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UBC Theses and Dissertations

Studies in radiation chemistry Shaede, Eric Albert 1971

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STUDIES IN RADIATION CHEMISTRY by ERIC ALBERT SHAEDE B.Sc., University of B r i t i s h Columbia, 1966 M.Sc, University of B r i t i s h Columbia, 1968 M.C.I.C., Chemical I n s t i t u t e of Canada, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Chemistry We accept t h i s thesis as conforming to the required standard The University of B r i t i s h Columbia A p r i l 1971 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or p u b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Chemistry The University of B r i t i s h Columbia Vancouver 8, Canada Date: A p r i l 7, 1971 ( i i ) ABSTRACT The e x p e r i m e n t a l w o r k p r e s e n t e d i n t h i s d i s s e r t a t i o n c o n s i s t s o f two s e p a r a t e p a r t s . F i r s t l y , a s t u d y o f t h e r e a c t i o n o f h y d r a t e d e l e c t r o n s w i t h m o l e c u l a r n i t r o g e n i s r e p o r t e d . S e c o n d l y , t h e r e s u l t s o f a n i n v e s t i g a t i o n o f t h e e f f e c t s o f ^ - r a d i a t i o n o n t h e p o l a r a p r o t i c s o l v e n t , p r o -p y l e n e c a r b o n a t e ; (a) i n t h e g l a s s y s o l i d s t a t e a t 77 °K, a n d (b) as a l i q u i d a t r o o m t e m p e r a t u r e , a r e p r e s e n t e d . H y d r a t e d e l e c t r o n s w e r e g e n e r a t e d b y < y ~ r a d i o l y s i s o f a q u e o u s s o l u t i o n s c o n t a i n i n g H^  and 0H~ a n d a l s o c o n t a i n i n g a t c o n c e n t r a t i o n s up t o 0.1 M (200 a t m p r e s s u r e ) . S i g n i f -i c a n t y i e l d s o f ammonia w e r e o b t a i n e d , b u t b y c o m p l e t e l y e l i m i n a t i n g t h e g a s s p a c e a b o v e t h e s o l u t i o n i t was shown t h a t t h e m a j o r i t y o f t h e NH^  a r o s e t h r o u g h " d i r e c t a c t i o n " o f t h e r a d i a t i o n o n d i s s o l v e d N^. A l t h o u g h t h e h y d r a t e d e l e c t r o n i s one o f t h e m o s t p o w e r f u l a n d r e a c t i v e r e d u c i n g a g e n t s , i t i s u n a b l e t o c a u s e r e d u c t i o n f i x a t i o n o f m o l e c u l a r n i t r o g e n . A n u p p e r l i m i t o f 18 M~"^ s~"'" was e s t i m a t e d f o r t h e r a t e c o n s t a n t o f t h e r e d u c t i o n r e a c t i o n . C o n v e r s i o n o f t h e h y d r a t e d e l e c t r o n s t o H atoms i n a c i d s o l u t i o n d i d n o t a f f e c t t h e ammonia y i e l d , i m p l y i n g t h a t H atoms a r e a l s o u n a b l e t o r e d u c e n i t r o g e n . I n t h e g l a s s y s o l i d s t a t e a t 77 °K, ^ - i r r a d i a t i o n o f p r o p y l e n e c a r b o n a t e p r o d u c e d s p e c i e s i d e n t i f i e d a s t r a p p e d e l e c t r o n s . T h e y w e r e c h a r a c t e r i s e d b y a n a r r o w (AH = 4.5 G ) , ( i i i ) Gaussian shaped ESR l i n e at g = 2 . 0 0 2 8 and an o p t i c a l absorp-t i o n band wi th X " 3 7 0 nm . The electrons were unstable max at 77 °K and decayed v i a a non-homogeneous process believed to be re a c t i o n with p o s i t i v e ions. Also formed by the *y - r a d i a t i o n were four u n i d e n t i f i e d trapped r a d i c a l s , a l l characterised by doublet ESR signals centered at g = 2 . 0 0 2 3 and with hyperfine s p l i t t i n g s of 4 2 , 5 8 , 8 3 , and 1 2 4 G . U l t r a v i o l e t photolysis of the i r r a d i a t e d glasses at 77 °K produced new r a d i c a l s i d e n t i f i e d as C O j , HCO and C H 3 . The CO^" r a d i c a l gave a singl e ESR l i n e at g ^ 2 . 0 1 5 and a broad v i s i b l e o p t i c a l absorption band with X ^ 6 0 0 nm . HCO was i d e n t i f i e d by i t s asymmetric doublet ESR s i g n a l with hyper-f i n e s p l i t t i n g of about 1 3 0 G and a m u l t i - l i n e v i b r o n i c absorption spectrum i n the 500 - 7 50 nm region. The methyl r a d i c a l s were unstable i n the matrix and were i d e n t i f i e d by t h e i r c h a r a c t e r i s t i c 1 : 3 : 3 : 1 quartet ESR spectrum with hyper-f i n e s p l i t t i n g of about 2 1 G . * y-radiolysis of l i q u i d propylene carbonate at 2 5 °C produced B.^ , CO and C 0 2 as the major gaseous "molecular" products with y i e l d s : G ^ = 0 . 7 5 ± 0 . 0 5 , G c 0 = 1 . 2 - 0 . 1 , and G Q Q 2 = 2 ~ 0 - 3 . Methane was also produced v i a a secondary process involving methyl r a d i c a l s with a y i e l d : G (CH ) = 0 . 2 0 - 0 . 0 2 . Scavenger experiments with N 2 0 , I , methanol and acid indicated that an anionic reducing species was formed by the r a d i a t i o n with a y i e l d of G - = 2 . 0 - 0 . 2 . This species was probably a solvated electron although the p o s s i b i l i t y of i t being a reactive molecular anion could not ~ (iv) be excluded on the basis of the steady state r a d i o l y s i s data. A transient o p t i c a l absorption at 630 nm was observed on pulse r a d i o l y s i s of propylene carbonate with 3 nsec pulses of 0.5 MeV electrons. However, e i t h e r solvated electrons or the C 0 3 ~ r a d i c a l ion could have been responsible for the absorbance. (v) TABLE OF CONTENTS CHAPTER PAGE I. INTRODUCTION TO RADIATION CHEMISTRY 1 A. Interaction of High Energy Radiation with matter 2 1. Electromagnetic r a d i a t i o n (X or ^ -rays) . . . 3 2. High energy electrons 6 B. Chemical Consequences of the Absorption of High Energy Radiation 10 1. Polar l i q u i d s 11 2. Frozen polar systems 15 3. Radiation chemical units and terms 17 C. S t a b i l i z e d Electrons 18 1. Electron s o l v a t i o n process 19 2. Electron trapping i n frozen systems 22 3. Properties of s t a b i l i z e d electrons 23 4. Yields of s t a b i l i z e d electrons 23 I I . AN ATTEMPT AT NITROGEN FIXATION UTILIZING HYDRATED ELECTRONS . . c 28 A. Introduction 28 1. Background to the problem 28 2. L i t e r a t u r e survey 29 3. The chemical system 31 B. Experimental 33 1. Reagents 33 2. Radiation source 34 3. Apparatus and techniques 34 (vi) I I . (CONTINUED) 4. A n a l y t i c a l p r o c e d u r e s 42 C. R e s u l t s 43 D. D i s c u s s i o n 4 3 E. S u g g e s t i o n s f o r F u r t h e r S t u d y 48 I I I . ASPECTS OF THE RADIATION CHEMISTRY OF PROPYLENE CARBONATE 50 PART I . R A D I O L Y S I S OF PROPYLENE CARBONATE I N THE S O L I D STATE 53 A. I n t r o d u c t i o n , 53 B. E x p e r i m e n t a l 55 1. R e a g e n t s 55 2. R a d i a t i o n s o u r c e 55 3. S a m p l e p r e p a r a t i o n a n d i r r a d i a t i o n 56 4. E l e c t r o n s p i n r e s o n a n c e m e a s u r e m e n t s . . . . 57 5. O p t i c a l a b s o r p t i o n m e a s u r e m e n t s 60 6. L i g h t s o u r c e s f o r p h o t o b l e a c h i n g e x p e r i m e n t s 6 1 C. R e s u l t s a n d D i s c u s s i o n 62 1. The t r a p p e d e l e c t r o n 62 (a) E l e c t r o n s p i n r e s o n a n c e o b s e r v a t i o n s . . . 62 (b) O p t i c a l a b s o r p t i o n s p e c t r u m 82 (c) Summary 87 2. T r a p p e d r a d i c a l s i n i r r a d i a t e d PC 89 (a) R a d i c a l s f o r m e d d u r i n g r a d i o l y s i s a t 77 °K 89 (b) R a d i c a l s f o r m e d b y UV p h o t o l y s i s o f t h e i r r a d i a t e d PC g l a s s e s a t 77 °K 96 ( v i i ) CHAPTER PAGE I I I . (CONTINUED) PART I I . RADIOLYSIS OF PROPYLENE CARBONATE IN THE LIQUID PHASE 105 A. I n t r o d u c t i o n 105 B. Experimental 107 1. Reagents 107 2. R a d i a t i o n source 107 3. Apparatus and techniques 108 (a) Sample c e l l and sample p r e p a r a t i o n . . . . 108 (b) Gaseous product a n a l y s i s I l l (c) Vacuum techniques and determination of the s o l u b i l i t y of n i t r o u s oxide 114 C. R e s u l t s 118 1. Pure PC - gaseous r a d i o l y s i s products . . . . 118 2. Scavenger experiments 118 D. D i s c u s s i o n 123 PART I I I . GENERAL CONCLUSION AND SUGGESTIONS FOR FURTHER STUDY OF THE RADIATION CHEMISTRY OF PROPYLENE CARBONATE 133 BIBLIOGRAPHY . 136 APPENDIX 1. FERROUS SULFATE DOSIMETRY 141 A. Theory 141 B. Procedure 142 C. R e s u l t s 143 1. N i t r o g e n f i x a t i o n experiments 143 2. R a d i o l y s i s of l i q u i d PC 144 3. Dose c o r r e c t i o n procedure - computer program 145 ( v i i i ) CHAPTER PAGE APPENDIX 2. INDOPHENOL BLUE AMMONIA ANALYSIS 149 A. Theory 149 B. Experimental Procedure . 149 1. Reagents 149 2. A n a l y s i s procedure 151 C. R e s u l t s 153 APPENDIX 3. ELECTRON SPIN RESONANCE 156 A. B a s i c Theory 156 B. A p p l i c a t i o n of ESR to amorphous systems . . . . 159 APPENDIX 4. PURIFICATION AND ANALYSIS OF PROPYLENE CARBONATE 161 APPENDIX 5. GAS CHROMATOGRAPHIC ANALYSIS SYSTEM . . . 169 VITA ( i x ) LIST OF TABLES TABLE PAGE 1. Approximate time s c a l e f o r the r a d i o l y s i s of l i q u i d water 13-14 2. S e l e c t e d p r o p e r t i e s of s t a b i l i z e d e l e c t r o n s formed by r a d i o l y s i s 24 3. Summary of n i t r o g e n f i x a t i o n experiments 44 4. C h a r a c t e r i s t i c s of e l e c t r o n s trapped i n propylene carbonate g l a s s e s at 77 °K 88 5. Summary of second scavenger experimental r e s u l t s . 124 (x) LIST OF FIGURES 1. Scheme of r e a c t i o n s i n r a d i a t i o n chemistry . . . . 1 2. D i s t r i b u t i o n of spurs and primary events along the t r a c k of a f a s t e l e c t r o n i n a l i q u i d 10 3. Primary processes i n the a c t i o n of r a d i a t i o n on matter 12 4. Q u a l i t a t i v e r e p r e s e n t a t i o n of the p o t e n t i a l energy of an e l e c t r o n as a f u n c t i o n of time a f t e r l o c a l -i z a t i o n i n a l i q u i d 21 5. Y i e l d of r a d i o l y t i c a l l y generated f r e e ions (G£^) as a f u n c t i o n of the s t a t i c d i e l e c t r i c constant (D ) s of a l i q u i d 26 6. Photograph of the modified 50 ml a l l - g l a s s s y ringe and attachments 36 7. Photograph of the s t a i n l e s s s t e e l high pressure c e l l used to p r e s s u r i z e the syringe s 37 8. Schematic diagram of the high pressure system . . . 38 9. Diagram of the apparatus used to f i l l the syring e s w i t h gas and l i q u i d samples, shown w i t h a s y r i n g e attached 40 10. One of the stereoisomers of propylene carbonate . 50 11. V a r i a b l e temperature ESR dewar 58 12. ESR spectrum of ' y - i r r a d i a t e d g l a s s y PC immediately f o l l o w i n g the i r r a d i a t i o n using the lowest operable microwave power (about 0.5 mW)c (Dose ^ 0.8 Mrad) . 63 (xi) FIGURE PAGE 13. ESR spectrum of the same sample as F i g u r e 12 only at about 10 mW microwave power 64 14. High r e s o l u t i o n ESR scan of l i n e "A" of Figu r e 12. This l i n e i s a t t r i b u t e d to trapped e l e c t r o n s i n PC 65 15. High r e s o l u t i o n ESR scan of the trapped e l e c t r o n l i n e showing the n o r m a l i z a t i o n method of l i n e shape a n a l y s i s . •=Gaussian, x=Lorentzian . . . . 67 16. ESR spectrum of ^ - i r r a d i a t e d g l a s s y PC at 77 °K c o n t a i n i n g 0.02 M i o d i n e 70 17. ESR spectrum of y - i r r a d i a t e d g l a s s y PC a t 77 °K c o n t a i n i n g 0.1 M napthalene 71 18. Isothermal spontaneous decay of trapped e l e c t r o n s i n ^ - i r r a d i a t e d g l a s s y PC at 77 °K as fol l o w e d by ESR. (Data are a r b i t r a r i l y normalized at "0" time.) 73 19. F i r s t order k i n e t i c analyses of the trapped e l e c t r o n decay data from Figure 18 7 5 20. Second order k i n e t i c analyses of the trapped e l e c t r o n decay data from Figure 18 7 6 21. ESR spectrum of y - i r r a d i a t e d g l a s s y PC a f t e r v i s i b l e p h o t o l y s i s to remove most of the trapped e l e c t r o n s 81 22. O p t i c a l a b s o r p t i o n s p e c t r a of y ~ i r r a d i a t e d PC at 77 °K (sample p a r t i a l l y c r y s t a l l i n e ) i n a 1 cm c e l l . Curve 1 was obtained immediately a f t e r the i r r a d i a t i o n and curve 2 was obtained 24 hours l a t e r . (Dose~1.3 Mrad) 83 ( x i i ) FIGURE PAGE 23. A b s o r p t i o n spectrum a t t r i b u t e d to trapped e l e c t r o n s i n PC at 77 °K as c onstructed by s u b t r a c t i o n of curve 2 from curve 1 of F i g u r e 22 . 85 24. ESR spectrum of y - i r r a d i a t e d g l a s s y PC a t ~110 °K a f t e r thermal decay of the trapped e l e c t r o n s and o t h er r a d i c a l s 90 25. ESR spectrum of y - i r r a d i a t e d g l a s s y PC at ~110 °K a f t e r p a r t i a l UV p h o t o l y s i s of the r a d i c a l r e s p o n s i b l e f o r doublet "D" 92 26. ESR spectrum of y - i r r a d i a t e d g l a s s y PC at 77 °K a f t e r standing f o r 200 hours i n the dark 93 27. ESR spectrum of y - i r r a d i a t e d g l a s s y PC at 77 °K a f t e r b r i e f UV p h o t o l y s i s . Samples dark blue i n c o l o u r 97 28. A b s o r p t i o n spectrum i n the 500 - 7 50 nm r e g i o n f o r y - i r r a d i a t e d PC a f t e r b r i e f UV p h o t o l y s i s . Spectrum a t t r i b u t e d to a combination of the HCO and c 0 3 ~ a b s o r p t i o n bands 98 29. ESR spectrum of y - i r r a d i a t e d g l a s s y PC at 77 °K a f t e r 20 minutes of i n t e n s e UV p h o t o l y s i s . Samples v i r t u a l l y c o l o u r l e s s . Arrows i n d i c a t e methyl r a d i c a l , (compare w i t h F i g u r e 27) 101 30. ESR spectrum of y - i r r a d i a t e d g l a s s y PC at 77 °K a f t e r 50 minutes of i n t e n s e UV p h o t o l y s i s . Arrows i n d i c a t e methyl r a d i c a l q u a r t e t 102 ( x i i i ) FIGURE PAGE 31. ESR spectrum of UV photolysed, ^ - i r r a d i a t e d o PC samples at 77 K; about 4 hours a f t e r p h o t o l -y s i s . Showing the decay of the methyl r a d i c a l s (arrows). (compare w i t h F i g u r e 30) 104 32. Photograph of the sample c e l l used f o r the l i q u i d phase r a d i o l y s i s of PC 109 33. Schematic diagram of the vacuum system used t o add n i t r o u s oxide t o the PC samples i n the "bubbler" c e l l 115 34. Concentration of d i s s o l v e d n i t r o u s oxide i n PC at room temperature (20 - 25 °C) as a f u n c t i o n of the n i t r o u s oxide pressure i n the "bubbler" c e l l . 117 35. Y i e l d s of gaseous r a d i o l y s i s products from PC ^ - i r r a d i a t e d a t 25 °C, as a f u n c t i o n of dose . . 119 36. N i t r o g e n y i e l d as a f u n c t i o n of accumulated sample dose f o r a constant n i t r o u s oxide c o n c e n t r a t i o n (0.05 M) 120 37. G ( N 2 ^ a s a f u n c t i ° n of n i t r o u s oxide c o n c e n t r a t i o n 18 l at constant dose (about 5 x 10 eV g ) 122 38. Data from F i g u r e 37 p l o t t e d as 1/G(N,,) versus 1/ [N 20] 128 39. P l o t of 1/G(N2) versus [ l 2 ] / [ N 2 Q ] f o r [N 20] = 0.077 M 130 (xiv) FIGURE PAGE +3 A l - 1 . P l o t of the absorbance of Fe at 304 nm as a f u n c t i o n of F e + ^ c o n c e n t r a t i o n at 25 °C . . . . 146 A l - 2 . Ferrous s u l f a t e dosimetry r e s u l t s f o r the "bubbler" c e l l used i n the l i q u i d phase r a d i o l y s i s of PC 147 A2-1. A b s o r p t i o n spectrum of the indophenol blue dye i n a l k a l i n e aqueous s o l u t i o n 150 A2-2. T y p i c a l c a l i b r a t i o n graph f o r the indophenol b l u e ammonia a n a l y s i s procedure 154 A3-1. T h e o r e t i c a l ESR d e r i v a t i v e l i n e shapes f o r amorphous samples when the r a d i c a l i s c h a r a c t e r -i z e d by: (a) an a x i a l l y symmetric g-tensor and no h y p e r f i n e s t r u c t u r e , (b) an asymmetric g-tensor and no h y p e r f i n e s t r u c t u r e , and (c) a x i a l l y symmetric g- and A-tensors w i t h the same symmetry axes and a l a r g e h y p e r f i n e s p l i t t i n g of a s p i n % nucleus 160 A4-1. Storage - d i s p e n s i n g f l a s k used t o keep the p u r i f i e d PC under a helium atmosphere 164 A4-2. Mass spectrum of doubly d i s t i l l e d propylene carbonate (PC) 165 A4-3. U l t r a v i o l e t a b s o r p t i o n s p e c t r a of Eastman Kodak p r a c t i c a l grade PC and i t s s i n g l y and doubly d i s t i l l e d f r a c t i o n s 166 (xv) FIGURE PAGE A4-4. 60 MHz nuclear magnetic resonance spectrum of doubly d i s t i l l e d PC 167 A4-5. I n f r a r e d a b s o r p t i o n spectrum of doubly d i s t i l l e d PC 168 A5-1. Schematic diagram of the modified V a r i a n Aerograph gas chromatography system 170 A5-2. T y p i c a l chromatogram produced by the GC system shown i n Figu r e A5-1 f o r a h y p o t h e t i c a l sample c o n t a i n i n g N 2, CH 4, CO and C0 2 173 A5-3. Detector response t o N 2, CO, CH 4 and C0 2 f o r the GC system shown i n Figure A5-1 174 ( x v i ) ACKNOWLEDG EMENT S The a u t h o r w o u l d l i k e t o e x p r e s s h i s g r a t i t u d e t o D r . D. C. W a l k e r . He e n c o u r a g e d h i s s t u d e n t s t o p u r s u e t h e i r own r e s e a r c h i n t e r e s t s a n d was a l w a y s a v a i l a b l e f o r c o n s u l t a t i o n a n d a d v i c e . S p e c i a l t h a n k s a r e due t o M r . N. S. D a l a i f o r g u i d a n c e i n t h e o p e r a t i o n o f t h e ESR s p e c t r o m e t e r a n d h e l p f u l d i s c u s -s i o n s r e g a r d i n g i n t e r p r e t a t i o n o f t h e s p e c t r a . E n l i g h t e n i n g d i s c u s s i o n s w i t h D r . F . G. H e r r i n g i n c o n n e c t i o n w i t h t h e ESR d a t a a r e a l s o a c k n o w l e d g e d . I n a d d i t i o n , t h e a u t h o r w o u l d l i k e t o t h a n k t h e U.B.C. C h e m i s t r y D e p a r t m e n t m e c h a n i c a l s h o p s a n d t h e g l a s s b l o w e r s f o r t h e i r e x c e l l e n t c r a f t s m a n s h i p i n d e s i g n i n g a n d c o n s t r u c t i n g some o f t h e a p p a r a t u s u s e d i n t h i s r e s e a r c h . F i n a n c i a l s u p p o r t b y t h e C h e m i s t r y D e p a r t m e n t i n t h e f o r m o f t e a c h i n g a s s i s t a n t s h i p s i s g r a t e f u l l y a c k n o w l e d g e d , a s i s t h e r e c e i p t o f a N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a p o s t g r a d u a t e s c h o l a r s h i p . ( x v i i ) In l o v i n g memory of my f a t h e r , ERIC ALBERT KURT SHAEDE (1904 - 1969) and a very dear f r i e n d , WILLIAM LYLE MACKEN (1883 - 1967). CHAPTER I INTRODUCTION TO RADIATION CHEMISTRY "Fundamental s t u d i e s i n r a d i a t i o n chemistry aim to i d e n t i f y the v a r i o u s SPECIES formed i n p a r t i c u l a r systems and to understand the PHYSICAL PROCESSES by which they a r i s e . The AMOUNTS formed f o r a g i v e n dose of r a d i a t i o n are measured. Then the CHEMISTRY of the species i n t h e i r r e a c t i o n s w i t h each other and other compounds present i s s t u d i e d . This i n v o l v e s i n v e s t i g a t i o n s of r e a c t i o n k i n e t i c s and mechanisms, and of int e r m e d i a t e s through to the f i n a l , s t a b l e end products."^ Thus the g e n e r a l scheme of r a d i a t i o n chemical i n v e s t i g a t i o n s may be represented by Figu r e 1. Radiat ion _ CHEMICAL S Y S T E M — W W l ^ t V . n n ^ WHAT S P E C I E S ? HOW MANY? chemical react ions t STABLE PRODUCTS - INTERMEDIATE SPECIES F i g u r e 1. Scheme of r e a c t i o n s i n r a d i a t i o n chemistry. ( a f t e r Figure 1.1, O'Donnell and Sangster, reference 2, page 2) - 2 -This thesis i s concerned with several of the funda-mental aspects of r a d i a t i o n chemistry outlined above. Specif-i c a l l y , the chemical reaction of the hydrated electron, an important primary species i n the r a d i o l y s i s of water, with molecular nitrogen was studied. In addition an i n v e s t i g a t i o n of the primary species formed by ^°Co ^ - r a d i a t i o n i n a polar organic l i q u i d (propylene carbonate) was conducted both d i r -e c t l y , i n the s o l i d state, by electron spin resonance and o p t i c a l spectroscopy; and i n d i r e c t l y , i n the l i q u i d phase, by competition k i n e t i c studies involving scavengers and analysis of the f i n a l stable products of the r a d i o l y s i s . Understanding r a d i a t i o n chemical phenomena requires a basic knowledge of the processes by which r a d i a t i o n i n t e r a c t s with matter since the chemical e f f e c t s are a d i r e c t consequence of the absorption of energy from the r a d i a t i o n . A. INTERACTION OF HIGH ENERGY RADIATION WITH MATTER Since the majority of investigations i n r a d i a t i o n chem-i s t r y involve the use of high energy photons or electrons as the r a d i a t i o n source, the present discussion w i l l be l i m i t e d to the in t e r a c t i o n s of these radiations with matter. In gen-e r a l the physics and chemistry of the interactions of other types of r a d i a t i o n , namely neutrons and charged p a r t i c l e s such as protons, alpha p a r t i c l e s and f i s s i o n fragments, are subst-a n t i a l l y d i f f e r e n t to that of electrons (and photons) and a complete discussion of a l l of the interactions i s beyond the scope of t h i s t h e s i s . -3-1. Electromagnetic R a d i a t i o n (X or ^ - r a y s ) Electromagnetic r a d i a t i o n i s absorbed by matter v i a f o u r p r i n c i p a l processes: (a) the p h o t o e l e c t r i c e f f e c t , (b) the Compton process, (c) p a i r p r o d u c t i o n , and (d)photo-n u c l e a r r e a c t i o n s . The importance of each of the f o u r r e a c t -i o n s depends p r i m a r i l y i n the energy of the i n c i d e n t photon and depends t o a l e s s e r e x t e n t on the atomic number of the absorbing m a t e r i a l . Since X or ^ - r a y s obey a b s o r p t i o n laws common to other e l e c t r o m a g n e t i c r a d i a t i o n (such as v i s i b l e l i g h t ) , they are not completely absorbed by a f i n i t e t h i c k n e s s of absorber. Thus a beam of h i g h energy photons of i n i t i a l i n t e n s i t y , I Q , w i l l have a f i n a l i n t e n s i t y , I , a f t e r passing through a t h i c k n e s s , x, of absorbing m a t e r i a l as g i v e n by the equation ( i ) , I = l Q • e " ^ X ( i ) where jX i s the t o t a l l i n e a r a t t e n u a t i o n c o e f f i c i e n t equal to the sum of the i n d i v i d u a l c o e f f i c i e n t s f o r each of the f o u r i n t e r a c t i o n processes mentioned above. This equation i s analagous to the Beer-Lambert law f o r the a b s o r p t i o n of l i g h t . The range of electromagnetic r a d i a t i o n i s o f t e n d i s c u s s e d i n terms of the " h a l f t h i c k n e s s " of an absorber, i . e . the t h i c k -ness r e q u i r e d to reduce the o r i g i n a l i n t e n s i t y by one h a l f . For 1 MeV photons the h a l f t h i c k n e s s i n water i s about 10 cm . The p h o t o e l e c t r i c e f f e c t i s the i n t e r a c t i o n of e l e c t r o -magnetic r a d i a t i o n w i t h an atom or molecule which r e s u l t s i n the complete a b s o r p t i o n of the photon and simultaneous e j e c t i o n - 4 -o f an e l e c t r o n w i t h k i n e t i c energy, E, g i v e n by e q u a t i o n ( i i ) , E = h j / - <f> ( i i ) where hV i s the photon energy and i s the " b i n d i n g energy" of the e l e c t r o n . ^ T h i s process i s important o n l y f o r comp-a r a t i v e l y low photon energi e s ( < 0.1 MeV) and i n c r e a s e s w i t h i n c r e a s i n g atomic number. An e l a s t i c c o l l i s i o n between a photon and a l o o s e l y bound or unbound e l e c t r o n , known as the Compton p r o c e s s , r e s u l t s i n the t r a n s f e r of some of the photon's energy to the e l e c t r o n . C o n s e r v a t i o n of momentum r e q u i r e s t h a t the photon must change d i r e c t i o n , i . e . i s s c a t t e r e d , and the amount of energy t r a n s f e r e d to the e l e c t r o n , E, i s g i v e n by the e q u a t i o n ( i i i ) / E = hi/ - hi/' ( i i i ) where hi/ and hi/' are the i n c i d e n t and s c a t t e r e d photon ener-g i e s r e s p e c t i v e l y . The energy of the s c a t t e r e d photon, hi/ , i s r e l a t e d to i t s o r i g i n a l energy and the s c a t t e r i n g a n g l e , Q , by the r e l a t i o n s h i p ( i v ) , ,,/_ hi/ ,. v hV = (iv) 1 + ( J ^ _ ) ( 1 - cosQ) m c^ o 2 o where m c i s the r e s t mass energy o f the e l e c t r o n . The o important aspects of Compton s c a t t e r i n g which are e v i d e n t from e q u a t i o n (iv) are: (a) the energy l o s t by a photon i n c r e a s e s • as the s c a t t e r i n g angle $ i n c r e a s e s , (b) the energy l o s s f o r -5-a g i v e n s c a t t e r i n g angle increases w i t h i n c r e a s i n g photon energy, and (c) the energy of a photon s c a t t e r e d at the maxi-mum angle ($=180) approaches a l i m i t i n g v a l u e of 0. 