c t i o n w i t h R O ( 3 . 6 1 ) . N 2 0 - + N 2 0 • N 2 0 2 + N 2 ( 3 R e c a l l t h a t the s t u d i e s on the r a d i o l y s i s o f R O i n gaseous hydroca rbons gave e v i d e n c e t h a t C 0 2 c o u l d scavenge the R0 2 i on (o r p o s s i b l y 0~ ) . U n f o r t u n a t e l y t h e r e i s ample e v i d e n c e to sugges t t h a t i n s o l u t i o n , C 0 2 may a l s o a c t as an e l e c t r o n s c a v e n g e r 1 8 4 ( 3 . 8 4 ) . e " + co • CO: i ' • s 2 2 (3.84) In the p r e s e n t s tudy o f HMPA r a d i o l y s i s , T a b l e VI shows t h a t the n i t r o g e n y i e l d f rom R O s o l u t i o n s was u n a f f e c t e d by the p re sence of C 0 2 > even i n amounts comparab le t o R O . A t f i r s t g l a n c e , t h i s appears c o n t r a r y to the o t h e r e v i d e n c e which p o i n t s to the i n vo l v emen t o f R0 2 ( o r l e s s l i k e l y , 0~) i on s i n the a n i o n i c - R O s c a veng i n g mechanism i n HMPA. However, i t i s s i g n i f i c a n t t h a t C 0 2 d i d not compete f o r the e l e c t r o n s d i r e c t l y . I f C0 2 behaved i n HMPA i n a way s i m i l a r to the wa te r or c y c l ohexane systems c i t e d , then the r e s u l t s n e c e s s i t a t e s t h a t the e l e c t r o n s p e c i e s (and s u b s e q u e n t l y 0 " or R O " ) be v e r y s h o r t - l i v e d i n HMPA. O the r e v i d e n c e sugges t s t h a t t h i s i s not the c a s e . One i s f o r c e d to the c o n c l u s i o n t h a t i n HMPA s o l u t i o n , - 188 -C 0 2 e i t h e r cannot compete f o r i o n i c s p e c i e s dn an equa l b a s i s w i t h o t h e r p r o ce s s e s as i t does i n o t h e r media o r e l s e the C 0 2 i on r e a d i l y g i v e s up i t s exces s e l e c t r o n to o t h e r s o l u t e s . I n f e l t a and S c h u l e r 1 7 0 had shown f o r example t h a t i n c y c l o -hexane s o l u t i o n , C 0 2 t r a n s f e r s an e l e c t r o n to CH^Br. One c o u l d f u r t h e r s p e c u l a t e on the sou rce o f t h i s a m b i g u i t y , but s u f f i c e i t to say t h a t C 0 2 i s not s u i t a b l e as a second s o l u t e f o r a t t empt s to e l u c i d a t e the R O s c a veng i n g mechanism i n HMPA. Thus , on the b a s i s o f i n f o r m a t i o n from R O i n pure HMPA, HjjO/HMPA m i x t u r e s , and s o l u t e s o f HMPA c o n t a i n i n g a second s o l u t e , some i n f o r m a t i o n has been ga i ned about the n a t u r e o f the e l e c t r o n s c a veng i n g mechanism by R O i n HMPA. The R O " i on formed i n i t i a l l y must be s h o r t - l i v e d , but probabj ly does no t decay to g i v e the o x i de i on 0". The R0~ i o n r e a c t s w i t h p o s i t i v e i o n s , s o l v e n t m o l e c u l e s or a d d i t i o n a l R O m o l e c u l e s to g i v e a m o l e c u l e o f n i t r o g e n and a p r oduc t ( o t h e r than 0") wh ich may or may not r e a c t f u r t h e r w i t h R O to produce a d d i t i o n a l n i t r o g e n . D i r e c t r e a c t i o n of R0~ i on s w i t h p o s i t i v e i o n s ( i . e . s i m p l e n e u t r a l i z a t i o n ) i s not l i k e l y to be o f major impo r t ance i n a system l i k e HMPA where seconda ry i o n i c r e a c t i o n s are i n d i c a t e d . F i r s t l y , be ing uncha rged , the s p e c i e s formed f rom n e u t r a l i z a t i o n are u n l i k e l y to l e ad to the subsequent r e a c t i o n n e c e s s a r y to produce ' s e c o n d a r y ' n i t r o g e n m o l e c u l e s . S e c o n d l y , even i f n e u t r a l i z a t i o n o c c u r r e d a t a r a t e l i m i t e d o n l y by d i f f u s i o n , the i ons are p r e s e n t at such a low s t eady s t a t e c o n c e n t r a t i o n t h a t they would s t i l l have l i f e t i m e s s u f f i c i e n t t o a l l o w e f f e c t i v e r e a c t i o n w i t h added s o l u t e s . I ndeed, Da in ton e t a l l 8 0 e s t i m a t e d - 1 8 9 -a second o r d e r r a t e c o n s t a n t o f about 2 X 1 0 1 * M~' s e c - 1 f o r n e u t r a l i z a t i o n o f NgO~ i n c y c l o h e x a n e s o l u t i o n . In HMPA the seconda ry i o n i c mechanism most l i k e l y i n v o l v e s the p a r t i c i p a t i o n of NgOg i on s a t some s t a g e . D e s p i t e the f a c t t h a t numerous approaches have been t r i e d , the p r e s e n t r e s e a r c h has f a i l e d to c l e a r l y d e f i n e the mechanism by which NgO scavenges a n i o n i c s p e c i e s produced d u r i n g s t e a d y s t a t e y - r a d i o l y s i s o f HMPA. C o n s e q u e n t l y , the a b s o l u t e v a l u e o f the s o l v a t e d e l e c t r o n or f r e e i o n y i e l d by s cavenge r methods has not been u n e q u i v o c a l l y e s t a b l i s h e d . I t s h o u l d be noted however t h a t s i m i l a r a m b i g u i t y and u n c e r t a i n t y e x i s t s i n many systems when s caveng i ng methods are i n v o l v e d . R e c e n t l y f o r example , two groups o f i n v e s t i g a t o r s s t u d i e d what was presumably the s i m p l e s t system o f t h i s t ype - - n i t r o u s o x i d e i n l i q u i d Xenon. Because of the l a r g e e l e c t r o n m o b i l i t y i n Xenon (2200 cm 2 V " 1 s e c - 1 ) 1 8 5 i t i s l i k e l y t h a t the r a d i o l y s i s would r e s u l t i n a l a r g e y i e l d of homogeneously d i s t r i b u t e d f r e e i o n s . Rzad and B a k a l e l 1 4 b a s i n g t h e i r arguments on c o n s i d e r a b l e d a t a , c o n v i n c i n g l y demons t ra ted the i n v o l v e m e n t o f s econda ry r e a c t i o n s o f NgO i n t h a t s y s tem. Rob inson and F r e e m a n 1 1 5 on the o t h e r hand, who were c o n d u c t i n g c o n c u r r e n t s t u d i e s , a p p l i e d Freeman ' s d i f f u s i o n m o d e l 1 4 6 and p roceeded t o d i s c u s s the same s y s t e m , e q u a l l y c o n v i n c i n g l y , c o m p l e t e l y i n terms o f f r e e and geminate e l e c t r o n s c a v e n g i n g . g) S o l u t i o n s of Sodium Meta l i n HMPA - 1 9 0 -As d i s c u s s e d i n some d e t a i l i n the i n t r o d u c t i o n , many p o l a r s o l v e n t s have been shown c a p a b l e o f d i s s o l v i n g a l k a l i meta l s to produce s t a b l e s o l u t i o n s . HMPA has been shown to be an e x c e l l e n t s o l v e n t f o r such pu rpo se s . The s o l u t i o n s compr i se a v a r i e t y o f s o l v a t e d s p e c i e s wh ich may i n c l u d e s o l v a t e d e l e c t r o n s , metal i ons and atoms, as w e l l as a number o f agg rega te s p e c i e s . The pa ramagne t i c s u s c e p t i b i l i t y , p a r a -magnet i c re sonance s p e c t r a , o p t i c a l a b s o r p t i o n s p e c t r a , and e q u i v a l e n t c o n d u c t i v i t y o f the s o l u t i o n s c o n s i s t e n t y i n d i c a t e t h a t the s o l v a t e d e l e c t r o n i s o f paramount impo r t ance i n those s y s tems . Thus , the d i s s o l u t i o n o f an a l k a l i m e t a l , M, i n a s o l v e n t , S, may w i t h r e a s o n a b l e c e r t a i n t y be i n i t i a l l y r e p r e s e n t e d by r e a c t i o n ( 3 . 8 5 ) . M + S * M s + V (3 .85) where the s u b s c r i p t , s , denotes s o l v a t e d s p e c i e s . A l k a l i meta l s o l u t i o n s o f known c o n c e n t r a t i o n s a re most d i f f i c u l t to p r e p a r e . I n i t i a l r e a c t i o n w i t h t r a c e i m p u r i t i e s and n a t u r a l decay d u r i n g the r e l a t i v e l y s low d i s s o l u t i o n o f bu l k metal p a r t i c l e s l e a d to u n c e r t a i n t y i n s p e c i e s c o n c e n t r a t i o n . The l a t t e r prob lem can be a l l e v i a t e d by t r a n s f e r r i n g the s o l v e n t to a v e s s e l c o n t a i n i n g a p repa red meta l m i r r o r wh ich d i s s o l v e s a lmos t i n s t a n t l y . However, such a p r ocedu re i s not a lways d e s i r a b l e and W a r s h a w s k y 1 8 6 has shown t h a t even under " s t i c k " vacuum c o n d i t i o n s the d i s t i l l a t i o n of a l k a l i me ta l s i n Py rex - 1 9 1 -g l a s swa re l e ad s t o p a r t i a l d e c o m p o s i t i o n due to r e a c t i o n w i t h s t r o n g l y absorbed water m o l e c u l e s . F i n a l l y , because of the d i v e r s i t y of s p e c i e s t h a t have been shown to e x i s t i n t he se s o l u t i o n s ( p o s s i b l y i n e q u i l i b r i u m w i t h each o t h e r ) one has l i t t l e hope i n p r e p a r i n g a s o l u t i o n of known c o m p o s i t i o n . S o l u t i o n s o f a l k a l i meta l s i n HMPA decay s l o w l y to g i v e p r e d o m i n a n t l y the s a l t s o f two a n i o n s , namely the d i m e t h y l amine an ion and d i a m i d o p h o s p h i t e an ion (3.86)} These i on s produce the ob se rved orange c o l o u r a t i o n of the " s p e n t " sodium s o l u t i o n s i n the p r e s e n t s t u d y . The decay was o r i -g i n a l l y thought to o ccu r v i a a s i m p l e r a d i c a l - a n i o n mechanism. In v iew o f the now accep ted major impo r t ance o f s o l v a t e d e l e c t r o n s i n these s y s tems , the decay mechanism more l i k e l y a r i s e s f rom i n i t i a l r e a c t i o n w i t h these s p e c i e s , w i t h r a d i c a l an i on s p o s s i b l y be i ng a r e a c t i v e i n t e r m e d i a t e . In HMPA the ob se r ved f o r m a t i o n of sma l l amounts of hydrogen and methane from decay o f the sodium s o l u t i o n sugges t s t h a t a number o f an i on s and r a d i c a l s may be formed such as sugges ted by r e a c t i o n s (3.87) t o (3.91). 0P(N(CH3)2)3 +.2M > • (CH3)2N'M+ + ((CH3)2N)2P0"M+ (3.86) Na + OP(N(CH3)2)3 + E ; * OP(N(CH3)2)3 (3.87) OP(N(CH3)2)3 * ^OP(N(CH'3)2)2N(CH3)CH2J + H (3.88) - 192 -Na + -> 0 P ( N ( C H 3 ) 2 ) 2 N C H 3 + CH 3 (3 .89) Na ~ 0 P ( N ( C H 3 ) 2 ) 2 + N ( C H 3 ) 2 (3 .90) Na " N ( C H 3 ) 2 + 0 P ( N ( C H 3 ) 2 ) 2 ( 3 .91 ) where 0 P ( N ( C H 3 ) 2 ) 3 may or may not r e p r e s e n t a r e a l s p e c i e s , and the r a d i c a l s may be s t a b a l i z e d v i a hydrogen a b s t r a c t i o n f rom HMPA o r b i m o l e c u l a r r e c o m b i n a t i o n . The f a c t t h a t n i t r o u s o x i d e was e f f i c i e n t l y and q u a n t i t a t i v e l y reduced to n i t r o g e n i n an exces s o f the b l ue s o l u t i o n o f sodium i n HMPA but c o m p l e t e l y u n a f f e c t e d by the decayed (orange sod ium-s a l t a n i on ) s o l u t i o n i s most s i g n i f i c a n t . I t demons t ra te s t h a t N 2 0 scavenges an e l e c t r o n s p e c i e s which e x h i b i t s an o p t i c a l a b s o r p t i o n i n the i n f r a - r e d r e g i o n o f the s p e c t r u m . That i s , N 2 0 was reduced e i t h e r by s o l v a t e d e l e c t r o n s t hemse l ve s or by some s o l v a t e d s p e c i e s wh ich c o n s i s t s s i m p l y o f a c l o s e a s s o c i a t i o n o f e l e c t r o n s w i t h sodium i on s and/or s o l v e n t m o l e c u l e s (HMPA - , ( N a + " e _ ) , Na , N a " , N a 9 ) - - but was not reduced by sodium c a t i o n s or any of the an ions o f decomposed HMPA. In o r d e r t o f a c i l i t a t e a more d e t a i l e d s t udy o f the N 2 0 s c a veng i n g mechanism, i t was n e c e s s a r y to i n t r o d u c e known amounts of sodium meta l t o a s o l u t i o n o f N 2 0 i n HMPA, w a t e r , and m i x t u r e s - 193 -o f the two. In a d d i t i o n , i t was n e c e s s a r y t o l i m i t the l o c a l c o n c e n t r a t i o n o f s o l v a t e d s p e c i e s at the p o i n t o f meta l i n j e c t i o n so as to a l l o w r e a c t i v e c o m p e t i t i o n and d i f f u s i o n by the NgO s o l u t e m o l e c u l e s . These p r e r e q u i s i t e s were r e a l i z e d th rough the use of l i q u i d sod ium/mercury amalgams. In pure w a t e r , where the i n t r o d u c t i o n o f pure sodium meta l i s e x p l o s i v e , the amalgams r e a c t e d s l o w l y ove r a p e r i o d o f hours to produce o n e - h a l f mole o f hydrogen f o r each mole of sodium atoms ( o r p resumably e~ ) p r e s e n t . T h i s o b s e r v a t i o n c o r r o b o r a t e s r e s u l t s p r e v i o u s l y r e p o r t e d 1 1 7 and obeys the a c c e p t e d s t o c h i o m e t r y o f r e a c t i o n (3.92) f o r wh ich the b i m o l e c u l a r c o m b i n a t i o n r a t e c o n s t a n t has a r e p o r t e d v a l u e k^ gg = 1 X 10^° s e c - 1 1 8 9 H ? 0 aq aq 2 aq [3 ,92 ) N i t r o u s o x i d e , p r e s e n t i n water a t a c o n c e n t r a t i o n of - 2 -6 X 10 M competed s u c c e s s f u l l y f o r e a q p roduced upon amalgam d i s s o l u t i o n . Hydrogen p r o d u c t i o n was s up re s sed and i n i t s p l a c e an e q u i v a l e n t amount o f n i t r o g e n o b t a i n e d . That i s , two moles of the r e d u c i n g s p e c i e s r e a c t e d w i t h NgO to produce a s i n g l e mole of n i t r o g e n . Shaede and W a l k e r 1 9 0 had ob se r ved t h i s unusua l b e h a v i o r e a r l i e r and a t t r i b u t e d the unexpected r e l a t i o n s h i p to i nhomogene i t y e f f e c t s . They sugges ted t h a t the r e d u c t i o n o f n i t r o u s o x i d e i n aqueous s o l u t i o n would r e s u l t i n the f o r m a t i o n o f a h y d r o x y l r a d i c a l th rough p r o t o n a t i o n o f the o x i d e i o n (3 .93) H 9 0 e " + N o 0 * • N 9 + OH + OH" (3 .93) - 1 9 4 -R e a c t i o n i s l o c a l i z e d at or near the amalgam/water i n t e r f a c e which would f a c i l i t a t e the s caveng i ng o f a second s o l v a t e d e l e c t r o n by the hydroxy ! r a d i c a l ( 3 . 9 4 ) . 