256 MeV (i.e.%m oc^) as the i n i t i a l photon energy i n c r e a s e s . Since i n t e r a c t i o n w i t h e l e c t r o n s i s i n v o l v e d i n the Compton process, i t s e f f e c t i v e n e s s i n c r e a s e s w i t h i n c r e a s i n g " e l e c t r o n d e n s i t y " ( i . e . r a t i o of atomic number, Z, t o atomic mass, A) of the absorber. Photons w i t h energy i n the range 0.2 t o 5 MeV are almost e x c l u s i v e l y absorbed by the Compton process f o r low atomic number m a t e r i a l s . P a i r p r o d u c t i o n r e s u l t s when a h i g h energy photon i s a n n i h i l a t e d i n the r e g i o n of an atomic nucleus w i t h the con-comitant production of an " e l e c t r o n " p a i r — a p o s i t i v e and a negative e l e c t r o n . This process has a threshold energy of 1.02 MeV, the energy r e q u i r e d t o produce two e l e c t r o n r e s t masses, and t h e r e f o r e the net k i n e t i c energy of the e l e c t r o n s , E + E , i s given by equation ( v ) , P e E + E = YiV - 2m c 2 = hi/- 1.02 MeV (v) p e o where hi/ i s the photon energy. P a i r p r o d u c t i o n becomes an important absorbing process o n l y f o r very h i g h photon energies (>10 MeV) . Photonuclear r e a c t i o n s a l s o r e q u i r e very h i g h photon energies i n order t o e j e c t neutrons or protons from atomic n u c l e i . The photon energy must exceed the "bin d i n g " energy of a nuclear p a r t i c l e and t y p i c a l l y i s i n the range of 6 to 18 MeV. However t h i s process makes a n e g l i g i b l e c o n t r i b u t i o n -6-to the t o t a l l i n e a r a t t e n u a t i o n c o e f f i c i e n t f o r electromagnetic r a d i a t i o n s normally used i n r a d i a t i o n chemical s t u d i e s . In summary, the most important i n t e r a c t i o n f o r photons 60 of moderate energy (e.g. Co ^ - r a y s at 1.17 and 1.33 MeV) i s the Compton process. Because t h i s a b s o r p t i o n e s s e n t i a l l y r e s u l t s i n the conversion of high energy photons to high energy e l e c t r o n s , i t i s necessary to consider i n d e t a i l the i n t e r a c t i o n of high energy e l e c t r o n s w i t h matter s i n c e they are the species d i r e c t l y r e s p o n s i b l e f o r r a d i a t i o n chemical e f f e c t s . 2. High Energy E l e c t r o n s U n l i k e electromagnetic r a d i a t i o n , e l e c t r o n s have a f i n i t e range i n an absorbing m a t e r i a l . They lo s e t h e i r energy v i a two important processes, namely r a d i a t i o n emission (Bremsstrahlung) and i n e l a s t i c c o l l i s i o n s w i t h other e l e c t r o n s i n the medium. When a high energy e l e c t r o n passes c l o s e to an atomic nucleus i t i s decelerated by the e l e c t r i c f i e l d . , According to c l a s s i c a l physics t h i s means t h a t the e l e c t r o n must r a d i a t e electromagnetic r a d i a t i o n (Bremsstrahlung) i n order t h a t the system conserve both energy and momentum. The r a t e at which 0 2 2 the e l e c t r o n loses energy, -dE/dx, i s p r o p o r t i o n a l .to e Z /m where e,Z are the e l e c t r o n i c and nuclear charges r e s p e c t i v e l y and m i s the e l e c t r o n mass. Bremsstrahlung emission i s n e g l -i g i b l e f o r energies below about 100 keV and becomes s i g n i f i c a n t o n l y above 1 MeV. Of course t h i s electromagnetic r a d i a t i o n -7-w i l l then be p a r t i a l l y re-absorbed i n the medium by the pro-cesses d i s c u s s e d i n the previous s e c t i o n . The predominant mechanism f o r energy l o s s by e l e c t r o n s w i t h energy l e s s than 1 MeV i s through i n e l a s t i c Coulombic 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 of the absorbing m a t e r i a l . I n t e r a c t i o n s of t h i s type produce the i o n i z a t i o n and e x c i t a t -i o n which leads t o chemical change i n the system. The r a t i o of energy l o s s by r a d i a t i o n emission t o t h a t l o s t by c o l l i s i o n i s g i v e n approximately by the formula ( v i ) , ( d E / d x ) r a d EZ Z ( v i ) ( d E / d x ) c o n 1600 mQcz where E i s the e l e c t r o n energy and Z i s the atomic number of 3 the m a t e r i a l . A t h i r d process which a f f e c t s the range of e l e c t r o n s i n an absorber i s t h e i r d e f l e c t i o n by the Coulombic f i e l d s of the atomic n u c l e i . This r e s u l t s o n l y i n a change i n d i r e c t i o n and i s g r e a t e s t f o r low e l e c t r o n energies and h i g h atomic number m a t e r i a l s . Thus e l e c t r o n s l o s e t h e i r energy and are d e f l e c t e d as they pass through a medium. The t o t a l i n i t i a l energy and the r a t e of energy l o s s consequently determine the range or penetra-t i o n d i s t a n c e of the e l e c t r o n . For monoenergetic e l e c t r o n s , a graph showing the number of e l e c t r o n s t r a n s m i t t e d at a g i v e n d i s t a n c e w i t h i n the bombarded m a t e r i a l , as a f u n c t i o n of d i s t -ance, i s n e a r l y l i n e a r w i t h a negative slope and f i n i s h i n g i n - 8 -a s m a l l t a i l . The e x t r a p o l a t e d or p r a c t i c a l range, R p, i s found by e x t r a p o l a t i n g the l i n e a r p o r t i o n of the curve. The maximum range, R Q, i s the p o i n t where the t a i l of the curve merges w i t h the background. For a beam of non-monoenergetic e l e c t r o n s (e.g. ^ - p a r t i c l e s or Compton e l e c t r o n s ) the curve does not have a l i n e a r r e g i o n and only a maximum range, R Q, can be determined. The range of e l e c t r o n s has been e m p i r i c a l l y r e l a t e d t o t h e i r energy f o r aluminum absorbers. For energies between 0.01 and 2.5 MeV the range of ^ - p a r t i c l e s w i t h maxi-mum energy, E, or the e x t r a p o l a t e d range of monoenergetic e l e c t r o n s of energy, E, i s g i v e n by equation ( v i i ) , Range = 412 E 1-265 - 0.0954 l n E ( v ± i ) where the range i s g i v e n i n mg cm~^. 3 This formula can a l s o be a p p l i e d t o most other low atomic number m a t e r i a l s s i n c e the range expressed i n u n i t s of mg cm~* v a r i e s o n l y s l i g h t l y w i t h atomic number. Thus the range of 1 MeV e l e c t r o n s as c a l c u l a t e d from equation ( v i i ) i s 412 mg cm~2 which corresponds to a t h i c k n e s s of 0.41 cm f o r water. The net r e s u l t of the passage of a h i g h energy e l e c t r o n through a condensed medium i s an i r r e g u l a r d i s t r i b u t i o n of i o n i z e d and e x c i t e d molecules. The path of the primary e l e c t r o n i s r e f e r r e d to as i t s " t r a c k " , which i s g e n e r a l l y near l i n e a r at h i g h energy but d e f l e c t i o n s become more common as the e l e c t -ron slows down. I o n i z a t i o n s and e x c i t a t i o n s which occur a t i r r e g u l a r i n t e r v a l s along the t r a c k are c a l l e d "primary events". The primary i o n i z a t i o n s w i l l produce secondary e l e c t r o n s w i t h s u f f i c i e n t energy t o escape recombination w i t h t h e i r p o s i t i v e - 9 -i o n and they may be c l a s s i f i e d i n t o two energy c a t e g o r i e s ; the low energy secondary « 100 eV) and the h i g h energy secondary e l e c t r o n s (> 100 eV). The low energy secondary e l e c t r o n s w i l l s u f f e r l a r g e d e f l e c t i o n s and form a r e g i o n of t e r t i a r y i o n i z a t i o n and e x c i t a t i o n c a l l e d a "spur"/ which t o a f i r s t approximation may be regarded as s p h e r i c a l i n shape. Each spur w i l l thus i n i t i a l l y c o n t a i n a number of e x c i t e d molecules, p o s i t i v e ions and very low energy e l e c t r o n s w i t h an average of somewhat l e s s than 100 eV deposited i n t h i s area. The more e n e r g e t i c secondary e l e c t r o n s have s u f f i c i e n t range t o form s h o r t t r a c k s of t h e i r own, c a l l e d "S-rays", and these e l e c t r o n s can be f u r t h e r c l a s s i f i e d i n t o two sub-groups. The most e n e r g e t i c secondary §-electrons w i l l form a t r u e t r a c k of t h e i r own along which there w i l l be f u r t h e r i o n i z a t i o n and the formation of spurs. Known as a "branch t r a c k " , the average s e p a r a t i o n of spurs along i t s path w i l l be s u f f i c i e n t so t h a t there w i l l be no o v e r l a p p i n g . In c o n t r a s t , the l e s s e n e r g e t i c ^ - e l e c t r o n s w i l l have o n l y enough energy to form a very s h o r t t r a c k and the spurs along i t w i l l o v e r l a p to form a k i n d of "super spur" which i s sometimes r e f e r r e d t o as a "blob". For a 1 MeV e l e c t r o n stopped i n water about 67% of the energy i s deposited i n i s o l a t e d spurs, 22% along the branch t r a c k s , and 11% i n b l o b s . 1 From the above d i s c u s s i o n s i t i s c l e a r 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 i s to produce an inhomogeneous d i s t r i b u t i o n of i o n i z e d and e l e c t r o n i c a l l y e x c i t e d molecules and very low -10-energy e l e c t r o n s i n the medium as may be d e p i c t e d by F i g u r e 2. F i g u r e 2. D i s t r i b u t i o n of spurs and primary events along the t r a c k of a f a s t e l e c t r o n i n a l i q u i d , ( a f t e r F i g u r e 2-8, Henley and Johnson, reference 2, page 31) The f a t e of these primary species governs the subsequent chemistry i n the system. B. CHEMICAL CONSEQUENCES OF THE ABSORPTION OF HIGH ENERGY  RADIATION "In g e n e r a l , most of the r e a c t i o n s which occur i n r a d i ~ a t i o n chemistry are o r d i n a r y thermal chemical r e a c t i o n s , a l t h -ough some i n v o l v e r a t h e r unusual chemical s p e c i e s . In some cases regions of excess energy - "hot spots" - may be present, and these regions can provide a c t i v a t i o n energies g r e a t e r than those a v a i l a b l e t h e r m a l l y . However, these r e a c t i o n s a l s o obey chemical laws, and there i s no need t o t r e a t r a d i a t i o n chemistry as a domain beyond the realms of chemistry because of the v a s t energies a v a i l a b l e . F o l l o w i n g the a b s o r p t i o n of high energy r a d i a t i o n , the e x c i t e d molecules and ions formed by the p h y s i c a l processes d e s c r i b e d i n the previous s e c t i o n undergo a v a r i e t y of changes and r e a c t i o n s . The ions may recombine, the e x c i t e d molecules may d i s s o c i a t e or luminesce and many other r e a c t i o n s may occur as the species d i f f u s e and become homogeneously d i s t r i b u t e d -11-throughout the medium. A ge n e r a l o u t l i n e of these chemical processes which occur f o l l o w i n g the p h y s i c a l stage of the r a d i o l y s i s i s s c h e m a t i c a l l y i l l u s t r a t e d i n Figure 3. The time s c a l e s on which these events occur, and indeed whether they occur, depend to a larg e extent on the p o l a r i t y of the medium and i t s p h y s i c a l s t a t e ( i . e . s o l i d , l i q u i d , or gas). The f o l l o w i n g d i s c u s s i o n w i l l be r e s t r i c t e d to the condensed phases, s p e c i f i c a l l y l i q u i d and f r o z e n p o l a r systems which w i l l be t r e a t e d s e p a r a t e l y . 1. P o l a r L i q u i d s An i n d i c a t i o n of the probable time s c a l e f o r the r a d -i a t i o n events i n a t y p i c a l d i e l e c t r i c l i q u i d , water, i s g i v e n i n Table 1. During the physiochemical stage of the r a d i o l y s i s , which begins w i t h i n 1 0 s e c o n d s a f t e r the in c i d e n c e of the r a d i a t i o n ; ion-molecule r e a c t i o n s occur, e l e c t r o n i c a l l y e x c i t e d molecules are i n v o l v e d i n energy t r a n s f e r processes or e l s e d i s s o c i a t i o n , and r a d i c a l d i f f u s i o n begins. In a d d i t -i o n the low energy e l e c t r o n s become t h e r m a l i z e d , t h a t i s they have energy e q u i v a l e n t to kT or '•'0.025 eV, and may enter i n t o ion-molecule r e a c t i o n s or e l s e become s o l v a t e d by causing a p o l a r i z a t i o n of the s o l v e n t d i p o l e s ( d i e l e c t r i c r e l a x a t i o n ) . Solvated e l e c t r o n s and t h e i r p r o p e r t i e s w i l l be discussed i n d e t a i l i n a l a t e r s e c t i o n . The d i s t i n c t i o n between the physiochemical and the true Physical stage * POSITIVE IONS • SLOW ELECTRONS (HIGH ENERGY PHOTONS) Compton ^ process FAST ELECTRONS Ionization, Excitation EXCITED MOLECULES Chemical stage M + Ion-molecule reactions I MH++ R-Pre-formed traps in solids fc— Geminate recombination ' ""= Ionic dissociat ion Dielectric relaxation in liquids M* h-9 uminescenc STABILIZED ELECTRONS diationless Dissociation H transitions I RADICALS Ion-molecule reactions > fV MH*,M or S M STABLE PRODUCTS F i g u r e 3 . P r imary processes i n the a c t i o n o f r a d i a t i o n on m a t t e r . TABLE 1 APPROXIMATE TIME SCALE FOR THE RADIOLYSIS OF LIQUID WATER (adapted from Table 8.3, reference 1, page 252-3) P h y s i c a l stage t A Time (sec) 10-18 10 -15 10 -14 Events Reactions E l e c t r o n of 1 MeV energy t r a v e r s e s molecule H 0-*<M*H 0 + + 2 2 Thermal e l e c t r o n (0.025 eV) t r a v -erses a molecule. Time between successive i o n i z a t i o n s by a MeV e l e c t r o n . Time f o r " v e r t i c a l " e x c i t a t i o n to an e l e c t r o n i c e x c i t e d s t a t e . J Ion molecule r e a c t i o n s . P e r i o d of molecular v i b r a t i o n . D i s s o c i a t i o n of molecules e x c i t e d to r e p u l s i v e s t a t e s . > H 0 — * A A > W - " H O 2 2 Species Present H„On 2 > J H 20 ++H 20 -*H30++OH-H O* 2 i H - + OH* ? H 20 ( l o c a l i z e d i n spurs or t r a c k zone) OH« l P h y s i o -chemical stage 10 -13 10 -12 Secondary e l e c t r o n s reduced to thermal energy. E l e c t r o n capture. I n t e r n a l conversion from higher t o lowest e l e c t r o n i c s t a t e . R a d i c a l moves one "jump" H30-+ •H20+H* i n d i f f u s i o n . H« 10~11 R e l a x a t i o n time f o r the o r i e n t a t i o n e~ *- e^q H* OH' ,e"" of the water d i p o l e s . ( e_ + H 2 0 ^ H . + o i r ) H 2 o*(?) a q ' (... continued on page 14) TABLE 1 c o n t -C h e m i c a l s t a g e T i me ( s e c ) 10 -10 10 10 -9 -8 10 - 5 10 - 4 10" 10 E v e n t s R e a c t i o n s S p e c i e s P r e s e n t Minimum t i m e f o r d i f f u s i o n c o n t r o l l e d r e a c t i o n s i n t h e b u l k o f t h e l i q u i d . E l e c t r o n w i t h i n i t i a l e n e r g y o f 1 MeV comes t o " r e s t " . R a d i a t i v e l i f e t i m e o f s i n g l e t e x c i t e d s t a t e s . F o r m a t i o n o f m o l e c u l a r p r o -d u c t s c o m p l e t e i n ' V-ray s p u r s . J R e a c t i o n t i m e f o r r a d i c a l s w i t h s o l -u t e s i n m o l a r c o n c e n t r a t i o n s . L i f e t i m e o f t h e h y d r a t e d e l e c t r o n . Time r e q u i r e d f o r a r a d i c a l t o . d i f f u s e t h e i n t e r - s p u r d i s t a n c e i n t h e t r a c k o f a n MeV e n e r g y e l e c t r o n . R a d i a t i v e l i f e t i m e o f t r i p l e t e x c i t e d s t a t e s . -C h e m i c a l r e a c t i o n s c o m p l e t e , H * + O H * 2 O H * — 2 H * > H * H 2 ° H 2 ° 2 H - 0 H « 2 e " » H 9 + 2 0 H " a q ^ c a q ' H 2 0 * ( ? ) , H 2 , y H 2 0 2 - ( a l l i n o r n e a r s p u r s a n d t r a c k ) R* + S J p r o d u c t s H o , H 2 0 2 a n d p r o d u c t s o f r a d i c a l r e a c t -w i t h s o l u t e s . i -15-chemical stages of the r a d i o l y s i s i s g e n e r a l l y drawn a t about —9 10 seconds, which a l s o happens to be the time r e q u i r e d to slow a 1 MeV e l e c t r o n to thermal v e l o c i t y . During the chemical stage, the r a d i c a l s and ions formed i n the spurs d i f f u s e out-ward and those which escape recombination e v e n t u a l l y become homogeneously d i s t r i b u t e d throughout the l i q u i d . Reactions of these "primary" species w i t h s o l u t e (scavengers) or s o l v e n t molecules can then occur to form s t a b l e "primary" r a d i o l y s i s products. Those species which r e a c t w i t h i n the spurs to form s t a b l e molecules, termed "molecular products", are g e n e r a l l y not scavengeable at normal s o l u t e c o ncentrations ( i . e . 10 M) , however i f v e r y h i g h s o l u t e c o ncentrations are used ( i . e . 1 M) the pr e c u r s o r s of these "molecular products" may be scavenged. When ve r y l a r g e r a d i a t i o n doses are g i v e n , the "primary" products may become i n v o l v e d i n r e a c t i o n s w i t h the primary r a d i c a l species to form d i f f e r e n t "secondary" species and products (e.g. i n water, H2°2 a m ° l e c u l a r product, e v e n t u a l l y reaches s u f f i c i e n t c o n c e n t r a t i o n to r e a c t w i t h e~ , OH*, and H* to form 0H~ + OH- , H- + H-,0, and OH* + H?0 r e s p e c t i v e l y ) . aq z ^ 2. Frozen P o l a r Systems By f r e e z i n g a l i q u i d and lowering the temperature s u f f i c i e n t l y , i t i s p o s s i b l e to slow down the chemical stages of r a d i o l y s i s l i s t e d i n Table 1. In many systems t h i s s i m p l i -f i e s the r e a c t i o n s which can occur and i n some cases, allows d i r e c t o b s e r v a t i o n of the primary species and intermediates i n v o l v e d . R a d i c a l s formed e i t h e r during the primary r a d i o l y s i s - 1 6 -p r o c e s s e s or from r e a c t i o n s of mobile primary s p e c i e s such as H atoms, become trapped i n low temperature m a t r i c e s . Ions may a l s o become s t a b i l i z e d ; the p o s i t i v e i o n s by t r a n s f e r r i n g a p r o t o n t o a neighbouring molecule w i t h r e s u l t i n g f o r m a t i o n o f an a d d i t i o n a l r a d i c a l , and the n e g a t i v e i o n s by becoming tr a p p e d i n d e f e c t s i n the m a t r i x where the s o l v e n t d i p o l e s are s u i t a b l y o r i e n t e d to p r o v i d e s t a b i l i z a t i o n . In p a r t i c u l a r , e l e c t r o n s a r e o f t e n trapped i n "pre-formed" p o s i t i v e " h o l e s " i n the medium. A l t e r n a t i v e l y , the i o n s may q u i c k l y induce e l e c t r o n i c p o l a r i z a t i o n o f the s u r r o u n d i n g molecules and s t a b -i l i z e themselves i n t h i s manner u n t i l d i p o l e r e l a x a t i o n can o c c u r to form a "deeper" t r a p . These trapped n e g a t i v e i o n s and r a d i c a l s are o f t e n paramagnetic and t h e r e f o r e may be observed by e l e c t r o n s p i n resonance s p e c t r o s c o p y . O p t i c a l s p e c t r o s c o p y a l s o may be used i n i d e n t i f i c a t i o n o f the s p e c i e s . Kevan^ has summarized the b a s i c o b j e c t i v e s of r e s e a r c h i n t o the r a d i o l y s i s of f r o z e n systems i n the f o l l o w i n g manner: (a) Which s p e c i e s may be d e t e c t e d and how do t h e i r y i e l d s r e l a t e to the o v e r a l l r a d i o l y s i s mechanism? (b) What r e a c t i o n s do trapped i n t e r m e d i a t e s undergo and how do these r e a c t i o n s compare to those o c c u r i n g i n the l i q u i d phase ? (c) What i s the nature o f the t r a p p i n g s i t e s o f the v a r i o u s s p e c i e s ? (d) What i s the s p a t i a l d i s t r i b u t i o n of the intermed-i a t e s and how does i t a f f e c t the c h e m i s t r y i n the system?' (e) Are new types of s p e c i e s generated r a d i o l y t i c a l l y i n f r o z e n systems due to m a t r i x s t a b i l i z a t i o n ? -17 -(f) Is energy s t o r e d i n i r r a d i a t e d f r o z e n systems? I f so, how e f f i c i e n t i s the process and can energy be s e l e c t i v e l y t r a n s f e r r e d from s i t e to s i t e ? Thus s t u d i e s on f r o z e n systems are o f t e n complimentary to l i q u i d phase i n v e s t i g a t i o n s and help i n s o l v i n g the o v e r a l l r a d i o l y s i s mechanism. 3. R a d i a t i o n Chemical U n i t s and Terms The y i e l d of a species or product formed by r a d i o l y s i s i s known as the "G value". I t i s de f i n e d as the number of molecules, i o n s , atoms or r a d i c a l s which are formed (or disr-apear) f o r every 100 eV of energy deposited by the r a d i a t i o n i n the system. For "primary" species or "molecular" products the n o t a t i o n " G " i s g e n e r a l l y used and G normally i s i n the range of 0-10 . Y i e l d s of products which are not formed s o l e l y from a primary process are i n d i c a t e d by " G(X) ". For secondary processes i n v o l v i n g c h a i n r e a c t i o n s i t i s not unusual f o r a G value to exceed 10. In order to o b t a i n an absolute measure of the G value of any species formed by r a d i a t i o n i t i s necessary to know the t o t a l amount of energy absorbed by the medium. This q u a n t i t y i s termed the "absorbed dose" and i t i s expressed i n many d i f f e r e n t u n i t s , the most common of which i s the "rad". One rad i s d e f i n e d as the d e p o s i t i o n of 100 ergs g ~ l . This u n i t i s sometimes converted to a more convenient q u a n t i t y , i . e . eV g~^, 13 -1 - l by the f a c t o r 6.24 x 10 eV g rad , s i n c e G value c a l c u l a t x o n s r e q u i r e knowing the t o t a l dose i n u n i t s of eV. There are -18-numerous methods used to determine dose (dosimetry). They v a r y from chemical r e a c t i o n s w i t h a c c u r a t e l y known y i e l d s + 2 (e.g. o x i d a t i o n of Fe i n 0.8N s u l f u r i c a c i d s o l u t i o n has G ( F e + 3 ) = 15.5 f o r 6 0 C o 'X-rays) to e l e c t r o n i c devices capable of measuring the number of charged p a r t i c l e s or ions produced and c a l o r i m e t r i c methods designed to measure the energy deposited when i t i s a l l converted to h e a t . 3 C. STABILIZED ELECTRONS5 I n the previous s e c t i o n s i t has been shown tha t the i n t e r a c t i o n of i o n i z i n g r a d i a t i o n w i t h matter produces high energy e l e c t r o n s which through a cascade of processes e v e n t u a l l y g i v e r i s e to numerous low energy e l e c t r o n s . (In f a c t a s i n g l e 1 MeV e l e c t r o n may produce between 10 and I O 3 secondary e l e c t r o n s . ) These low energy e l e c t r o n s (<10 eV) l o s e most of t h e i r remaining energy by e x c i t a t i o n of e l e c t r o n i c t r a n s i t i o n s and v i b r a t i o n a l s t a t e s of the medium. When t h e i r energy reaches or drops below ^0.5 eV, the " s u b - e x c i t a t i o n " e l e c t r o n s become th e r m a l i z e d , which means t h a t t h e i r v e l o c i t y decreases to 7 -1 **10 cm s (at room temperature) and t h e i r energy i s f u r t h e r reduced to**»0.03 eV. The s e p a r a t i o n d i s t a n c e between the c a t i o n s and e l e c t r o n s i n c r e a s e s during the l a t t e r stage of energy l o s s and the t h e r m a l i z e d e l e c t r o n may leave i t s spur and p o s s i b l y overcome the Coulombic f i e l d of the c a t i o n s . Charge n e u t r a l i z a t i o n does e v e n t u a l l y occur however i f the e l e c t r o n i s unable to escape the i n f l u e n c e of i t s p o s i t i v e i o n and t h i s process i s c a l l e d "geminate recombination". The p r o b a b i l i t y t h a t the t h e r m a l i z e d e l e c t r o n s w i l l escape from the i n f l u e n c e -19-of the Coulombic f i e l d depends on the p o l a r i t y of the medium ( i t s d i e l e c t r i c . c o n s t a n t ) , i t s degree of order ( l i q u i d , c r y s t a l l i n e or g l a s s y amorphous s t a t e ) , and the temperature ( v i s c o s i t y ) . I f the thermal e l e c t r o n s do escape geminate recombin-a t i o n then they may be s t a b i l i z e d by the surrounding medium, e i t h e r through a s o l v a t i o n process i n p o l a r l i q u i d s or by t r a p p i n g i n pre-formed "holes" i n f r o z e n systems. An a l t e r n -a t i v e , f a t e f o r these thermal e l e c t r o n s i s of course to r e a c t w i t h a s o l v e n t molecule and t h i s r e a c t i o n w i l l depend on the e l e c t r o n a f f i n i t y of the s o l v e n t . 