6 a q + 0 H V 0 H aq : ( 3 .94 ) R e a c t i o n (3.94) i s known to be ve r y f a s t 3 7 ( k 3 g 4 = 3 X i n 1 0 M _ 1 s e c - 1 ) and c o u l d w e l l a c coun t f o r the obse rved s t o i c h i o m e t r y and absence o f m o l e c u l a r oxygen. I t was hoped t h a t the a d d i t i o n of sodium amalgams t o s o l u t i o n s o f R O i n HMPA would r e s u l t i n the f o r m a t i o n o f two moles o f n i t r o g e n f o r each mole of sodium (or s o l v a t e d e l e c t r o n s ) i n j e c t e d . Such a r e s u l t would p r o v i d e d i r e c t e v i d e n c e f o r the secondary i o n i c s c a veng i n g mechanism proposed from the r a d i o l y s i s e x p e r i m e n t s . The i n v o l v e m e n t o f the o x i d e a n i o n , 0~ , has been shown most u n l i k e l y i n the HMPA scheme so t h a t t he f o r m a t i o n and subsequent r e a c t i o n of h y d r o x y l r a d i c a l s ( f rom r e a c t i o n s ana lagous to (3 .93 ) and ( 3 .94 ) ) was d o u b t f u l . However, as can be seen from the data p r e s e n t e d i n T a b l e V I I I , the a d d i t i o n o f sodium amalgam to RO/HMPA s o l u t i o n s produced n i t r o g e n y i e l d s l e s s than or a t bes t equa l to those o b t a i n e d f rom the aqueous s o l u t i o n s In o t h e r words , assuming t h a t R O scavenges a l l the r e d u c i n g s p e c i e s , these r e s u l t s i m p l y t h a t two e l e c t r o n s ( o r sodium atoms) are r e q u i r e d to reduce a s i n g l e R O m o l e c u l e to R i n the amalgam/ RO/HMPA sy s tem. These r e s u l t s a re not a t a l l c o m p a t i b l e w i t h the r a d i o l y s i s data and c e r t a i n l y show no e v i d e n c e o f seconda ry - 195 -i o n i c r e a c t i o n s l e a d i n g t o Ng f o r m a t i o n . I t may be t h a t s c a veng i n g r e a c t i o n s on amalgam a d d i t i o n take p l a c e t o t a l l y at the amalgam s u r f a c e th rough a un ique mechanism and as such do not r e f l e c t the p r o p e r t i e s o f the • s o l v e n t or s o l u t e s i n v o l v e d . One f e a t u r e of the amalgam e x p e r i m e n t s t h a t s u p p o r t s t h i s c o n j e c t u r e i s the f a c t t h a t the amalgams were c o m p l e t e l y s t a b l e i n pure HMPA, r e a c t i n g o n l y when NgO was added. T h i s sugges t s t h a t s o l v a t i o n energy f o r e l e c t r o n s i n HMPA i s s m a l l - -c e r t a i n l y l e s s than the s t a b i l i z a t i o n energy p r o v i d e d by the amalgam. Now, such a c o n c l u s i o n i s not u n r e a s o n a b l e , f o r as has been s t a t e d , HMPA i s known to be an e x t r e m e l y poor s o l v a t o r o f a n i o n s . I ndeed , i t came as an unexpected s u r p r i s e t h a t s o l v a t e d e l e c t r o n s were a t a l l s t a b l e i n t h i s s o l v e n t . I t was noted t h a t when the amalgams were p repa red c o n s i d e r a b l e heat was g e n e r a t e d , showing the p roce s s to be e x o t h e r m i c . The e x a c t n a t u r e o f the amalgams i s unknown. The sodium atoms may be i o n i z e d , u n d i s s o c i a t e d or bonded i n a Na-Hg compound, a l t h o u g h e l e c t r i c a l c o n d u c t i v i t y d a t a 1 9 1 on d i l u t e amalgams tends to r u l e out the fo rmer c a s e . R e a c t i o n of NgO a t the amalgam s u r f a c e c o u l d then i n v o l v e Na atoms d i r e c t l y . In the gas phase , s u c h . r e a c t i o n i s known to o ccu r to g i v e NagO ( 3 . 9 5 ) . 2Na + NgO • NagO + Ng ^ 3 I t i s i n t e r e s t i n g to note t h a t i n the gas phase , two Na atoms are 196 -requ.i red to reduce a./s ing le NgO m o l e c u l e . : I t was n o t i c e d i n the p r e s e n t s tudy that .NgO c o u l d i n f a c t r e a c t w i t h sodium amalgam i n the absence o f s o l v e n t . T h i s , , t aken w i t h the f a c t t h a t amalgams were s t a b l e i n HMPA u n t i l NgO was i n t r o d u c e d , ' s t r o n g l y sugges t s t h a t s u r f a c e r e a c t i o n a l s o o c c u r r e d i n s o l u t i o n . P o s s i b l y t h e n , the u n e x p e c t a b l y low Ng y i e l d s f rom such s o l u t i o n a r i s e f rom r e a c t i o n s (3 .96 ) and (3.97) i n which two Na atoms a re r e q u i r e d f o r each N o 0 r educed . Na/Hg + NgO NaO/Hg + Na/Hg where /Hg r e f e r s t o s p e c i e s i n or a t the amalgam s u r f a c e . T h e r e f o r e , w h i l e the s t u d i e s i n v o l v i n g sodium meta l i n HMPA suppo r t the g e n e r a l c o n j e c t u r e t h a t NgO r e a c t s w i t h s o l v a t e d e l e c t r o n s i n HMPA, the da ta u n f o r t u n a t e l y cannot f u r t h e r e l u c i d a t e the e x a c t s c aveng i ng mechanism. C. PULSE RADIOLYSIS STUDIES OF SOLVATED ELECTRONS IN HMPA As had been a n t i c i p a t e d , the s t eady s t a t e gamma r a d i o l y s i s and sodium metal s o l u t i o n s t u d i e s showed HMPA to be a v e r y u s e f u l medium f o r i n v e s t i g a t i n g s o l v a t e d e l e c t r o n s . However, •> NaO/Hg + N 2 ; 3 . o f i , * N a 2 ° / H 9 , (3.97) - 197 -the work s u f f e r s a major drawback i n t h a t most o f the s p e c i f i c c o n c l u s i o n s p e r t a i n i n g to the s o l v a t e d e l e c t r o n come i n d i r e c t l y from o b s e r v a t i o n s o f r e a c t i o n p r o d u c t s . As a r e s u l t j a number o f a m b i g u i t i e s a ro se and i t has proven d i f f i c u l t to d e c i d e i n many cases whether the prob lem o r i g i n a t e s w i t h a p r i m a r y s p e c i e s o r some r e a c t i v e i n t e r m e d i a t e i n the p r o d u c t f o r m a t i o n scheme. Scavenger s t u d i e s i n d i c a t e d t h a t the r a d i a t i o n p roduced s o l v a t e d e l e c t r o n i n HMPA has a n a t u r a l l i f e t i m e o f the o r d e r of m i c r o s e c o n d s — l ong enough to be d i r e c t l y o b s e r v a b l e w i t h modern p u l s e r a d i o l y s i s a p p a r a t u s . C l e a r l y , d i r e c t o b s e r v a t i o n o f the e l e c t r o n ' s spect rum and r e a c t i o n s would complement the s t eady s t a t e work v e r y w e l l . I t would be p a r t i c u l a r l y v a l u a b l e to o b t a i n an i ndependent v a l u e o f the e l e c t r o n y i e l d . A l s o , o b s e r v a t i o n s o f the r e a c t i o n s between s o l v a t e d e l e c t r o n s and s cavenger s ( n o t a b l y R O , ace tone and H^O) might c o n f i r m the p roposed r e a c t i o n schemes. The r e l a t i v e i m p o r t a n c e of geminate i on s caveng i ng i n t h i s system might a l s o be r e s o l v e d f rom s t u d i e s u s i n g both e l e c t r o n and p o s i t i v e i on s c a v e n g e r s . The p u l s e r a d i o l y s i s f a c i l i t i e s a t U.B.C. were not s u i t a b l e f o r , t h e t ypes o f e xpe r imen t s t h a t would g i v e u s e f u l i n f o r m a t i o n on the HMPA system.. F o r t u n a t e l y , an o p p o r t u n i t y a ro se t o make use (on a s h o r t term b a s i s ) o f the p u l s e r a d i o l y s i s f a c i l i t i e s at The Ohio S t a t e U n i v e r s i t y i n Co lumbus, Oh io . Those f a c i l i t i e s were i d e a l l y s u i t e d f o r the p a r t i c u l a r e xpe r imen t s d e s i r e d , so ar rangements were made f o r two f i e l d t r i p s to t h a t e s t a b l i s h m e n t . The knowledge ga ined about the HMPA system from the s t e a d y s t a t e - 198 -work at U. B. C. f a c i l i t a t e d the p l a n n i n g of s p e c i f i c e x p e r i m e n t s and c o n d i t i o n s such t h a t a g r e a t dea l of u s e f u l i n f o r m a t i o n c o u l d be o b t a i n e d i n the s h o r t t ime a v a i l a b l e . The a b s o r p t i o n s p e c t r a f o r the wave l eng th r e g i o n 300 -2300 nm o b t a i n e d i n pure HMPA at the end o f 5 Krad e l e c t r o n p u l s e s i s shown i n F i g u r e 111-17. HMPA i t s e l f e x h i b i t s a number o f s t r o n g a b s o r p t i o n bands over t h i s s p e c t r a l r e g i o n . These can be seen i n F i g u r e 111-18 which shows the s o l v e n t a b s o r p t i o n s p e c t r a as measured d i r e c t l y i n a r a d i o l y s i s c e l l hav ing a 20 mm path l e n g t h . The t r a n s i e n t a b s o r p t i o n spec t rum o f F i g u r e 111-17 a c t u a l l y r e p r e s e n t s the n o r m a l i z e d r e s u l t s o f s e v e r a l i ndependent s e r i e s o f e x p e r i m e n t s u s i n g c e l l s h a v i n g o p t i c a l path l e n g t h s of 3 and 20 mm. The i n t e n s i t y and decay c h a r a c t e r i s t i c s o f t h i s band were s t u d i e d as a f u n c t i o n o f w a v e l e n g t h , r a d i a t i o n dose and sample p u r i t y . I t was r e v e a l l e d t h a t a second s p e c i e s c o n t r i b u t e d to the a b s o r p t i o n beloW 500 nm. Th i s s p e c i e s , wh ich a ccoun ted f o r the i n c r e a s e i n absorbance i n the UV r e g i o n as can be seen i n F i g u r e I I I - l 7 , was much l o n g e r l i v e d than the s p e c i e s a b s o r b i n g i n the i n f r a - r e d . A number o f s o l u t e s i n c l u d i n g e l e c t r o n and p o s i t i v e i on s cavenger s were added to HMPA samp le s . The e f f e c t s of these upon the i n t e n s i t y and decay o f these bands was i n v e s t i g a t e d . When new a b s o r p t i o n s appeared t h e i r i n t e n s i t y and decay were m o n i t o r e d . The s p e c i f i c d e t a i l s and r e s u l t s of a l l these expe r imen t s are be s t p r e s e n t e d as t h e i r i m p l i c a t i o n s are d i s c u s s e d . 1.2 1.0 0 . 8 x (TJ | 0 . 6 < 0 . 4 0 . 2 0 3 0 0 7 0 0 1 1 0 0 1 5 0 0 W A V E L E N G T H ( n m ) 1 9 0 0 c o CD 2 3 0 0 F i g u r e • 111 -1:7 . T r a n s i e n t a b s o r p t i o n spect rum i n p u l s e i r r a d i a t e d HMPA. A r e p r e s e n t s the abso rbance a t 2200 nm. m a x W A V E L E N G T H ( n m ) gure 111-18. A b s o r p t i o n spect rum o f pure HMPA e x h i b i t e d i n a r a d i o l y s i s c e l l hav ing an : o p t i c a l path l e n g t h of 20 mm. - 201 -D. DISCUSSION: PULSE RADIOLYSIS EXPERIMENTS 1 . T r a n s i e n t A b s o r p t i o n S p e c t r a The t r a n s i e n t a b s o r p t i o n spect rum o b t a i n e d from the p u l s e r a d i o l y s i s of HMPA as shown i n F i g u r e 111-17 i s s i m i l a r o ve r the wave leng th range 1000 - 2300 nm to t h a t a t t r i b u t e d by Brooks and D e w a l d 7 5 to the s o l v a t e d e l e c t r o n i n s o l u t i o n s o f sodium i n HMPA. T h e i r s p e c t r a e x h i b i t s a b s o r p t i o n s below 1000 nm a t t r i b u t e d to s p e c i e s ' i n v o l v i n g a l k a l i meta l c a t i o n s . No such a b s o r p t i o n appears i n the p r e s e n t r a d i o l y s i s s t u d y , but when the two s p e c t r a a re n o r m a l i z e d a t 1500 nm they c o r r e s p o n d v e r y w e l l o ve r the r e g i o n 1000 nm to 2300 nm ( the d e t e c t i o n l i m i t f o r the p r e s e n t s t u d y ) . In a d d i t i o n , the spec t rum i s i n g e n e r a l agreement w i t h two r e c e n t s t u d i e s o f the p u l s e r a d i o l y s i s o f H M P A 1 9 3 ' 1 9 4 wh ich suggest t h a t a b s o r p t i o n band maximum o c c u r s at X >1500 nm. As can be seen f rom F i g u r e 111-17, the max 3 p r e s e n t s tudy i n d i c a t e s t h a t the a b s o r p t i o n maximum i s near 2200 nm. However, the i n t e n s e s o l v e n t a b s o r p t i o n , the dec rea sed i n t e n s i t y o f the a n a l y z i n g l i g h t , and the reduced s e n s i t i v i t y of the p h o t o - d i o d e i n t h i s r e g i o n p r e c l u d e d i n u n e q u i v o c a l d e t e r m i n a t i o n of * m a x > Jou e t a l l 9 5 have shown e m p i r i c a l l y t h a t f o r b i n a r y m i x t u r e s of e t h y l e n e d i a m i n e (EDA) w i t h v a r i o u s e t h e r s , ^max c n a n 9 e s m o n o t o n i c a l l y between the maxima f o r the pure l i q u i d s . E x t r a p o l a t i o n of p l o t s of m i x t u r e c o m p o s i t i o n ve r su s band maximum energy to the pure e t h e r case gave e x c e l l e n t agreement w i t h the a c t u a l band maximum energy ob se r ved from the pure l i q u i d . - 202 -The band maxima i n pure EDA o c c u r s a t 1360 nm. M i x t u r e s o f HMPA/EDA would be e xpec t ed to g i v e maxima between 1360 - 2200 nm - - a r e g i o n where the p h o t o d e t e c t i o n system employed i s much more s e n s i t i v e . E x t r a p o l a t i o n of an e n e r g y / c o m p o s i t i o n p l o t to pure HMPA would thus p r o v i d e a i ndependent e s t i m a t i o n of the band maximum f o r t h a t s y s tem. F i g u r e 111-19 shows such a p l o t f o r HMPA/EDA m i x t u r e s . M i x t u r e c o m p o s i t i o n i s e x p r e s s e d i n terms of both mole and volume f r a c t i o n . Jou and c o - w o r k e r s used mole f r a c t i o n i n t h e i r d i s c u s s i o n , but t h e r e seems l i t t l e j u s t i f i c a t i o n f o r so d o i n g . For m i s c i b l e l i q u i d s , t h e r e a re reasons f o r e x p e c t i n g the c o m p o s i t i o n o ve r wh ich the s o l v a t e d e l e c t r o n ' s d e n s i t y i s d i s t r i b u t e d to be g i v e n by the volume f r a c t i o n a l c o m p o s i t i o n . T h i s c o n j e c t u r e i s s uppo r ted by the s u p e r i o r l i n e a r i t y e x h i b i t e d by the p l o t of energy of the m i x t u r e maxima ve r s e s volume f r a c t i o n as opposed to mole f r a c t i o n . When the m o l e c u l a r we i gh t s o f the components a re comparab le (as i n the Jou e x p e r i m e n t s ) i t w o u l d n ' t m a t t e r wh ich f r a c t i o n i s u sed. E x t r a p o l a t i o n o f F i g u r e 111 —19 g i v e s a v a l u e of X ,„ = J max 2150 ± 100 nm f o r pure HMPA, which i s i n e x c e l l e n t agreement w i t h the d i r e c t measurements. Dye and c o - w o r k e r s 1 9 6 have shown t h a t s o l v a t e d e l e c t r o n s p e c t r a can be m a t h e m a t i c a l l y r ep roduced by a combined G a u s s i a n / L o r e n t z i a n f u n c t i o n . For a g i ven s p e c t r a l e n e r g y , v, the r e l a t i v e ab so rbance , y ( v ) , has been found to be w e l l app rox imated by ( x l v i i i ) : 8 0 0 0 E 6 0 0 0 U J 4 0 0 Q F i g u r e 111-19 0.5 F R A C T I O N E D A A b s o r p t i o n maxima f o r e j i n HMPA/EDA m i x t u r e s as a f u n c t i o n o f s o l u t i o n c o m p o s i t i o n exp re s sed as mole ( O ) and volume ( • ) f r a c t i o n . - 204 -In 2 ( v 2 y ( v ) .= exp max f o r v . < v max f o r v . > v L 2 + ( v . - v ) 2  V I max ; max (x 1 v i i i ) where G and L are h a l f o f the i n d e p e n d e n t l y v a r i a b l e " h a l f -w i d t h s " f o r the Gau s s i an and L o r e n t z i a n f u n c t i o n s , v max i s the energy .(•cm"'') c o r r e s p o n d i n g to the a b s o r p t i o n maximum. Tak i n g v m a - = 4550 c m " 1 ( c o r r e s p o n d i n g to A. = 2200 nm) f o r max max the HMPA s y s tem, the f u n c t i o n was f i t t e d t o the da ta of F i g u r e 111-17. The r e s u l t a n t cu rve i s shown a l ong w i t h the da ta p l o t t e d on an energy s c a l e i n F i g u r e 111-20. A l s o i n c l u d e d i n t h i s f i g u r e , r e p r e s e n t e d by the f i l l e d c i r c l e s , i s d a t a ove r the r e g i o n 1000 - 3200 nm taken from the spect rum o b t a i n e d by Brooks and D e w a l d 7 5 f o r sodium s o l u t i o n s i n HMPA. From t h i s s p e c t r a l d a t a a l one one can w i t h r e a s o n a b l e c o n f i d e n c e a s s i g n the ob se rved band to the s o l v a t e d e l e c t r o n i n HMPA. As w i l l be d e t a i l e d l a t e r , the b e h a v i o r o f the s p e c i e s r e s p o n s i b l e f o r the a b s o r p t i o n i n the p re sence o f added s cavenge r s c o r r o b o r a t e s t h i s c o n c l u s i o n . The h a l f - w i d t h o f the a b s o r p t i o n band i n HMPA appears i n F i g u r e 111-2 0 to be o n l y 3600 c m " 1 (0 .45 eV) - - a much s m a l l e r v a l u e than has been g e n e r a l l y found i n o t h e r s y s tems . For i n s t a n c e i n the a l c o h o l s , amines and e t h e r s , h a l f - w i d t h s i n the range of 0.7 to 1.5 eV are common. A l s o , the a b s o r p t i o n maximum 0, 2 0 0 0 "50 6 O O O 1 0 0 0 0 1 4 0 0 0 W A V E N U M B E R (cm" 1) 1 8 0 0 0 F i g u r e 111-20. Combined G a u s s i o n / L o r e n t z i a n f i t to the spect rum o f on an energy s c a l e . P u b l i s h e d da ta from Na s o l u t i o n i n c l u d e d . G and L a re e x p l a i n e d i n the t e x t . F i g u r e 111-17 i n HMPA ( • ) i p i o t t e d s - 206 -a t 2200 nm co r re spond s to a t r a n s i t i o n energy o f o n l y 0.57 eV. These f a c t s t o g e t h e r sugges t t h a t s o l v a t e d e l e c t r o n s are v e r y weak ly bound i n HMPA, and t h a t the c a v i t i e s a re l a r g e and not too d i s s i m i l a r i n s i z e . These r e s u l t s had been a n t i c i p a t e d from c o n s i d e r a t i o n s o f the s i z e and n a t u r e o f the HMPA d i p o l e s . A s i m p l e c a l c u l a t i o n of the s m a l l e s t c a v i t y p o s s i b l e from a t e t r a h e d r a l c o n f i g u r a t i o n o f f o u r HMPA m o l e c u l e s p o i n t i n g " h e a d - i n " i n d i c a t e d a r a d i u s o f about 5A. I ndeed , t a k i n g ^max = 2200 nm, a c a l c u l a t i o n based on J o r t n e r ' s c a v i t y con t i nuum model has been m a d e 1 9 7 and a v a l u e o f 4.3 A f o r the c a v i t y r a d i u s o b t a i n e d . These v a l u e s are l a r g e i n compar i son to s i m i l a r c a l c u l a t i o n s f o r s e v e r a l a l c o h o l s , e t h e r s and amines © where r a d i i from 1 - 3 A were i n d i c a t e d } 9 8 2. Mo la r A b s o r p t i v i t y and Band O s c i l l a t o r S t r e n g t h The a b s o l u t e i n t e n s i t y o f the t r a n s i e n t a b s o r p t i o n band was measured by c o n v e r t i n g the s o l v a t e d e l e c t r o n s t o an i on s o f known mo la r a b s o r p t i v i t y . A r o m a t i c h yd roca rbon s e x h i b i t r e l a t i v e l y h i gh e l e c t r o n a f f i n i t i e s 1 9 9 and t hey r e a c t t o produce s t a b l e an ions wh ich absorb s t r o n g l y i n the v i s i b l e r e g i o n o f the s pec t r um. Through e l e c t r o n t r a n s f e r r e a c t i o n s w i t h a known amount o f the d i m e r i c d i a n i o n o f 1,1-d i phenyl e t h y l e n e , G i l l e t a l 2 0 0 p repa red s o l u t i o n s o f the an ions o f pyrene (Py) and an th racene (An) i n t e t r a h y d r o f u r a n (THF) . They r e p o r t e d v a l u e s 4 -1 -1 f o r the maximum molar a b s o r p t i v i t i e s o f 5.0 X 10 M cm f o r Py" at 495 nm and 1.0 X 1 0 4 M _ 1 c m " 1 f o r An " a t 740 nm r e s p e c t i v e l y . - 207 -When pyrene or an th r acene were p r e s e n t i n p u l s e i r r a d i a t e d HMPA the e l e c t r o n spect rum was r e p l a c e d by bands c o r r e s p o n d i n g to the a r o m a t i c an ions as shown i n F i g u r e 111-21 .• In o r d e r t o v e r i f y t h a t t he se a r o m a t i c an i on s po s se s s e q u i v a l e n t m o l a r a b s o r p t i v i t i e s i n HMPA and THF, t h e i r s p e c t r a i n the two systems were compared. F o r t u n a t e l y , the appa r a t u s had been used some-t ime e a r l i e r f o r a s tudy of pyrene i n T H F 2 0 1 In F i g u r e 111-21, the s o l i d l i n e o f the pyrene an i on s p e c t r a a c t u a l l y r e p r e s e n t s the s p e c t r a o b t a i n e d i n THF, n o r m a l i z e d a t the band maximum w i t h the HMPA da ta ( c i r c l e s ) . The s p e c t r a , undoub ted l y a r i s i n g from (TT ->• TT ) e l e c t r o n i c t r a n s i t i o n s i n v o l v i n g the c o n j u g a t e d a r o m a t i c r i n g s y s t em, would not be e xpec ted to be g r e a t l y i n f l u e n c e d by the s o l v e n t . The f a c t t h a t the pyrene a n i o n s p e c t r a appear i d e n t i c a l i n the two s o l v e n t s s u p p o r t s the i d e a t h a t e q u i v a l e n t t r a n s i t i o n s t a t e s a re i n v o l v e d . Thus , s i n c e the s o l v e n t s i n q u e s t i o n have comparab le r e f r a c t i v e i n d i c e s , one can make the r e a s o n a b l e a s sumpt ion t h a t the o s c i l l a t o r s t r e n g t h f o r the band i s a p p r o x i m a t e l y the same i n both med ia . Then, the c o n c l u s i o n i s reached t h a t the v a l ue f o r the maximum mo la r a b s o r p t i v i t y o f the pyrene an ion i n THF a l s o a p p l i e s f o r t h a t an ion i n HMPA. An ana logous s i t u a t i o n most p r o b a b l y ho l d s f o r the an th r acene an ion as w e l l . I t s hou ld be p o i n t e d out t h a t both the a r o m a t i c c a t i o n s 2 0 2 and e x c i t e d a r o m a t i c m o l e c u l e s 2 0 3 a re a l s o known t o absorb i n the same r e g i o n o f the spect rum as do the a r o m a t i c a n i o n s . F o r t u n a t e l y , i t was e a s i l y proven t h a t no such i n t e r f e r e n c e had o c c u r r e d . The r e l a t i v e l y l ong l i f e - t i m e s of the a b s o r p t i o n s - 203 -0 . 8 h if) o O 0 . 4 h 0 F i g u r e 111-21 5 0 0 5 5 0 ' 7 0 0 7 5 0 8 0 0 W A V E L E N G T H ( h m ) at ( i ) py rene and R a d i a t i o n produced t r a n s i e n t s ob se rved 470 - 520 nm f o r (2.6 ± 0.1) X 1 0~5M ( i i ) 710 - 780 nm f o r (6 .3 ± 0.5) X 1 n~TM an th racene i n HMPA. The s p e c i e s are a t t r i b u t e d to th'» a r o m a t i c n e g a t i v e i o n s . - 209 -a t t r i b u t e d to the a r o m a t i c an i on s p r e c l u d e d any c o n t r i b u t i o n from e x c i t e d s p e c i e s . In the p re sence o f ^ 0.1M R O (a s o l u t e known to scavenge n e g a t i v e but hot p o s i t i v e i o n s ) samples _ 3 c o n t a i n i n g < 10 M Py o r An, the a r o m a t i c an ion y i e l d was reduced by more than 95%. Th i s s t r o n g l y sugges t s t h a t the a r o m a t i c c a t i o n s P y + o r A n + a l s o d i d not c o n t r i b u t e a p p r e c i a b l y to the obse rved a b s o r p t i o n s . F u r t h e r , over the c o n c e n t r a t i o n - 4 - 1 range from 10 to 10 M, the y i e l d o f a r o m a t i c an i on s i n HMPA was found to be e s s e n t i a l l y i ndependent o f s o l u t e c o n c e n t r a t i o n . On the b a s i s o f these c o n s i d e r a t i o n s i t seems r e a s o n a b l e to a c c e p t the q u a n t i t a t i v e c o n v e r s i o n o f e ^ M p ^ to the a r o m a t i c a n i o n s . The magnitude o f the t r a n s i e n t a b s o r p t i o n a t 1000 nm i n p u l s e i r r a d i a t e d pure HMPA was compared w i t h the maximum of the r a d i c a l a n i o n a b s o r p t i o n i n the same sample a f t e r the a d d i t i o n i n vacuo o f s u f f i c i e n t amounts of Py o r An to ensu re comp le te eHMPA s c a v e n 9 i n 9 • Tab l e XI I shows t y p i c a l d a t a . The compar i son s gave a v a l u e f o r the molar a b s o r p t i v i t y f o r the t r a n s i e n t i n HMPA a t 1000 nm o f (4 .75 + 0.4) X 1 0 3 M " 1 s e c " 1 . U s i ng the measured r a t i o of 6.7 ± 0.4 f o r the abso rbance at 2200 nm to t h a t a t 1000 nm, the maximum mola r a b s o r p t i v i t y f o r the s o l v a t e d e l e c t r o n i n HMPA was c a l c u l a t e d to be £ e HMPA = (3 .2 ± 0.5) X 4 _ i _ i m a x 10 M sec . T h i s v a l u e i s s u b s t a n t i a l l y l a r g e r than t h a t r e c e n t l y r e p o r t e d by Nauta and Van H u i s 1 9 3 who found £ e HMPA ^ 4 - 1 - 1 -V - l 1 . 6 X 1 0 M cm us inn hi nhpnvl anH a n + h v>a r o n o a+• in c i n 1 In a d d i t i o n , M a l ' t s e v and V a n n i k o v 1 s 1 9 3 r e s u l t o f g eHMPA = 2.8 X 3 -1 -1 3 ? 1 6 0 0 "10 M cm based on an th r a cene at 10 to 10 M i s o n l y one T A B L E XII Compar ison of abso rbances i n a 20 mm c e l l o f e ^ p ^ at 1 0 0 0 nm w i t h the r a d i c a l an ion maximum a f t e r a r o m a t i c hydrocarbon a d d i t i o n . For each p a i r o f r ead i n g s e q u i v a l e n t r a d i a t i o n pu l s e s were used. . An th racene Pyrene A eHMPA = 0 . 3 6 5 ± 0 . 0 0 2 1 0 0 0 An" A J J Q = 0 . 1 7 2 ± 0 . 0 0 1 |~An] = 6.3 ± 0.5 X 1 0 " 4 M S^Q = 1.0 X 1 0 4 M " 1 c m " 1 AeHMPA = 0 . 0 3 2 5 ± 0 . 0 0 1 1 0 0 0 A J ^ 5 = 0.34 ± 0.01 [py] = 2.6 ± 0.1 X 1 0 " 3 M G ^ 5 = 5.0 X 1 0 4 M " 1 c m " 1 c a l c e 6 H M P A = 4.71 ± 0 . 2 0 X 1 0 3 M" 1 1 0 0 0 r m - l . cm c a l c eeHMPA = 4 . 7 8 ± 0 . 3 0 X 1 0 3 M _ 1 c m " 1 1 0 0 0 - 2 1 1 -t e n t h the c o r r e s p o n d i n g v a l u e £ HMPA = (2 .0 ± 0.2) X 10 M _ 1 1600 cm found i n t h i s work. A p o s s i b l e e x p l a n a t i o n f o r the l owe r v a l u e s found i n the o t h e r s t u d i e s i s the f a c t t h a t t hey used c o m p a r a t i v e l y l o ng r a d i a t i o n p u l s e s d u r i n g wh ich a s i g n i f i c a n t f r a c t i o n o f the s o l v a t e d e l e c t r o n s would have decayed . No i n d i c a t i o n was g i v e n t h a t c o r r e c t i o n s were made f o r such decay i n those s t u d i e s . The h i g h e r v a l u e f o r the maximum mo la r a b s o r p t i v i t y g e HMPA = 3.2 X 1 0 4 M " 1 c m " 1 found i n t h i s work i s max s uppo r ted by a c a l c u l a t i o n of the o s c i l l a t o r s t r e n g t h f o r the a b s o r p t i o n band. The o s c i l l a t o r s t r e n g t h of an a b s o r p t i o n band, f , i s a measure of the p r o b a b i l i t y o f e l e c t r o n i c t r a n s i t i o n s o c c u r r i n g . I t i s a measure o f the degree o f d i p o l a r o s c i l l a t i o n o f the e l e c t r o n between o r b i t a l s i n v o l v e d i n t r a n s i t i o n s — the s o -c a l l e d d i p o l e s t r e n g t h o f the t r a n s i t i o n s . The symmetr ie s and m u l t i p l i c i t i e s o f s t a t e s n e c e s s a r i l y a f f e c t the o s c i l l a t o r s t r e n g t h , but a f u l l y a l l o w e d t r a n s i t i o n has f = 1. I t can be shown t h a t 2 0 4 f = 2.303 mc 2 p £ d v b e 2 / ( x l i x ) where m and e a re the mass and charge o f an e l e c t r o n , c i s the v e l o c i t y of l i g h t , b i s the number of s o l u t e m o l e c u l e s per c c , and F i s a term r e l a t e d to the r e f r a c t i v e i n d e x , n, o f the medium. - 212 -S e v e r a l s u g g e s t i o n s have been made as to the a p p r o p r i a t e 7 7 7 r e f r a c t i v e index term to u s e 2 0 5 such as n, n , or 9n/(n + 2) . S i n c e t h e r e i s no g e n e r a l agreement and s i n c e each o f the e x p r e s s i o n s u s u a l l y y i e l d s a number c l o s e to u n i t y , the term i s o f t e n o m i t t e d . In t h a t c a s e , e x p r e s s i o n ( x l i x ) reduces to ( 1 ) . f = 4.32 X 1 0 " 9 / Gdv (1) where the i n t e g r a l i s s i m p l y the a rea under the t o t a l . a b s o r p t i o n band p l o t t e d on an energy s c a l e . As a f i r s t a p p r o x i m a t i o n to the IR a b s o r p t i o n i n HMPA, the Gaus s i an - L o r e n t z i a n f u n c t i o n ( x l v i i i ) was taken as r e p r e s e n t a t i v e of the e l e c t r o n a b s o r p t i o n band. I t has been s h o w n 1 9 5 t h a t i n t e g r a t i o n o f t h a t f u n c t i o n l e a d s to the f o l l o w i n g e x p r e s s i o n ( l i ) f o r the e n c l o s e d a r e a : y(S)<Jv = (^ZL G + * n G m a x 2 ^TnT 2 = ( 1 . 0 6 5 G • 1 . 