1. E l e c t r o n S o l v a t i o n Process During the e a r l y i n v e s t i g a t i o n s of the r a d i a t i o n chem-i s t r y of water, by f a r the most thoroughly s t u d i e d system to date, s e v e r a l t h e o r i e s were advanced to account f o r the observed chemistry, and p a r t i c u l a r l y the p r o d u c t i o n of hydrogen. Samuel and Magee proposed t h a t the s u b e x c i t a t i o n e l e c t r o n s would be moderated to thermal energies w i t h i n 10""13 seconds and w h i l e s t i l l i n the Coulombic f i e l d of the parent i o n s . Since the d i e l e c t r i c r e l a x a t i o n time of water i s 10~H seconds, the d i p o l e s would not be able to r e o r i e n t themselves q u i c k l y enough to s o l v a t e the e l e c t r o n . Therefore geminate recombination would occur to produce an e x c i t e d water molecule which would then d i s s o c i a t e to g i v e an H atom and an OH r a d i c a l w i t h s u f f i c i e n t energy to escape the s o l v e n t cage. Thus the primary species i n the Samuel-Magee theory are H and OH r a d i c a l s formed i n the immediate v i c i n i t y of the spur. - 2 0 -An a l t e r n a t i v e model of Lea^ and Gray^ suggested t h a t the secondary e l e c t r o n would move about 150 A* from the parent p o s i t i v e i o n and thus beyond the e f f e c t of i t s e l e c t r o s t a t i c f i e l d . Under these circumstances the e l e c t r o n and the p o s i t i v e i o n would r e a c t independently w i t h the s o l v e n t . I t was p o s t -u l a t e d t h a t the e l e c t r o n r e a c t s to giv e an H atom and OH", q w h i l e the p o s i t i v e i o n produces an OH r a d i c a l . Platzman e s s e n t i a l l y agreed w i t h the Lea-Gray theory about the e l e c t r o n escaping the Coulombic f i e l d of H^ O"^ , however he poi n t e d out th a t the r e a c t i o n of thermal e l e c t r o n s w i t h water i s r e l a t i v e l y slow. He concluded t h a t the e l e c t r o n might not undergo t h i s r e a c t i o n but could s u r v i v e to become s o l v a t e d . The s o l v a t e d e l e c t r o n would then r e a c t i n a manner s i m i l a r to the H atom. Platzman's p o s t u l a t e was l a t e r proven to be c o r r e c t when i t was e x p e r i m e n t a l l y determined t h a t an i o n i c reducing species w i t h u n i t negative charge was i n v o l v e d i n the r a d i o l y s i s of water.' L 0 This species was subsequently i d e n t i f i e d as the hydrated e l e c t r o n by i t s i n t e n s e o p t i c a l a b s o r p t i o n i n the red which was s i m i l a r i n c h a r a c t e r i s t i c s to the o p t i c a l a b s o r p t i o n band of c h e m i c a l l y produced s o l v a t e d e l e c t r o n s i n l i q u i d ammonia.^ Since these " e a r l y " s t u d i e s , numerous r e p o r t s have been made on the formation of s o l v a t e d e l e c t r o n s i n a v a r i e t y of l i q u i d s . 5 The exact d e t a i l s of the mechanism of the s o l v a t i o n process are s t i l l s p e c u l a t i v e s i n c e no one has yet been able t o , a c t u a l l y observe the formation of s o l v a t e d e l e c t r o n s d e s p i t e the f a c t t h a t time r e s o l u t i o n i n t o the 10 second r e g i o n has -21-been achieved. The most g e n e r a l l y accepted q u a l i t a t i v e p i c t u r e of the s o l v a t i o n process i n v o l v e s the thermal e l e c t r o n i n i t i a l l y ( w i t h i n 10~1^ seconds) e l e c t r o n i c a l l y p o l a r i z i n g the surrounding molecules. This e l e c t r o n i c p o l a r i z a t i o n i s thought to e f f e c t -i v e l y "immobilize" the e l e c t r o n w h i l e the slower atomic and d i p o l a r p o l a r i z a t i o n s can occur to s o l v a t e i t . A schematic r e p r e s e n t a t i o n of these events f o r water and a comparatively non-polar l i q u i d , n-hexane, i s shown i n F i g u r e 4. " T R A P P E D " E L E C T RON 1 3 1 2 11 L O G t (sec) S O L V A T E D E L E C T R O N F i g u r e 4. Q u a l i t a t i v e r e p r e s e n t a t i o n of the p o t e n t i a l energy of an e l e c t r o n as a f u n c t i o n of time a f t e r l o c a l i z a t i o n i n a l i q u i d , ( a f t e r F i g u r e 4, Freeman, reference 12, page 23) Sol v a t e d e l e c t r o n s are formed i n both systems, however the much g r e a t e r p o t e n t i a l energy g a i n by the e l e c t r o n i n water due to d i p o l e o r i e n t a t i o n makes the hydrated e l e c t r o n a more " s t a b l e " s p e c i e s . In a d d i t i o n , because of the d i e l e c t r i c constant d i f f e r e n c e s between n-hexane and water, the y i e l d of e l e c t r o n s which escape geminate recombination w i l l be much l a r g e r f o r water. The e s s e n t i a l d i f f e r e n c e between the "trapped" e l e c t r o n and the s o l v a t e d one i s simply t h a t the d i e l e c t r i c has reached i t s e q u i l i b r i u m p o i n t i n the case of the s o l v a t e d e l e c t r o n , -22-w h e r e a s i t h a s n o t c o m p l e t e l y r e l a x e d i n t h e " t r a p p e d " c a s e . The s o l v a t e d e l e c t r o n i s i n t h e r m a l e q u i l i b r i u m w i t h t h e medium w h i l e t h e " t r a p p e d " s t a t e i s n o t . 2. E l e c t r o n T r a p p i n g i n F r o z e n S y s t e m s I n t h e c a s e o f f r o z e n s y s t e m s , on t h e o t h e r hand, t h e t h e r m a l i z e d e l e c t r o n s a r e t h o u g h t t o be s t a b i l i z e d b y w h o l l y o r p a r t i a l l y p r e - f o r m e d t r a p s w h i c h e x i s t i n t h e l o w t e m p e r a t u r e m a t r i x . I n g l a s s y amorphous s y s t e m s , t h e randomness o f t h e m o l e c u l a r o r i e n t a t i o n s o f t h e l i q u i d s t a t e i s " f r o z e n i n " by q u i c k c o o l i n g . C o n s e q u e n t l y t h e r e i s a l a r g e c o n c e n t r a t i o n o f p r e - f o r m e d " t r a p s " ( i . e . d i p o l e - o r i e n t e d m o l e c u l e s f o r m i n g an e f f e c t i v e p o s i t i v e h o l e ) a v a i l a b l e t o compete w i t h t h e p a r e n t i o n ' s C o u l o m b i c a t t r a c t i o n . T h i s i s i n c o n t r a s t t o c r y s t a l l i n e s y s t e m s where s h o r t r a n g e o r d e r p r e v a i l s . Thus f o r example, g l a s s y a l c o h o l s t r a p e l e c t r o n s v e r y e f f i c i e n t l y ( G p - = 2 . 5 t r f o r m e t h a n o l ) w h e r e a s t h e same m a t e r i a l i n a p o l y c r y s t a l l i n e f o r m does n o t . ^ The t r a p p i n g p r o c e s s i s t h o u g h t t o be s i m i l a r t o t h e s o l v a t i o n p r o c e s s d i s c u s s e d a bove, a l t h o u g h t h e a d v a n t a g e o f a t l e a s t p a r t i a l p r e - o r i e n t a t i o n o f t h e d i p o l e s a l l o w s f o r a much g r e a t e r t r a p p i n g e f f i c i e n c y . N o r m a l l y h i g h e r y i e l d s o f t r a p p e d e l e c t r o n s a r e o b s e r v e d r e l a t i v e t o t h e s o l v a t e d e l e c t r o n y i e l d i n t h e same m a t e r i a l . T h i s i s p r o b a b l y a d i r e c t c o n s e q u e n c e o f t h e f a c t t h a t t h e t r a p p e d e l e c t r o n s i n p o l a r m e d i a a r e s t a b i l i z e d w i t h i n t h e i m m e d i a t e v i c i n i t y o f t h e s p u r ( i . e . t h e y d o n ' t t r a v e l a s f a r i n t h e s o l i d as i n t h e l i q u i d ) . Thus t h e s p a t i a l d i s t r i b u t i o n o f t r a p p e d e l e c t r o n s -23-i s l i k e l y to be very d i f f e r e n t to that of s o l v a t e d ones. 3. P r o p e r t i e s of S t a b i l i z e d E l e c t r o n s S t a b i l i z e d e l e c t r o n s formed by r a d i o l y s i s have p r o p e r t i e s v e r y s i m i l a r to those of s o l v a t e d e l e c t r o n s formed by d i s -s o l u t i o n of a l k a l i metals i n ammonia and amines. They are very powerful reducing s p e c i e s , r e a c t i n g o f t e n w i t h d i f f u s i o n con-t r o l l e d r a t e constants i n s o l u t i o n , and consequently have r e l a t i v e l y s h o r t l i f e t i m e s i n i r r a d i a t e d l i q u i d systems ( i . e . 10""^ seconds or l e s s ) . S o l v e n t - s t a b i l i z e d e l e c t r o n s are c h a r a c t e r i z e d by broad i n t e n s e o p t i c a l a b s o r p t i o n s p e c t r a i n the v i s i b l e or near i n f r a r e d r e g i o n s . This i s t h e i r most important property as f a r as d e t e c t i o n and k i n e t i c s t u d i e s are concerned. The species are a l s o paramagnetic and t h e r e f o r e e l e c t r o n s p i n resonance (ESR) i s e x t e n s i v e l y used to study the long l i v e d trapped e l e c t r o n s i n f r o z e n systems. Recent advances i n ESR techniques have a l s o enabled the 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 s produced by pulse r a d i o l y s i s of l i q u i d s to be s t u d i e d . ^ S t a b i l i z e d e l e c t r o n s are now known to e x i s t i n a whole v a r i e t y of media from the non-polar l i q u i f i e d noble gases to h i g h l y p o l a r s o l v e n t s such as water and a l c o h o l s . Table 2 contains some of the s p e c t r a l data f o r e l e c t r o n s s t a b i l i z e d i n a few of the more e x t e n s i v e l y s t u d i e d systems. 4. Y i e l d s of S t a b i l i z e d E l e c t r o n s I t has been reasonably w e l l e s t a b l i s h e d t h a t the y i e l d TABLE 2 SELECTED PROPERTIES OF STABILIZED ELECTRONS FORMED BY RADIOLYSIS O p t i c a l Data ESR Data Y i e l d Medium T(°K) X max (nm) 6 (M"1cm~1) q - f a c t o r G e " 7N NaOH g l a s s - 77 580 19,000 2.0006 15 2.0 Water 78.2 293 7 20 17,000 2.0002 0.5 2.8 77 640 17,000 2.0009 10.6 0.0003 Heavy water 78. 5 293 77 700 630 20,200 20,200 2.0007 2.7 0.0006° Methanol 32.6 293 77 630 526 17,000 11,000 2.0018 11.2 1.1 2.7 e Isopropanol 18.6 293 77 820 615 13,000 15,300 2.0018 10 1.0 1.9 e Methyl t e t r a -hydrofuran 4.6 293 77 1250 16,800 2.0011 4.5 0.23 2.6 e Ethylene G l y c o l 38.7 293 77 580 513 14,000 -10-15 1'2e 2.6 e Footnotes: a. Data from references 4, 5d, and 5e. . b. S t a t i c d i e l e c t r i c constant a t room temperature. c. Line width between p o i n t s o f maximum slope. d. P o l y c r y s t a l l i n e s o l i d , e. Glassy amorphous s o l i d . -25-o f s o l v a t e d e l e c t r o n s c o r r e l a t e s w i t h the s t a t i c d i e l e c t r i c c o n s t a n t of the l i q u i d . T h i s e m p i r i c a l c o r r e l a t i o n i s i l l u s -t r a t e d g r a p h i c a l l y i n F i g u r e 5 which c o n t a i n s data from s e v e r a l sources and o b t a i n e d by a v a r i e t y of t e c h n i q u e s . For some o f the s o l v e n t s , c o n c l u s i v e p r o o f t h a t the i o n i c r e d u c i n g e n t i t y i s i n f a c t a s o l v a t e d e l e c t r o n does not e x i s t . Indeed, i n a few cases the s p e c i e s c o u l d w e l l be a r a d i c a l a nion formed by an i o n-molecule r e a c t i o n o f e i t h e r the thermal e l e c t r o n o r a v e r y s h o r t l i v e d s o l v a t e d one w i t h the s o l v e n t . For t h i s r e a s o n the term " f r e e i o n y i e l d " , G f^, i s used i n F i g u r e 5 because i t i s more u n i v e r s a l l y a p p l i c a b l e and i n essence i t r e p r e s e n t s the number of thermal e l e c t r o n s which escape geminate recombination. The y i e l d s o f t rapped e l e c t r o n s i n f r o z e n systems, however, do not seem to c o r r e l a t e w e l l w i t h any p a r t i c u l a r p h y s i c a l p r o p e r t y of the m a t r i x , o t h e r than i t s degree o f o r d e r . In most g l a s s y s o l i d s the y i e l d s o f trapped e l e c t r o n s are h i g h , and i n many cases h i g h e r than the y i e l d of s o l v a t e d e l e c t r o n s i n the l i q u i d m a t e r i a l . T h i s i s l i k e l y to be the r e s u l t o f the f a c t t h a t one o r more of the f o l l o w i n g f a c t o r s may be o p e r a t i n g i n the g l a s s y s t a t e to lower the p r o b a b i l i t y 17 o f prompt geminate rec o m b i n a t i o n : (a) " s u b - i o n i z a t i o n " e l e c t r o n s may have a lower c r o s s - s e c t i o n f o r energy l o s s per c o l l i s i o n i n the s o l i d m a t r i x than i n the l i q u i d because of fewer v i b r a t i o n a l and r o t a t i o n a l s t a t e s to which energy can be t r a n s f e r r e d , (b) the t r a p depth r e s u l t i n g from e i t h e r s e l f t r a p p i n g by s u i t a b l y o r i e n t e d d i p o l e s or from i m p e r f e c t i o n s i n the m a t r i x may be s u f f i c i e n t t o compete w i t h the Coulombic 1 • 1 • • 1 - • _ ^ • O - o -o S 0 0 • = Alcohols and waters ; reference 5e. o / • • o = Nitrites, ketones, ethers, ami des, pyridine, — dimethyl sulfoxide, propylene carbonate, — hydrocarbons; reference 14. X : Formamide ; reference 15. / ° D D = Ammonia; reference 16. o 1 I 1 1 . t . 1 20 40 CL 60 80 100 i to 0> F i g u r e 5. Y i e l d of r a d i o l y t i c a l l y generated f r e e ions (Gfi) as a f u n c t i o n of the s t a t i c d i e l e c t r i c constant (D s) of the l i q u i d . -27-force of the p o s i t i v e ion, and (c) as the trapped charge con-centration increases with dose, the presence of competing e l e c -t r i c f i e l d s from several p o s i t i v e ions weakens the d i r e c t i o n a l e f f e c t on the trapped electron. Thus f o r example, the y i e l d of trapped electrons i n glassy methanol at 77°K i s G = 2.5 e t r whereas the y i e l d of solvated electrons i n the same material at room temperature i s only G _ = 1.1 . e s -28-CHAPTER II AN ATTEMPT AT NITROGEN FIXATION UTILIZING HYPRATED ELECTRONS A. INTRODUCTION 1. Background to the Problem Nitrogen f i x a t i o n * has always been of p a r t i c u l a r i n t e r e s t to s c i e n t i s t s because of i t s importance to the evolution of l i f e on the earth. In current b i o l o g i c a l systems, nitrogen i s taken from the atmosphere and incorporated with the aid of micro-organisms, v i a ammonia, into organic nitrogen compounds under very mild conditions. In contrast to t h i s , temperatures of 300-600 °C and several hundred atmospheres pressure of nitrogen are required i n the i n d u s t r i a l Haber-Bosch process to overcome the inertness of the molecule and convert nitrogen to ammonia. Thus a c r u c i a l question to understanding evolution i s how nitrogen was reduced i n p r e b i o l o g i c a l times when only the basic ingredients; nitrogen, hydrogen, and methane existed i n a r e l a t i v e l y mild environment. - Although the problem of b i o l o g i c a l nitrogen f i x a t i o n has been, and currently i s , one of the major areas of research i n biology and biochemistry; i t was only recently that the chemists' i n t e r e s t i n non-enzymatic nitrogen f i x a t i o n was rekindled by A l l e n and Senoff's discovery of a t r a n s i t i o n metal *footnote 1. Nitrogen f i x a t i o n i s used here i n i t s broadest sense to include any process which converts molecular nitrogen to some other molecule. -29-18 compl e x o f m o l e c u l a r n i t r o g e n . T h i s d i s c o v e r y and s u b s e q u e n t 19 r a p i d g r o w t h i n t h e f i e l d o f n i t r o g e n c o m p l e x c h e m i s t r y , w i t h t h e a d d i t i o n a l d i s c o v e r y t h a t t h e n i t r o g e n l i g a n d s c o u l d be 20 r e d u c e d i n c e r t a i n s y s t e m s , s t i m u l a t e d i n t e r e s t i n t h i s l a b -o r a t o r y i n t h e p o s s i b i l i t y o f r a d i a t i o n c h e m i c a l f i x a t i o n o f m o l e c u l a r n i t r o g e n . 21 — The h y d r a t e d e l e c t r o n (e ) , a p r i m a r y s p e c i e s f o r m e d aq i n t h e r a d i o l y s i s o f w a t e r , i s known t o be an e x t r e m e l y p o w e r f u l r e d u c i n g a g e n t w i t h a s t a n d a r d e l e c t r o d e p o t e n t i a l o f -2.7 V. T h i s makes i t c o m p a r a b l e i n r e d u c i n g s t r e n g t h t o m e t a l l i c s o d i u m i n w a t e r . I n a d d i t i o n , i t i s t h e i d e a l n u c l e o p h i l e f o r s i m p l e 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 and i t shows e x t r e m e l y h i g h r a t e c o n s t a n t s f o r r e a c t i o n s w i t h a w i d e v a r i e t y o f compounds, some 22 o f w h i c h h a d p r e v i o u s l y b e e n r e g a r d e d as n o n - r e d u c i b l e . F u r t h e r m o r e , a l m o s t a l l o f t h e s e r e a c t i o n s p r o c e e d w i t h v i r t u a l l y no e n e r g y o f a c t i v a t i o n , a l t h o u g h i n some c a s e s t h e r e a c t i o n 23 r a t e i s d i m i n i s h e d by a s m a l l p r e - e x p o n e n t i a l f a c t o r . Thus t h e a i m o f t h i s i n v e s t i g a t i o n was t o d e t e r m i n e w h e t h e r r e a c t i o n (1) o c c u r s w i t h a n o b s e r v a b l e r a t e . e + N~ • aq 2 i z J aq L i t e r a t u r e S u r v e y [ N 2 ~ ] a *• NH 3 ( o r N 2 H 4 o r NH 20H) (1) N o n - e n z y m a t i c n i t r o g e n f i x a t i o n h a s b e e n a c h i e v e d i n a v a r i e t y o f ways i n a d d i t i o n t o t h o s e i n v o l v i n g t r a n s i t i o n m e t a l c o m p l e x e s . I n a d d i t i o n t o t h e H a b e r - B o s c h p r o c e s s f o r t h e 24 c a t a l y t i c c o n v e r s i o n o f N 2 / H 2 m i x t u r e s t o ammonia, g a s pi o x i d a t i v e a n d / o r r e d u c t i v e n i t r o g e n f i x a t i o n h a s b e e n -30-25 accomplished by: (a) r a d i o l y s i s of N /0 or N /H gas mixtures, 2 2 2 2 26 27 (b) s o n o l y s i s of a i r , (c) gas discharge methods , and (d) heterogeneous r e a c t i o n of molecular n i t r o g e n w i t h some group I and I I metals to g i v e i o n i c n i t r i d e s which can be hydrolysed to T O g i v e ammonia . The e l e c t r o l y t i c r e d u c t i o n of molecular n i t r o g e n i n an a p r o t i c s o l v e n t using an a l k a l i metal or the napthalene anion r a d i c a l i n c o n j u n c t i o n w i t h a t r a n s i t i o n metal i o n has 29 been reporte d and the r a d i a t i o n chemical f i x a t i o n of molecular n i t r o g e n i n ^ - i r r a d i a t e d organic compounds has been c l a i m e d 3 0 . In aqueous s o l u t i o n , the o x i d a t i o n of n i t r o g e n by e x c i t e d s i n g l e t oxygen molecules was r e p o r t e d ^ and a process f o r hetero-geneous c a t a l y t i c f i x a t i o n of n i t r o g e n or a i r i n an aqueous s o l u t i o n u t i l i z i n g high energy r a d i a t i o n has been patented-'''. In a d d i t i o n to the l i t e r a t u r e mentioned above, and p a r t i c u l a r l y r e l e v a n t to the present study, are a number of p u b l i c a t i o n s concerned w i t h the r a d i o l y s i s of aqueous s o l u t i o n s 33 3 3 ci b of n i t r o g e n and a i r . Dmitriev and P s h e z h e t s k i i ' reported t h a t the ^ - r a d i o l y s i s of n e u t r a l aqueous s o l u t i o n s c o n t a i n i n g e i t h e r pure n i t r o g e n or nitrogen-oxygen mixtures produced f i x e d n i t r o g e n i n the form of n i t r i t e , n i t r a t e , and ammonia. The y i e l d s of these products increased w i t h i n c r e a s i n g gas pressure above the s o l u t i o n and the presence of oxygen apparently had l i t t l e e f f e c t on the y i e l d s . A value of GdsTH^^O.! was quoted f o r a pressure of one atmosphere of e i t h e r pure n i t r o g e n or an 80/20 mixture of n i t r o g e n and oxygen, w i t h t h i s r i s i n g to G(NH 3)~0.7 at 150 atmospheres. An attempt was made to e x p l a i n these r e s u l t s on the b a s i s of r e a c t i o n s of H, OH and H0 2 -31-r a d i c a l s w i t h n i t r o g e n ; however, on the b a s i s of the now known r e a c t i o n r a t e constants of H (e~ ) and OH w i t h oxygen, i t i s aq v e r y d i f f i c u l t to understand how n i t r o g e n at 10~^ M (1 atm) would be able to e f f i c i e n t l y compete w i t h any oxygen or impur-33c i t i e s present and g i v e ammonia. Hammar et a l . obtained v e r y c o n t r a r y r e s u l t s f o r nitrogen-hydrogen mixtures i n n e u t r a l water using a r e a c t o r r a d i a t i o n source and t a k i n g care to e l i m i n a t e the gas space above the l i q u i d . T h e i r r e s u l t s of G N2(NH 3) = 2.6 ± 1.3 f o r 75% N 2 - 25% H 2 are at l e a s t four orders of magnitude lower than those of D m i t r i e v and P s h e z h e t s k i i s i n c e the G^2 (NH3) n o t a t i o n r e f e r s to the y i e l d c a l c u l a t e d on the b a s i s of the energy absorbed d i r e c t l y by the d i s s o l v e d n i t r o g e n . At one atmosphere l e s s than 1/10,000 of the energy deposited i n the s o l u t i o n would be absorbed d i r e c t l y by the 33d d i s s o l v e d n i t r o g e n . Sato and Steinb e r g e s s e n t i a l l y agreed w i t h the r e s u l t s of Hammar e_t a_l. . They s t u d i e d the * y ~ r a d i o l -y s i s of a i r i n aqueous s o l u t i o n and found t h a t G a i r(NH 3) = 2.3 at n e u t r a l pH, w i t h G a i r(NH^) v a r y i n g s i g n i f i c a n t l y w i t h pH and f l o w r a t e of a i r through t h e i r apparatus. 3. The Chemical System The experiments r e p o r t e d i n t h i s d i s s e r t a t i o n were designed to study i n d e t a i l j u s t one aspect of the r a d i a t i o n chemistry of aqueous n i t r o g e n ; namely the r e a c t i o n of the hydrated e l e c t r o n w i t h d i s s o l v e d n i t r o g e n . For t h i s reason the experimental c o n d i t i o n s were chosen to opti m i z e the chances of observing the r e a c t i o n s i n c e i t c e r t a i n l y would not be expected to be very f a s t . - 3 2 -In a l k a l i n e s o l u t i o n s c o n t a i n i n g hydrogen, the primary r a d i o l y s i s species other than e~ are converted to hydrated ag e l e c t r o n s v i a the s e r i e s of r e a c t i o n s (2), ( 3 ) , and (4). O H + H 2 —* H + H 2 0 (2) k 2= 4 x l 0 7 M" 1s~ 1 H + 0 H ~ —> e~ q + H 2 0 ( 3 ) k 3= 2 x l 0 7 M~ 1s" 1 H 2 0 2 + e~^—• O H + O H " (4) k 4= 1 . 2 x l 0 1 0 M ^ s " 1 Furthermore tr a c e q u a n t i t i e s of oxygen are a l s o e l i m i n a t e d q u i c k l y by r e a c t i o n (5). Thus the H 2 / O H ~ system produces a °2 + e a q ~ * 02~~*"H2°2 ( 5 ) k 5 = L ^ x l O 1 0 M" 1s" 1 very " c l e a n " source of hydrated e l e c t r o n s w i t h a s u b s t a n t i a l y i e l d G(e~ )*»»6 . Although the b i m o l e c u l a r r e a c t i o n of the hydrated e l e c t r o n (6) has a high r a t e constant, at the low dose S a q + e a q - * H 2 + 2 0 H a q <6> V 5xl°9 M " l s _ 1 r a t e s used, the steady s t a t e c o n c e n t r a t i o n of hydrated e l e c t r o n s w i l l be so low tha t t h i s r e a c t i o n may be disregarded. Nominally then, i n the H 2 / O H ~ system c o n t a i n i n g d i s s o l v e d n i t r o g e n , r e a c t i o n (1) w i l l be i n competition o n l y w i t h N 2 + e a q " ' L"2 J aq [ N 2 ~ ] A ^ - * - N H 3 (or N H 2 O H or N ^ ) (1) r e a c t i o n s of the hydrated e l e c t r o n s w i t h i m p u r i t i e s or hydrogen peroxide generated v i a the "molecular" process (G T T n = 0.8) 2. H 2 0 2 A second advantage of the presence of molecular hydrogen was to supply the s t o i c h i o m e t r i c H necessary f o r the u l t i m a t e r e d u c t i o n of N 2 to N H 3 . I t seemed reasonable to assume tha t once the b a r r i e r to r e d u c t i o n had been overcome i n the formation of N 2 or N 2 H , then i t s eventual conversion to ammonia ( or at l e a s t hydrazine or hydroxylamine) would be i n e v i t a b l e i n the reducing environment. Indeed both N 2 ~ and N 2 H have been pro-posed as f e a s i b l e intermediates on the b a s i s of t h e o r e t i c a l - 3 3 -. 