5 7 1 L) 6 m a x Va lues f o r the h a l f - w i d t h s , G and L o f 1600 c m - 1 and 2000 c m - 1 r e s p e c t i v e l y , were taken from the be s t f i t cu rve as shown i n F i g u r e 111 -2 0. S u b s t i t u t i o n i n t o e q u a t i o n (1) gave a v a l u e f o r the o s c i l l a t o r s t r e n g t h o f the s o l v a t e d e l e c t r o n band i n HMPA of f = 0.7 ± 0 .2 . Th i s l a r g e v a l u e i s i n a c c o r d w i t h s i m i l a r d e t e r m i n a t i o n s f o r o t h e r s o l v a t e d e l e c t r o n s y s t e m s 2 0 6 ' 2 0 7 - 213 -and sugges t s t h a t the v a l u e f o r the maximum mo la r a b s o r p t i v i t y ee H M P A = (3.2 ± 0.5) X 1 0 4 M"1 s e c " 1 o b t a i n e d i n t h i s work i s . max not u n r e a s o n a b l e . F u r t h e r e v i d e n c e which tends to c o r r o b o r a t e t h i s v a l u e f o r HMPA comes f rom compar i son w i t h the s o l v a t e d e l e c t r o n s p e c t r a r e p o r t e d f o r o t h e r s o l v e n t s . To a good a p p r o x i m a t i o n , max Hi ( I i i ) where u, i s the h a l f band w i d t h . F i g u r e 111 - 2 2 shows s o l v a t e d s p e c t r a f o r a number o f s o l v e n t s . Below each cu r ve i s the v a l u e of £ • •<*>, f o r each max h s pec t r um. As can be s e e n , the v a l u e f o r t h e . s o l v a t e d e l e c t r o n spect rum i n HMPA i s comparab le to the o t h e r systems when the v a l ue f o r g eHMPA found i n t h i s work i s u sed, max 3. S o l v a t e d E l e c t r o n Y i e l d i n HMPA A p a r t i c u l a r l y u s e f u l p i e c e of i n f o r m a t i o n t h a t can be l e a r n e d from a p u l s e r a d i o l y s i s e xpe r imen t i s the y i e l d o f some a b s o r b i n g s p e c i e s . I f one knows the dosage r e p r e s e n t e d by the r a d i a t i o n p u l s e and e i t h e r knows o r can a s c e r t a i n the mo la r a b s o r p t i v i t y o f the s p e c i e s i n q u e s t i o n then the t e c h n i q u e has obv ious advantages over the i n d i r e c t methods r e q u i r e d i n s t e a d y -s t a t e r a d i o l y s i s s t u d i e s . O f t e n the two t e c h n i q u e s are 1/A. ( c m " 1 x 1 0 + 5 ) F i g u r e 111-22. O s c i l l a t o r s t r e n g t h compar i son f o r es~ i n s e v e r a l s o l v e n t s . The f i g u r e s i n b r a c k e t s r e p r e s e n t v a l u e s o f 6_ - u ^ X 1 O ^ ' c r r f 1 as e x p l a i n e d i n the t e x t . - 215 -complementary . In pure HMPA, no f a s t geminate decay of the s o l v a t e d e l e c t r o n s c o u l d be ob se rved w i t h the f a s t e s t a v a i l a b l e t ime r e s o l u t i o n o f 5. ns . Th i s would be e xpec t ed f o r a l i q u i d o f h i gh d i e l e c t r i c c o n s t a n t because the r e s u l t i n g sma l l Onsager escape r a d i u s would l e a d t o v e r y s h o r t l i f e t i m e s f o r geminate i on p a i r s . Thus , i t can be r e a s o n a b l y a c c e p t e d t h a t the a b s o r p t i o n o b s e r v a b l e a f t e r 5 ns c o r r e s pond s a lmos t e n t i r e l y to f r e e i o n ' s - - - p a r t i c u l a r l y i n l i g h t of the f a c t t h a t the h a l f - l i f e f o r decay was on the us t i m e s c a l e . The y i e l d s o f the a r o m a t i c an ions o f pyrene and a n t h r a c e n e were de te rm ined f o r v a r i o u s c o n c e n t r a t i o n s o f t ho se s c a v e n g e r s . R a d i a t i o n d o s i m e t r y was per fo rmed under i d e n t i c a l a c c e l e r a t o r o p e r a t i n g c o n d i t i o n s and i n . i d e n t i c a l c e l l s u s i n g wa te r or KCNS s o l u t i o n s . The e l e c t r o n a b s o r p t i o n was c o m p l e t e l y e l i m i n a t e d from s o l u t i o n s o f the a r o m a t i c s c a v e n g e r s , so i t was assumed t h a t ; q u a n t i t a t i v e c o n v e r s i o n of free e l e c t r o n s to an ions had taken p l a c e . As w i l l be shown l a t e r i n a d i s c u s s i o n of the r e a c t i o n s of the s o l v a t e d e l e c t r o n s , c a r e f u l e x a m i n a t i o n o f the e l e c t r o n decay and a r o m a t i c an i on bui ld-up from d i l u t e s cavenger s o l u t i o n s c o n f i r m e d t h i s assumed o n e - t o - o n e co r r e spondence Tab le X I I I shows the r a d i a t i o n y i e l d s of the a r o m a t i c an i on s found f o r v a r i o u s s cavenger c o n c e n t r a t i o n s . Based on the da ta from t h i s t a b l e , i t i s conc luded t h a t the f r e e i on or s o l v a t e d e l e c t r o n y i e l d i n HMPA i s G ( e ~ M D f l ) = 2.3 ± 0.4. - 216 -TABLE X I I I R a d i a t i o n y i e l d of a r o m a t i c an i on s from An th racene and Pyrene at v a r i o u s c o n c e n t r a t i o n s i n HMPA f o r ^ 1 krad p u l s e s . S o l u t e C o n c e n t r a t i o n Y i e l d An th racene (6 ± 2) X 1 0 " 5 M (6.3. ± 0.5) X 1 0 " 4 M G(An" ) 1.9 ± 0.4 2.3 ± 0.4 Pyrene (3.2 ± 0 . 3 ) X 1 0 " 4 M (2.6 ± 0.1) X 10 " 3 M (1.2 ± 0.1 ) X 1 0 _ 1 M G (Py " ) 2.1 ± 0.4 2.3 ± 0.4 2.4 ± 0.4 Th i s y i e l d i s i n good agreement w i t h the v a l u e o f 2.4 ± 0.3 measured at about the same t ime by Nauta and van H u i s 1 9 1 \ but t w i c e t h a t r e p o r t e d by M a l ' t s e v e t a l l 9 3 The v a l u e i s i n e x c e l l e n t a c co rd w i t h the v a l u e o f G ( e ^ M p A ) = 2.2 ± 0.2 proposed f rom n i t r o g e n y i e l d s f rom the 6 0 C o s t eady s t a t e r a d i o l y s i s s t u d i e s o f n i t r o u s o x i d e s o l u t i o n s i n HMPA p r e s e n t e d e a r l i e r i n t h i s Chap te r . 4. K i n e t i c s S t u d i e s a) Decay of P r i m a r y S p e c i e s - 217 -i ) The S o l v a t e d E l e c t r o n The 1 i f e t i m e o f the s o l v a t e d e l e c t r o n i n pure i r r a d i a t e d HMPAwas s e n s i t i v e to sample p u r i f i c a t i o n and p r e p a r a t i o n t e c h n i q u e s . G e n e r a l l y , the l i f e t i m e i n i t i a l l y i n c r e a s e d w i t h t o t a l absorbed dose per samp le , p resumably due to the r e a c t i o n and removal o f sma l l amounts of i m p u r i t i e s . For a g i v en samp le , the e l e c t r o n l i f e t i m e was g r e a t l y dependent upon the r a d i a t i o n dose per p u l s e . T y p i c a l o s c i l l o s c o p e t r a c e s a re shown i n F i g u r e 111 - 2 3. By u s i n g ve r y low dose p u l s e s 300 r ad s ) i n v e r y c a r e f u l l y deoxygenated s amp le s , f i r s t h a l f - l i v e s a p p r o a c h i n g 50 ysec c o u l d be a c h i e v e d ( F i g u r e I I I - 2 3 ( a ) ) . For the m a j o r i t y of the expe r imen t s r e p o r t e d h e r e , medium dose p u l s e s (^ 2000 r ad s ) were u t i l i z e d which gave l i f e t i m e s on the o r d e r of 5 ysec ( F i g u r e I I I - 2 3 ( b ) ) . Very l a r g e dose p u l s e (^ 10,000 r a d s ) gave l i f e t i m e s of o n l y about 2 y sec ( F i g u r e I I I - 2 3 ( c ) ) . The e l e c t r o n decay da ta were examined to see i f i t f o l l o w e d s i m p l e 1 s t o r 2 n c * o r d e r k i n e t i c s . F i g u r e 111 -2 4 shows the r e s u l t s o b t a i n e d f o r the da ta f rom F i g u r e I I I - 2 3 ( a ) and ( c ) - -i . e . f o r e l e c t r o n s o f both l o n g and s h o r t l i f e t i m e s . As can be s een , the very low dose data cu r ve f i t s a second o r d e r p l o t over at l e a s t the two f i r s t h a l f - l i v e s w i t h an appa ren t b i m o l e c u l a r r a t e c o n s t a n t o f 2.5 X 10 1 ( ^ M _ 1 s e c - 1 . On the o t h e r hand, the data from a l a r g e dose p u l s e i s n e a r l y l i n e a r on a l o g a r i t h m i c p l o t over the f i r s t two h a l f l i v e s w i t h a p s e u d o - f i r s t o r d e r 5 - 1 c o n s t a n t of about 3 X 1 0 sec . The l a t t e r r e s u l t i s c o n s i s t e n t w i t h v a l u e s r e p o r t e d by the o t h e r i n v e s t i g a t o r s 1 9 3 ' 1 9 1 * who - 218 -F i g u r e 111 - 2 3 . L i f e t i m e of e H Mp» as a f u n c t i o n o f r a d i a t i o n dose per p u l s e . Doses of (a) 300, (b) 2 ,000 , (c ) 10,000 rads gave e l e c t r o n f i r s t h a l f - l i v e s of 50, 5 and 2 usee r e s p e c t i v e l y . O 2 4 6 O 2 4 6 — ' 1 — 1 . I 1— 1_ I - i i r r — I , -, r — — — r -10 2 0 3 0 4 0 O 10 2 0 3 0 4 0 t ( ^ s e c ) t ( / x s e c ) F i g u r e 111-24. F i r s t (a) and second o r d e r (b) p l o t s f o r the data of F i g u r e 111-23 a and c . Data f o r s h o r t - l i v e d e l e c t r o n s ( o ) and l o n g - l i v e d e l e c t r o n s ( • ) are shown. - 220 -a p p a r e n t l y used o n l y l a r g e dose p u l s e s . Data from moderate pu l s e s f i t n e i t h e r f i r s t nor second o r d e r p l o t s . These r e s u l t s suggest t h a t both appa ren t f i r s t and second o r d e r p r o ce s s are i n v o l v e d i n the decay mechanism, and t h a t the f i r s t o r d e r component i n v o l v e s , r e a c t i o n o f e j ^ p ^ w i t h r a d i a t i o n produced s p e c i e s . A method e x i s t s f o r a n a l y z i n g mixed f i r s t and second o r d e r decays where the r e a c t a n t c o n c e n t r a t i o n can be m o n i t o r e d . 2 0 8 For the e l e c t r o n decay d a t a , the method a p p l i e s to a mechanism i n v o l v i n g a p s e u d o - f i r s t o r d e r r e a c t i o n (3 .98) and b i m o l e c u l a r combi n a t i o n ( 3 . 9 9 ) . k 3 . 9 8 eHMPA + X * X (3 .98) 2 k 3 . 9 9 eHMPA + eHMPA . * P ™ d u c t s (3 .99) In t h i s c a s e , X r e p r e s e n t s u n s p e c i f i e d i m p u r i t i e s , both i n t r i n s i c and r a d i a t i o n p r oduced , wh ich are a t c o n s i d e r a b l y h i g h e r c o n -c e n t r a t i o n t h a t e ^ M p A - . The method u t i l i z e s a p l o t o f In ((A^. + a)/A^.) ve r su s t i m e . A t i s the e l e c t r o n abso rbance a t t ime t and a i s a d i m e n s i o n l e s s pa rameter g i v e n by e q u a t i o n ( l i i i ) : a - k 3 > 9 8 / 2 k 3 . 9 9 £ 1 ( l i i i ) where £ i s the mo la r a b s o r p t i v i t y of the s o l v a t e d e l e c t r o n - 221 -a t the wave leng th s t u d i e s and 1 i s the o p t i c a l path l e n g t h . A computer program was w r i t t e n wh ich when g i v e n decay da ta and some a r b i t r a r y v a l u e f o r a , c a l c u l a t e s t h rough an i t e r a t i v e p r oce s s the v a l u e o f a t h a t g i v e s the be s t l e a s t squares f i t to the d a t a . The p s e u d o - f i r s t o r d e r r a t e c o n s t a n t i s g i v e n d i r e c t l y by the s l o p e , m, of the p l o t o f In ( ( A t + a ) / A^.) ve r su s t ime : k 3 . 9 8 = m =^ s l ° P e ( l i v ) and the b i m o l e c u l a r r a t e c o n s t a n t , 2 k 3 g g f rom p v - m £ 1 ^ K 3 . 9 9 " a ( l v ) F i g u r e 111-25 shows the r e s u l t s of such r e g r e s s i o n a n a l y s i s f o r the e l e c t r o n decay from both the l a r g e and s m a l l dose ca se s from F i g u r e r 11 - 2 3. A n a l y s e s o f t h i s t ype c o n s i s t e n t l y g i v e v a l u e s o f 2 k 3 g g = (1.9 ± 0.4) X I D 1 0 M - 1 s e c " 1 ; but kg g 8 v a l u e s were found to va r y between 7 X 10 and 1 X 10 sec depend ing on p u l s e dose and sample o r i g i n . T a b l e XIV shows t y p i c a l r e s u l t s f o r a s e r i e s of e x p e r i m e n t s where the dose per p u l s e v a r i e d from 400 to 12,000 r a d s . - 222 -O 2 4 6 I— 1 1 L • — i 1 1 r -O 10 2 0 3 0 4 0 F i g u r e III-2-5'. Mixed f i r s t p l u s second o r d e r p l o t f o r the data of F i g u r e 111 - 24. The method i s e x p l a i n e d i n the t e x t . - 223 -TABLE XIV K i n e t i c a n a l y s i s o f e l e c t r o n decay da t a i n a sample o f HMPA f o r which the dose per p u l s e v a l u e was v a r i e d . The combined f i r s t p l u s second o r d e r t r e a t m e n t d e s c r i b e d i n the t e x t was used to c a l c u l a t e the p a r a m e t e r s , Q R a n d 2k^ Q q . Dose ' 3 .98 2 k 3 . 9 9 ( r ad s ) (X 1 0 " 4 s e c - 1 ) (X 1 0 " 1 0 M - 1 s e c - 1 ) 400 0.96 ± 0.1 2.1 ± 0.2 600 0.96 ± 0.1 2.1 ± 0.2 800 1 .6 ± 0.2 1.9 ± 0.2 2,000 3.5 ± 0.4 1.6 ± 0.2 12,000 12.0 ± 1 . 2 2.0 ± 0.2 Now, the f o r e g o i n g r e a c t i o n scheme i g n o r e s the n e u t r a l i -z a t i o n r e a c t i o n (3 .100) t h a t c o u l d o c c u r i n HMPA. k 3 . 1 0 0 eHMPA + S ~~* S (3 .100) where S r e p r e s e n t s the p o s i t i v e c o u n t e r - i o n or some o x i d i z i n g r a d i c a l s p e c i e s formed t h e r e f r o m . The computer was programmed to s i m u l a t e e l e c t r o n decay da ta from mechanisms i n v o l v i n g p s e u d o - f i r s t o r d e r decay (3 .98) and (A) b i m o l e c u l a r c o m b i n a t i o n (3.99) or (B) b i m o l e c u l a r r e c o m b i n a t i o n - 224 -( 3 . 1 0 0 ) . F i g u r e 111 - 2 6 shows a t y p i c a l compar i son between an a c t u a l ob se r ved decay ( l i n e ) and tho se s i m u l a t e d f rom mechanisms (A) ( f i l l e d c i r c l e s ) and (B) (open c i r c l e s ) t a k i n g k 3 . 9 8 = 3 X ] ° 3 M _ 1 s e c _ 1 ? 2 k 3 99 = 2 X 1 0 1 0 M " 1 s e c " 1 ; and k 3 100 = 2 , 5 X l O 1 ^ M " 1 s e c " 1 . No doubt o t h e r mechanisms a re a l s o p o s s i b l e , but one can c onc l ude t h a t e l e c t r o n decay v i a r e a c t i o n s (3 . 98) , ( 3 . 99) and/or (3.100) i s e n t i r e l y c o n s i s t e n t w i t h the d a t a . That a s i n g l e s p e c i e s c o n t r i b u t e d to the a b s o r p t i o n ove r the wave leng th range from 500 to 2300 nm was c o n f i r m e d from k i n e t i c s t u d i e s . W i t h i n e x p e r i m e n t a l e r r o r the decay c h a r a c t e r -i s t i c s of t r a n s i e n t a b s o r p t i o n s over t h i s r e g i o n were i d e n t i c a l and the band has been a t t r i b u t e d to the s o l v a t e d e l e c t r o n i n HMPA. T h i s k i n e t i c c o n s i s t e n c y a l s o p r e v a i l e d i n the p re sence o f a number of s o l u t e s wh ich e x h i b i t e d v a r y i n g dec r ee s o f r e a c t i v i t y towards the t r a n s i e n t . i i ) The UV Ab so rb i n g S p e c i e s Below 500 nm, as noted p r e v i o u s l y \ 9 " a t l e a s t one o t h e r s p e c i e s c o n t r i b u t e s to the a b s o r p t i o n . T h i s i s e v i d e n c e d by the f a c t t h a t a f t e r the i n f r a - r e d e l e c t r o n a b s o r p t i o n i n pure i r r a d i a t e d HMPA had decayed , some a b s o r p t i o n p e r s i s t e d a t wave leng th s s h o r t e r than 500 nm. I ndeed , t h i s second s p e c i e s had a l i f e t i m e on the o r d e r o f 100 y s e c . The UV a b s o r p t i o n band was much weaker than t h a t found i n the i n f r a - r e d , s u g g e s t i n g t h a t e i t h e r t h i s l o n g e r - l i v e d s p e c i e s was produced w i t h a 0 0 F i gu r e M I - 2 6 2 5 S i m u l a t i o n o f the e l e c t r o n decay mechanisms i n v o l v i n g b i m o l e c u l a r r e c o m b i n a t i o n ( O ) . t ( p s e c ) 5 0 at 1 000 nm in HMPA (-c o m b i n a t i o n ( # ) and •) by means o f b i m o l e c u l a r - 226 -c o r r e s p o n d i n g l y l ower y i e l d or e l s e i t pos ses sed a s m a l l e r mo lar a b s o r p t i v i t y a t the wave leng th s s t u d i e d . The f i g u r e would tend to s uppo r t the l a t t e r i d e a f o r , as can be seen i n F i g u r e 111-17, i t i s v e r y p o s s i b l e t h a t o n l y the low energy t a i l o f the a b s o r p t i o n band was be ing m o n i t o r e d . Now, the e x p e r i m e n t a l appa ra tu s was not de s i g ned f o r , nor w e l l s u i t e d t o f t h e s tudy of t r a n s i e n t s p e c i e s wh ich abso rb o n l y i n the u l t r a v i o l e t r e g i o n of the s pec t r um. C o n s e q u e n t l y , q u a n t i t a t i v e s t u d i e s of t h i s second s p e c i e s - - p a r t i c u l a r l y k i n e t i c a n a l y s e s - - were cumbersome and s u b j e c t t o g ro s s u n c e r t a i n t i e s . In a d d i t i o n to the prob lem o f i n t e n s e s o l v e n t a b s o r p t i o n , o b s e r v a t i o n s i n t h a t r e g i o n were c o m p l i c a t e d by the p re sence o f Cerenkov r a d i a t i o n e m i s s i o n and the p o s s i b l e p h o t o l y s i s o f s p e c i e s i n the c e l l by the a n a l y z i n g l i g h t beam. A l s o , the e l e c t r o n band may have c o n t r i b u t e d s i g n i f i c a n t l y to the absorbance i n the UV r e g i o n . By m o n i t o r i n g the e l e c t r o n decay i n the i n f r a - r e d , one c o u l d d e t e r m i n e the n a t u r e o f the e l e c t r o n decay then s u b t r a c t the a p p r o p r i a t e c o n t r i b u t i o n f rom the UV decay. However, i t proved d i f f i c u l t t o d e t e r m i n e the e xac t p r o p o r t i o n a l i t y between the e l e c t r o n a b s o r p t i v i t y f o r the two r e g i o n s . In any e v e n t , the r e s u l t i n g " c o r r e c t e d " UV decay da ta must then c o n t a i n a c o r r e s p o n d i n g l y g r e a t e r u n c e r t a i n t y . A f u r t h e r c o m p l i c a t i o n a ro se because o f the much l o n g e r l i f e t i m e of the UV s p e c i e s . R e c a l l t h a t the a n a l y z i n g l i g h t sou rce was ope r a t ed i n a p u l s e d mode so as t o i n c r e a s e i t s i n t e n s i t y . Such o p e r a t i o n r e s t r i c t e d the t ime p e r i o d over which r e a c t i o n s c o u l d be s t u d i e d to a few hundred y s e c . R e s o r t i n g to - 227 -a s t e a d y - s t a t e mode o f lamp o p e r a t i o n would have f a c i l i t a t e d l o n g e r t i m e s c a l e s t u d i e s , but the r e s u l t i n g enormous l o s s i n the s i g n a l - t o - n o i s e r a t i o would r ende r r e s u l t s more ambiguous. The c o n t r i b u t i o n o f the s o l v a t e d e l e c t r o n s t o the UV a b s o r p t i o n c o u l d be e l i m i n a t e d th rough the a d d i t i o n o f e l e c t r o n s cavenge r s such as n i t r o u s o x i d e or oxygen. Under those c o n d i t o n s , an a b s o r p t i o n i n the UV r e g i o n p e r s i s t e d w i t h a l i f e t i m e o f the same o r d e r of t h a t found i n the pure s y s tem. However, one c o u l d not be c e r t a i n t h a t the y i e l d o f r e a c t i v i t y o f the UV s p e c i e s was u n a f f e c t e d under tho se c o n d i t i o n s . In a d d i t i o n , most s o l u t e s produced new a b s o r p t i o n s i n the UV or v i s i b l e r e g i o n s wh ich masked the a b s o r p t i o n be i ng s t u d i e d . The p re sence of e l e c t r o n s cavenger s i n the sys tem had l i t t l e o b s e r v a b l e e f f e c t on the UV s p e c i e s . C e r t a i n l y , i t s l i f e t i m e was not s i g n i f i c a n t l y a l t e r e d . As w i l l be d e t a i l e d , t h e r e i s e v i d e n c e t h a t the t r a n s i e n t r e a c t e d to some e x t e n t w i t h the s o l v a t e d e l e c t r o n s o f the sy s tem. These f a c t s tend to sugges t t h a t the s p e c i e s may i n f a c t be the p r i m a r y o x i d i z i n g s p e c i e s , S. Th i s s p e c i e s would be e xpec t ed to be e i t h e r t he s o l v a t e d p o s i t i v e c o u n t e r i o n , HMPA g , or e l s e one o f i t s decay p r o d u c t s - -p o s s i b l y some r a d i c a l s p e c i e s . C o o p e r J 1 7 f o r examp le , has i d e n t i f i e d a band c e n t e r e d a t 550 nm as the p o s i t i v e i on i n i r r a d i a t e d d i m e t h y l s u l p h o x i d e (DMSO). DMSO, a l s o be ing a p o l a r a p r o t i c s o l v e n t , i s s i m i l a r t o HMPA i n r e a c t i v i t y and many o f i t s p r o p e r t i e s . A l s o , S im i c and H a y o n 2 1 0 have shown t h a t r a d i c a l s and r a d i c a l an ions of many amides e x h i b i t a b s o r p t i o n maxima below 300 nm. - 2 2 8 -Low c o n c e n t r a t i o n s o f wate r or m e t h a n o l , both known t o r e a c t w i t h p o s i t i v e i o n s , had no e f f e c t on the UV s p e c i e s . However, the s caveng i ng r e a c t i o n s o f both wate r (3 .101) and methanol (3.102) are known to o c c u r v i a p r o t on t r a n s f e r f rom the po s i t i ve i on. „ 2 0 + R H + ^ R + H 3 0 + ( 3 C „ 3 0 H • RH + . R • C H 3 0 H 2 ( 3 1 0 2 ) Because o f the e x c e p t i o n a l l y a p r o t i c n a t u r e o f HMPA, the f a i l u r e o f the UV s p e c i e s to r e a c t w i t h methanol or water does not r u l e out the p o s s i b i l i t y o f i t be i n g the p o s i t i v e i o n . The s p e c i e s was a l s o u n r e a c t i v e towards d i s s o l v e d hydrogen gas. S t r o n g l y o x i d i z i n g , r a d i c a l s p e c i e s r e a c t w i t h hydrogen ( 3 0 5 ) , R + H 2 . R H + H . and the f a c t t h a t the UV s p e c i e s d i d n o t , does tend to r u l e out the l i k e l i h o o d o f i t be ing of t h i s n a t u r e . i i i ) R e a c t i o n Between T r a n s i e n t S p e c i e s That the two t r a n s i e n t s p e c i e s r e a c t e d t o g e t h e r was demons t ra ted by a s e r i e s of e xpe r imen t s conduc ted w i t h a sample o f HMPA c o n t a i n i n g a sma l l amount o f some unknown e l e c t r o n - 229 -s caveng i ng i m p u r i t y . F i g u r e I I I - 2 7 d e t a i l s the p e r t i n e n t t r a n s i e n t a b s o r p t i o n s observed at 1000 nm ( a , b , c , ) and 350 nm ( d , e , f , ) . A f t e r the f i r s t i r r a d i a t i o n p u l s e the e l e c t r o n , mon i t o r ed at 1000 nm, decayed r a p i d l y (a) because of r e a c t i o n w i t h d i s s o l v e d i m p u r i t y . The f i r s t h a l f - l i f e was o n l y 3 y sec i n t h a t case and the. e l e c t r o n a b s o r p t i o n t o t a l l y u n o b s e r v a b l e 50 usee a f t e r the p u l s e . With each s u c c e e d i n g p u l s e , the e l e c t r o n l i f e t i m e i n c r e a s e d as the t r a c e i m p u r i t y c o n c e n t r a t i o n d e c r e a s e d . A f t e r f i v e p u l s e s , the f i r s t h a l f - l i f e had doub led to about 6 y s e c , ( b ) . 'The e l e c t r o n a b s o r p t i o n was aga in absent a f t e r 50 ysec s u g g e s t i n g t h a t i t s decay s t i l l i n v o l v e d a s i g n i f i c a n t f i r s t o r d e r component. F o l l o w i n g 30 p u l s e s , the f i r s t h a l f - l i f e f o r the e l e c t r o n had not s i g n i f i c a n t l y i n c r e a s e d , ( c ) , but i t s decay c h a r a c t e r i s t i c s had changed. A s i g n i f i c a n t a b s o r p t i o n remained 50 ysec a f t e r the p u l s e , i n d i c a t i n g t h a t f i r s t o r d e r components to the decay were n e g l i g i b l e a f t e r t h a t p e r i o d . I t i s i n t e r e s t i n g to compare t h i s w i t h t h a t b e h a v i o r of the UV a b s o r b i n g s p e c i e s ( d , e , f , ) m o n i t o r e d a t 350 nm from a p a r a l l e l s e r i e s o f e x p e r i m e n t s . Those r e s u l t s were o b t a i n e d a f t e r the s m a l l (1 ml) volume of l i q u i d t h a t had been " p u r i f i e d " by i r r a d i a t i o n was mixed w i t h the bu l k of the s o l u t i o n (50 ml) and r e p l a c e d w i t h a f r e s h sample c o n t a i n i n g the t r a c e i m p u r i t y . F i g u r e 111- 2 7 ( d , e , f , ) show the decays ob se rved f o r the UV s p e c i e s under s i m i l a r c o n d i t i o n s to those d e s c r i b e d f o r the e l e c t r o n decay s t u d i e s . Because the UV s p e c i e s i s l o n g e r l i v e d , the t i m e s c a l e i s i n c r e a s e d f o u r - f o l d i n those d i a g r ams . C o n t r a r y to the - 230 -20 [ISGC 100— * i 1 U U — T-— I 1 95- 1 I i 0/ T d Vo 1 • 20 ysec 100-i l i jt 95-i y o/ T f /o I -100-95-%T 20 ysec 100-i oc i i i ab — 0/ T e % 1 hoo-95-%T 5 jJSGCf^-— • « — I 1 1 1 1 ;/ b 5 , ysec 4— i nn — IUU ' } ! 95-o/ T C /o 1 F i ~\:r~ 111 - 2 : Interaction b e t w e e n eHUP* o b s e r v e d a t 1 000 nm ( a , b , c ) and t h e UV s p e c i e s a t . 3 5 0 nm ( d , e , f ) , as a f u n c t i o n of sam'pln e x p l a i n e d i n t h e t e x t . t y . The: v a r i o u s e f f e c t s on d e c a y s a r e v:""'."' • - 2 3 1 -e l e c t r o n c a s e , removal o f the t r a c e i m p u r i t y r e s u l t e d i n a c c e l e r a t i o n o f the decay o f the UV a b s o r b i n g s p e c i e s . A f t e r the f i r s t p u l s e the UV s p e c i e s had a f i r s t h a l f - l i f e o f about 90 ysec ( d ) . A f t e r 5 and 30 p u l s e s t h i s v a l u e had dec r ea s ed to y 45 ysec and 20 y sec r e s p e c t i v e l y . In a d d i t i o n , the decay i n those l a t t e r ca se s c l e a r l y o c c u r r e d v i a a t l e a s t two p r o c e s s e s on v a s t l y d i f f e r e n t t i m e s c a l e s . In the l a t t e r e x p e r i m e n t the f a s t decay of the UV s p e c i e s c o r r e sponded v e r y w e l l to the e l e c t r o n decay. On a q u a l i t a t i v e b a s i s , t he se o b s e r v a t i o n s a r e c o n s i s t e n t w i t h a mechanism i n which the two t r a n s i e n t s r e a c t w i t h each o t h e r i n c o m p e t i t i o n w i t h o t h e r s p e c i e s . Attempts, to e s t a b l i s h a f e a s i b l e r e a c t i o n scheme and t h e r e b y o b t a i n q u a ! i t a t i v e i n f o r m a t i o n about such r e a c t i o n s , however, proved f r u i t l e s s . Undoubted l y the l a r g e u n c e r t a i n t i e s a s s o c i a t e d w i t h the UV s p e c i e s a b s o r p t i o n i s r e s p o n s i b l e f o r the d i f f i c u l t i es . i v ) E l e c t r o n Decay i n I r r a d i a t e d Sodium Meta l S o l u t i o n In an a t tempt t o f u r t h e r e l u c i d a t e the mechanism f o r the e l e c t r o n decay , some expe r imen t s were pe r fo rmed on the p u l s e r a d i o l y s i s o f b l ue s o l u t i o n s o f Na meta l d i s s o l v e d i n HMPA. These s o l u t i o n s c o n t a i n e d s u b s t a n t i a l (but unknown) c o n c e n t r a t i o n s of s o l v a t e d e l e c t r o n s and p o s s i b l y o t h e r i o n i c s p e c i e s . F i g u r e 111-2 8 shows a t y p i c a l o s c i l l o s c o p e t r a c e a t 750 nm o b t a i n e d when such a s o l u t i o n was s u b j e c t e d to two r a d i a t i o n p u l s e s , about 30 seconds a p a r t . - 232 -F i g u r e .111-28." E f f e c t o f e l e c t r o n p u l s e s on a s o l u t i o n o f Na i n HMPA at 750 nm. The second p u l s e , b, f o l l o w e d 30 sec a f t e r t he f i r s t and showed t h a t the p u l s e s gave an i n i t i a l i n c r e a s e i n e^Mpa c o n c e n t r a t i o n but a ne t permanent l o s s o f the. s p e c i e s . S i m i l a r r e s u l t s were found a t 1550 nm. As can be s e e n , each r a d i a t i o n p u l s e i n i t i a l l y c r e a t e d a d d i t i o n a l s o l v a t e d e l e c t r o n s but the o v e r a l l p r o ce s s e s l e d to a net l o s s i n c o n c e n t r a t i o n of eHJVJp/\- T n e l ° s s w a s comparab le i n magni tude to the r a d i a t i o n y i e l d o f s o l v a t e d e l e c t r o n s . F u r t h e r m o r e , the l o s s was permanent as e v i d e n c e d by the f a c t t h a t 30 seconds l a t e r , j u s t p r i o r t o the second r a d i a t i o n p u l s e , the r e s i d u a l c o n c e n t r a t i o n of e ^ p ^ had remained a t the l ower v a l u e . I f the e l e c t r o n s p e c i e s formed v i a r a d i a t i o n i nduced i o n i z a t i o n and sodium meta l d i s s o l u t i o n are i d e n t i c a l , then e l e c t r o n decay th rough b i m o l e c u l a r c o m b i n a t i o n (3 ,99) can be r u l e d out as an i m p o r t a n t s t ep i n t h i s s y s tem. - 233 -eHMPA + eHMPA "* P^ducts (3.99) Even i f such a reaction involved an equilibrium with some dimeric (^HMPA s P e c i e s > t n e observations could not be reconciled. It is'significant that neutralization recombination reaction between electrons and the co-produced positive ions could not account for the net loss in gHMPA" I n o r d e r that the charge balance be preserved, these two species must be produced in equal yield during radiolysis, therefore, the i n i t i a l and final concentrations of e ^ M p A would have been the same. Since the stable Na solutions obviously cannot contain impurities which react with e^ MPA' o n e m u s t conclude that other radiation produced species, such as free radicals, contribute to the electron decay. Recall that this conclusion had been proposed to account for the dependence of the electron decay on the radiation dose per pulse. b) Behavior of Transient Species in the Presence of  Added Solutes i) Electron Scavengers i . 