34 e l e c t r o n a f f i n i t y s t u d i e s . A number o f p r e l i m i n a r y e x p e r i m e n t s were p e r f o r m e d i n t h i s l a b o r a t o r y by W a l k e r and E d w a r d s J J i n a n a t t e m p t t o t e s t t h e s y s t e m and r e a c t i o n ( 1 ) . T h e y u s e d a s t a i n l e s s s t e e l h i g h p r e s s u r e c e l l w h i c h c o n t a i n e d a g l a s s v e s s e l o f v a r i a b l e v olume t o h o l d t h e aqueous s o l u t i o n . By p r e s s u r i z i n g t h e c e l l t o s e v e r a l h u n d r e d a t m o s p h e r e s a s i g n i f i c a n t d i s s o l v e d n i t r o g e n c o n c e n t r a t i o n ( ~ 0 . 1 M) c o u l d be a c h i e v e d . Ammonia a n a l y s i s was p e r f o r m e d on t h e s o l u t i o n a f t e r ^ - r a d i o l y s i s u s i n g t h e N e s s l e r s p e c t r o p h o t o m e t r i c t e c h n i q u e . A l t h o u g h s u b s t a n t i a l y i e l d s o f ammonia were o b s e r v e d , t h e r e s u l t s were n o t r e p r o d u c i b l e . I n a d d i t i o n , i t was v i r t u a l l y i m p o s s i b l e t o d e t e r m i n e f r o m t h e i r d a t a how much, i f any, o f t h e ammonia was p r o d u c e d v i a r e a c t i o n (1) s i n c e a t a l l t i m e s a s i g n i f i c a n t r a d i a t i o n d o s e was a b s o r b e d b y t h e N 2 / H 2 g a s m i x t u r e above t h e s o l u t i o n and t h e ammonia p r o d u c e d b y g a s p h a s e r a d i o l y s i s was a l s o b e i n g m e a s u r e d . To a l l e v i a t e t h i s p r o b l e m , a t e c h n i q u e was d e v e l o p e d i n w h i c h c o m p l e t e i s o l a t i o n o f t h e n i t r o g e n s a t u r a t e d l i q u i d was p o s s i b l e . B. EXPERIMENTAL 1. R e a g e n t s A l l c h e m i c a l s u s e d were a n a l y t i c a l g r a d e o r b e t t e r . S o l u t i o n s o f v a r i o u s pH were made u s i n g " A n a l a r " s u l f u r i c a c i d and s o d i u m h y d r o x i d e . H y d r o g e n and n i t r o g e n were M a t h e s o n p r e p u r i f i e d g r a d e . F u r t h e r o x y g e n r e m o v a l f r o m t h e h y d r o g e n -n i t r o g e n m i x t u r e s was a c h i e v e d b y i n c o r p o r a t i o n o f a "Deoxo" -34-p a l l a d i u m c a t a l y s t i n the gas t r a i n . Compositions of the gas mixtures used were determined using a V a r i a n Aerograph A90-P-2 gas chromatograph w i t h a 20 f o o t 13X molecular s i e v e column and WX thermal c o n d u c t i v i t y d e t e c t o r s . Water was p u r i f i e d by three consecutive d i s t i l l a t i o n s ; the f i r s t from tap water, the second from a c i d i f i e d dichromate, a f t e r which i t was ^ - i r r a d i a t e d w i t h a 0.5 Mrad dose to remove t r a c e o r g a n i c i m p u r i t i e s . F i n a l l y i t was placed under c o n t i n -uous c y c l i c r e f l u x d i s t i l l a t i o n from a l k a l i n e permanganate _7 u n t i l used. The water was shown to c o n t a i n l e s s than 5 x 10 M ammonium ions by the a n a l y s i s procedure to be d i s c u s s e d l a t e r —6 and i t probably contained much l e s s than 10" M r e a c t i v e o r g a n i c i m p u r i t i e s . 2. R a d i a t i o n Source 60 A Co Gammacell 220 r a d i a t i o n source was used which had an a c t i v i t y of 6170 Curies when loaded i n June, 1967. The dose r a t e was determined by the F r i c k e f e r r o u s s u l f a t e procedure which i s discussed i n d e t a i l i n Appendix 1. The dose r a t e i n s i d e the high pressure c e l l was determined t o be 2500 rads m i n ~ l (1.6 x l O 1 ^ eV g^min"-1-) and the day to day v a r i a t i o n i n dose r a t e caused by decay of the source was c o r r e c t e d f o r using a computer program a l s o discussed i n Appendix 1. 3. Apparatus and Techniques A syringe technique by which the s o l u t i o n c o n t a i n i n g a hig h c o n c e n t r a t i o n of d i s s o l v e d n i t r o g e n could be e f f e c t i v e l y -35-i s o l a t e d to prevent gas phase r a d i o l y s i s was developed. 50 ml a l l - g l a s s s y r i n g e s ( Becton, D i c k i n s o n and Co.) were modified by c u t t i n g o f f the f l a r e d end of the b a r r e l and g r i n d -i n g the tapered g l a s s t i p to f i t a modified B7 socket ( the f l a r e d ends of the B7 sockets were a l s o cut o f f ). The plungers of the syr i n g e s were a l s o cut l e a v i n g a s e c t i o n about 1% inches long to f i t i n s i d e the modified s y r i n g e b a r r e l . A B14 socket was sealed i n place " i n s i d e " the plunger s e c t i o n to f a c i l i t a t e o p e r a t i o n of the syr i n g e by i n s e r t i n g a s e c t i o n of g l a s s tubing w i t h a B14 cone on the end. In a d d i t i o n a modi f i e d B7 socket "cap" was ground to f i t each s y r i n g e t i p i n order to provide a l i q u i d t i g h t s e a l . A photograph of the modi f i e d s y r i n g e and attachments appears i n Fi g u r e 6. The modified s y r i n g e s were designed to f i t snugly i n s i d e the s t a i n l e s s s t e e l high pressure apparatus which i s shown i n Figur e 7. This c e l l was manufactured from a s i n g l e piece of #304 s t a i n l e s s s t e e l and i t s dimensions were 88 mm O.D. by 17 2 mm long w i t h the bore f o r the sy r i n g e being 35 x 145 mm. The top, 25 mm t h i c k , was h e l d down by e i g h t 3/8 i n c h by 1% i n c h cap screws and the pressure s e a l was made by a rubber "0" r i n g . 1/8 i n c h t h i c k c i r c u l a r rubber pads were placed on the i n s i d e of the c e l l to p r o t e c t the syringes during o p e r a t i o n . High pressure n i t r o g e n was fed i n t o the c e l l v i a 1/4 i n c h s t a i n l e s s s t e e l h i g h pressure tubing and valve which were f i t t e d to the s i d e of the c e l l by a standard high pressure c o u p l i n g . A schematic of the pressure system i s shown i n Figure 8. Before use, the apparatus was pressure t e s t e d to F i g u r e 6. Photograph of the m o d i f i e d 50 ml a l l - g l a s s syringes and attachments. F i g u r e 7 . Photograph of the s t a i n l e s s s t e e l high pressure c e l l used t o p r e s s u r i z e the s y r i n g e s . regulator (0-6000 psig ) 1/4"stainless steel high pressure tubing>ir va I ve I—X—I valve | ^ vent high pressure cell 6000 psig gas cylinder F igure 8 . Schematic diagram of the h i g h p r e s s u r e sys tem. -39-10,000 p s i g which was more than twice the normal o p e r a t i n g pressure. The s y r i n g e s were f i l l e d w i t h a nitrogen-hydrogen gas mixture and aqueous s o l u t i o n using the apparatus i l l u s t r a t e d i n F i g u r e 9. The s y r i n g e was attached v i a the B7 socket and l u b r i c a t e d w i t h the s o l u t i o n . By t i l t i n g the apparatus i n the a p p r o p r i a t e d i r e c t i o n s ; f i r s t the gas sample could be admitted to the s y r i n g e and t h i s f o l l o w e d by the gas s a t u r a t e d aqueous s o l u t i o n . Then a f t e r q u i c k l y removing the s y r i n g e from the B7 socket, w h i l e f l u s h i n g a p o r t i o n of the l i q u i d out, a B7 cap was placed over the end. Using t h i s procedure i t was p o s s i b l e to f i l l the s y r i n g e s without any a p p r e c i a b l e atmos-p h e r i c oxygen contamination. When the s y r i n g e was then p r e s s u r i z e d , the gas i n s i d e was compressed and the plunger was pushed down to contact the l i q u i d s u r f a c e s i n c e most of the gas present d i s s o l v e d i n the s o l u t i o n a t the h i g h pressure. A s m a l l g l a s s p l a t e was used as a mixer i n the s y r i n g e s and the s o l u t i o n s were made homogeneous by i n v e r t i n g the h i g h pressure c e l l d u r i n g e q u i l i b r a t i o n . In a t y p i c a l experimental s e r i e s , a l l of the g l a s s apparatus would f i r s t be s c r u p u l o u s l y cleaned by i n i t i a l l y soaking i n permanganic a c i d (KMnO^ i n 95% E^SO^), f o l l o w e d by r i n s i n g w i t h d i s t i l l e d water, soaking i n a n i t r i c a c i d s o l u t i o n of hydrogen peroxide t o remove tr a c e s of Mn0 2, and f i n a l l y r i n s i n g w e l l w i t h s i n g l y , doubly and t r i p l y d i s t i l l e d water s u c c e s s i v e l y . The apparatus was then d r i e d i n a s p e c i a l o r g a n i c f r e e oven at 100 °C and then assembled. Corning s i l i c o n e Figure 9 . Diagram of the apparatus used to f i l l the syringes with gas and l i q u i d samples, shown with a syringe attached. -41-grease was used on the stopcocks which c o n t r o l l e d the gas flow; a l l other j o i n t s were l u b r i c a t e d w i t h the experimental s o l u t i o n . With the s y r i n g e disconnected and a B7 plug s u b s t i t u t e d , the s o l u t i o n was deoxygenated by bubbling h i g h p u r i t y hydrogen through the apparatus f o r about one hour. Then, w i t h the stopcocks c l o s e d , the s o l u t i o n was " p r e - i r r a d i a t e d " f o r 30 5 minutes (~2 x 10 rads) under the hydrogen atmosphere. This p r e - i r r a d i a t i o n was intended to remove any t r a c e s of r e d u c i b l e o r g a n i c or i n o r g a n i c contaminants i n the s o l u t i o n s . F o l l o w i n g the p r e - i r r a d i a t i o n , a weighed s y r i n g e was attached to the app-aratus and l u b r i c a t e d w i t h a b i t of the s o l u t i o n . A f t e r f l u s h i n g the e n t i r e system w i t h the ^2/^2 m i x t u r e f ° r about 2-3 hours, the s y r i n g e was f i l l e d w i t h ~30 ml of gas and 20 ml of l i q u i d , then removed and capped using the procedure d e s c r i b e d above. The syr i n g e was then weighed to o b t a i n the weight of s o l u t i o n and p r e s s u r i z e d i n the h i g h pressure c e l l u s ing pure n i t r o g e n a t 3000 p s i g (200 atm). At t h i s pressure v i r t u a l l y a l l of the gas would d i s s o l v e i n the s o l u t i o n and to ensure t h a t t h i s occured, the p r e s s u r i z e d sample was l e f t to e q u i l i b r a t e f o r about one hour, during which time i t was o c c a s i o n a l l y i n v e r t e d to cause the mixing p l a t e to " s t i r " the s o l u t i o n . The e q u i l i b r a t e d sample was then i r r a d i a t e d i n the Gammacell f o r v a r i o u s times ranging from 60 - 12,000 minutes. A f t e r the i r r a d i a t i o n , the c e l l was s l o w l y depressurized ( f a s t d e p r e s s u r i z a t i o n i n v a r i a b l y l e d to a s h a t t e r e d s y r i n g e caused by the plunger becoming jammed) and the sy r i n g e was removed and the s o l u t i o n analysed. I t was noted t h a t when the sy r i n g e s were removed (about 5 minutes a f t e r d e p r e s s u r i z a t i o n ) o n l y - 4 2 -a b o u t 10 - 1 5 ml o f g a s was o b s e r v e d above t h e s o l u t i o n w h i c h was e v o l v i n g g a s c o n t i n u o u s l y . By s h a k i n g t h e s y r i n g e and a l l o w i n g s u f f i c i e n t t i m e , t h e g a s volume s o o n i n c r e a s e d t o i t s o r i g i n a l v a l u e . T h e s e o b s e r v a t i o n s i n d i c a t e d t h a t d u r i n g t h e p r e s s u r i z a t i o n s t a g e no l e a k a g e o f t h e s y r i n g e o c c u r e d and a l s o t h a t most o f t h e g a s h a d d i s s o l v e d i n t h e l i q u i d u n d e r p r e s s u r e . 4 . A n a l y t i c a l P r o c e d u r e s The i r r a d i a t e d s o l u t i o n s were a n a l y s e d f o r ammonia u s i n g t h e v e r y s e n s i t i v e " i n d o p h e n o l b l u e " method. The p r o -c e d u r e u s e d was s i m i l a r t o t h e one d e v e l o p e d b y T e t l o w and 37 W i l s o n and c o m p l e t e d e t a i l s o f t h e t e c h n i q u e and c a l x b r a t i o n d a t a a r e g i v e n i n A p p e n d i x 2 . B a s i c a l l y , t h e p r o c e d u r e i n v o l v e d c o n v e r t i n g t h e ammonia t o t h e i n d o p h e n o l b l u e dye u s i n g p h e n o l and s o d i u m h y p o c h l o r i t e i n v e r y a l k a l i n e s o l u t i o n . The c o n c e n -t r a t i o n o f ammonia was r e l a t e d t o t h e a b s o r b a n c e o f t h e dye a t 6 3 0 nm . The s e n s i t i v i t y o f t h e t e s t was a b o u t 5 x 10 M ammonium i o n s , however i n t e r f e r e n c e o f h y d r o g e n p e r o x i d e as d i s c u s s e d i n t h e A p p e n d i x l i m i t e d t h e w o r k i n g s e n s i t i v i t y t o —6 a b o u t 5 x 10 M . I n a n a l y s i s o f a t y p i c a l e x p e r i m e n t , t h e i r r a d i a t e d s o l u t i o n was d i v i d e d i n t o two p a r t s ; one o f w h i c h was a n a l y s e d i n t h e n o r m a l manner and t o t h e o t h e r p o r t i o n a known q u a n t i t y o f NH^ + s o l u t i o n was added as an " i n t e r n a l " s t a n d a r d t o c h e c k on any p o s s i b l e e f f e c t s o f o t h e r r a d i o l y s i s p r o d u c t s ( s u c h as h y d r o g e n p e r o x i d e ) . Q u a l i t a t i v e s p o t c h e c k s f o r t h e p r e s e n c e o f h y d r a z i n e and h y d r o x y l a m i n e were p e r f o r m e d u s i n g t r i n i t r o - b e n z e n e - s u l f o n i c -43-C. RESULTS The p e r t i n e n t data from the " s u c c e s s f u l " experiments are l i s t e d i n Table 3. (Several experiments were u n s u c c e s s f u l because of s h a t t e r e d syringes.) The two methods used to c a l -c u l a t e the y i e l d s of ammonia, G w ( N H o ) and G N2 ( N H ) , are based J 3 on the dose absorbed by the water and d i r e c t l y by the n i t r o g e n molecules, r e s p e c t i v e l y . Experiment A-4 was done to t i e i n 35 w i t h the p r e l i m i n a r y experiments of Walker and Edwards . In t h i s experiment the plunger was removed from the s y r i n g e so t h a t the s o l u t i o n was i n contact w i t h a l a r g e gas volume and gas phase r a d i o l y s i s could occur. In a l l cases the spot t e s t s f o r hydrazine and h y d r o x y l -amine were negative i n d i c a t i n g t h a t these " i n t e r m e d i a t e s " were not present i n s i g n i f i c a n t y i e l d s at the time of a n a l y s i s . The o v e r a l l r e s u l t s of these experiments may be summar-i z e d as f o l l o w s : the y i e l d of ammonia from an i s o l a t e d s o l u t i o n of n i t r o g e n at 0.1 M was G W(NH 3) ~> 0.002 and t h i s y i e l d was independent of the pH of the sample. D. DISCUSSION The f a c t t h a t the y i e l d of ammonia based on the dose absorbed by the water was G W ( N H 3 ) ~ 0.002 means t h a t d e s p i t e the favourable c o n d i t i o n s f o r r e a c t i o n of hydrated e l e c t r o n s w i t h n i t r o g e n , i t does not occur to a s i g n i f i c a n t extent. The value of G N2(NH 3) = 0.7 f o r the n e u t r a l s o l u t i o n of n i t r o g e n i s i n reasonable agreement w i t h t h a t measured by Hammar et a l . TABLE 3 SUMMARY OF NITROGEN FIXATION EXPERIMENTS Exp PH Dissolved Gas a(Mxl0 2) Dose -20 (eVxlO ) GW(NH,) b GN2(NH N [H2] M Water N 2 (MxlO 6) A-1 11 1.8 0.38 <0.001 1.6 0.0036 < 5 <0.03 <1.4 2 11 8.9 1.6 <0.001 1.5 0.0036 < 5 r ) <0.03 <1.4 3 11 8.9 1.6 <0.001 23 0.060 $ 5 d $0,002 $0.8 4 e 11 13 0.27 < 0.001 1.5 1.9 84 0.51 0.42 B - l 7 7.5 <0.001 2.2 2.5 0.0052 < 5 f <0.02 <10 2 7 9.2 <0.001 <0.001 2.6 0.0067 <5 <0.02 <10 3 7 7.1 0.79 <0.001 2.6 0.0052 <5 <0.02 <10 C- l 7 8.0 <0.001 2.4 23 0.051 « 5 d $0,002 «1.0 2 7 9.2 <0.001 <0.001 24 0.062 $0,002 $1.0 3 7 8.0 1.2 < 0.001 26 0.056 $ 5 d $0,002 $1.0 4 7 9.6 0.72 <0.001 280 0.64 51 0.0016 0.70 D-l? 7 7.9 0.79 <0.001 25 0.056 $ 5 d $ 0.002 $1.0 2 2 7.1 0.79 < 0.001 25 0.050 $ 5 d $ 0.002 $ 1.0 Footnotes: a. The concentrations of dissolved gases were calculated from the s o l u b i l i t y data - ref. 39. b. G W(NH 3) i s the y i e l d of ammonia based on the energy absorbed by the water, c. G 2(NH3) i s the y i e l d of ammonia based on the energy absorbed d i r e c t l y by nitrogen, d. Ammonia'concentrations given as an upper l i m i t because of the very small absorbance (»**0.03) which could have been caused by peroxide. e. Plunger removed from the syringe, large gas volume above the so l u t i o n . f. Severe hydrogen peroxide interference due to presence of oxygen, g. About 4 g of sta i n l e s s s t e e l chips added to the so l u t i o n . (G N 2(NH 3) = 1.8 - 0 . 9 ) 3 3 ° and by Sato and Steinberg ( G a i r ( N H 3 ) = 2.3)^"^. This ammonia i s v e r y l i k e l y produced by the d i r e c t a c t i o n of the ^ - r a d i a t i o n on the d i s s o l v e d n i t r o g e n molecules to g i v e or N^* which then r e a c t w i t h the s o l v e n t or d i s -s o l v e d hydrogen to e v e n t u a l l y g i v e ammonia. This process apparently proceeds w i t h a s i m i l a r e f f i c i e n c y to the gas phase r e a c t i o n s i n c e G^2(NH^) = 0.7 was obtained by Cheek and Linnenbom f o r the gas phase r a d i o l y s i s of nitrogen-hydrogen 25e N? , » mixtures. A f u r t h e r s i g n i f i c a n c e of the G ^(NH^) value i s t h a t once the r e d u c t i o n process has been i n i t i a t e d , the end product i s ammonia. This agrees w i t h the r e s u l t s of the q u a l i t a t i v e t e s t s f o r hydrazine and hydroxylamine which i n d i c a t e d t h a t these intermediates were not formed i n a s i g n i f i c a n t amount. (The l i m i t of d e t e c t i o n of the t e s t was about 10~ 5 M _3 f o r hydrazine and about 10 M f o r hydroxylamine which g i v e s corresponding upper l i m i t s of G W(N 2H 4) ^  0. 005 and G w(NH 2OH) ^  0. 5). The f a c t t h a t some, i f not a l l , of the ammonia formed i n these experiments a r i s e s from d i r e c t a c t i o n of r a d i a t i o n on the d i s s o l v e d n i t r o g e n means t h a t an upper l i m i t of G W ( N H 3 ) ^ 10" can be placed on the y i e l d of ammonia produced through the r e a c t i o n of n i t r o g e n w i t h hydrated e l e c t r o n s . This allows a c a l c u l a t i o n of an upper l i m i t f o r the r a t e constant of r e a c t i o n (1). Assuming the a l t e r n a t i v e f a t e of the hydrated e l e c t r o n s i s r e a c t i o n w i t h u n s p e c i f i e d i m p u r i t y , X, a t the d i f f u s i o n c o n t r o l l e d r a t e of 1 0 ^ M \ then an estimate of the con-c e n t r a t i o n of X w i l l a l l o w c a l c u l a t i o n of k^. P u r i f i c a t i o n of the water and i t s deoxygenation probably reduced the i m p u r i t y -46-l e v e l to about 10~^ M . Since l a r g e r a d i a t i o n doses were used i n the presence o f m o l e c u l a r hydrogen, most o f t h i s i n i t i a l i m p u r i t y would be e l i m i n a t e d w i t h i n the f i r s t few minutes of the i r r a d i a t i o n . In a d d i t i o n , hydrogen p e r o x i d e produced by the "molecular" processes would be d e s t r o y e d by e~^ almost as f a s t as i t was being produced and i t would reach a "steady s t a t e " c o n c e n t r a t i o n of v e r y much l e s s than 10 ^ M . There-f o r e a v e r y c o n s e r v a t i v e estimate o f the steady s t a t e r e a c t i v e r i —6 i m p u r i t y c o n c e n t r a t i o n , [xj , would be 10 M . A p p l y i n g the steady s t a t e k i n e t i c treatment t o e : aq r a t e o f p r o d u c t i o n o f e~ = G(e~ ) • I * aq aq rate of loss of e" = k |X]„_ Te" | aq x L J s s I aqj where I is the radiation intensity, implies: ss [*a-q] ss = G ( e a q > • 1 k [ x ] ss x ss The r a t e o f ammonia p r o d u c t i o n i s g i v e n by: r a t e o f p r o d u c t i o n of NH 3 = k^  ^ N2j £eaqJ = G W(NH 3) • I Sin c e G(e~ ) = G _ + G R + G n „ = 5.7 a t h i g h pH then f o r a a 3 e a q U H n i t r o g e n c o n c e n t r a t i o n of 0.1 M i t f o l l o w s t h a t : kx = G W(NH 3) • I = G W(NH 3) • I H [Saq]ss [N2] ' G(e^) ' I *x [*]s -47-k. < 1Q"3 ^ 18 M-V1 -1 10 • 5.7 10 _6 10 • 10 ° Thus a c o n s e r v a t i v e upper l i m i t f o r the r e a c t i o n r a t e constant f o r p r o d u c t i o n of ammonia from hydrated e l e c t r o n s r e a c t i n g w i t h n i t r o g e n i s k ^ 18 M "*"s~^. This i s one of the slowest 1 r a t e constants estimated f o r hydrated e l e c t r o n s . ( I f r e a c t i o n (1) gave hydroxylamine i n s t e a d of ammonia, the upper l i m i t on 3 -1 -1 the r a t e constant would be about 5 x 10 M s , however because of the i n s e n s i t i v i t y of the t e s t t h i s c a l c u l a t i o n i s c o n s i d e r a b l y l e s s c e r t a i n than t h a t f o r ammonia.) The s i g n i f i c a n c e of the experiment at pH 2 (D-2) i s t h a t the hydrogen atom r e a c t i o n w i t h n i t r o g e n i s a l s o extremely slow. At pH 2, a l l of the hydrated e l e c t r o n s would be con-v e r t e d to H atoms v i a r e a c t i o n (7). e~ + H*—*- H + H,0 (7) k_ = 2 x l 0 1 0 M " 1 s ~ 1 acj a<j £ I This r e s u l t i s not p a r t i c u l a r l y s u p r i s i n g s i n c e hydrogen atoms are much l e s s powerful reducing agents than hydrated e l e c t r o n s and t h e i r r e a c t i o n r a t e s are g e n e r a l l y slower. Experiment D-l i n which the s o l u t i o n contained s t a i n l e s s s t e e l chips i n d i c a t e d t h a t no c a t a l y t i c enhancement of the r e a c t i o n could be achieved by i t s presence. In c o n c l u s i o n , these experiments have r e i t e r a t e d t h a t the n i t r o g e n molecule i s extremely s t a b l e towards r e d u c t i o n , e i t h e r by hydrated e l e c t r o n s or hydrogen atoms. I t t h e r e f o r e seems u n l i k e l y that the f i x a t i o n of n i t r o g e n i n nature (even - 4 8 -i n t h e p r e - b i o t i c t i m e s ) c o u l d e v e r a r i s e t h r o u g h " i n d i r e c t a c t i o n " o f h i g h e n e r g y r a d i a t i o n . E. SUGGESTIONS FOR FURTHER STUDY S i n c e t h e h y d r a t e d e l e c t r o n i s u n a b l e t o r e d u c e " f r e e " m o l e c u l a r n i t r o g e n , a n i n t e r e s t i n g q u e s t i o n i s w h e t h e r i t c a n f i x m o l e c u l a r n i t r o g e n i n a b o u n d f o r m . A s was d i s c u s s e d i n t h e i n t r o d u c t i o n t o t h i s s t u d y , i t h a s b e e n r e p o r t e d t h a t some o f t h e t r a n s i t i o n m e t a l c o m p l e x e s a p p a r e n t l y c a n b e r e d u c e d 20 i n o r g a n i c s o l v e n t s t o g i v e ammonia. Thus i t m i g h t b e e x p e c t e d t h a t t h e h y d r a t e d e l e c t r o n w o u l d be a b l e t o i n i t i a t e r e d u c t i o n o f c o o r d i n a t e d n i t r o g e n . U n f o r t u n a t e l y m o s t o f t h e known t r a n s i t i o n m e t a l com-p l e x e s o f n i t r o g e n a r e u n s t a b l e i n a q u e o u s s o l u t i o n . One o f t h e o smium compounds, h o w e v e r , was t h o u g h t t o b e s t a b l e a n d i n c o n j u n c t i o n w i t h P r o f e s s o r B. R. James a n d D r . E. O c h i a i I I a n e x p e r i m e n t was p e r f o r m e d t o s e e i f Os (NH^) ^ N2C^"2 c o u ^ b e r e d u c e d . T h i s e x p e r i m e n t p r o v e d t o b e a f a i l u r e s i n c e t h e c o m p l e x was f a i r l y r a p i d l y h y d r o l y s e d i n t h e a l k a l i n e s o l u t i o n a n d some o f t h e ammonia l i g a n d s w e r e a p p a r e n t l y r e p l a c e d b y OH~ o n e s . I n l i g h t o f t h e a b o v e r e m a r k s , a s u g g e s t i o n f o r f u r t h e r s t u d i e s o f t h e r e d u c i n g p o w e r o f t h e h y d r a t e d e l e c t r