1) Pyrene and Anthracene As has been discussed previously, the pyrene (Py) and anthracene (An) react with in HMPA to yield the aromatic anions. The aromatic hydrocarbons the reducing species reactions appeared to - 234 -be ve ry f a s t - - the e l e c t r o n decay be i ng comple te i n ns f o r s o l u t i o n s c o n t a i n i n g moderate c o n c e n t r a t i o n s (> 10 M) o f the a r o m a t i c s c a v e n g e r s . The r e a c t i o n between s o l v a t e d e l e c t r o n s and an t h r a cene was s t u d i e d by p r e p a r i n g a sample c o n t a i n i n g o n l y (6 ± 2) X -5 10 M an th r a cene i n HMPA. Upon i r r a d i a t i o n , both the e l e c t r o n decay a t 1000 nm and the c o r r e s p o n d i n g an th r a cene an i on b u i l d -up at 740 nm c o u l d be o b s e r v e d . F i g u r e 111-29. Decay o f e H M p f l a b s o r p t i o n a t 1000 nm (A) and s i m u l t a n e o u s b u i l d - u p o f A n " a b s o r p t i o n a t 740 nm (B) i n a s o l u t i o n o f (6 ± 2) X 10~*M an th racene i n HMPA f o l l o w i n g a 200 ns r a d i a t i o n p u l s e . F i g u r e 111-2 9(a) shows the a c c e l e r a t e d e l e c t r o n decay , mon i t o red at 1000 nm. ' I n i t i a l l y , the a r o m a t i c an i on i s ab sent and thus does not mask the e l e c t r o n decay . The a r o m a t i c an ion i s much l o n g e r l i v e d than the e l e c t r o n , so i t s maximum c o n t r i b u t i o n - 235 -to the ab so rp t i on at 1000 nm i s g iven by the s teady non -zero absorbance at long time ( in t h i s case t > 2 y s e c ) . S ince t h i s ab so rp t i on represented l e s s than 5% of the i n i t i a l e l e c t r o n a b s o r p t i o n , and s ince i t s c o n t r i b u t i o n would a c t u a l l y be much sma l l e r dur ing the i n i t i a l s tages of the e l e c t r o n decay, i t was i gnored in the k i n e t i c a n a l y s i s of the da ta . On the o ther hand, as shown in F igure 111-17, the s o l v a t e d e l e c t r o n in HMPA abso rp t i on band encompasses the whole o f the v i s i b l e spectrum. Thus, the b u i l d - u p of the anthracene a n i o n , monitored at 740 nm, shown in F i gure I I I -29(b) con ta in s a s i g n i f i c a n t c o n t r i b u t i o n from s o l v a t e d e l e c t r o n s . The problem i s compounded by the f a c t tha t e l e c t r o n a b s o r p t i o n i s l a r g e s t i n i t i a l l y when the anthracene anion component i s s m a l l . However, the c o n t r i b u t i o n by the e l e c t r o n to the t o t a l absorbance at 740 nm i s determined s imply from the cor respond ing va lues observed at 1000 nm, m u l t i p l i e d by the r a t i o of the e l e c t r o n e x t i n c t i o n c o e f f i c i e n t s at these two wavelengths. From F i gure 111-17 the r a t i o i s found to be: eeHMPA / eeHMPA = 0 . 5 6 740 1000 ( I v i ) - 5 Even at (6 ± 2) X 10 M c o n c e n t r a t i o n , the anthracene c o n c e n t r a t i o n would be about ten times that expected f o r s o l v a t e d e l e c t r o n s produced in HMPA by a 200 ns p u l s e . T h e r e f o r e , b imo lecu l a r r e a c t i o n between those spec ie s would be expected to appear as a p s e u d o - f i r s t order p roce s s . F i gure 111-30 shows f i r s t order p l o t s f o r the e l e c t r o n decay at 1000 nm and the 236 -< I t ( j j s e c ) F i g u r e I I I - 3 0 F i r s t o r d e r k i n e t i c p l o t s f o r the eTyw/i decay at 1000 nm C O ) and the " c o r r e c t e d " an th r a cene an ion a b s o r p t i o n b u i l d - u p at 740 nm ( • ) from an i r r a d i a t e d s o l u t i o n of (6 ± 2) X 10"fM An i n HMPA. - 237 -" c o r r e c t e d " an th racene an ion b u i l d - u p a t 740 nm. As e x p e c t e d , the e l e c t r o n decay d e v i a t e d from f i r s t o r d e r b e h a v i o r a t l ong t imes ( t > 1.5 y sec ) because o f a sma l l but s i g n i f i c a n t c o n t r i b u t i o n to the t o t a l a b s o r p t i o n by an t h r a cene a t t h a t p o i n t . For the An" b u i l d - u p , the p l o t i s In (A,, ^ - A^) a g a i n s t t ime,where Ag ^ i s the abso rbance 2.5 y sec a f t e r the pu l s e ( p l a t e a u v a l u e ) and A^ . i s the abso rbance a t t ime t , c o r r e c t e d f o r the e l e c t r o n c o n t r i b u t i o n . I t i s most s i g n i f i c a n t t h a t the two p l o t s e x h i b i t n e a r l y i d e n t i c a l s l o p e s . That i s , from the s l o p e s one c a l c u l a t e s p s e u d o - f i r s t o r d e r r a t e c o n s t a n t s f o r the e l e c t r o n decay and an i on b u i l d - u p o f (1 .5 + 0.2) X 1 0 6 s e c " 1 and (1.7 + 0.2) X 1 0 6 s e c " 1 r e s p e c t i v e l y . Th i s r e s u l t a f f o r d s r a t h e r c o n v i n c i n g e v i d e n c e t h a t the s o l v a t e d e l e c t r o n i n HMPA r e a c t s on a o n e - t o -one b a s i s w i t h an th r acene m o l e c u l e s to produce one s i n g l y c h a r g e d a n i o n ( 3 . 1 0 3 ) . k 3 . 1 0 3 , • eHMPA +. A n " * A n . (3 .103) Combin ing the p s e u d o - f i r s t o r d e r r a t e c o n s t a n t s w i t h the c o n c e n t r a t i o n o f an th r acene p r e s e n t ( (6 ± 2) X 10~ 5 M) one c a l c u l a t e s the r a t e c o n s t a n t f o r r e a c t i o n (3 .103) t o be = (3 ± 1) X 1 0 1 0 M " 1 s e c " 1 . S i m i l a r , b e h a v i o r was found f o r s o l u t i o n s of pyrene i n HMPA. However, the e l e c t r o n and Py " a b s o r p t i o n s appeared to m u t u a l l y i n t e r f e r t o a g r e a t e r e x t e n t making a n a l y s i s more d i f f i c u l t . - 238 -Al so , the -.1 owest c o n c e n t r a t i o n s t u d i e d , (3 .2 ± 0.3) X 1 0 " 4 M Py, r e s u l t e d i n the e l e c t r o n decay and Py" b u i l d - u p be ing too; f a s t to be s t u d i e d i n d e t a i l . To a f i r s t a p p r o x i m a t i o n , the s caveng ing r e a c t i o n between pyrene and s o l v a t e d e l e c t r o n s (3.104) o ccu r s at a r a t e comparab le to t h a t measured f o r . a n t h r a c e n e . eHMPA + * py~ (3 .104) The a r o m a t i c an i on s were r ema r kab l y l o n g - l i v e d i n HMPA. F i g u r e 111- 31 shows the decay o f the a b s o r p t i o n o f (a) An " at 740 nm and (b) Py" at 495 nm. F i g u r e .111-31. Decay o f a r o m a t i c an i on s produced v i a e l e c t r o n s caveng i ng i n i r r a d i a t e d HMPA. (A) A n " a t 740 nm from (6 ± 2) X 1O^M An i n HMPA, (B) P y " a t 495 nm from (3.2 ± 0.3) X 10"VM Py i n HMPA. - 239 -I t can be seen t h a t these decays a re complex as each c l e a r l y e x h i b i t s two d i s t i n c t decay r e g i o n s . Such b e h a v i o r c o u l d a r i s e from the f o r m a t i o n and subsequent decay o f a d i m e r i c a r o m a t i c s p e c i e s or p o s s i b l y some n e u t r a l i z a t i o n complex . In any c a s e , e l e c t r o n s caveng i ng by An or Py i n HMPA l e a d s t o the f o r m a t i o n of a b s o r b i n g s p e c i e s w i t h l i f e t i m e s a p p r o a c h i n g seconds . Smal l amounts o f the e l e c t r o n s cavenge r n i t r o u s o x i d e were added to samples o f an th r acene or pyrene i n HMPA. Upon i r r a d i a t i o n , the i n i t i a l y i e l d o f a r o m a t i c an i on s was s ub -s t a n t i a l l y dec rea sed - - but the decay r a t e o f A n " or Py " was e s s e n t i a l l y unchanged. These f i n d i n g s are c o n s i s t e n t w i t h t h e r e be ing a c o m p e t i t i o n between NgO and the hyd roca rbons f o r s o l v a t e d e l e c t r o n s . . - I t f u r t h e r i m p l i e s t h a t t h e r e i s no charge exchange between An " (o r Py " ) and NgO or between [NgO~] and An ( o r P y ) . By c o n t r a s t , the p re sence of oxygen i n a Py s o l u t i o n a f f e c t e d both the y i e l d o f Py" and i t s r a t e o f decay . In t h a t c a s e , the Py " decay was g r e a t l y a c c e l e r a t e d , s u g g e s t i n g t h a t , Og r e a c t e d , w i t h t he : a r o m a t i c an ion ( 3 . 1 0 5 ) . -, y - r + o 2 — + o- C 3 - . 1 0 S i n c e the a r o m a t i c hydrocarbons produced o b s e r v a b l e t r a n s i e n t s p e c i e s upon r e a c t i o n w i t h s o l v a t e d e l e c t r o n s , they p r o v i d e d a means whereby the p o s s i b l e i n v o l v e m e n t o f geminate i on s c a veng -i ng c o u l d be s t u d i e d d i r e c t l y . The s c a veng i n g r a t e of a n t h r a cene (and presumably py rene) has been shown to be f a s t enough t h a t one would expec t t h a t a t h igh c o n c e n t r a t i o n the a r o m a t i c h y d r o -- 2 4 0 -carbons c o u l d compete e f f e c t i v e l y w i t h geminate r e c o m b i n a t i o n f o r the e l e c t r o n s . I f such were the c a s e , one would a n t i c i p a t e the f o l l o w i n g b e h a v i o r . As the c o n c e n t r a t i o n o f An or Py was i n c r e a s e d from the normal s cavenge r l e v e l (10~ t o 10" M) t o say 10 " 1 M one would e xpec t to see a c o r r e s p o n d i n g i n c r e a s e i n the i n i t i a l an ion y i e l d - - due to geminate e l e c t r o n s c a veng i n g ( 3 . 106 ) . HMPA + 'HMPA + Py HMPA + Py" (3 .106) However, as a r e s u l t of the s t r o n g cou l omb i c a t t r a c t i o n o f the nearby p o s i t i v e i o n , t h i s e x t r a a b s o r b i n g an ion would a lmos t c e r t a i n l y i t s e l f undergo a r a p i d geminate n e u t r a l i z a t i o n r e a c t i o n ( 3 . 1 0 7 ) , r e s u l t i n g i n a dec rea sed ab s o rbance . HMPA + Py -] HMPA - Py (3 .107) s caveng ing r e a c t i o n i s (1.2 ± 0.2) X 1 0 1 1 M 1 s e c " 1 •4, J o u 2 0 8 has ob se rved j u s t such b e h a v i o r i n t e t r a h y d r o f u r o n (THF) s o l u t i o n , where the r a t e c o n s t a n t f o r the e l e c t r o n I r r a d i a t i o n o f THF c o n t a i n i n g up to 3 X l O ^ M pyrene gave r e s u l t s s i m i l a r to those r e p o r t e d here f o r HMPA. At h i g h e r Py c o n c e n t r a t i o n s Jou found an i n c r e a s e d i n i t i a l Py " y i e l d t h a t decayed r a p i d l y (over ns) to the l ower v a l ue a t which p o i n t the much s l o w e r decay p r e v a i l e d . No such e v i d e n c e was found i n HMPA. R e c a l l from Tab l e XVI - 241 -t h a t f r o m a s a m p l e c o n t a i n i n g ( 1 . 2 ± 0 . 1 ) X 1 0 - 1 M p y r e n e i n HMPA t h e i n i t i a l P y " y i e l d was G ( P y " ) = 2 . 4 ± 0 . 4 . T h i s v a l u e i s n o t s i g n i f i c a n t l y h i g h e r t h a n t h a t o b s e r v e d f r o m s o l u t i o n s c o n t a i n i n g o r d e r s o f m a g n i t u d e l e s s p y r e n e . A l s o , a t t h e f a s t e s t t i m e s c a l e o b s e r v a b l e (5 n s ) , no f a s t d e c a y o f P y " c o u l d b e - d e t e c t e d . T h e s e f i n d i n g s t e n d t o r u l e o u t t h e i n v o l v e m e n t o f g e m i n a t e e l e c t r o n r e a c t i o n s t h e s c a v e n g i n g p r o c e s s e s ' i n i r r a d i a t e d HMPA. B e c a u s e o f i t s 1 i m i t e d s o l u b i l i t y i n HMPA, a n t h r a c e n e c o u l d n o t be u s e d a t a h i g h e n o u g h c o n c e n t r a t i o n t o p r o v i d e c o r r o b o r a t i n g e v i d e n c e . i . 2 ) N i t r o u s O x i d e N i t r o u s o x i d e , N g O , was u s e d e x t e n s i v e l y as an e l e c t r o n s c a v e n g e r i n . t h e s t e a d y s t a t e gamma r a d i o l y s i s s t u d i e s . As has been d i s c u s s e d , t h o s e e x p e r i m e n t s i n d i c a t e d t h a t t h e s c a v e n g i n g r e a c t i o n ( 3 . 6 8 ) was r a p i d and l e d u l t i m a t e l y t o t h e p r o d u c t i o n o f m o l e c u l a r n i t r o g e n . k e H M P A + N 2 ° 3 , 6 8 » ( N 2 0 " ) • N g + o t h e r p r o d u c t s ( 3 g 8 ) I t was t h u s i m p e r a t i v e t h a t t h e p u l s e r a d i o l y s i s w o r k i n c l u d e e x p e r i m e n t s t o d i r e c t l y c o n f i r m t h a t NgO s c a v e n g e s e J j M p A a n d i f p o s s i b l e t o m e a s u r e t h e r a t e o f s u c h a r e a c t i o n . F i g u r e 1 1 1 - 3 2 shows t y p i c a l f a s t t i m e s c a l e o s c i l l o s c o p e - 242 -t r a c e s o b t a i n e d a t 1000 nm upon i r r a d i a t i o n (a) o f pure HMPA and (b) c o n t a i n i n g (7 .9 ± 0.2) X 1 0 " 4 M N o 0 . — • 200 nsec 100- - _ V.T A n u pul se le ngtt-200 nsec 100 -1 KJ\J %T B 7 5 -pul se le ngtl-F i g u r e I I 1 - 3 2 . T y p i c a l f a s t t ime s c a l e o s c i l l o s c o p e t r a c e s o b t a i n e d a t 1000 nm f o l l o w i n g 700 nsec r a d i a t i o n p u l s e s i n pure HMPA (A) and HMPA c o n t a i n i n g (7 .9 ± 0.2) X 10"yM R0 ( B ) . As can be c l e a r l y seen i n the f i g u r e , the e l e c t r o n decay was g r e a t l y a c c e l e r a t e d i n the p re sence o f R O . F i g u r e 111-3 3 i s a f i r s t o r d e r p l o t o f the e l e c t r o n decay i n the p re sence o f RO ( f rom F i g u r e I I I - 3 2 ( b ) ) i n which the end of the p u l s e i s t aken as t ime z e r o . W i t h i n the u n c e r t a i n t y o f the d a t a , the p l o t i s l i n e a r , and from the s l o p e , a p s e u d o - f i r s t o r d e r r a t e c o n s t a n t equal to (1.1 ± 0.3) X 1 0 7 s e c " 1 was o b t a i n e d . For [RO| / x -4 (7.9 ± 0.2) X 10 M, t h i s c o r r e spond s t o a b i m o l e c u l a r r a t e c o n s t a n t ^ 3 6 B > = (1-4 ± 0.4) X 1 0 1 0 M " 1 s e c - 1 f o r the r e a c t i o n between RO and e ^ p ^ ( 3 . 6 8 ) . F i g u r e 111-3 2 shows t h a t the b u i l d - u p o f the s o l v a t e d - 243 -6 < I 4 • o i g u r e 111-3 3, 0 . 0 5 0 . 1 0 t ( p s e c ) 0.15 F i r s t o r de r k i n e t i c p l o t f o r r a n i d e'^p/i decay in the p re sence of (7 .9 ± 0.2) X 10~¥M 1^0 from F i g u r e I I I - 3 2 ( b ) . T i ne z e r o co r r e spond s to the end of the r a d i a t i o n - u 1 s e . - 244 -e l e c t r o n c o n c e n t r a t i o n i s v e r y d i f f e r e n t i n the p re sence and absence o f R O . i n F i g u r e 111-34 the da ta are p l o t t e d i n absorbance u n i t s . As a consequence o f the f a c t t h a t the r a d i a t i o n p u l s e s were e s s e n t i a l l y square waves ( F i g u r e 1 1 - 1 2 ) , the e l e c t r o n absorbance i n c r e a s e d s t e a d i l y t h r oughou t the d u r a t i o n of the p u l s e . That i s , the r a t e o f f o r m a t i o n of s o l v a t e d e l e c t r o n s was a c o n s t a n t , K: d e HMPA = d ( A / e l ) = K dt d t ( l v i i ) where A i s the e l e c t r o n a b s o r b a n c e , e i s i t s mo la r a b s o r p t i v i at the wave leng th s t u d i e s and 1 i s the o p t i c a l path l e n g t h . Now, i n the p re sence of N 2 O , the e l e c t r o n decay i s g r e a t l y a c c e l e r a t e d th rough r e a c t i o n ( 3 . 6 8 ) . The r e s u l t i s t h a t d u r i n g the r a d i a t i o n p u l s e a p o i n t i s reached where the r a t e s of e l e c t r o n f o r m a t i o n and decay are equa l and e l e c t r o n absorbance remains, c o n s t a n t - - at a s o - c a l l e d s t eady s t a t e v a l u e , A g s . Under those c o n d i t i o n s , k 3 . 6 8 ( I v i i i ) S u b s t i t u t i o n o f the a p p r o p r i a t e v a l u e s from F i g u r e 111- 34 i n t o ( l v i i i ) g i v e s k 3 g 8 = (1.5 + 0.2) X 1 0 1 0 M " 1 s e c " 1 as the r a t e c o n s t a n t f o r s c a veng i n g o f s o l v a t e d e l e c t r o n s by n i t r o u s o x i d e - 245 -0 . 5 0 , 4 UJ U < 0 . 3 CQ m O ui CQ < 0 . 