o n t o w a r d s n i t r o g e n w o u l d be t o e x a m i n e t h e a q u e o u s r a d i a t i o n c h e m i s t r y o f s u i t a b l e n i t r o g e n c o m p l e x e s when compounds w h i c h a r e n o t h y d r o l y s e d h a v e b e e n s y n t h e s i z e d . T h i s a r e a o f r e s e a r c h c o u l d - 4 9 -have coiranercial p o t e n t i a l i f a su i t a b l e system were found where a t r a n s i t i o n metal ion would f i r s t coordinate the nitrogen and then a f t e r the i n i t i a t i o n of reduction, complex another nitrogen molecule. A continuous c y c l i c process can be envisaged wherein the t r a n s i t i o n metal ion acts e s s e n t i a l l y as a homo-geneous c a t a l y s t , with nitrogen and hydrogen flowing through an a l k a l i n e s o l u t i o n under r a d i o l y s i s , and thus producing ammonia. CHAPTER III ASPECTS OF THE RADIATION CHEMISTRY OF PROPYLENE CARBONATE Propylene carbonate (4-methyl-2-dioxolone, which may be abbreviated as PC i n the following discussions) i s a very polar aprotic solvent with some extraordinary physical and chemical properties. I l l u s t r a t e d i n Figure 10, propylene u V 3 U / \ <V° M o Figure 10. One of the stereoisomers of propylene carbonate. carbonate i s a f i v e membered he t e r o c y c l i c r i n g system which has two stereoisomers. Among i t s unusual f e a t u r e s ^ 0 are: (a) a very large l i q u i d range from less than -70 °C to +240 °cf° a (b) a moderately high d i e l e c t r i c constant of 65 at room temp-erature which i s very temperature dependent, r i s i n g to greater than 90 below -60 °C , (c) a corresponding very large —18 40d permanent dipole moment of 4.94 x 10 esu cm although t h i s does not appear to cause strong intermolecular association i n the solvent as evidenced by a Kirkwood c o r r e l a t i o n factor 40b, e of near unity and NMR .and IR data ' , (d) u l t r a v i o l e t trans-41 \ parency above 225 nm which indicates that the ,0=0 i s not an "organic" carbonyl but rather that the electron system i s del o c a l i z e d as i n the CO^ ion, and (e) chemical s t a b i l i t y ; PC does not react with strong reducing agents such as the -51-40g,41 a l k a l i metals (or d i s s o l v e them), nor i s i t e a s i l y 41 o x i d i z e d by strong oxidants such as permanganate. In a d d i t i o n , PC i s r e a d i l y p u r i f i e d by simple vacuum d i s t i l l a t i o n 42 and i t i s not t o x i c or hygroscopic. The major disadvantage of PC i s o t h a t i t i s hydrolysed by a c i d s and bases, although 40a i t s n e u t r a l aqueous s o l u t i o n s are s t a b l e . In l i g h t of the p a r t i c u l a r l y i n t e r e s t i n g and unusual aspects of propylene carbonate, the present i n v e s t i g a t i o n was i n i t i a t e d t o examine some of the f a c e t s of the r a d i a t i o n chemistry of PC. In p a r t i c u l a r , i t was of i n t e r e s t t o d e t e r -mine whether e l e c t r o n s could be s t a b i l i z e d i n the system, s i n c e t h i s l a b o r a t o r y has been p a r t i c u l a r l y i n v o l v e d w i t h : i c 15 43a s o l v a t e d e l e c t r o n s , both i n water and i n p o l a r a p r o t i c s o l v e n t s of h i g h d i e l e c t r i c constant, such as formamide 43b and d i m e t h y l s u l f o x i d e . A g e n e r a l i n v e s t i g a t i o n of the r a d i o l y s i s of PC would a l s o provide o r i g i n a l i n f o r m a t i o n about a c l a s s of compounds whose r a d i a t i o n chemistry i s e s s e n t i a l l y unexplored. A thorough survey of the l i t e r a t u r e r e v ealed t h a t although the e l e c t r o c h e m i s t r y of PC had been s t u d i e d t o a l i m i t e d e x t e n t , i t s r a d i a t i o n chemistry has never been i n v e s t -i g a t e d . * Of the other c y c l i c organic carbonates ( l i t e r a l l y dozens of v a r i a t i o n s of the b a s i c f i v e membered r i n g system co u l d be synthesized) o n l y ethylene carbonate and i t s t e t r a -44 phenyl d e r i v a t i v e have been b r i e f l y examined. *f o o t n o t e 2. A f t e r t h i s study was begun, Hayon reported the f r e e i o n y i e l d f o r PC as obtained by pulse r a d i o l y s i s of i t s anthracene s o l u t i o n . - 5 2 -During the i n i t i a l stages of t h i s i n v e s t i g a t i o n , i t was d i s c o v e r e d t h a t propylene carbonate formed an e x c e l l e n t g l a s s y s o l i d when cooled q u i c k l y i n l i q u i d n i t r o g e n . Subsequent ^ - i r r a d i a t i o n of these g l a s s e s at 77 °K and examination by e l e c t r o n s p i n resonance revealed some very unusual r a d i c a l s p e c t r a . Because of t h i s , the emphasis of the i n i t i a l l y proposed r a d i a t i o n chemical i n v e s t i g a t i o n was s h i f t e d from the l i q u i d phase to the g l a s s y s o l i d s t a t e . Thus the m a j o r i t y of the r e s u l t s presented here w i l l be concerned w i t h the r a d i o l y s i s of the low temperature g l a s s e s . Although a s i g n i f -i c a n t amount of data was obtained from the l i q u i d phase experiments, t h i s study was not as extensive as was o r i g i n a l l y planned. -53-PART I - RADIOLYSIS OF PROPYLENE CARBONATE IN THE SOLID STATE A. INTRODUCTION Studies of the r a d i o l y s i s of f r o z e n p o l a r systems have been mainly concerned w i t h water, i t s s o l u t i o n s and the 4 45 a l c o h o l s , although ketones and e s t e r s have been examined and comprehensive i n v e s t i g a t i o n s of the very p o l a r a c e t o n i t r i l e 46 compounds have been pub l i s h e d . The weakly-polar f r o z e n systems which have been the most e x t e n s i v e l y s t u d i e d are the 47 g l a s s y e t h e r s , such as methyl-tetrahydrofuran In v i r t u a l l y every compound which forms a g l a s s y s o l i d (amorphous s t a t e i n which the random o r i e n t a t i o n of the l i q u i d phase i s thought to be "frozen in") r a d i o l y t i c a l l y generated 5 species i d e n t i f i e d as trapped e l e c t r o n s have been discovered . These e n t i t i e s are c h a r a c t e r i s e d by i n t e n s e , n e a r - i n f r a r e d or v i s i b l e 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 s i n g l e narrow, Gaussian shaped, e l e c t r o n s p i n resonance l i n e s near the f r e e s p i n g-value of 2.0023. The ESR l i n e i s normally q u i t e e a s i l y power s a t u r a t e d . This s a t u r a t i o n does not cause the us u a l l i n e broadening but r a t h e r i s an "inhomogeneous" type of s a t u r a t i o n where the l i n e shape does not change w h i l e i t s i n t e n s i t y decreases uni f o r m l y . Photobleaching of the trapped e l e c t r o n s w i t h l i g h t i n the r e g i o n corresponding to t h e i r o p t i c a l a b s o r p t i o n spectrum a l s o occurs. The species may a l s o be e l i m i n a t e d by added s o l u t e s o f high e l e c t r o n a f f i n i t y , i . e . e l e c t r o n scavengers, such as napthalene. Though normally s t a b l e f o r long periods of time i n very p o l a r g l a s s e s at 77 °K, - 5 4 -trapped e l e c t r o n s i n weakly p o l a r systems (such as methyl-tetrahydrofuran) are not s t a b l e and they decay v i a processes at l e a s t p a r t i a l l y a t t r i b u t a b l e to "geminate recombination" 17 w i t h t h e i r parent p o s i t i v e ions Trapped r a d i c a l s produced by r a d i o l y s i s are however, not so e a s i l y i d e n t i f i e d . Since h i g h energy r a d i a t i o n i s not very d i s c r i m i n a t i n g i n i t s primary e x c i t a t i o n , there i s seldom s p e c i f i c decomposition, but r a t h e r numerous d i f f e r e n t r a d i c a l s are g e n e r a l l y formed i n a s i n g l e system. The combination of proton h y p e r f i n e s p l i t t i n g of the i n d i v i d u a l r a d i c a l ESR s p e c t r a and a n i s o t r o p y of the s i g n a l s , together w i t h the o v e r l a p p i n g o f the l i n e s u s u a l l y prevents p o s i t i v e i d e n t i f i -c a t i o n of the species i n a l l but the v e r y s i m p l e s t systems, such as water and methanol. A b r i e f d i s c u s s i o n of the b a s i c theory of ESR and i t s a p p l i c a t i o n s to amorphous s o l i d s i s g i v e n i n Appendix 3. O p t i c a l a b s o r p t i o n spectroscopy i s a l s o of o n l y l i m i t e d v a l u e i n i d e n t i f y i n g o rganic r a d i c a l s s i n c e most absorb i n the u l t r a v i o l e t and the s p e c t r a are again u s u a l l y broad and o v e r l a p p i n g . V a r i a b l e temperature s t u d i e s , scavenger techniques, photobleaching experiments, and ESR r . f . power s a t u r a t i o n e f f e c t s sometimes h e l p to u n r a v e l the complexity of i r r a d i a t e d organic systems. In most cases however, the r a d i c a l species defy p o s i t i v e i d e n t i f i c a t i o n unless they can be generated i n an unambiguous manner, such as OH r a d i c a l s by the p h o t o l y s i s of ^202' Since propylene carbonate i s a very p o l a r compound and because g l a s s y p o l a r systems are known to t r a p e l e c t r o n s very 4 5 e f f i c i e n t l y ' the present i n v e s t i g a t i o n was begun. I t s aim -55-was t o determine whether e l e c t r o n s could be s t a b i l i z e d i n g l a s s y PC, and i f so, t o study t h e i r p r o p e r t i e s both by ESR and o p t i c a l techniques. Any i n f o r m a t i o n obtained w i t h regard to the r a d i c a l species produced would be considered an added b e n e f i t of the i n v e s t i g a t i o n . B. EXPERIMENTAL 1. Reagents Eastman Kodak p r a c t i c a l grade propylene carbonate was p u r i f i e d by f r a c t i o n a l d i s t i l l a t i o n under vacuum as d e s c r i b e d i n Appendix 4. The p u r i f i e d s o l v e n t was s t o r e d under dry h e l i u m i n a d i s p e n s i n g apparatus which i s a l s o discussed i n Appendix 4 along w i t h the p h y s i c a l analyses. A l l other chemicals used were a n a l y t i c a l reagent grade or e q u i v a l e n t . Helium used f o r deoxygenation was s u p p l i e d by Canadian L i q u i d A i r and i t was d r i e d by passing through a long copper c o i l immersed i n l i q u i d n i t r o g e n . 2. R a d i a t i o n Source 60 A Co Gammacell 220 was used f o r the i r r a d i a t i o n s . The source a c t i v i t y was 6170 Curies when loaded i n June 1967. An approximate dose r a t e of 4000 rads min (2.5 x 10^eV min"^") was estimated from f e r r o u s s u l f a t e dosimetry done f o r other experimental apparatus. T o t a l doses ranged from 2 x 10 4 t o 1 x 10 6 rads (1 x 1 0 1 8 to 8 x 1 0 1 9 eV g ' 1 ) . - 5 6 -3. Sample P r e p a r a t i o n and I r r a d i a t i o n E l e c t r o n s p i n resonance measurements were made on g l a s s y " b a l l s " of PC, 2-3 mm i n diameter, prepared by dropping the deoxygenated PC i n t o a dewar of l i q u i d n i t r o g e n as described 48 by A l g e r . The samples prepared i n t h i s way were n e a r l y p e r f e c t spheres, completely t r a n s p a r e n t , f r e e from bubbles and cr a c k s , and mechanically sound. Attempts to prepare a g l a s s i n s m a l l diameter quartz tubing always gave a p o l y c r y s t a l l i n e sample. For the o p t i c a l s t u d i e s , the sample of PC was sealed i n a 1 cm square " s p e c t r o s i l " quartz spectrophotometer c e l l a f t e r thorough degassing by m u l t i p l e freeze-pump-thaw c y c l e s . When immersed i n l i q u i d n i t r o g e n , these sample u s u a l l y became p o l y c r y s t a l l i n e . On occ a s i o n , however, a reasonably t r a n s -parent " g l a s s " was formed w i t h r e l a t i v e l y few cracks and these samples were used. I t was noted t h a t the a d d i t i o n of even very s m a l l q u a n t i t i e s of water to the PC d i d not improve the q u a l i t y of the g l a s s . In f a c t , a d d i t i o n of o n l y 0.1% water caused the samples to become cloudy and v i r t u a l l y opaque. A l l glassware used i n handl i n g the propylene carbonate samples, i . e . beakers, p i p e t t e s , s p e c t r o - c e l l s e t c . , was always s c r u p u l o u s l y cleaned u s i n g the r o u t i n e permanganic a c i d - p e r o x i d e - d i s t i l l e d water treatment. The sample b a l l s of PC f o r the ESR s t u d i e s were i r r a d -i a t e d i n a sm a l l pyrex dewar c o n t a i n i n g l i q u i d n i t r o g e n . In some cases, when k i n e t i c s t u d i e s and y i e l d measurements were made, the samples were placed i n a s m a l l beaker l i n e d w i t h -57-aluminum f o i l to prevent p o s s i b l e b l e a c h i n g by the blue f l u o r e s c e n c e which pyrex emits under r a d i o l y s i s at l i q u i d n i t r o g e n temperature. S i m i l a r precautions were taken w i t h the samples f o r the o p t i c a l s t u d i e s . 4. E l e c t r o n Spin Resonance Measurements A l l e l e c t r o n s p i n resonance s p e c t r a were measured using a V a r i a n A s s o c i a t e s E-3 spectrometer which operated at 9.3 GHz (X-band) and w i t h 100 kHz f i e l d modulation. The s p e c t r a a t 77 °K were recorded w i t h the samples contained i n a quartz dewar f i l l e d w i t h l i q u i d n i t r o g e n . For measurements above 77 °K, the custom-made quartz dewar shown i n F i g u r e 11 was used. The sample b a l l was supported i n the center of t h i s dewar on the end of the t h i n evacuted quartz tube which a l s o contained the thermocouple. Coo l i n g was achieved using n i t r o g e n f l o w i n g through a long copper c o i l immersed i n a l a r g e dewar of l i q u i d n i t r o g e n . In p r a c t i c e , the dewar was f i r s t cooled to the minimum p o s s i b l e temperature (85-90 °K) and the gas f l o w was then momentarily i n t e r r u p t e d w h i l e a sample b a l l was q u i c k l y t r a n s f e r r e d from a l i q u i d n i t r o g e n bath and dropped i n t o the top of the dewar. Continuous monitoring of the temp-e r a t u r e w i t h a chart recorder i n d i c a t e d t h a t during the t r a n s -f e r process the samples probably warmed to about 95 °K before the gas f l o w was r e s t a r t e d . The temperature was v a r i e d simply by a l t e r i n g the f l o w r a t e of c o l d n i t r o g e n through the dewar. A g i v e n temperature could be maintained to 1 2 °K by t h i s method which proved to be adequate f o r the purposes of the experiments reported here. - 5 8 -•EVACUATED DEWAR SAMPLE BALL THERMOCOUPLE COLD NITROGEN B 7 CONE / SOCKET JOINT EVACUATED SAMPLE SUPPORT TUBE * WAX SEAL THERMOCOUPLE LEADS Figure 11. Varia b l e temperature ESR dewar - 5 9 -The magnetic f i e l d s t r e n g t h and f i e l d scan l i n e a r i t y were c a l i b r a t e d w i t h a proton-probe gaussmeter and the micro-wave frequency was checked w i t h a Hewlett-Packard model 52 55A d i g i t a l frequency counter. The microwave power l e v e l s were not c a l i b r a t e d and the values quoted are those read d i r e c t l y from the power c o n t r o l d i a l of the instrument. Spectroscopic s p l i t t i n g f a c t o r s , g-values, were d e t e r -mined using a f i n e l y powdered sample of DPPH (diphenyl p i c r y l -hydrazyl) sealed i n a t h i n quartz tube which was placed i n the dewar along w i t h the sample. 9DPPH = 2.0036 was used and the unknown g-values were c a l c u l a t e d u s i n g the formula ( v i i i ) g = 2.0036 ( 1 - AH ) ( v i i i ) H r where AH i s the d i f f e r e n c e i n the magnetic f i e l d between the center of the unknown resonance and t h a t of the DPPH and H. r 48 i s the absolute magnetic f i e l d of the " r " resonance. R a d i a t i o n y i e l d s (G values) were estimated using i r r a d -i a t e d methanol as a standard. Both the PC sample and methanol were prepared under i d e n t i c a l c o n d i t i o n s ; i . e . the samples were made as c l o s e to the same s i z e as p o s s i b l e and both were i r r a d i a t e d s imultaneously i n the same dewar to provide i d e n t i -c a l absorbed doses (about 1.2 Mrad). The trapped e l e c t r o n s i n methanol were photo-converted t o the CH^OH r a d i c a l to avoid the power s a t u r a t i o n problems of t h i s species and G(CH^0H)= 6.7 was used f o r the t o t a l y i e l d of C^OH r a d i c a l s 4 . The ESR s p e c t r a of both PC and methanol were recorded f o r a s i n g l e b a l l i n i d e n t i c a l p o s i t i o n s i n the c a v i t y and under i d e n t i c a l spectrometer c o n d i t i o n s . The areas under the absorption curves -60-were estimated by double i n t e g r a t i o n (see r e f . 63, page, 442 ) and the r e l a t i v e G values were c a l c u l a t e d using the r a t i o of the areas. These G values were s u b j e c t t o con s i d e r a b l e e r r o r due t o the u n c e r t a i n t y i n the r e l a t i v e diameters of the sample b a l l s , i n the i n t e g r a t i o n technique and i n G(CH^OH). A l l the ESR s p e c t r a reproduced i n t h i s t h e s i s were recorded d i r e c t l y onto the drawing paper by the E-3 s p e c t r o -meter using reduced g a i n and incre a s e d f i e l d sweep. 5. O p t i c a l Absorption Measurements O p t i c a l a b s o r p t i o n s p e c t r a were recorded using a Cary model 14 spectrophotometer. The sample c e l l was maintained at 77 °K i n a. quartz dewar with o p t i c a l windows. L i q u i d n i t r o g e n bubbling i n the dewar caused some "noise" problems but by using a slow scan r a t e and maximum pen damping, a reasonable s i g n a l - t o - n o i s e r a t i o was obtained. A sample of u n - i r r a d i a t e d PC i n a 1 cm c e l l a t room temperature was used i n the reference compartment and the instrument was ."balanced" u s i n g n e u t r a l d e n s i t y f i l t e r s i n the reference beam. This was r e q u i r e d because the p a r t i a l c r y s t a l l i n i t y of the sample caused a p p r e c i a b l e s c a t t e r i n g of the sample beam. Bla c k cardboard b a f f l e s were used t o b l o c k out most of the s c a t t e r e d l i g h t . For measurements i n the i n f r a r e d r e g i o n the sample was pro t e c t e d from bl e a c h i n g by the v i s i b l e l i g h t of the tungsten I R source lamp by p l a c i n g a t h i c k U V - v i s i b l e c u t o f f f i l t e r (Corning #2-61) between the source and the sample. (When -61-the Cary 14 i s o p e r a t i n g i n the IR mode, the e n t i r e emission of a tungsten lamp passes through the sample before reaching the monochromator and detector.) 6. L i g h t Sources f o r Photobleaching Experiments An u n f i l t e r e d low pressure mercury vapour lamp (Hanovia # 687A45) was g e n e r a l l y used f o r the u l t r a v i o l e t p h o t o l y s i s experiments. This lamp had a "Vycor" envelope and t h e r e f o r e t r a n s m i t t e d o n l y wavelengths above 220 nm. In the ESR experiments, the samples were e i t h e r bleached d i r e c t l y i n the c a v i t y by s h i n i n g the l i g h t through the g r i l l of the c a v i t y or e l s e the dewar c o n t a i n i n g the sample b a l l s was placed d i r e c t l y a g a i n s t the lamp envelope and both were surrounded w i t h aluminum f o i l . When photolysed i n the c a v i t y , the l i g h t i n t e n s i t y was r a t h e r low but changes i n the s p e c t r a could be f o l l o w e d d i r e c t l y ; whereas i r r a d i a t i o n o u t s i d e the c a v i t y was c o n s i d e r a b l y more in t e n s e and caused r a p i d changes. An o r d i n a r y 100 watt tungsten lamp was used f o r the v i s i b l e p h o t o l y s i s experiments and the samples were u s u a l l y bleached d i r e c t l y i n the c a v i t y . I s o l a t i o n of v a r i o u s wave-le n g t h regions was achieved by using Corning g l a s s f i l t e r s . In each case these were checked to ensure t h a t an u n d e s i r ^ a b l e UV or IR "window" was not present. C. RESULTS AND DISCUSSION 1. The Trapped E l e c t r o n (a) E l e c t r o n s p i n resonance observations ^ - i r r a d i a t i o n of g l a s s y propylene carbonate at l i q u i d n i t r o g e n temperature w i t h a 1 Mrad dose imparted only a very pale g r e e n i s h c o l o u r to the samples. (This i s i n c o n t r a s t to most other g l a s s y m a t e r i a l s which become i n t e n s e l y coloured when i r r a d i a t e d at low temperature.) The ESR d e r i v a t i v e spectrum obtained using the lowest operable microwave power ( about 0.5 mW ) immediately f o l l o w i n g the i r r a d i a t i o n i s shown i n F i g u r e 12. This spectrum c o n s i s t s of nine p r i n c i p a l l i n e s spread over a r e g i o n some 130 G wide and centered at the " f r e e - s p i n " g-value. The most s t r i k i n g f e a t u r e of t h i s group i s the narrow c e n t r a l s i n g l e l i n e , l a b e l e d "A". (The other e i g h t l i n e s i n F i g u r e 12 w i l l be examined i n d e t a i l i n P a r t 2 of t h i s s e c t i o n which w i l l d e a l w i t h r a d i c a l s other than e"^ .) As the microwave power l e v e l was i n c r e a s e d above 0.5 mW the i n t e n s i t y of l i n e "A" began to s l o w l y decrease at f i r s t u n t i l i t was v i r t u a l l y completely s a t u r a t e d at about 10 mW as i n d i c a t e d i n F i g u r e 13. This s a t u r a t i o n e f f e c t was of the "inhomogeneous" type where the l i n e width remained e s s e n t i a l l y constant w i t h only the peak amplitude decreasing u n i f o r m l y . A higher r e s o l u t i o n t r a c e of l i n e "A" appears i n F i g u r e 14. The l i n e was determined to be at g = 2.0028 ± 0.0002 when measured r e l a t i v e to DPPH ( t h i s value compares favour-a b l y w i t h g = 2.0030 c a l c u l a t e d from the measured f i e l d and frequency). The l i n e - w i d t h as measured between p o i n t s of Figure 12. ESR spectrum of ^ - i r r a d i a t e d glassy PC immediately following the i r r a d i a t i o n using the lowest operable microwave power (about 0.5 mW) . (Dose-'0.8 Mrad) F i g u r e 13. ESR spectrum of the same sample as Figure 12 o n l y at about 10 mW microwave power. F i g u r e 14. High r e s o l u t i o n ESR scan of l i n e "A" of F i g u r e 12. This l i n e i s a t t r i b u t e d t o trapped e l e c t r o n s i n PC. - 6 6 -maximum s l o p e , i . e . " p e a k - t o - p e a k " on t h e d e r i v a t i v e , was f o u n d t o be AH _ = 4.5 ± 0.1 G . A n a l y s i s o f t h e l i n e - s h a p e , b o t h ms J 48 b y t h e method o f s l o p e s and t h e n o r m a l i z a t i o n method , i n d i c -a t e d i t t o be v e r y n e a r l y G a u s s i a n w h i c h i s i n a g r e e m e n t w i t h t h e o b s e r v e d s a t u r a t i o n c h a r a c t e r i s t i c s . The n o r m a l i z a t i o n t e c h n i q u e o f l i n e - s h a p e a n a l y s i s i s a p p l i e d t o l i n e "A" i n F i g u r e 15 where b o t h t h e G a u s s i a n and L o r e n t z i a n l i n e - s h a p e s a r e i l l u s t r a t e d . On t h e b a s i s o f t h e G a u s s i a n l i n e - s h a p e , t h e " f r e e - s p i n " g - v a l u e and t h e power s a t u r a t i o n c h a r a c t e r i s t i c s , a l l o f w h i c h a r e v e r y s i m i l a r t o t h o s e o f t r a p p e d e l e c t r o n s i n 4 5 o t h e r m e d i a ' , t h i s s i n g l e l i n e , "A", was a s s i g n e d t o t r a p p e d e l e c t r o n s i n PC. The much n a r r o w e r l i n e o f i n PC as compared t o o t h e r p o l a r g l a s s e s ( e . g . a l c o h o l s and i c e s where AH = 10 - 15 G) c a n be e x p l a i n e d by t h e a b s e n c e o f h y d r o x y l i c h y d r o g e n atoms i n PC. I n t h e a l c o h o l s a n d i c e s , l i n e b r o a d e n i n g i s t h o u g h t t o be c a u s e d by h y p e r f i n e i n t e r a c t i o n s o f t h e OH p r o t o n s s u r r o u n d i n g t h e t r a p p i n g s i t e s . T h e r e f o r e i n t h i s r e g a r d , PC s h o u l d be compared t o t h e g l a s s y e t h e r s where ESR l i n e - w i d t h s 5 o f 3 - 4 G a r e o b s e r v e d f o r t r a p p e d e l e c t r o n s . The i n i t i a l y i e l d o f t r a p p e d e l e c t r o n s i n g l a s s y PC s a m p l e s was e s t i m a t e d u s i n g g l a s s y m e t h a n o l as t h e s t a n d a r d as d e s c r i b e d i n t h e e x p e r i m e n t a l s e c t i o n . C o m p a r i s o n o f t h e a r e a u n d e r t h e a b s o r p t i o n c u r v e o f l i n e "A" w i t h t h e t o t a l a r e a u n d e r t h e C H „ O H t r i p l e t g a v e G _ =0.3 i 0.2 u s i n g 2 e t r G ( C H „ 0 H ) = 6.7 „. (The e r r o r i n d i c a t e d h e r e f o r G^- o n l y z e t r i n c l u d e s t h e u n c e r t a i n t i e s i n GCCH^OH) , i n t h e r e l a t i v e s a m p l e s i z e s and i n t h e i n t e g r a t i o n t e c h n i q u e . ) T h i s y i e l d i s F i g u r e 15. High r e s o l u t i o n ESR scan of the trapped e l e c t r o n l i n e showing the n o r m a l i z a t i o n method of l i n e shape a n a l y s i s . •= Gaussian, x= L o r e n t z i a n . -68-somewhat lower than was expected on the b a s i s of the p o l a r i t y of the s o l v e n t and the " v i s u a l q u a l i t y " of the g l a s s . For example, i n most g l a s s y a l c o h o l s and i c e s = 2 - 3 and even i n the r e l a t i v e l y non-polar methyl-tetrahydrofuran g l a s s G _ =2.6,. In a d d i t i o n , the room temperature l i q u i d phase e t r r a d i o l y s i s r e s u l t s to be discussed l a t e r i n t h i s t h e s i s and 14 those of Hayon i n d i c a t e that the y i e l d of s o l v a t e d e l e c t r o n s (or f r e e ions) i s G~2, which would p r e d i c t a y i e l d of trapped e l e c t r o n s g r e a t e r than 2 f o r the low temperature g l a s s , s i n c e i n most other systems G _ > G (e.g. i n methanol G - = 2.7 •®tr e- e t r as compared w i t h G _ = 1.1). A probable e x p l a n a t i o n of t h i s e s discrepancy i s t h a t at the microwave power l e v e l used a sub-s t a n t i a l degree of s a t u r a t i o n was o c c u r r i n g t o g i v e a misleading r e s u l t . In any event, y i e l d measurements by ESR are subject to so many u n c e r t a i n t i e s t h a t t h e i r absolute values are not p a r t i c u l a r l y r e l i a b l e . The r e l a t i v e values obtained are, however, s i g n i f i c a n t and i n t h i s case the value of G _ •** 0.3 e t r means t h a t e l e c t r o n s are trapped reasonably e f f i c i e n t l y i n the g l a s s y PC. Determination of the exact degree of trapping e f f i c i e n c y w i l l r e q u i r e a much more accurate G value obtained by some method other than ESR. A spectrophotometric technique whereby the e l e c t r o n s are converted to a s t r o n g l y absorbing species w i t h a known molar a b s o r p t i v i t y , such as the napthalene anion, together w i t h absolute dosimetry, would g i v e a "good" G value. Attempts to scavenge the trapped e l e c t r o n s i n g l a s s y PC w i t h s i l v e r n i t r a t e , carbon monoxide and carbon d i o x i d e were u n s u c c e s s f u l . This was l i k e l y due t o t h e i r l i m i t e d _2 s o l u b i l i t y i n PC, which was l e s s than 10 M ; whereas much l a r g e r s o l u t e concentrations are normally r e q u i r e d to scavenge e l e c t r o n s i n g l a s s y s o l i d s . A d d i t i o n of water and aqueous s o l u t i o n s of hydrogen peroxide or formaldehyde gave cloudy p o l y c r y s t a l l i n e samples which were not s t u d i e d . However, i o d i n e a t 0.02 M d i d completely remove the trapped e l e c t r o n ESR l i n e as w e l l as modify the r a d i c a l spectrum somewhat as shown i n F i g u r e 16. This i s i n accord w i t h the known e f f i c i e n c y of i o d i n e as an e l e c t r o n scavenger, which f o r example r e a c t s 10 -1 -1 22 w i t h the hydrated e l e c t r o n a t k 2 = 5 x 10 M s . Iodine has a l s o been reporte d t o be a trapped e l e c t r o n scavenger i n 49 g l a s s y e t h a n o l . The absence of ESR s i g n a l s f o r the expected 50 product I i o n probably can be e x p l a i n e d on the b a s i s of 2 d i s s o c i a t i o n of I 2 to form I and I * , w i t h the I r a d i c a l then undergoing secondary r e a c t i o n s which r e s u l t i n the observed m o d i f i c a t i o n of the ESR spectrum of the other r a d i c a l s . Napthalene, at 0.1 M , a l s o removed the trapped e l e c t r o n ESR s i g n a l , r e p l a c i n g i t w i t h an i n c o m p l e t e l y r e s o l v e d m u l t i - l i n e s i g n a l as shown i n F i g u r e 17. This new paramagnetic species was most probably the napthalene anion, an assignment which i s supported by the o b s e r v a t i o n t h a t these samples were green i n 48 c o l o u r which i s c h a r a c t e r i s t i c of the napthalene anion Napthalene has been used as an e l e c t r o n scavenger i n the g l a s s y 49 51 a l c o h o l s ' although the species formed was b e l i e v e d to be the protonated anion i n these systems. The o b s e r v a t i o n t h a t the trapped e l e c t r o n s were not s t a b l e i n the i r r a d i a t e d PC g l a s s e s at 77 °K was perhaps the most p u z z l i n g d i s c o v e r y of t h i s study. The f a i r l y r a p i d F i g u r e 17. ESR spectrum of ^ - i r r a d i a t e d g l a s s y PC a t 77 °K c o n t a i n i n g 0.1 M napthalene. -72-spontaneous decay of the ESR l i n e was f i r s t n o t i c e d i n a sample t h a t had stood i n the dark f o r s e v e r a l hours f o l l o w i n g the i r r a d i a t i o n . The i n t e n s i t y of l i n e "A" r e l a t i v e to the r a d i c a l spectrum was found to be g r e a t l y reduced. This n a t u r a l decay d i d not appear to produce any new paramagnetic species nor was there any n o t i c e a b l e change i n the i n t e n s i t y of the other r a d i c a l s p e c t r a . ( I t would however be d i f f i c u l t to d e t e c t an o v e r a l l change i n the broad r a d i c a l spectrum s i n c e the change i n area would o n l y be about 10%.) In subsequent experiments, the n a t u r a l decay of e^ r was f o l l o w e d d i r e c t l y by ESR f o r s e v e r a l hours a f t e r the i r r a d i a t i o n . These s t u d i e s r e v e a l e d the decay c h a r a c t e r i s t i c s i l l u s t r a t e d i n F i g u r e 18, where data obtained f o r three d i f f e r e n t t o t a l doses i s g i v e n . The peak-to-peak h e i g h t of the ESR d e r i v a t i v e s i g n a l i s p l o t t e d as a f u n c t i o n of time. Since the peak w i d t h , A H , d i d not ms change s u b s t a n t i a l l y d u r i n g the i n t e r v a l over which the decay was monitored, the d e r i v a t i v e peak height i s p r o p o r t i o n a l to the t o t a l number of spins present. (In a c t u a l f a c t , a very slow, smooth decrease i n of about 8% occured over an i n t e r v a l of 350 minutes. This was not considered to be s i g n i f -i c a n t however, s i n c e when a c o r r e c t i o n was made f o r t h i s decrease i t d i d not a l t e r the r e s u l t s of the k i n e t i c analyses.) Because data f o r the very e a r l y stages of the decay ( f i r s t 10 minutes) could not be e x p e r i m e n t a l l y obtained, the curves p l o t t e d i n F i g u r e 18 were normalized u s i n g an e x t r a p o l a t e d "zero time" peak h e i g h t . This was obtained from the second order k i n e t i c p l o t s by e x t r a p o l a t i o n of the i n i t i a l l i n e a r r e g i o n (see F i g u r e 20). TIME (minutes) F i g u r e 18. Isothermal spontaneous decay of trapped e l e c t r o n s i n ^ - i r r a d i a t e d g l a s s y P C 77 °K as followed by ESR. (Data are a r b i t r a r i l y normalized a t "0" time.) - 7 4 -F i g u r e 19 shows the f i r s t order k i n e t i c analyses of decay data i n which the l o g a r i t h m of the peak heights are p l o t t e d versus time. A l l three s e t s of data g i v e smooth curves which i n d i c a t e s t h a t the decay mechanism does not f o l l o w simple f i r s t order k i n e t i c s . F i r s t order decay would be expected i f the e l e c t r o n s were simply r e a c t i n g w i t h one of the s o l v e n t molecules c o n s t i t u t i n g t h e i r t r a p s . Second order a n a l y s i s of the data i s g i v e n i n Figure 20 where the i n v e r s e of the normalized peak heights are p l o t t e d versus time. These graphs a l l have two l i n e a r regions; an i n i t i a l " f a s t e r " decay whose d u r a t i o n depends on the length of the i r r a d i a t i o n , f o l l o w e d by a "slower" decay which i s f o l l o w e d f o r a t l e a s t one " h a l f - l i f e " . Data not shown i n these Figures was obtained at 500 and 1500 minutes a f t e r a 300 minute i r r a d -i a t i o n and these p o i n t s f i t the e x t r a p o l a t e d second order p l o t i n F i g u r e 20 w i t h i n the experimental e r r o r i n v o l v e d i n removing the dewar from the ESR c a v i t y . This i n d i c a t e d that the "slower" decay process was f o l l o w e d f o r a t l e a s t two " h a l f - l i v e s " . T h i s apparent second order decay i s not c o n s i s t e n t w i t h a homogeneous r e a c t i o n of the e l e c t r o n s w i t h p o s i t i v e ions (these s p e c i e s would be present i n equal concentration) because the slopes of the second order k i n e t i c p l o t s are apparently d i f f e r -ent , i m p l y i n g a dose-dependent r a t e constant. The d i f f e r e n c e i n the slopes i s p a r t i a l l y due t o the n o r m a l i z a t i o n f a c t o r s used, although the un-normalized data a l s o gave d i f f e r e n t s l o p e s . I t should be noted however, t h a t the data were obtained u s i n g d i f f e r e n t spectrometer c o n d i t i o n s (gain and sample s i z e ) and thus absolute s p i n concentrations could not be measured. • = 300 min irradiation X= 30 min irradiation A = 5 min irradiation 100 200 TIME (minutes) 300 Figure 19. F i r s t order k i n e t i c analyses of the trapped electron decay data from Figure 18. TIME (minutes) Figure 20. Second order k i n e t i c analyses of the trapped electron decay data from Figure 18 -77-I t i s t h e r e f o r e p o s s i b l e t h a t the d i f f e r e n c e s i n slope of the l i n e s i n Figu r e 20 may be a consequence of the experimental methods used. In any event, i f the r e a c t i o n process i n v o l v e d a homogeneous d i s t r i b u t i o n of e l e c t r o n s , then the i n i t i a l t r a n s i e n t d e v i a t i o n would have to be a t t r i b u t e d to a non-homo-g e n e i t y of the system during the e a r l y stages f o l l o w i n g the i r r a d i a t i o n . On t h i s b a s i s , the lowest dose experiment should have taken the longest time t o become homogeneous because the spur s e p a r a t i o n d i s t a n c e would be g r e a t e s t . In a d d i t i o n , F i g u r e 18 shows t h a t the o v e r a l l decay i s f a s t e s t f o r the lowest dose, but t h i s may a r i s e from the f a c t t h a t a much l a r g e r p r o p o r t i o n of the species observed a f t e r the long i r r a d -i a t i o n are " l o n g - l i v e d " ones. I t i s a l s o d i f f i c u l t t o envisage e i t h e r s p ecies being s u f f i c i e n t l y mobile to d i f f u s e r a p i d l y through the h i g h l y p o l a r low temperature m a t r i x and become homogeneously d i s t r i b u t e d . T h e i r i n i t i a l d i s t r i b u t i o n would c e r t a i n l y not be homogeneous. Simple homogeneous second order decay would t h e r e f o r e seem to be r u l e d out unless some very unusual process f o r charge m i g r a t i o n e x i s t s i n t h i s system. A t h i r d p o s s i b i l i t y f o r the spontaneous decay mechanism i s t h a t the e l e c t r o n s are "pre-destined" to r e a c t w i t h a p a r t -i c u l a r p o s i t i v e i o n . In t h i s case the k i n e t i c s may not f o l l o w any simple r e a c t i o n order but r a t h e r the decay would depend on the s e p a r a t i o n - d i s t a n c e d i s t r i b u t i o n s of the e l e c t r o n s and c a t i o n s . This type of decay i s thought t o occur i n 3-methyl 17 pentane and met h y l - t e t r a h y d r o f u r a n g l a s s e s . The i n i t i a l decay r a t e s of the trapped e l e c t r o n s i n these systems were found to be d i r e c t l y p r o p o r t i o n a l to dose and by n o r m a l i z a t i o n -78-of the data the decay curves were found t o be superimposable f o r a t l e a s t the f i r s t 50% of the decay and over a c o n s i d e r -a b l e v a r i a t i o n i n dose. In these systems the decay of each e l e c t r o n i s a p p a r e n t l y by a process which i s independent of the t o t a l number of p o s i t i v e charges i n the m a t r i x . I t i s not easy t o see i f t h i s i s the case f o r e~ r i n PC g l a s s e s because the i r r a d i a t i o n times and the decay " h a l f - l i v e s " are comparable. The e l e c t r o n decay k i n e t i c s i n PC as shown by F i g u r e 18 most probably represent the species r e a c t i n g w i t h p o s i t i v e i o n s v i a a t o t a l l y non-homogeneous process. The r e s u l t s of the 5 minute i r r a d i a t i o n experiment i n d i c a t e t h a t 50% of the e l e c t r o n s produced are l o s t w i t h i n 15 minutes a f t e r the end of the i r r a d i a t i o n . Therefore a l a r g e f r a c t i o n of those pro-duced d u r i n g the e a r l y stages of the 30 and 300 minute i r r a d -i a t i o n s have decayed before the i r r a d i a t i o n was f i n i s h e d . The i n i t i a l t r a n s i e n t f e a t u r e s i n d i c a t e d i n the second order p l o t s (Figure 20) are t h e r e f o r e due to decay of the " s h o r t -l i v e d " s p e c ies formed during the l a s t few minutes of the i r r a d i a t i o n . These species may be those trapped c l o s e enough to t h e i r parent p o s i t i v e i o n t h a t they are a l r e a d y e f f e c t i v e l y "captured" and thus have no choice but to r e a c t . The remainder of the e l e c t r o n s are those which are s u f f i c i e n t l y d i s t a n t from a p o s i t i v e i o n t h a t the t r a p energy i s comparable to or g r e a t e r than the Coulombic f o r c e s and t h e r e f o r e they have a r e l a t i v e l y long l i f e t i m e governed p r i m a r i l y by d i f f u s i o n . I n t e r p r e t a t i o n of the k i n e t i c s of t h i s system w i l l be a complex mathematical problem. A g e n e r a l t h e o r e t i c a l treatment of - 7 9 -b i m o l e c u l a r r e a c t i o n s i n s o l i d and l i q u i d systems where d i f -52 f u s i o n i s the r a t e c o n t r o l l i n g f a c t o r has been made by Waite . S o l u t i o n s of the problem are however e a s i l y obtained only i f the d i s t r i b u t i o n of the r e a c t i n g species i s assumed to be random and i f no long-range f o r c e s are i n v o l v e d . These assump-t i o n s would not be expected to be v a l i d i n the case of i o n i c r a d i c a l s produced i n i r r a d i a t e d low temperature s o l i d s since Coulombic i n t e r a c t i o n s may be i n v o l v e d i n a d d i t i o n to a non-random d i s t r i b u t i o n . Although Waite's gen e r a l treatment has p r o v i s i o n s f o r extension to i n c l u d e non-random d i s t r i b u t i o n s and i n t e r - i o n i c f o r c e s , s o l u t i o n s are extremely complex and not e a s i l y a p p l i e d to the experimental data. In a d d i t i o n , because the i n i t i a l d i s t r i b u t i o n of the species i s most o f t e n unknown, t h i s would r e q u i r e a su c c e s s i v e approximation method to f i t the theory to the experimental data using v a r i o u s assumed d i s t r i b u t i o n s . One important i m p l i c a t i o n of the decay s t u d i e s i s th a t the y i e l d of trapped e l e c t r o n s estimated by comparison w i t h methanol i s o b v i o u s l y low. Since the experiment i n v o l v e d a long i r r a d i a t i o n time (300 minutes) i n order t o o b t a i n a reasonable s i g n a l l e v e l , the e l e c t r o n s measured were c l e a r l y o n l y the " l o n g - l i v e d " ones. Most of the s h o r t - l i v e d species would have already decayed duri n g the i r r a d i a t i o n . Thus the value of G0_ = 0.3 i s undoubtedly low (perhaps by as much e t r as a f a c t o r of 10) . I t must t h e r e f o r e r e f l e c t o n l y a lower l i m i t f o r the primary y i e l d of trapped e l e c t r o n s i n g l a s s y propylene carbonate. -80-As would be expected, the trapped e l e c t r o n s were t h e r m a l l y unstable above l i q u i d n i t r o g e n temperature. When a sample was warmed above 90 °K, the ESR l i n e disappeared almost immediately. A broad s i g n a l showing unresolved hyper-f i n e s t r u c t u r e r e p l a c e d the e~ r s i g n a l but t h i s new species was b e l i e v e d to o r i g i n a t e from the decay of one of the other r a d i c a l species and not from the e l e c t r o n . The electron's f a t e on warming was most l i k e l y the same as i t s f a t e at l i q u i d n i t r o g e n temperature, onl y the r e a c t i o n irate i n c r e a s i n g w i t h temperature. Photobleaching experiments using a tungsten lamp and v a r i o u s o p t i c a l f i l t e r s were performed on a sample i n the ESR c a v i t y . The most s i g n i f i c a n t r e s u l t of these experiments was t h a t the n a t u r a l decay of the trapped e l e c t r o n s was not a f f e c t e d by l i g h t w i t h X>500 nm but i t was s u b s t a n t i a l l y a c c e l e r a t e d by l i g h t i n the 300 - 500 nm r e g i o n . As w i l l be d i s c u s s e d i n the next s e c t i o n , t h i s r e s u l t supports the assignment of an a b s o r p t i o n band i n the v i o l e t r e g i o n t o the trapped e l e c t r o n s . I t i s a l s o i n agreement w i t h the v i s u a l o b s e r v a t i o n of o n l y a p a l e green c o l o u r i n d i c a t i n g t h a t there are no broad absorp-t i o n bands i n the v i s i b l e or near i n f r a r e d r e g i o n s . As i n the n a t u r a l decay at 77 °K, p h o t o l y s i s d i d not produce any new paramagnetic species as shown by the "photobleached" spectrum i n F i g u r e 21. The s i g n a l s remaining near the center of t h i s spectrum ( i n a d d i t i o n to a s m a l l amount of trapped e l e c t r o n s remaining) were most l i k e l y there i n i t i a l l y , o n l y obscured by the more inte n s e e^ r l i n e . Again the f a t e of the trapped e l e c t r o n s on p h o t o l y s i s i s probably the same as the F i g u r e 21. ESR spectrum of ^ - i r r a d i a t e d g l a s s y PC a f t e r v i s i b l e p h o t o l y s i s to remove most of the trapped e l e c t r o n s . -82-n a t u r a l decay, i . e . r e a c t i o n w i t h a p o s i t i v e i o n , the process merely being a c c e l e r a t e d by the energy inp u t of the l i g h t , probably v i a a p h o t o i o n i z a t i o n process. •(b) O p t i c a l a b s o r p t i o n spectrum As mentioned at the beginning of the previous s e c t i o n , i r r a d i a t e d g l a s s y PC b a l l s became pale green i n c o l o u r . This e f f e c t was more e a s i l y seen f o r the samples i n the 1 cm spectrophotometer c e l l s , which were a dark green f o l l o w i n g i r r a d i a t i o n w i t h a 1 Mrad dose. The o p t i c a l a b s o r p t i o n spectrum recorded immediately a f t e r the i r r a d i a t i o n i s represented by curve 1 of F i g u r e 22. There was a strong a b s o r p t i o n band i n the UV w i t h a maximum below 280 nm. This was probably due t o r a d i c a l s o r r a d i o l y s i s products having an organic c a r b o n y l group s i n c e PC does not absorb above 22 5 nm. The more i n t e r e s t i n g p a r t s of t h i s spectrum are the shoulder on the UV band i n the 300 - 500 nm r e g i o n and the complex band showing " v i b r a t i o n a l " s t r u c t u r e between 500 and 7 50 nm . As w i l l be d i s c u s s e d i n a l a t e r s e c t i o n the 500 - 7 50 nm a b s o r p t i o n can be a t t r i b u t e d to a s m a l l i n i t i a l y i e l d of CO^ and HCO r a d i c a l s both of which absorb i n t h i s r e g i o n . A search of the near i n f r a r e d r e g i o n revealed no a b s o r p t i o n beyond 7 50 nm out to 1300 nm f o r a sample i r r a d i a t e d with, a 1 Mrad dose. When the i r r a d i a t e d PC was s t o r e d i n the dark a t 77 °K f o r 24 hours, the shoulder of the UV band was g r e a t l y reduced i n i n t e n s i t y as i n d i c a t e d by curve 2 of F i g u r e 22 and the F i g u r e 22. O p t i c a l a b s o r p t i o n s p e c t r a of ^ - i r r a d i a t e d PC at 77 °K (sample p a r t i a l l y c r y s t -a l l i n e ) i n a 1 cm c e l l . Curve 1 was obtained immediately a f t e r the i r r a d i a t i o n , curve 2 was obtained 24 hours l a t e r . (Dose ~ 1.3 Mrad) -84-sample appeared pale blue i n c o l o u r due to the remaining a b s o r p t i o n i n the red re g i o n . Since the n a t u r a l decay of the trapped e l e c t r o n s was a c c e l e r a t e d by p h o t o l y s i s w i t h blue l i g h t and s i n c e about 90% of the trapped e l e c t r o n s would have decayed n a t u r a l l y i n the 24 hour p r e i o d (as i m p l i e d from the ESR s t u d i e s ) , a l o g i c a l assignment of the shoulder of the UV band i s to e~ r. The spectrum constructed by s u b t r a c t i n g curve 2 from curve 1 of F i g u r e 22 i s a broad band w i t h a X ^ 370 nm max (3.4 eV) as shown i n Figu r e 23. The width of t h i s band a t h a l f maximum, W , i s approximately 0.9 eV, which i s of the same order of magnitude as t h a t of trapped e l e c t r o n s i n other media. The l a c k of any asymmetry on the high energy s i d e of the ab s o r p t i o n band, which i s normally c h a r a c t e r i s t i c of trapped and s o l v a t e d e l e c t r o n s p e c t r a , i s probably due to the method used to c o n s t r u c t the spectrum. In the 300 - 350 nm re g i o n d i f f e r e n c e s between two la r g e absorbances > 1 were i n v o l v e d and t h e r e f o r e there i s a considerable u n c e r t a i n t y i n the net absorbance due to e~ r. I t i s a l s o p o s s i b l e , i f not probable, t h a t the n a t u r a l decay of the trapped e l e c t r o n s does not i n v o l v a Uniform l o s s of e l e c t r o n s from the d i f f e r e n t depth t r a p s , i . e . the e l e c t r o n s i n the lowest energy traps may be l o s t p r e -f e r e n t i a l l y . Since the ab s o r p t i o n s p e c t r a of trapped e l e c t r o n s are b e l i e v e d to represent a p o p u l a t i o n d i s t r i b u t i o n of the d i f f e r e n t t r a p depths, the above method f o r c o n s t r u c t i n g the spectrum would then g i v e a f a l s e p i c t u r e of the ab s o r p t i o n band shape and i t s Xmax. Obviously Figure 23 may not r e p r e -sent the true a b s o r p t i o n band, however i t does g i v e a F i g u r e 23. Absorption spectrum a t t r i b u t e d to trapped e l e c t r o n s i n PC at 77 °K as constructed by s u b t r a c t i o n of curve 2 from curve 1 of F i g u r e 22. -86-q u a l i t a t i v e p i c t u r e of the spectrum. The assignment of t h i s band to trapped e l e c t r o n s i s 5d c o n s i s t e n t w i t h the high d i e l e c t r i c constant of PC. Ekstrom presented some "unpublished" data f o r trapped e l e c t r o n s i n a formamide + 15% water g l a s s at 77 °K where an a b s o r p t i o n w i t h X *" 400 nm-was apparently observed. (Pure formamide has max a d i e l e c t r i c constant of 109 and the presence of 15% water would probably reduce the e f f e c t i v e d i e l e c t r i c constant con-s i d e r a b l y . ) In a d d i t i o n i f the c o r r e l a t i o n between the room temperature d i e l e c t r i c constant and the a b s o r p t i o n maxima f o r o 5d e l e c t r o n s trapped i n various g l a s s y media at 77 UK i s extended, n a X trapped e l e c t r