2 0.1 0 radiation pulse duration o F i g u r e 111-34 In pure HMPA 2 0 0 4 0 0 t ( n s e c ) 6 0 0 8 0 0 B u i l d - u p - o f e 700 nsec " ( O ) r a t e Hfttfl absorbance at 1000 nm d u r i n g s qua re " r a d i a t i o n p u l s e s of F i g u r e 111-3 2 o f f o r m a t i o n was c o n s t a n t , dA/dt = (5.2 ± 1) X l O ^ s e c - ' . For HMPA c o n t a i n i n g (7.9 ± 0.2) X ) a steady s t a t e , absorbance A,.^ = 0.044 ± 0.002 s.s. 10"VM was reached N,0 - 246 -i n HMPA. Th i s r e s u l t i s i n e x c e l l e n t agreement w i t h t h a t o b t a i n e d d i r e c t l y from the e l e c t r o n decay and t h e r e f o r e l e n d s c redence to t h a t v a l u e ; b u t , s i n c e i t was not o b t a i n e d c o m p l e t e l y i n d e p e n d e n t l y , does not c o n f i r m i t s p r e c i s i o n . The r a t e c o n s t a n t f o r the c o r r e s p o n d i n g r e a c t i o n i n wa te r i s 0.56 X 1 0 1 0 M " 1 s e c " 1 ? 7 The h i g h e r r a t e i n HMPA i s u n d e r s t a n d a b l e i n terms of the ve r y weak s o l v a t i o n energy o f the e l e c t r o n i n HMPA and p o s s i b l y i t s h i g h e r e l e c t r o n m o b i l i t y . 1.3) Oxygen Oxygen appears to r e a c t r a p i d l y (and e x p e c t e d l y ) w i t h eHMPA i s e v i ' d e n c e d by the f a c t t h a t when i r r a d i a t e d HMPA was exposed to a i r at one atm p r e s s u r e the e l e c t r o n ab so rbance was g r e a t l y reduced and s h o r t - l i v e d . 1.4) Acetone Another proven e l e c t r o n s c a v e n g e r , a c e t o n e , had been used i n the s t eady s t a t e 6 0 C o r a d i o l y s i s s t u d i e s o f HMPA - - u s u a l l y i n c o n j u n c t i o n w i t h NgO. Under these c o n d i t i o n s , i t was expec ted t h a t acetone would scavenge e l e c t r o n s (3 .107) i n c o m p e t i t i o n w i t h n i t r o u s o x i d e (3 .68) w i t h the r e s u l t be ing a l ower y i e l d of n i t r o g e n f rom the 1 a t t e r r e a c t i o n . eHMPA + C H 3 C 0 C H 3 * (CH 3 C0CH 3 ) (3 .107) - 247 -e HMPA * N 0 + p r o d u c t s (3 .68) In p o i n t of f a c t however, the n i t r o g e n y i e l d was found to be i n c r e a s e d i n those s t u d i e s . The a m b i g u i t y was a t t r i b u t e d to a charge t r a n s f e r r e a c t i o n (3 .43) between the ace tone an i on and NoO. In t h a t c a s e , the i n c r e a s e d n i t r o g e n y i e l d would s i m p l y be a r e f l e c t i o n o f the i n c r e a s e d t o t a l s c a venge r c o n c e n t r a t i o n . In o r d e r to c o n f i r m the se s u s p i c i o n s , a s e r i e s of p u l s e r a d i o l y s i s e xpe r imen t s were c o n d u c t e d . As can be seen f rom Tab le XV, the i n i t i a l e l e c t r o n y i e l d i n i r r a d i a t e d HMPA (as measured from the abso rbance a t 1550 nm i m m e d i a t e l y f o l l o w i n g the e l e c t r o n p u l s e ) was c o n s i d e r a b l y l ower i n the p re sence o f a c e t o n e . In the case o f a s o l u t i o n c o n t a i n i n g (1 .6 ± 0.2) X 1 0 _ 1 M a c e t o n e , the dec rea se amounted to 96%. A new broad t r a n s i e n t a b s o r p t i o n appeared i n the v i s i b l e r e g i o n o f the spect rum from i r r a d i a t e d s o l u t i o n s of HMPA c o n t a i n i n g a c e t o n e . F i g u r e 111-3 5 shows the s p e c t r a ob se r ved between 400 and 2050 nm from a s o l u t i o n c o n t a i n i n g (1.6 ± 0.2) _ 2 X 10 M a c e t o n e . S p e c t r a p r e s e n t i m m e d i a t e l y a f t e r , and 2 y sec a f t e r , the r a d i a t i o n pu l s e a re g i v e n . The a b s o r p t i o n from about 1300 nm t o 2050 nm i s a s s i g n e d to the s o l v a t e d e l e c t r o n . The e l e c t r o n s p e c t r a would a l s o c o n t r i b u t e to some e x t e n t below 1300 nm 2 ( C H 3 C 0 C H 3 ) " + N 2 0 • No + p r o d u c t s ( 3 .43 ) TABLE XV E f f e c t of ace tone on the e l e c t r o n absorbance a t 1550 nm i n i r r a d i a t e d HMPA. [«.] A o R e l a t i v e I n i t i a l Absorbance (8 1 550 nm Pe rcen tage Decrease i n A 0 From Pure HMPA F i r s t Hal f-L i f e ( e " M p A ) ( y s ec ) 0 (3.2 ± 0.5) X 10 " 2 M (1.6 ± 0.2) X 1 0 " 1 M 0.708 ± 0.010 0.135 ± 0.005 0.031 + 0.003 0 81 % 96% 2.3- + 0.2 2.2 ± 0.2 - 1 . 5 ± 0.4 W A V E L E N G T H ( n m ) F i g u r e 111-35. S p e c t r a o b t a i n e d i n i r r a d i a t e d HMPA c o n t a i n ine (1.6 ± 0 . 2 ) X 1 0 - 1 M ace tone . i m m e d i a t e l y ( O ) and 2 ysec ( • ) a f t e r the Du l s e . - 250 -but the major i n c r e a s e i n the v i s i b l e and UV r e g i o n s i s a t t r i b u t e d to the p r oduc t of r e a c t i o n ( 3 . 1 0 7 ) , perhaps to the acetone a n i o n , (CHgCOCHg) - . V i d y a t h i 1 3 7 has ob se r ved an a b s o r p t i o n due to t h i s an ion o r i t s p r o t o n a t e d r a d i c a l i n w a t e r , and has r e p o r t e d t h a t i t s a b s o r p t i o n maximum o c c u r s below 250 nm. I t s h o u l d be noted t h a t the e l e c t r o n s p e c t r a p e r s i s t e d 2 psec a f t e r the p u l s e . The s i g n i f i c a n c e o f t h i s f a c t i s more e v i d e n t from the. da ta i n Tab le XV. The t a b l e shows t h a t w h i l e the i n i t i a l e l e c t r o n abso rbance was g r e a t l y reduced i n the p re sence o f a c e t o n e , the e l e c t r o n decay f o l l o w i n g the p u l s e was e s s e n t i a l l y u n a f f e c t e d . These f a c t s i m p l y t h a t ace tone does not i n f a c t scavenge s o l v a t e d e l e c t r o n s i n HMPA but r a t h e r a t t a c k s the p r e c u r s o r of t h a t s p e c i e s , e~ ( 3 . 1 0 8 ) . HMPA / V W V V • + p r o d u c t s (3 .14) e s (3 .23) + CH3COCH3 — • (CH3COCH3)- ( 3 1 0 8 ) Th i s p r o p o s i t i o n i s s uppo r t ed by e v i d e n c e found b y : i n v e s t i g a t i n g the s o l v a t e d e l e c t r o n f o r m a t i o n d u r i n g the r a d i a t i o n p u l s e i n both the p re sence and absence o f a c e t o n e . F i g u r e 111-36 shows the b u i l d - u p s observed at 1550 nm. Note t h a t i n both pure HMPA -2 and i n the p re sence o f (3.2 ± 0.5) X 10 M acetone the e l e c t r o n 251 -i d U < CD-ca O CO cn < 2 0 0 4 0 0 t ( n s e c ) 6 0 0 8 0 0 F i g u r e I I 1 -36 B u i l d - u p of e'M„PA abso rbance at 1 550 nm i n i r r a d i a t e d samples of HMPA ( O ) and HMPA c o n t a i n i n g (3,2 ± 0.5) X in" 2M acetone ( • ) . - . • 252 -absorbance b u i l d - u p was l i n e a r , w i t h d_A = (1.1 ± 0.1) X 10 -_ 1 c _i dt sec and (2.1 ± 0 . 1 ) X 10 sec r e s p e c t i v e l y . That i s , i n the p re sence o f (3.2 ± 0.5) X 10 " M ace tone the r a t e o f s o l v a t e d e l e c t r o n f o r m a t i o n was o n e - f i f t h t h a t i n pure HMPA. S ince. such a dec rea sed r a t e would i t s e l f a ccoun t f o r the 80% drop i n e l e c t r o n c o n c e n t r a t i o n at p u l s e end between the s amp le s , r e a c t i o n between acetone and e ^ p ^ i s not i n d i c a t e d . I n s t e a d , acetone appears to scavenge f o u r - f i f t h s o f the s o l v a t e d e l e c t r o n p r e c u r s o r s i n t h a t c a s e . C o n t r a s t these r e s u l t s w i t h ' t h o s e f rom a sample c o n t a i n i n g a f i f t y - f o l d l ower c o n c e n t r a t i o n o f N 2 0 .((7.9 + 0.2) X 10 " 4 M) i n HMPA. In t h a t case a l s o , the e l e c t r o n abso rbance was s i m i l a r l y reduced a t t he p u l s e end (by 87%) but d u r i n g the p u l s e the e l e c t r o n c o n c e n t r a t i o n had q u i c k l y reached a c o n s t a n t ( s t e a d y - s t a t e ) v a l u e ! C l e a r l y d i f f e r e n t p r o c e s s e s were o p e r a t i v e f o r . t h e two s c a v e n g e r s . The p re sence o f (7.9 ± 0 . 2 ) X 1 0 " 4 M R0 i n a s o l u t i o n o f (3.2 ± 0.5) X 10 " 2 M. ace tone i n HMPA r e s u l t e d i n the near t o t a l removal of a l l a b s o r p t i o n s a t 1000 nm. T h i s o b s e r v a t i o n c o n f i r m s the e a r l i e r c o n j e c t u r e t h a t R O r e a c t e d w i t h the a n i o n i c acetone s p e c i e s i n HMPA ( 3 . 4 3 ) . ( C H 3 C 0 C H 3 ) " + R O 3 , 4 3 • p r o d u c t s 4 3 ^ Based on a crude " s t e a d y - s t a t e " c a l c u l a t i o n , the p r e s e n t e xpe r imen t puts a. Tower l i m i t f o r the r a t e c o n s t a n t o f k.g 4 3 > 9 _ ] _ i • . " 10 M sec • f o r the r e a c t i o n between R O and Ac . - 253 -i i ) P o s i t i v e Ion Scavengers i i . 1) Water and Methanol As might be e x p e c t e d , even l a r g e c o n c e n t r a t i o n s o f wa te r o r methanol had l i t t l e e f f e c t on the s o l v a t e d e l e c t r o n a b s o r p t i o n i n HMPA. These mo l e cu l e s are known to scavenge p o s i t i v e i on s th rough p r o ton t r a n s f e r r e a c t i o n s . An upper l i m i t f o r the r a t e c o n s t a n t f o r b i m o l e c u l a r c o m b i n a t i o n w i t h 5 - 1 - 1 e HMPA ' k < 10 M sec was e s t a b l i s h e d from s o l u t i o n s c o n t a i n i n g up to 4M wate r and up to 2M m e t h a n o l . Most l i k e l y , t h a t l i m i t i s governed by i m p u r i t y l e v e l s i n t ho se s o l u t e s . i i . 2) Bromide Ions i i . Z i ) Y i e l d o f O x i d i z i n g S p e c i e s I r r a d i a t i o n of HMPA samples c o n t a i n i n g bromide i on s r e s u l t e d i n an i n c r e a s e i n both the y i e l d and l i f e t i m e o f s o l v a t e d e l e c t r o n s . For examp le , from 0.14M L i B r or 0.17M NaBr, the e l e c t r o n y i e l d was 35% g r e a t e r than from pure HMPA a l o n e . In a d d i t i o n , a new t r a n s i e n t a b s o r p t i o n w i t h X ,„ ^ r max 360 nm was obse rved on i r r a d i a t i o n o f the bromide s o l u t i o n s . The t r a n s i e n t s p e c t r a can be seen i n F i g u r e -1.11-37. Th i s new v i s i b l e band was s i m i l a r to t h a t r e p o r t e d by G ro s swe i ne r and M a t h e s o n 2 0 9 f rom p h o t o l y z e d aqueous bromide s o l u t i o n s . . They. a t t r i b u t e d the a b s o r p t i o n to the d i b r o m i d e 0 . 6 W A V E L E N G T H ( n m ) i g u r e 111-37. S p e c t r a o b t a i n e d i n i r r a d i a t e d HMPA c o n t a i n i n g (2.0 ± 0.2) X 1 0 " 1 M L i B r ( o ) compared to pure HMPA ( )•. A l s o shown a re the l o n g - l i v e d t r a n s i e n t ob se r ved on the a d d i t i o n of (1.3 ± 0.1) X 10 " 'M'N ,0 ( • ) and the s h o r t - l i v p d eHMPA c o n t r i b u t i o n from t h a t s o l u t i o n ( • ) . • c ' - 255 -a n i o n , B r " . S u b s e q u e n t l y , t h a t s p e c i e s has been ob se r ved and i d e n t i f i e d i n , a number of p u l s e r a d i o l y s i s s t u d i e s ? 1 1 Many i n v e s t i g a t o r s c o n c l u d e d t h a t B r " i on s a r i s e f rom the s caveng i ng o f p o s i t i v e i on s by B r " ( 3 . 109 , 3 .110 ) . B r " + S + • Br + S (3 .109) Br + B r " • * B r l , „ 1 1 A . 2 (3 .110) Such p roce s se s were proposed to account f o r an obse rved i n c r e a s e i n s o l v a t e d e l e c t r o n y i e l d from i r r a d i a t e d s o l u t i o n s o f B r " i on s i n e t h a n o l and m e t h a n o l ? 1 2 That i s , t hey sugges t t h a t p o s i t i v e i on s caveng ing leads to a dec r ea se i n n e u t r a l i z a t i o n p roce s se s ( 3 . 1 1 1 ) . [ R O H + , e " + B r " • [ R O H , B r , e~] (3.111) The observed i n c r e a s e i n e ^ M p f t y i e l d and l i f e t i m e i n HMPA c o n t a i n i n g l a r g e c o n c e n t r a t i o n s of B r " has two i m p o r t a n t c on se -quences on t h a t s y s tem. F i r s t l y , i t s u p p o r t s the p o s t u l a t e the n e u t r a l i z a t i o n w i t h p o s i t i v e i on s c o n t r i b u t e s to the complex e l e c t r o n decay mechanism. S e c o n d l y , the r e s u l t s i n d i c a t e some i n v o l v e m e n t of geminate i on s c a veng i n g i n HMPA. I f l a r g e c o n c e n t r a t i o n s o f B r " i on s can scavenge p o s i t i v e geminate i on s i n i r r a d i a t e d HMPA, then t h e y would be e x p e c t e d to have scavenged a l l f r e e p o s i t i v e i on s i n the p r o c e s s . S i n c e such r e a c t i o n l e ad s to an o b s e r v a b l e p r o d u c t , B r ^ , i t s hou l d be - 256 -p o s s i b l e to measure the p o s i t i v e f r e e i o n y i e l d . T h i s would g i ve ano the r i ndependent measure o f the s o l v a t e d e l e c t r o n y i e l d i f one assumes a o n e - t o - o n e co r r e spondence between tho se s p e c i e s . From the measured d o s i m e t r y and the abso rbance a t 360 nm f o r a 0.14M L i B r s o l u t i o n i n HMPA, one c a l c u l a t e s G ( B r 2 ) £ 2 = a i l l m a x (2.1 ± 0.1) X 10^ M~ c m " 1 m o l e c u l e (100 e v ) " 1 . A prob lem a r i s e s as t o whether or not the abso rbance a t 360 nm from t h i s s o l u t i o n a r i s e s s o l e l y f rom B r 2 a n i o n s . In pure HMPA, s t u d i e s i n d i c a t e d t h a t e ^ M p ^ and a t l e a s t one o t h e r s p e c i e s , S, (which may be the p o s i t i v e i o n or i t s d e r i v a t i v e ) a l s o absorb a t 360 nm. Assuming t h a t both s p e c i e s c o n t r i b u t e i n the bromide s o l u t i o n and based on e x t r a p o l a t i o n and compar i son w i t h the e l e c t r o n absorbance a t 1000 nm one f i n d s t h a t the 4 c o n t r i b u t i o n from B r 2 i s reduced to (1.2 ± 0.1) X 10 . I f o n l y one o f the o t h e r two s p e c i e s c o n t r i b u t e s to the abso rbance a t 360 nm, t h i s v a l ue becomes (1 .5 ± 0.1) X 1 0 4 o r (1.7 ± 0.1) X 4 10 f o r the e l e c t r o n and UV s p e c i e s r e s p e c t i v e l y . U n f o r t u n a t e l y , the Br^ s p e c i e s i t s e l f c o m p l e t e l y masked the r e g i o n o ve r wh ich the UV s p e c i e s was o b s e r v a b l e i n t ho se e x p e r i m e n t s . Thus i t was i m p o s s i b l e t o f u r t h e r d e f i n e i t s c o n t r i b u t i o n to the abso rbance by d i r e c t means. The s o l v a t e d e l e c t r o n and d i b r o m i d e an i on decays o c c u r r e d o ve r comparab le t i m e s c a l e s , so t h a t t h e i r r e l a t i v e components c o u l d not be - 3 r e s o l v e d d i r e c t l y e i t h e r . N i t r o u s o x i d e at (1 .3 ± 0.1) X 10 M c o n c e n t r a t i o n was i n t r o d u c e d i n t o a sample c o n t a i n i n g (2 .0 ± 0.2) X 10~ 1 M L i B r . F i g u r e 111-38 d e p i c t s a s e r i e s of n o r m a l i z e d o s c i l l o s c o p e t r a c e s ob se rved at v a r i o u s wave leng th s between 360 - . 