o n s i n PC w i t h i t s d i e l e c t r i c constant of 65. then a A i n the r e g i o n of 400 nm may be expected f o r Further evidence supporting the above assignment i s an approximate c a l c u l a t i o n of the molar a b s o r p t i v i t y , € f o r the a b s o r p t i o n band. Assuming t h a t the y i e l d of e l e c t r o n s i n PC i s G - = 0-3 ( i . e . that measured by ESR), then 6 i s  e t r max c a l c u l a t e d t o be 9 x 10 3 M~lcm -^. This i s only s l i g h t l y lower than the € m a x observed f o r trapped e l e c t r o n s i n other media where €• _„ ranges from 8 x 10 3 to 2 x 10 4 M - 1cm - 1. max 3 The y i e l d measured by ESR i s undoubtedly not accurate as discussed above. In a d d i t i o n the samples used f o r the o p t i c a l s t u d i e s were not "good" glasses but r a t h e r p a r t i a l l y c r y s t -a l l i n e and thus the trapped e l e c t r o n y i e l d i n these samples could have been c o n s i d e r a b l y lower than t h a t f o r the g l a s s e s used i n the ESR measurements. Consequently a more meaningful q u a n t i t y to c a l c u l a t e i s G - x € . and t h i s has a value e t r max of about 3 x 10 M cm (electrons/100 eV). (c) Summary In summary, the above data presents a stro n g case f o r the e x i s t e n c e of trapped e l e c t r o n s i n g l a s s y propylene carbonate. T h e i r p r o p e r t i e s , as l i s t e d i n Table 4, are i n accord w i t h the p r o p e r t i e s of trapped e l e c t r o n s i n other media (see Table 2 and references 4,5), w i t h the exception of the mysterious i n s t a b i l i t y of the s p e c i e s . This i n s t a b i l i t y i s p a r t i c u l a r l y d i f f i c u l t to r a t i o n a l i z e on the b a s i s of the assignment of the a b s o r p t i o n band w i t h Xmax = 370 nm to the s p e c i e s . I f t h i s assignment i s c o r r e c t , i t means t h a t the average t r a p depth f o r the e l e c t r o n s i s about 3.5 eV. This i n d i c a t e s t h a t the e l e c t r o n i s ve r y s t r o n g l y bound i n i t s t r a p as would be expected from the very l a r g e d i p o l e moment of -18 PC (4.9 x 10 esu cm). Consequently to t r y to e x p l a i n the n a t u r a l decay on the b a s i s of "thermal d i f f u s i o n " of the e l e c t r o n s from t r a p to t r a p i s not p o s s i b l e . In a d d i t i o n , the h i g h d i e l e c t r i c constant of the medium should weaken the i n t e r i o n i c f o r c e s between the c a t i o n s and the e l e c t r o n s unless the e l e c t r o n s are trapped immediately adjacent t o a p o s i t i v e i o n . -88-TABLE 4 CHARACTERISTICS OF ELECTRONS TRAPPED IN PROPYLENE CARBONATE GLASSES AT 77 °K ESR g - f a c t o r 2.0028 ± 0.0002 A H 4.5 ± 0.1 G ms line-shape. Gaussian OPTICAL X ~370 nm (3.4 eV) max W, ^0.9* eV h YIELD G _ >0.3 electrons/100 eV e t r STABILITY Not s t a b l e at 77 °K; spontaneous decay v i a a non-homo-geneous process b e l i e v e d to be r e a c t i o n w i t h p o s i t i v e i o n s . Approximate f i r s t " h a l f - l i f e " f o r 1 Mrad dose i s about 4 hours; w i t h the apparent " h a l f - l i f e " d ecreasing w i t h dose at a g i v e n dose r a t e (0.3 Mrad/hr) - 8 9 -2. Trapped R a d i c a l s i n I r r a d i a t e d PC (a) R a d i c a l s formed during r a d i o l y s i s at 77 K The r a d i c a l s formed during the i n i t i a l r a d i o l y s i s of g l a s s y PC at 77 °K gave an ESR spectrum which c o n s i s t e d of e i g h t p r i n c i p a l l i n e s centered at e s s e n t i a l l y the f r e e - s p i n g-value and spread over a r e g i o n some 130 G wide as shown i n F i g u r e 12. No i n d i c a t i o n of any s i g n a l s w i t h a s p l i t t i n g of 500 G a t t r i b u t a b l e to H atoms was found. One p a i r of the l i n e s shown i n F i g u r e 12, l a b e l e d "C", was e a s i l y power s a t u r a t e d as the microwave i n t e n s i t y was increased as i n d i c a t e d i n F i g u r e 13. This broad doublet, w i t h a s p l i t t i n g of 58 - 3 G, was t h e r m a l l y unstable above about 90 °K and at about 110 °K i t decayed r a p i d l y to completely r e v e a l the u n d e r l y i n g s i x l i n e s , "B", "D", "E", as i l l u s t r a t e d i n F i g u r e 24. A new paramagnetic species w i t h incompletely r e s o l v e d s m a l l hyper-f i n e s p l i t t i n g "grew" i n at the center of the spectrum as the broad doublet decayed. The r a d i c a l r e s p o n s i b l e f o r the broad doublet "C" could not be c o n c l u s i v e l y i d e n t i f i e d although a s p e c u l a t i v e guess would be the r a d i c a l : C H 3 H which might be expected to give a very broad ESR doublet w i t h a l a r g e proton h y p e r f i n e s p l i t t i n g . i f i c a t i o n as doublet "C". However, w i t h the a i d of photo-b l e a c h i n g , i s o t h e r m a l decay and scavenger experiments, i t was p o s s i b l e to show that the s i x l i n e s were r e a l l y three s e t s of \ H The other s i x l i n e s were j u s t as e l u s i v e i n t h e i r i d e n t -D F i g u r e 24. ESR spectrum of ^ - i r r a d i a t e d g l a s s y PC at <*110 °K a f t e r thermal decay of the trapped e l e c t r o n s and other r a d i c a l s . -91-doublets w i t h h y p e r f i n e s p l i t t i n g of 42 i 2, 8 3 - 3 , and 124 - 4 G r e s p e c t i v e l y . A l l three doublets were centered at g = 2.0023 ± 0.0003. When a sample was f i r s t warmed to about 110 °K to remove the broad doublet "C" (which obscured the r e s o l u t i o n of l i n e s "B" and "D") and then photolysed w i t h the u n f i l t e r e d mercury lamp, the middle doublet "D" was observed to decrease i n i n t e n s i t y f a i r l y r a p i d l y w i t h a concomitant p r o d u c t i o n of a new i n t e n s e l i n e , "F", at the center of the spectrum. L i n e "F" appeared superimposed on the e x i s t i n g cen-t r a l multicomponent s i g n a l . This i s i l l u s t r a t e d i n F i g u r e 25 where i t i s c l e a r t h a t doublet "D" has decreased markedly i n i n t e n s i t y a f t e r only a few minutes of UV i r r a d i a t i o n . Doublet "E" was a l s o s l i g h t l y reduced i n i n t e n s i t y f o l l o w i n g photo-l y s i s but the r e l a t i v e l y s m a l l e r decrease suggested t h a t t h i s was a separate r a d i c a l . In a d d i t i o n , i f a f i l t e r was used t o e l i m i n a t e the UV l i g h t below 240 nm, doublet "E" was not s i g -n i f i c a n t l y a f f e c t e d by p h o t o l y s i s but doublet "D" s t i l l decayed although at a reduced r a t e . Doublet "B" d i d not seem to be i n f l u e n c e d at a l l by the u l t r a v i o l e t l i g h t . Further evidence to support the i n d i v i d u a l i t y of doublet "B" was obtained by a l l o w i n g a sample to stand i n the dark f o r s e v e r a l hundred hours at l i q u i d n i t r o g e n temperature. During t h i s time the r e l a t i v e i n t e n s i t y of doublet "B" was s u b s t a n t i a l l y reduced as shown i n F i g u r e 26. In a d d i t i o n , F i g u r e 16 i n d i c a t e d t h a t i o d i n e scavenged t h i s r a d i c a l as w e l l as the e l e c t r o n s , s i n c e doublet "B" i s conspicuously absent from t h i s spectrum. This scavenging could have occured by a secondary process i n v o l v i n g the I atoms formed from decomposition. A l t e r n a t i v e l y , i t F i g u r e 25. ESR spectrum of ^ - i r r a d i a t e d g l a s s y PC at ~110 °K a f t e r p a r t i a l UV p h o t o l y s i s of the r a d i c a l r e s p o n s i b l e f o r doublet "D". F i g u r e 26. ESR spectrum of ^ - i r r a d i a t e d g l a s s y PC at 77 °K a f t e r standing f o r 200 hours i n the dark. -94-may have i n v o l v e d a d i r e c t r e a c t i o n w i t h the i o d i n e molecule s i n c e i s known to be a good r a d i c a l scavenger as w e l l as e l e c t r o n scavenger. A l l of the r a d i c a l s formed by the r a d i o l -y s i s decayed very r a p i d l y i f the temperature of the sample o was r a i s e d above 150 K, although they were r e l a t i v e l y s t a b l e a t about 110 °K (except f o r the doublet "C" r a d i c a l ) and were i n d e f i n i t e l y s t a b l e at 77 °K (except f o r the doublet "B" r a d i c a l ) . Thus the r a d i o l y s i s of g l a s s y PC at 77 °K appears to g i v e r i s e t o f o u r major r a d i c a l s p e c i e s . These are a l l char-a c t e r i s e d by very l a r g e doublet h y p e r f i n e s p l i t t i n g which c l e a r l y must r e s u l t from c o u p l i n g of the unpaired e l e c t r o n w i t h Ctprotons. The t o t a l r a d i c a l y i e l d as estimated from the comparison w i t h i r r a d i a t e d methanol was = 4 - 2 . None of these primary species can be c o n c l u s i v e l y i d e n t i f i e d because of t h e i r unusual ESR s p e c t r a , s p e c i f i c a l l y the v e r y l a r g e proton h y p e r f i n e s p l i t t i n g and the i s o t r o p i c appearance of the l i n e s which would not be expected from very simple r a d i c a l s such as HCO, HC02, OH, e t c . A very s p e c u l a t i v e assignment of the r a d i c a l formed on UV p h o t o l y s i s of the r a d i c a l "D" i s to the C02~ r a d i c a l i o n . CO~ i s known to g i v e a narrow ESR l i n e very near the f r e e - s p i n g (g = 2 . 0 0 1 ) 5 4 av and l i n e "F" i s e s s e n t i a l l y i n t h i s p o s i t i o n . I f t h i s a s s i g n -ment i s c o r r e c t , then a p o s s i b l e candidate f o r the r a d i c a l "D" would be the HCO2 r a d i c a l ( i . e . t h i s may be p h o t o d i s s o c i a t e d to g i v e H + and C02~) . This s p e c u l a t i o n i s supported to some extent by the l i q u i d phase r a d i o l y s i s r e s u l t s to be d i s c u s s e d i n a l a t e r s e c t i o n , where a l a r g e y i e l d of "molecular" C02 -95-was found. CO^ might be expected t o o r i g i n a t e from a r a d i c a l such as HCO or to produce HCXX, by scavenging r e a c t i o n s . 54 However Nazhat e_t a l . have reported observing the r a d i c a l HCO2 i n i r r a d i a t e d f r o z e n sodium formate s o l u t i o n s . They a t t r i b u t e d a s i n g l e l i n e t o i t at g = 2.0121 i n d i c a t i n g t h a t the unpaired e l e c t r o n i s l o c a l i z e d on the oxygen atoms. In summary, the most unusual f e a t u r e of the r a d i c a l s p e c i e s formed by the primary a c t i o n of r a d i a t i o n on g l a s s y PC i s the apparent absence of "normal" r a d i c a l s w i t h s m a l l CL,/? proton c o u p l i n g such as i s observed i n most other i r r a d -i a t e d o r g a n i c systems. For example, one might have expected H atom a b s t r a c t i o n t o occur i n t h i s system to g i v e r a d i c a l s such as: c ^ Q „ o p 0 0 \ r Y ti H o o but t h i s a p p a r e n t l y does not occur w i t h a s i g n i f i c a n t y i e l d , a t l e a s t not d u r i n g the primary r a d i o l y s i s . (Radicals of t h i s type could account f o r the m u l t i l i n e spectrum i n the center of F i g u r e 24 caused by thermal r e a c t i o n of some of the primary r a d i c a l s w i t h the solvent.) Instead of t h i s type of r a d i c a l ; at l e a s t f o u r d i f f e r e n t s p e c i e s , a l l a p p a r e ntly w i t h o n l y Cl protons, were formed. I t i s hoped t h a t f u r t h e r s t u d i e s of PC, i n c l u d i n g i t s deuterated forms, w i l l h e l p to r e v e a l the i d e n t i t y of the paramagnetic species generated by the r a d i o l y s i s . -96-(b) R a d i c a l s formed by UV p h o t o l y s i s of the i r r a d i a t e d  PC g l a s s e s a t 77 °K U l t r a v i o l e t p h o t o l y s i s of the ^ - i r r a d i a t e d g l a s s e s at l i q u i d n i t r o g e n temperature produced r a d i c a l s which could be p o s i t i v e l y i d e n t i f i e d . F i g u r e 27 shows the ESR spectrum of an i r r a d i a t e d PC sample a f t e r a very s h o r t exposure t o the f u l l i n t e n s i t y of the mercury lamp. The samples underwent a v e r y dramatic c o l o u r change and became a deep blue . As shown i n F i g u r e 27, a new intense asymmetric l i n e , l a b e l e d "G", appeared down-field from the f r e e - s p i n g-value and a value of g ^ 2.015 was measured from the center of t h i s l i n e r e l a t i v e to DPPH. By f o l l o w i n g the changes i n the ESR spectrum u s i n g low i n t e n s i t y UV i l l u m i n a t i o n of the sample d i r e c t l y i n the ESR c a v i t y , i t was concluded t h a t at l e a s t p a r t of the blue c o l o u r a t i o n was due t o the new paramagnetic species a s s o c i a t e d w i t h l i n e "G". In a d d i t i o n , t h i s r a d i c a l was apparently generated by the p h o t o l y s i s of the species respon-s i b l e f o r the broad doublet "C" of F i g u r e 12. This o b s e r v a t i o n was supported by the f a c t t h a t n e i t h e r the blue c o l o u r nor the narrow ESR l i n e "G" could be produced by UV p h o t o l y s i s of the samples at 110 °K a f t e r the broad doublet had decayed t h e r m a l l y . The blue c o l o u r was due to a very l a r g e i n c r e a s e i n the 500 - 7 50 nm a b s o r p t i o n band t h a t was shown i n Figure 22. UV p h o t o l y s i s of the samples used f o r the o p t i c a l s t u d i e s produced the same blue c o l o u r and the i n t e n s i t y of the 500 - 7 50 nm band was g r e a t l y i n c r e a s e d . F i g u r e 28 shows t h i s band i n more d e t a i l , p a r t i c u l a r l y what appears to be v i b r a t i o n a l F i g u r e 27. ESR spectrum of ^ - i r r a d i a t e d g l a s s y PC at 77 °K a f t e r b r i e f UV p h o t o l y s i s . Samples were dark blue i n c o l o u r . i 1 1 r 5 5 0 6 0 0 6 5 0 7 0 0 W A V E L E N G T H (nm) F i g u r e 28. A b s o r p t i o n spectrum i n the 500 - 750 nm r e g i o n f o r " ^ - i r r a d i a t e d PC a f t e r b r i e f UV p h o t o l y s i s . Spectrum a t t r i b u t e d to a combination of the HC0 and CO^ " bands. -99-f i n e s t r u c t u r e . On the b a s i s of the blue c o l o u r and the s i n g l e ESR l i n e at g~2.015, t h i s new r a d i c a l can be assigned to the CO^ r a d i c a l anion. CO~ has been observed s p e c t r o s c o p i c a l l y i n aqueous s o l u t i o n where a broad asymmetric a b s o r p t i o n w i t h X.max*' 600 nm was detected on f l a s h p h o t o l y s i s and pulse r a d -55 56 i o l y s i s of aqueous carbonate s o l u t i o n s . Ershov et a l . r e p o r t e d e s s e n t i a l l y the same o p t i c a l spectrum f o r CO" i n f r o z e n aqueous carbonate g l a s s e s i r r a d i a t e d a t 77 °K . They a l s o a t t r i b u t e d an asymmetric ESR l i n e a t g_„ = 2.011 to the s p e c i e s . I t i s t h e r e f o r e evident t h a t what appears t o be v i b r a t i o n a l f i n e s t r u c t u r e on the 500 - 7 50 nm band i s r e a l l y a b s o r p t i o n bands due t o another species superimposed on the broad s t r u c t u r e l e s s band of CO^- . The o n l y l i k e l y s pecies which could g i v e a m u l t i p l e l i n e v i b r o n i c spectrum i n t h i s system i s HCO. HCO i s known to show a t l e a s t seven narrow a b s o r p t i o n bands i n the r e g i o n between 500 and 7 50 nm when trapped i n a carbon monoxide m a t r i x o 57 at 20 K . Although the p o s i t i o n s of the bands observed i n PC at 77 °K ( ~690, ~650, *- 630, ^610, ~590, and ^ 57 5 nm) do not agree p r e c i s e l y w i t h those observed i n the CO m a t r i x a t 20 °K (670, 635, 605, 579, 555, 533, and 510 nm) the d i f f -erences are probably caused by the overlapping of the C0~ spectrum and r a t h e r poor r e s o l u t i o n . The presence of HCO was 58 d e f i n i t e l y e s t a b l i s h e d by i t s ESR spectrum which i s the asymmetric doublet, l a b e l e d "H", i n Fig u r e 27. (A l a r g e i s o t r o p i c h y p e r f i n e s p l i t t i n g f a c t o r of 137 G i s r e s p o n s i b l e f o r the doublet; w h i l e g = 2.0041, g f = 2.0027 and g„= 1.9960 j^Ji. yy Z Z together w i t h the a n i s o t r o p i c p a r t of the h y p e r f i n e t e n s o r , -100-B = -4.2 G, B = -0.8 G, B = 5.8 G and B = 5 G ( g i v e xx yy Y 2 2 2 r i s e t o the complex l i n e s h a p e . 5 8 b ) These s i g n a l s are i n e s s e n t i a l l y the same p o s i t i o n as doublet "E" but these species are a p p a r e n t l y d i f f e r e n t r a d i c a l s . On f o l l o w i n g the p h o t o l y s i s a t low l i g h t i n t e n s i t y , doublet "E" was observed to f i r s t decrease i n i n t e n s i t y and then s l o w l y change shape and i n c r e a s e i n i n t e n s i t y as the new species "grew" i n . P o s i t i v e i d e n t i f -i c a t i o n was made by comparison w i t h HCO generated by UV photo-4, 59 l y s i s of i r r a d i a t e d methanol. A l s o evident i n F i g u r e 27 are s i g n a l s due t o other paramagnetic species which i n c r e a s e d i n i n t e n s i t y as the UV p h o t o l y s i s was prolonged. A f t e r about 20 minutes of i n t e n s e p h o t o l y s i s , the samples became n e a r l y c o l o u r l e s s w i t h complete l o s s of the COj s i g n a l a t g "2.015. The HCO r a d i c a l was a l s o bleached t o some degree by the u l t r a v i o l e t l i g h t . (Both the C0 3 and HCO s i g n a l s could a l s o be bleached by red l i g h t from a helium neon l a s e r . ) The ESR spectrum recorded a f t e r the i n t e n s e UV i l l u m i n a t i o n i s shown i n F i g u r e 29 where i t i s c l e a r t h a t s e v e r a l new paramagnetic species are present. Remaining p o r t i o n s of r a d i c a l s "B" and "D" are i n d i c a t e d a p p r o p r i a t e l y . In a d d i t i o n t o these and an u n i d e n t i f i e d s i g n a l " I " , f o u r narrow l i n e s are evident as marked by the arrows. At f i r s t i t was thought t h a t they were u n r e l a t e d but when the sample was UV i r r a d i a t e d f o r a f u r t h e r 30 minutes, a l l f o u r s i g n a l s i n c r e a s e d e q u a l l y as shown i n F i g u r e 30 i n d i c a t i n g t h a t they were indeed r e l a t e d . Since the s p l i t t i n g of the q u a r t e t was 21 - 2 G and s i n c e they were centered at e s s e n t i a l l y the f r e e - s p i n g-value w i t h an i n t e n s i t y r a t i o of 1:3:3:1, t h i s o Figure 30. ESR spectrum of y - i r r a d i a t e d glassy P C at 77 °ir , n . Photolysis. A r r o £ s i n d i c a t e ^ J£ ll7^" - t e r 50 m.nutes of intense uv - 1 0 3 -4,59 s p e c i e s i s undoubtedly the methyl r a d i c a l . T h i s a s s i g n -ment i s confirmed by the r e l a t i v e i n s t a b i l i t y o f the r a d i c a l as d e t e c t e d by the slow decrease i n s i g n a l i n t e n s i t y as the sample stood f o r s e v e r a l hours f o l l o w i n g the p h o t o l y s i s . {A h a l f - l i f e o f about 2 hours was e s t i m a t e d from a q u a l i t a t i v e decay study.) The decay produced an i n c r e a s e i n the i n t e n s i t y o f the broad asymmetric s i g n a l " I " a t the c e n t e r o f the spectrum as i n d i c a t e d i n F i g u r e 3 1 . T h i s s p e c i e s i s probably a r a d i c a l w i t h o n l y ^ p r o t o n s and thus v e r y s m a l l h y p e r f i n e s p l i t t i n g , formed by H atom a b s t r a c t i o n by the r e a c t i v e methyl r a d i c a l s . In summary, UV p h o t o l y s i s o f i r r a d i a t e d PC g l a s s e s a t 7 7 °K produced r a d i c a l s i d e n t i f i e d as C 0 3 , HCO and CH 3 ; the l a t t e r being u n s t a b l e i n the m a t r i x a t l i q u i d n i t r o g e n temperature. -105-PART I I - RADIOLYSIS OF PROPYLENE CARBONATE IN THE LIQUID PHASE A. INTRODUCTION This s e c t i o n r e q u i r e s l i t t l e i n t r o d u c t i o n i n a d d i t i o n to t h a t g i v e n i n Chapter I and i n the g e n e r a l remarks made at the beginning of the c u r r e n t Chapter. Propylene carbonate as a " s a t u r a t e d " organic compound would be expected to be r e l a t i v e l y u n r e a c t i v e w i t h thermal e l e c t r o n s generated by r a d i o l y s i s . I t s i n e r t n e s s towards 40g,41 r e a c t i o n w i t h the a l k a l i metals tends to support t h i s p r e d i c t i o n . The high d i e l e c t r i c constant of the medium (65) should a l l o w a l a r g e f r a c t i o n of the thermal e l e c t r o n s to escape geminate recombination. The l a r g e permanent d i p o l e —18 moment of PC (4.9 x 10 esu cm) would be s u f f i c i e n t to provide ample s o l v a t i o n energy i f the thermal e l e c t r o n s s u r -v i v e long enough to become s o l v a t e d . Thus on the b a s i s of i t s known p h y s i c a l and chemical p r o p e r t i e s , one might p r e d i c t t h a t s o l v a t e d e l e c t r o n s would be generated i n a r e l a t i v e l y l a r g e 14 y i e l d i n t h i s system. Indeed, Hayon r e c e n t l y p u b l i s h e d a d e t e r m i n a t i o n of t h i s y i e l d by an i n d i r e c t method which gave G _ = 2.25. However, by h i s technique of measuring the e s y i e l d of anthracene anions generated i n the pulse i r r a d i a t e d l i q u i d , i t i s not p o s s i b l e to say c o n c l u s i v e l y whether the reducing species measured was a s o l v a t e d e l e c t r o n . A r e a c t i v e s o l v e n t anion could c o n c e i v a b l y a l s o undergo a f a s t charge t r a n s f e r r e a c t i o n w i t h anthracene to g i v e the anthracene anion. -106-Support f o r t h i s a l t e r n a t e mechanism was r e c e n t l y obtained by a c o l l e a g u e i n t h i s l a b o r a t o r y f o r the r a d i o l y s i s 43b of d i m e t h y l s u l f o x i d e , where the reducing species does appear to be a s o l v e n t anion formed i n a y i e l d equal to t h a t expected f o r s o l v a t e d e l e c t r o n s and measured as such by Hayon^f Experience i n t h i s l a b o r a t o r y w i t h another h i g h d i e l e c t r i c 15 s o l v e n t , formamide , a l s o i n d i c a t e d t h a t s o l v e n t anions c o u l d w e l l be the r e a c t i v e reducing s p e c i e s , although compet-i t i o n k i n e t i c s t u d i e s f o r r e d u c t i o n r e a c t i o n s w i t h e l e c t r o n scavengers such as N 20, H , and Ag gave r a t e constant r a t i o s v e r y s i m i l a r t o those known f o r s o l v a t e d e l e c t r o n s i n water. I t can be i n f e r r e d from the above d i s c u s s i o n t h a t scavenger s t u d i e s i n i r r a d i a t e d systems can produce v a l u a b l e i n f o r m a t i o n about the f r e e i o n y i e l d i n a s o l v e n t . However, they cannot always be r e l i e d upon to i d e n t i f y the species unambiguously. Only by d i r e c t o b s e r v a t i o n , u s i n g ESR or o p t i c a l techniques i n c o n j u n c t i o n w i t h pulse r a d i o l y s i s (or f l a s h p h o t o l y s i s ) , can s o l v a t e d e l e c t r o n s be p o s i t i v e l y i d e n t i f i e d i n a medium. With the above thoughts i n mind, the b a s i c o b j e c t i v e of the c u r r e n t study was t o i n v e s t i g a t e the r a d i o l y s i s of l i q u i d propylene carbonate, both pure and w i t h added s o l u t e s . By determining the r a d i o l y t i c a l l y generated product y i e l d s and observing the e f f e c t s of the added scavengers on these y i e l d s and the y i e l d s of products from scavenger r e a c t i o n s , i t was hoped t h a t the y i e l d of reducing species could be deduced. -107-B. EXPERIMENTAL 1. Reagents Eastman Kodak p r a c t i c a l grade propylene' carbonate (PC) was p u r i f i e d by f i r s t d r y i n g over Linde 4A molecular s i e v e s and then double vacuum f r a c t i o n a t i o n as d e s c r i b e d i n Appendix 4. The dry p u r i f i e d s o l v e n t was s t o r e d under dry helium i n a g l a s s d i s p e n s i n g apparatus which i s a l s o d i s cussed i n Appendix 4, along w i t h the p h y s i c a l analyses of the s o l v e n t . N i t r o u s oxide, used as an e l e c t r o n scavenger, was obtained from Matheson and p u r i f i e d by " t r a p - t o - t r a p " d i s t -i l l a t i o n i n a h i g h vacuum system, where i t was subsequently s t o r e d i n a f i v e l i t r e f l a s k u n t i l r e q u i r e d . A l l other chemicals used as scavengers were a n a l y t i c a l reagent grade or e q u i v a l e n t and were used as r e c e i v e d . Water was p u r i f i e d as d e s c r i b e d i n Chapter I I . Helium used f o r deoxygenation of the samples and f o r chromatography was s u p p l i e d by Canadian L i q u i d A i r . I t was d r i e d by passing through a long copper c o i l immersed i n a l i q u i d n i t r o g e n bath. 2. R a d i a t i o n Source The Co Gammacell 220 was used f o r the i r r a d i a t i o n s . Ferrous s u l f a t e dosimetry done i n the sample c e l l used i n the c u r r e n t study, as d e t a i l e d i n Appendix 1, i n d i c a t e d t h a t the dose r a t e f o r the f e r r o u s s u l f a t e s o l u t i o n was 4500 rads min""^ (2.84 x 1 0 1 7 eV g " 1 m i n - 1 ) on August 1, 1970. C o r r e c t i o n s f o r -108-t h e " e l e c t r o n d e n s i t y " d i f f e r e n c e b e t w e e n PC a n d t h e d o s i m e t e r s o l u t i o n a n d f o r t h e n a t u r a l d e c a y o f t h e s o u r c e a c t i v i t y w e r e made as d e s c r i b e d i n A p p e n d i x 1. 