2 5 7 -I 2 0 % Transmittance 1000 nm li \ 750 nm 450 nm V 400 nm 360 nm o 2 5 t ( p s e c ) 5 0 F i g u r e 111-38. No rma l i z ed o s c i l l o s c o p e t r a c e s l e n g t h s from the (2 .0 ± 0.2) X c o n t r i b u t i o n s o f e~u 1 0" . . ... i rn and B? i r r a d i a t i o n of a s o l u t i o n L i B r and (1.3 + 0.1 ) X V showing the r e l a t i v e at v a r i o u s wave-o f HMPA c o n t a i n i n g 1 0"*M N 20. - 258 -and 1000 nm. Under t he se c o n d i t i o n s , the e l e c t r o n a b s o r p t i o n was g r e a t l y reduced and ve r y s h o r t l i v e d , w h i l e t h a t of t he B r " i o n was n e a r l y u n a f f e c t e d . Thus the e f f e c t of N^O a d d i t i o n was to f a c i l i t a t e the r e s o l u t i o n o f the two a b s o r b i n g s p e c i e s over t h i s wave l eng th r e g i o n . S h o r t e n i n g the e l e c t r o n l i f e t i m e i n t h i s way had i t s p r i c e however, f o r the i n i t i a l e l e c t r o n y i e l d was reduced to a s t e a d y - s t a t e v a l u e o n l y o n e - s i x t h t h a t found i n the o r i g i n a l , making a c c u r a t e r e s o l u t i o n d i f f i c u l t . The most s t r i k i n g and obv i ou s c o n c l u s i o n t h a t one reache s f rom the se da ta i s t h a t the e l e c t r o n does not c o n t r i b u t e to the abso rbance below .400 nm and t h e r e f o r e cannot i n t e r f e r e w i t h the B r " a b s o r p t i o n a t 360 nm. However, as can be seen i n F i g u r e 111 - 3 7 the p re sence o f N^O r e s u l t e d i n a d e c r e a s e i n the Br^ maximum at 360 nm. These r e s u l t s a re not c o m p a t i b l e . T h e r e f o r e , u n f o r t u n a t e l y , the huge range i n the G - gva l ue f o r Br^ f rom (1.2 ± 0.1 ) X 1 0 4 to (2.1 ± 0.1) X 1 0 4 c o u l d not be d e f i n i t i v e l y nar rowed down. A second major c o n t r o v e r s y which adversely a f f e c t s the u n c e r t a i n t y of a Brg y i e l d c a l c u l a t i o n i s t h a t r e g a r d i n g the magnitude o f the maximum molar a b s o r p t i v i t y o f the d i b r o m i d e a n i o n , p- 2. Va lue s have been r e p o r t e d i n the 1 i t e r a t u r e 2 1 1 ' 2 1 3 * 2 1 * 360 , , f o r t h i s c o n s t a n t r a n g i n g f rom 8200 to 12000 M cm i n aqueous s o l u t i o n . Thus, the p r e s e n t s tudy g i v e s a v a l u e f o r the y i e l d of d i b r o m i d e i on s from 0.14M L i B r i n HMPA, G^Br^) = 1.8 ± 0 .8 . Th i s v a l ue which s hou ld be e q u i v a l e n t to the p o s i t i v e i on y i e l d i s much lower than the c o r r e s p o n d i n g e l e c t r o n y i e l d , G ( e ^ M P f l ) = 3.1 ± 0 .4 , i n the same s o l u t i o n . C e r t a i n l y i t i s - 259 -s i g n i f i c a n t l y l a r g e r than the i n c r e a s e i n the e l e c t r o n y i e l d caused by the p re sence of B r " . I t i s p o s s i b l e t h a t the o x i d i z i n g s p e c i e s i n HMPA have r a p i d a l t e r n a t i v e decay paths and thus are not a l l scavenged by 0.14M Br~ i o n s . For example , f rom a s o l u t i o n c o n t a i n i n g (6 ± 2) X 10 M L i B r , the B r 2 y i e l d was o n l y 3 or 4 p e r c e n t t h a t of the 0.14M s o l u t i o n . From (1.7 ± 0.3) X 10~ 2 M L i B r , the y i e l d o f Br^ was about h a l f t h a t o f t he more c o n c e n t r a t e d s o l u t i o n , but a l l the absorbance g r e w - i n d u r i n g the r a d i a t i o n pu l s e and was c e r t a i n l y comp le te i n a m i c r o s e c o n d . These f i n d i n g s suppo r t the i d ea t h a t the p o s i t i v e i on s have a l t e r n a t e f a t e s wh ich r ende r s them r e l a t i v e l y u n r e a c t i v e . T h i s p o s s i b i l i t y c o u l d e x p l a i n why the a r o m a t i c an i on s Py~ and An " were so l ong l i v e d i n HMPA. Indeed - - such a mechanism might w e l l be the major c o n t r i b u t i n g f a c t o r r e s p o n s i b l e f o r the l a r g e y i e l d of s t a b l e s o l v a t e d e l e c t r o n s found i n i r r a d i a t e d HMPA. i i . 2 . i i.) K i n e t i c C o n s i d e r a t i o n s As m e n t i o n e d , i n the p re sence of 0.14M L i B r , the s o l v a t e d e l e c t r o n decay was d i m i n i s h e d . The f i r s t h a l f - l i f e i n c r e a s e d from 3.5 ysec to 5.0 y s e c . Tab le XVI g i v e s the r e s u l t s o b t a i n e d from f i r s t p l u s second o r d e r r e g r e s s i o n a n a l y s e s of the e l e c t r o n decay under the two c o n d i t i o n s . As one would e x p e c t , bromide i on s had no e f f e c t on the appa ren t f i r s t o r d e r p o r t i o n of the e l e c t r o n decay. The c a l c u l a t e d second o r d e r component of i t s decay however was h a l v e d . Th i s i s a t t r i b u t e d to the - 260 -removal by bromide ions o f p o s i t i v e i on s t h a t would o t h e r w i s e n e u t r a l i z e s o l v a t e d e l e c t r o n s . TABLE XVI R e s u l t s o f f i r s t p l u s second o r d e r r e g r e s s i o n . a n a l y s e s f o r the e l e c t r o n decay at 1000 nm i n pure HMPA and the same sample c o n t a i n i n g 0.14M L i B r . Sample k ' - - C a l c u l a t e d HMPA F i r s t Order Component o f e~ decay ( s e c - 1 ) k - - C a l c u l a t e d HMPA Second Order Component o f e~ decay ( M " 1 s e c " 1 ) pure HMPA 0.14M L i B r (1.2 ± 0.1) X 1 0 5 . (1 .1 ± 0.1) X 1 0 5 (2 .0 ± 0.1) X l O 1 0 (8.6 ± 0.1) X 1 0 9 The d i b r o m i d e an ion decay proved to be much s i m p l e r than t h a t of the e l e c t r o n . In F i g u r e 111- 3 9 t h e B r^ decay a t 360 nm f rom the same 0.14M L i B r s o l u t i o n (open c i r c l e s ) was found t o . conform to a second o r d e r p l o t . From the s l o p e , one o b t a i n s a second o r d e r r a t e c o n s t a n t , k R - = (8.6 ± 0 . 1 ) X 1 0 5 c B r 2 M " 1 2 36 0 s e c " 1 . Tak i n g g 3 6 0 = (1 .0 ± 0.2) X 1 0 4 M " 1 c m " 1 one c a l c u l a t e s B r " a second o r d e r r a t e c o n s t a n t f o r the Br^ decay , k R r - = (8 .5 ± 1 . 5 ) X 1 0 9 M " 1 s e c " 1 - 261 -t ( | j s e c ) 0 5 1 0 1 5 1 : — i r r -R i _ J i _ i l p 0 5 0 1 0 0 1 5 0 . t ( p s e c ) F iQure T i r - 3 9 . Second o r d e r k i n e t i c p l o t s f o r the decay o f the BrJ a b s o r p t i o n c t 360 nm i n i r r a d i a t e d HMPA c o n t a i n i n g (0.17 ±0 .03 ) M L i B r . Decay was examined ove r both s h o r t ( O ) and l o n g t ime s c a l e s ( • ) . - 262 -Comparing t h i s . v a l u e w i t h t h a t o b t a i n e d f o r the second o r d e r component of the e l e c t r o n decay , k^- = (8.6 ± 0.1) X 1 0 9 M " 1 s e c - 1 i t s ugges t s t h a t t he se s p e c i e s i n f a c t r e a c t t o g e t h e r ( 3 . 1 1 1 ) . k - - 3 111 B r 2 + eHMPA : * 2 B r (3 .111) Such a r e a c t i o n has been proposed to o c cu r i n aqueous s o l u t i o n 2 1 1 0 a v a l ue f o r the b i mol e c u l ar . rate c o n s t a n t k = (1.3 ± 0.5) X 1 0 1 0 M" 1 s e c - 1 be ing r e p o r t e d . I t s hou ld be remembered t h a t about h a l f o f the e l e c t r o n a b s o r p t i o n decay o c c u r s s i m u t a n e o u s l y v i a f i r s t o r d e r p r o c e s s e s . T h e r e f o r e , i f r e a c t i o n (3 .111) i s the dominant decay p r oce s s f o r d i b r o m i d e i o n s , about h a l f the Br^ a b s o r p t i o n would remain when the e l e c t r o n a b s o r p t i o n had d i s a p p e a r e d . T h i s was found to be so f o r the 0.14M L i B r s o l u t i o n . • From a s o l u t i o n c o n t a i n i n g (2.0 ± 0.2) X 1 0 _ 1 M L i B r , the Br^ decay was mon i t o r ed over a t ime p e r i o d of 150 y sec by u s i n g the a n a l y z i n g lamp i n i t s s t eady DC mode. T h i s a l l o w e d i n v e s -t i g a t i o n of the Br^ decay a f t e r e ^ p ^ had c o m p l e t e l y decayed . As can be seen i n the p l o t . o f F i g u r e 111 - 3 9 , the s low Br^ decay ( f i l l e d c i r c l e s ) a l s o f o l l o w e d second o r d e r k i n e t i c s w i t h an 9 -1 -1 appa ren t b i m o l e c u l a r r a t e c o n s t a n t k = ( 4 . 0 + 1.2) X 10 M cm In aqueous s o l u t i o n a s i m p l e b i m o l e c u l a r c o m b i n a t i o n r e a c t i o n (3.112) has been proposed w i t h k 3 ] -| 2 o n t h e o r d e r o f (2 ' t o '3) X - 263 -sec k 3.112 B r 0 + Br * B r 0 + Br (3 .112) The t r i b r o m i d e an ion i s r e p o r t e d to absorb below 300 nm? 2 11 a an i n a c c e s s i b l e r e g i o n i n the p r e s e n t s t udy of HMPA because o f s t r o n g s o l v e n t a b s o r p t i o n . The removal o f s o l v a t e d e l e c t r o n s t h rough the a d d i t i o n of R O would be expec ted to s u r p r e s s r e a c t i o n (3 .111) and t h e r e b y ex tend the l i f e t i m e o f B r ^ i o n s - - p a r t i c u l a r l y over the f i r s t 10 - 20 y s e c . However, the f i r s t h a l f - l i f e p e r i o d ' o f 18 y sec f o r 0.14M L i B r i n HMPA was reduced to 14 ysec and 13 ysec i n the p re sence o f R O . a t (1.3 + 0.1) X 1 0 " 3 M and (1.3 + 0.1) X 10 M r e s p e c t i v e l y . Wh i le i t i s p o s s i b l e t h a t some i m p u r i t y c o u l d have been i n t r o d u c e d a l o n g w i t h the R O ' , t he se r e s u l t s would tend to suggest t h a t Br^ r e a c t s w i t h . R O " or some o t h e r s p e c i e s formed from i t ( R O ^ , 0 " , 0^* e t c . ) . Such r e a c t i o n might w e l l s u r p r e s s secondary i o n i c r e a c t i o n s of R O t o such an e x t e n t so as to account f o r the o n l y m a r g i n a l i n c r e a s e i n n i t r o g e n y i e l d from n i t r o u s o x i d e s o l u t i o n s of HMPA i n the p re sence o f bromide i o n s . Thus , w h i l e the s o l v a t e d e l e c t r o n y i e l d • i n c r e a s e d up to 35% th rough geminate p o s i t i v e i on s c a veng i n g by B r " t he i n c r e a s e was not n e c e s s a r i l y r e f l e c t e d i n the n i t r o g e n y i e l d from N o 0 . 2 - 264 -i i . 2 . i i i ) Na" Spectrum For s o l u t i o n s c o n t a i n i n g Na-Br, the spect rum was s t u d i e d c a r e f u l l y over the wave l eng th r e g i o n 700 to 1000 nm. In both sodium meta l s o l u t i o n s t u d i e s and o t h e r p u l s e r a d i o l y s i s i n v e s t i g a t i o n s , a s t r o n g a b s o r p t i o n a t t r i b u t e d to Na has been r e p o r t e d . The end of p u l s e spect rum from 0.17M NaBr i n HMPA was e s s e n t i a l l y i d e n t i c a l to t h a t f rom pure HMPA. However, some 30 p sec a f t e r the r a d i a t i o n p u l s e when ^ 97% of the s o l v a t e d e l e c t r o n s had decayed , t h e r e was some e v i d e n c e of an a b s o r p t i o n band w i t h a maximum i n the r e g i o n of 750 nm. The a b s o r p t i o n was ve ry .weak and p r o b a b l y amounted to l e s s than 5% of the i n i t i a l e l e c t r o n a b s o r p t i o n i n t h i s r e g i o n . C e r t a i n l y the da ta does not i n d i c a t e the p re sence o f a s i g n i f i c a n t a b s o r p t i o n due to some a n i o n i c sodiurn s p e c i e s . R a t h e r , t h i s sma l l c o n t r i b u t i o n i s p r o b a b l y a s s o c i a t e d w i t h the Br~^ a n i o n . Rabarn has r e p o r t e d a second a b s o r p t i o n a t 700 nm from aqueous L i B r s o l u t i o n s and a t t r i b u t e d i t . to B r g . I ndeed, a c l o s e r l ook at the Br^ spect rum from (2.0 + 0.2) X 10 " 1 M L i B r s o l u t i o n i n HMPA where e ' ^ p ^ had been removed by NgO ( F i g u r e 111-3 7) r e v e a l s the p re sence of such a peak i n the p r e s e n t work. E. Summary Th i s i n v e s t i g a t i o n has p r o v i d e d a w e a l t h of i n f o r m a t i o n about the r a t h e r r emarkab l e phenomenon o f the s o l v a t e d e l e c t r o n i n HMPA. The expe r imen t s were de s i gned i n such a way so as - 265 -t o . t h o r o u g h l y probe t h a t c u r i o u s and complex system th rough the use of s e v e r a l i n v e s t i g a t i v e t e c h n i q u e s . As p r e d i c t e d , s o l v a t e d e l e c t r o n s proved to be s u f f i c i e n t l y l o n g - l i v e d i n HMPA so as to be p o s i t i v e l y i d e n t i f i e d from t h e i r s p e c t r a , b e h a v i o r and r e a c t i v i t y w i t h a ho s t o f s o l u t e s . The r a d i o l y s i s y i e 1 d , G ( e ~ M p A ) = 2.2 + 0.2 e s t a b l i s h e d from two q u i t e d i f f e r e n t r a d i o l y s i s t e c h n i q u e s ; the na t u r e and o p t i c a l p r o p e r t i e s of the a b s o r p t i o n s p e c t r u m ; and the d i v e r s i t y o f the k i n e t i c b e h a v i o r a t t r i b u t e d to the s o l v a t e d e l e c t r o n i n HMPA are c o m p l e t e l y c o m p a t i b l e w i t h the e x t r a o r d i n a r y p r o p e r t i e s of t h a t s o l v e n t . There i s ample e v i d e n c e to s uppo r t the s u p p o s i -t i o n t h a t s o l v a t e d e l e c t r o n s i n HMPA do not undergo f a s t geminate decay and t h e r e f o r e the ob se rved y i e l d does i n f a c t r e p r e s e n t the f r e e - i o n y i e l d . Taken i n t o t o , the r e s u l t s s t r o n g l y s uppo r t the i m p o r t a n t c o n c l u s i o n t h a t the h i gh n i t r o g e n y i e l d s ob se rved from s t e a d y -s t a t e s t u d i e s i n v o l v i n g the use o f n i t r o u s o x i d e as e l e c t r o n scavenger a r i s e from seconda ry i o n i c p r o c e s s e s . Perhaps as a consequence of some of the unusual p r o p e r t i e s o f HMPA, a number of such unique f i n d i n g s ( p a r t i c u l a r l y i n v o l v i n g added s o l u t e s ) were o b t a i n e d . I t may w e l l be t h a t some o f the i n n o v a t i v e approaches taken i n t h i s work may be a p p l i c a b l e to o t h e r s o l v e n t systems where a m b i g u i t i e s e x i s t . 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