3. A p p a r a t u s a n d T e c h n i q u e s (a) S a m p l e c e l l a n d s a m p l e p r e p a r a t i o n The p y r e x g l a s s s a m p l e c e l l a n d e x p e r i m e n t a l t e c h n i q u e s u s e d i n t h i s s t u d y w e r e s i m i l a r t o t h o s e o r i g i n a l l y u s e d b y 60 H e a d . The s a m p l e c e l l i s shown i n F i g u r e 3 2 . A PC s a m p l e was i n s e r t e d i n t o t h i s a p p a r a t u s v i a t h e B7 s o c k e t o p e n i n g u s i n g a p i p e t t e w i t h a f i n e , d r a w n o u t , t i p w h i c h w o u l d f i t t h r o u g h t h e c a p i l l a r y t u b i n g a b o v e t h e s a m p l e s e c t i o n o f t h e c e l l . The B7 c o n e a n d s o c k e t j o i n t was g r e a s e d s p a r i n g l y w i t h A p i e z o n N h i g h v a c u u m g r e a s e a n d h e l d t o g e t h e r w i t h t w o s t a i n l e s s s t e e l s p r i n g s . The f o u r - w a y s t o p c o c k was a l s o g r e a s e d w i t h A p i e z o n N a n d h e l d i n p l a c e w i t h a n a l u m i n u m s t o p c o c k r e t a i n e r a s shown i n t h e p h o t o g r a p h o f F i g u r e 3 2 . (The s t o p c o c k r e t a i n e r a n d t h e s p r i n g s w e r e r e q u i r e d b e c a u s e t h e c e l l was p r e s s u r i z e d t o 20 - 30 p s i g d u r i n g t h e a n a l y s i s p r o c e d u r e . ) The s a m p l e c e l l was c o n n e c t e d e i t h e r t o t h e e x t e r n a l s a m p l e " l o o p " o f t h e g a s c h r o m a t o g r a p h y s y s t e m o r t o t h e v a c u u m l i n e v i a S13 b a l l j o i n t s o n t h e o t h e r two arms o f t h e f o u r - w a y s t o p c o c k . B y t u r n i n g t h e s t o p c o c k i n a s u i t a b l e d i r e c t i o n , t h e PC s a m p l e i n t h e c e l l c o u l d e i t h e r be i s o l a t e d o r e x p o s e d t o t h e v a c u u m l i n e o r GC l o o p . The f r i t t e d d i s k a t t h e b o t t o m o f t h e s a m p l e s e c t i o n p r o v i d e d a f i n e s t r e a m o f g a s b u b b l e s t h r o u g h t h e l i q u i d f o r d e o x y g e n a t i o n o r f l u s h i n g t h e v o l a t i l e r a d i o l y s i s p r o d u c t s i n t o t h e GC. F i g u r e 32. P h o t o g r a p h o f t h e s a m p l e c e l l u s e d f o r t h e l i q u i d p h a s e r a d i o l y s i s o f PC. -110-Th e sample c e l l and as s o r t e d a u x i l i a r y glassware ( i . e . beakers, p i p e t t e s , etc.) were r o u t i n e l y cleaned a f t e r each experiment (or s e r i e s ) using a standard procedure. The sample c e l l was f i r s t degreased u s i n g hexane and then was thoroughly f l u s h e d w i t h hexane and then d i s t i l l e d water, using a water a s p i r a t o r t o suck the s o l v e n t s through the c e l l . F o l l o w i n g t h i s i n i t i a l washing, the c e l l was e i t h e r annealed i n the g l a s s b l o w e r s ' oven or baked i n a 300 °C l a b oven t o remove the brown c o l o u r a t i o n produced by the r a d i a t i o n i n the g l a s s . A f t e r a n nealing, the c e l l and other glassware were soaked f o r at l e a s t 12 hours i n permanganic a c i d , then r i n s e d w i t h d i s t i l l e d water and soaked f o r s e v e r a l hours i n a con-c e n t r a t e d hydrogen peroxide - n i t r i c a c i d s o l u t i o n t o remove t r a c e s of Mn02. F o l l o w i n g t h i s , the apparatus was thoroughly r i n s e d , f i r s t w i t h s i n g l y d i s t i l l e d water and then w i t h t r i p l y d i s t i l l e d water. F i n a l l y the glassware was d r i e d i n a "cle a n " oven a t 250 °C f o r s e v e r a l hours before use. In a t y p i c a l experimental s e r i e s , the pre-weighed c l e a n c e l l was f i l l e d w i t h a sample of p u r i f i e d PC which had been dispensed from the g l a s s storage f l a s k i n t o a s m a l l beaker. The same f i l l i n g p i p e t t e was used f o r a l l experiments and i t d e l i v e r e d 17.9 ml of the s o l v e n t i n t o the c e l l . The weight of each PC sample was however determined a c c u r a t e l y by weighing the c e l l on a beam balance t o the nearest 1/100*-*1 of a gram. This sample weight was then used i n subsequent dose c a l c u l a t i o n s . F o l l o w i n g weighing and gr e a s i n g of the c e l l , the PC sample was deoxygenated by f l u s h i n g w i t h dry helium f o r about 30 minutes, a f t e r which the c e l l stopcock was turned about 45° and the - I l l -sample was thereby sealed i n the c e l l under a helium atmos-phere. (The stopcock was onl y turned 45° so t h a t the sample would remain above the f r i t t e d g l a s s d i s k . I f i t was turned 90°, the sample tended to f l o w through the d i s k and f i l l the opposite s i d e of the c e l l . The l a t t e r was not d e s i r e a b l e s i n c e the dosimetry had been done w i t h the f e r r o u s s u l f a t e s o l u t i o n above the d i s k . ) A f t e r deoxygenation, the c e l l was e i t h e r i r r a d i a t e d or e l s e attached t o the vacuum system f o r a d d i t i o n of n i t r o u s oxide to the sample p r i o r t o i r r a d i a t i o n . S o l i d and l i q u i d scavengers were always added d i r e c t l y to the PC i n a s m a l l beaker before the sample was put i n the c e l l . In a l l cases, i r r a d i a t i o n s were performed w i t h the sample c e l l maintained a t 25 °C i n a c i r c u l a t i n g a l c o h o l bath which f i t i n s i d e the Gammacell c a v i t y . (b) Gaseous product a n a l y s i s F o l l o w i n g i r r a d i a t i o n the sample c e l l was attached to the e x t e r n a l loop of the gas chromatograph. In the m a j o r i t y of the experiments, a s p e c i a l l y m o d i f i e d V a r i a n Aerograph A-90-P2 GC was used f o r a n a l y s i s of the gaseous r a d i o l y s i s products (N 2, CH^, CO and C 0 2 ) . D e t a i l s of t h i s system are g i v e n i n Appendix 5 along w i t h o p e r a t i n g and c a l i b r a t i o n data. B r i e f l y , the chromatograph was equipped w i t h d u a l columns connected i n s e r i e s w i t h the d e t e c t o r s . The c a r r i e r gas, a f t e r f l u s h i n g through the sample c e l l , passed through a 2 f o o t by 1/8 i n c h Porapak Q "pre-column" vapour t r a p whose purpose was t o prevent PC vapour from e n t e r i n g the GC system. (This pre-column was back-flushed a f t e r each experiment t o -112-remove the PC vapour.) The c a r r i e r gas then flowed through a 1 f o o t by 1/4 i n c h copper c o i l immersed i n a l i q u i d n i t r o g e n bath where the condensible gases, i . e . C0 2 and N^O, were trapped out. F o l l o w i n g t h i s , the helium passed through an 8 f o o t by 1/8 i n c h Porapak Q column at 0 C before reaching the "sample" s i d e of the thermal c o n d u c t i v i t y d e t e c t o r s w i t h WX f i l a m e n t s . The permanent gases (N 2, CO and CH^) were not separated by t h i s column and a l l e l u t e d at the same time to g i v e a s i n g l e peak on the recor d e r . The gas then flowed i n t o a 20 f o o t by 1/4 i n c h 13X molecular s i e v e column at 100 °C where the N^, CO and CH^ were separated before e n t e r i n g the " r e f e r e n c e " s i d e of the d e t e c t o r s . The sample c e l l was c o n t i n u o u s l y f l u s h e d by the c a r r i e r gas u n t i l a f t e r the e l u t i o n of methane (about 15 minutes from i n i t i a l exposure a t 60 ml/min f l o w r a t e ) ; at t h i s p o i n t , the sample loop was bypassed to prevent o v e r l o a d i n g of the pre-column vapour t r a p . About f i v e minutes a f t e r the sample loop was c l o s e d CO e l u t e d and was measured. Subsequently, CO^ was determined by warming the t r a p and v a p o u r i z i n g the gas condensed there. The 8 f o o t Porapak Q column at 0 °C h e l d the C0 2 back f o r s e v e r a l minutes and thus allowed the "pressure peak", caused by warming the t r a p , t o pass through the d e t e c t o r s . I f n i t r o u s oxide was present both CO^ and N 20 e l u t e d n e a r l y simultaneously and t h i s p r e -vented measurement of C0 2 i n the presence of N 20. I t was found t h a t w i t h a 60 ml/min f l o w r a t e , t a i l i n g of the N 2» CH^ and CO peaks was not s e r i o u s . Since C0 2 was trapped out, i t gave a very narrow peak w i t h no t a i l i n g . In a d d i t i o n , "double f l u s h " experiments on the same i r r a d i a t e d -113-sample showed t h a t during the 15 minute exposure to the c a r r i e r gas, g r e a t e r than 99% of the permanent gases were removed from the c e l l and about 98% of the CC^ was removed. A t y p i c a l chromatogram and s e n s i t i v i t y data are a l s o presented i n Appendix 5. The s e n s i t i v i t y was determined by i n j e c t i n g known amounts of the v a r i o u s gases w i t h an " i n - l i n e " sample loop of c a l i b r a t e d volume. Peak areas were measured by manual t r i a n g u l a t i o n . The l i n e a r i t y of d e t e c t o r response was checked by i n j e c t i n g known q u a n t i t i e s of the gases w i t h a second sample loop f i l l e d on a vacuum l i n e to v a r i o u s p r e s s -ures. The day-to-day v a r i a t i o n i n s e n s i t i v i t y was monitored by i n j e c t i n g a n i t r o g e n "standard" w i t h the " i n - l i n e " sample loop before each experiment. Since the v a r i a t i o n i n these n i t r o g e n standard peak areas was always l e s s than - 5% of the mean, no c o r r e c t i o n s were made to the experimental data. The 5% d e v i a t i o n could be accounted f o r on the b a s i s of changes i n atmospheric pressure and temperature as w e l l as u n c e r t a i n t y i n t r o d u c e d by the manual i n t e g r a t i o n technique. Because the hydrogen response of the A-90-P2 system w i t h helium as the c a r r i e r gas was i n s u f f i c i e n t t o d e t e c t the q u a n t i t i e s of hydrogen formed during the r a d i o l y s i s , t h i s product was measured us i n g a V a r i a n Aerograph 1720 GC w i t h argon as the c a r r i e r gas. This chromatograph was equipped w i t h 13X molecular s i e v e columns and WX thermal c o n d u c t i v i t y d e t e c t o r s . The same e x t e r n a l sample loop system was used, the e s s e n t i a l o p e r a t i n g d i f f e r e n c e s being a d i f f e r e n t f l o w r a t e and column temperature. Only H 2 (and N_ f o r a n i t r o u s -114-oxide s e r i e s ) was measured because of the e x c e s s i v e l y long r e t e n t i o n times f o r CH„ and CO. 4 (c) Vacuum techniques and determination of the  s o l u b i l i t y of n i t r o u s oxide Deoxygenated PC samples were vacuum degassed by a t t a c h -i n g the c e l l to the vacuum system i l l u s t r a t e d i n F i g u r e 33. The S13 b a l l j o i n t s of the c e l l were connected to the two S13 s o c k e t s , each of which was i s o l a t e d from the vacuum l i n e by a s m a l l stopcock, and S^. A t h i r d stopcock, S^, f u r t h e r separated the e x t e r n a l connections from the main vacuum manifold . I n i t i a l l y , w i t h the four-way stopcock of the c e l l turned so as t o i s o l a t e the sample from the vacuum, stopcocks S^, S 2 and were opened and the l i n e up to and i n c l u d i n g the "bore" of the four-way stopcock was pumped t o a good vacuum —6 (10 mm of Hg) using a three stage mercury d i f f u s i o n pump backed by a r o t a r y o i l pump. Then w i t h the stopcock S 2 c l o s e d , the four-way stopcock was s l o w l y r o t a t e d 90° u n t i l the gas i n s i d e the c e l l s t a r t e d to "bubble" out v i a S^. This p a r t of the procedure r e q u i r e d c o n s i d e r a b l e care. I f the c e l l was opened too q u i c k l y , the sample would bubble e x c e s s i v e l y and s p l a s h i n t o the vacuum l i n e . Once most of the helium was pumped out of the c e l l v i a S^, was c a u t i o u s l y opened and the t r a c e s of gas remaining on t h a t s i d e of the s i n t e r e d d i s k were removed. The c e l l and sample were f i n a l l y pumped down to a good vacuum, a f t e r which, S , S and S were c l o s e d . 3 N i t r o u s oxide, which had p r e v i o u s l y been trapped out i n t r a p T^ (using a l i q u i d n i t r o g e n bath) and w e l l degassed, Figure 3 3 . Schematic diagram of the vacuum system used to add nitrous oxide to the PC samples i n the "bubbler" c e l l . -116-was then vapourized i n t o the evacuated l i n e t h a t was i s o l a t e d from the vacuum pumps by stopcock S^. The i n i t i a l N 20 p r e s s -ure was read on the mercury manometer and from the known volume of the l i n e between and S^, the i n i t i a l amount of N^O was c a l c u l a t e d using the i d e a l gas law; PV =nRT. Then S was opened, f o l l o w e d by S which was s l o w l y opened to a l l o w the N 20 t o bubble through the PC sample. S 1 remained c l o s e d at t h i s stage. Again i t was necessary t o use great care i n opening S 2, otherwise the sample would s p l a s h i n t o the l i n e . The was allowed t o bubble s l o w l y through the sample u n t i l e q u i l i b r i u m was reached (about 30 minutes). Then was opened, the four-way stopcock c l o s e d by t u r n i n g 45° and the f i n a l pressure was read from the manometer. From the c a l -i b r a t e d t o t a l volume of the system, the f i n a l amount of gaseous N 0 was c a l c u l a t e d . (A s u i t a b l e c o r r e c t i o n was made f o r the 2 manometer volume change.) The d i f f e r e n c e between the i n i t i a l and f i n a l amounts of gaseous n i t r o u s oxide gave the q u a n t i t y which had d i s s o l v e d i n the PC sample. Using the known volume of PC the c o n c e n t r a t i o n of ^ O was c a l c u l a t e d . The v a r i a t i o n of n i t r o u s oxide c o n c e n t r a t i o n w i t h e q u i l i b r a t e d pressure i s p l o t t e d i n F i g u r e 34. From the slope of the l i n e through these data, the s o l u b i l i t y of N O i n PC at room temperature —4 1 (20-25 C) was estimated to be 1.5 x 10 M mm . The s l i g h t s c a t t e r i n the data most probably i s a r e s u l t of temperature v a r i a t i o n s from experiment to experiment. The "non-zero" i n t e r c e p t i s b e l i e v e d to be due to the presence of the s i n t e r e d d i s k i n the apparatus. F i g u r e 34. Concentration of d i s s o l v e d n i t r o u s oxide i n PC at room temperature (20 - 25 as a f u n c t i o n of the n i t r o u s oxide pressure i n the "bubbler" c e l l . -118-C. RESULTS 1. Pure PC - Gaseous R a d i o l y s i s Products The four gaseous products formed w i t h s i g n i f i c a n t y i e l d i n PC i r r a d i a t e d at 25 °C were H^, CH^, CO and C0 2. T h e i r y i e l d was found to be independent of dose up t o at l e a s t 0.3 Mrad (1.9 x 1 0 1 9 eV g" 1) f o r H 2 and at l e a s t 1.4 Mrad (8.6 x 1 0 1 9 eV g" 1) f o r CH^, CO and C0 2. ( Y i e l d s f o r l a r g e r doses than these were not measured.) Some of these data are p l o t t e d i n Fi g u r e 35, where from the slopes of the l i n e s the G values were c a l c u l a t e d t o be: G(H 2) = 0.75 i 0.05 G(CH 4)= 0.20 ± 0.02 G(CO) =1.2 ± 0.1 G(C0 2)= 3.3 ± 0.2 No oxygen was produced by the r a d i o l y s i s and non-gaseous products were not measured. 2. Scavenger Experiments S e v e r a l s e r i e s of experiments were performed w i t h n i t r o u s oxide present as a scavenger. The y i e l d of n i t r o g e n produced by scavenging r e a c t i o n s at a f i x e d n i t r o u s oxide c o n c e n t r a t i o n was found t o be dependent on dose as i l l u s t r a t e d i n F i g u r e 36. For a n i t r o u s oxide c o n c e n t r a t i o n of 0.05 M, the i n i t i a l G(N 2) = 1.8 decreased to G(N 2) = 0.9 a f t e r the 19 -1 sample had absorbed a t o t a l of about 0.8 Mrads (5 x 10 eV g ). This decrease i n G(N 2) was b e l i e v e d to be caused by a n o n - v o l a t i l e product b u i l d i n g up i n the system and competing - 1 1 9 -Figure 35. Yields of gaseous r a d i o l y s i s products from PC *y - i r r a d i a t e d at 25 °C, as a function of dose. I NJ O I Figure 36. Nitrogen y i e l d as a function of accumulated sample dose for a constant nitrous oxide concentration (0.05 M). -121-w i t h n i t r o u s oxide f o r the reducing s p e c i e s . This c o n c l u s i o n i s based on the o b s e r v a t i o n t h a t the n i t r o g e n y i e l d measured f o r a s m a l l dose g i v e n to a sample p r e v i o u s l y i r r a d i a t e d w i t h a l a r g e dose, but f l u s h e d t o remove the v o l a t i l e products, was much s m a l l e r than t h a t obtained f o r a s m a l l dose g i v e n to a f r e s h sample. Because of t h i s dependence of G(N ) on dose, a l l other experiments i n v o l v i n g n i t r o u s oxide were done on a new sample us i n g the s m a l l e s t p r a c t i c a l dose of about 18 —1 0.08 Mrad (5 x 10 eV g ) f o r which a reasonably accurate G(N 2) could be determined. N i t r o u s oxide d i d not a f f e c t the hydrogen or carbon monoxide y i e l d s or s i g n i f i c a n t l y lower the methane y i e l d . At the low doses used i n most experiments, accurate methane det e r m i n a t i o n was not p o s s i b l e because of i t s low y i e l d and " t a i l i n g " ' of the n i t r o g e n peak. However, the y i e l d c a l c u l a t e d f o r l a r g e dose experiments w i t h N 20 were i n agreement w i t h G(CH 4) obtained f o r pure PC and the y i e l d s measured at low doses were never l e s s than G(CH^) = 0.1 . Carbon d i o x i d e could not be measured w i t h n i t r o u s oxide present because of the i n t e r f e r e n c e mentioned i n the experimental s e c t i o n . The y i e l d of n i t r o g e n , G(N 2), f o r constant dose exper-iments, p l o t t e d as a f u n c t i o n of n i t r o u s oxide c o n c e n t r a t i o n i s shown i n F i g u r e 37. G ( N 2 ^ i n c r e a s e d r a p i d l y between 0 and 0.01 M n i t r o u s oxide and then increased more s l o w l y , e v e n t u a l l y reaching a near p l a t e a u value of G(N 2) = 2.0 above 0.07 M . (The maximum e x p e r i m e n t a l l y o b t a i n a b l e n i t r o u s oxide concen-t r a t i o n was 0.1 M, which corresponded to 7 50 mm pressure.) »122--123-S e v e r a l experiments were performed w i t h second scav-engers present i n a d d i t i o n to n i t r o u s oxide. These data are l i s t e d i n Table 5, along w i t h data f o r the e f f e c t s of the second scavengers alone. I o d i n e , a very 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 scavenger, d i d not a f f e c t the carbon monoxide or carbon d i o x i d e y i e l d s . I t d i d , however, reduce the methane y i e l d by a f a c t o r of at l e a s t 10. In the presence of 0.077 M n i t r o u s oxide, i o d i n e a t 0.001 M competed e f f e c t i v e l y f o r the pr e c u r s o r of n i t r o g e n and reduced G(N 2) by a f a c t o r of 2. As the i o d i n e c o n c e n t r a t i o n was incr e a s e d 70 f o l d to 0.068 M i t o n l y decreased G(N 2) by a f u r t h e r f a c t o r of 2, c o n t r a r y t o t h a t expected on the b a s i s of simple competition k i n e t i c s . Water at 0.22 M d i d not a f f e c t the n i t r o g e n y i e l d or t h a t of any of the other gaseous products. Methanol at 0.18 M a l s o d i d not a f f e c t G(N 2) or G(CH 4) and G(CO). S e v e r a l experiments were performed w i t h s u l f u r i c a c i d although i t was known to cause h y d r o l y s i s of the propylene carbonate; producing CO,,, 40a propylene oxide and propionaldhyde among other products. At 0.33 M H + the n i t r o g e n y i e l d was reduced by more than a f a c t o r of 2 w i t h n i t r o u s oxide at 0.07 5 M. I t d i d not appear to s i g n i f i c a n t l y a f f e c t the hydrogen y i e l d although i f 0.2 M methanol was added, the hydrogen y i e l d d i d increase s l i g h t l y to G(H 2) = 0.95 . D. DISCUSSION Since the hydrogen, carbon monoxide and carbon d i o x i d e y i e l d s were not a f f e c t e d by the presence of the e l e c t r o n and TABLE 5 Second SUMMARY OF SECOND SCAVENGER EXPERIMENTAL RESULTS G(CO) G(CO S 2 (M) N 2 0 (M) G(H 2) G(N 2) G(CH 4) Scavenqer N i l N i l N i l 0 . 7 5 N i l 0 . 2 0 1 . 2 3 . 3 N i l N i l 0 . 0 9 4 0 . 7 6 2 . 0 a a a N i l N i l 0 . 0 7 7 a 2 . 0 0 . 1 5 1 . 1 a Iodine 0 . 0 5 3 N i l a N i l 0 . 0 2 1 . 1 3 . 4 0 . 0 0 1 0 0 . 0 7 7 a 1 . 1 0 . 0 3 1 . 1 a 0 . 0 0 4 6 0 . 0 7 7 a 0 . 8 6 0 . 0 1 1 . 1 a 0 . 0 1 2 0 . 0 7 7 a 0 . 7 5 0 . 0 1 1 . 2 a 0 . 0 2 2 0 . 0 7 7 a 0 . 6 5 0 . 0 3 1 . 1 a 0 . 0 6 8 0 . 0 7 7 a 0 . 5 5 0 . 0 2 1 . 1 a Water 0 . 2 2 N i l 0 . 7 7 N i l 0 . 1 7 1 . 2 3 . 5 II 0 . 2 2 0 . 0 7 7 0 . 7 7 1 . 9 a a a Methanol 0 . 1 8 0 . 0 5 1 a 1 . 7 0 . 1 4 1 . 2 a H + ( H , S O j b 0 . 3 3 0 . 0 7 5 a 0 . 6 0 0 . 1 4 1 . 1 a » 2 4 0 . 2 9 N i l 0 . 7 8 N i l a a a n • 0 . 2 9 c N i l 0 . 9 5 N i l a a a Footnotes: a. Not determined, b. A c i d caused slow h y d r o l y s i s of the PC producing C 0 2 among other products. This may have i n f l u e n c e d the r e s u l t s , c. 0 . 2 M methanol added i n a d d i t i o n to the a c i d . -125-r a d i c a l scavengers, n i t r o u s oxide and i o d i n e at g r e a t e r than 10 M, these products are most l i k e l y formed v i a molecular processes. A probable mechanism f o r the formation of CO and C 0 2 i s through "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 propylene carbonate molecules, although r a p i d decomp-o s i t i o n of p o s i t i v e ions could a l s o produce these products. Indeed, the mass spectrum of PC (see Appendix 4) i n d i c a t e s the PC p o s i t i v e i o n i s v e r y unstable i n the gas phase and i t does appear to l o s e CO and CO2 very r e a d i l y . The molecular hydrogen, on the other hand, i s b e l i e v e d to r e s u l t from f a s t H atom a b s t r a c t i o n r e a c t i o n s . The precursor of methane was r e a d i l y scaveng/ed by _3 i o d i n e , even a t 10 M, which i n d i c a t e s the methane a r i s e s from r e l a t i v e l y l o n g - l i v e d methyl r a d i c a l s formed i n the r a d i o l y t i c decomposition processes of propylene carbonate. The f a c t t h a t n i t r o u s oxide d i d not s i g n i f i c a n t l y lower G(CH^) as compared w i t h i o d i n e , most probably r e f l e c t s the much slower r e a c t i o n r a t e constants f o r r a d i c a l r e a c t i o n s of N 2 0 5 as compared w i t h i o d i n e . I o d i n e , f o r example, r e a c t s 4 x 10 22 times f a s t e r w i t h H atoms i n aqueous s o l u t i o n . The s i g n i f i c a n c e of the water experiments i s to i n d i c a t e t h a t e i t h e r water does not scavenge the precursors of the molecular products, H , CO, CO and C H , or the p r e c u r s o r of 2 ^ 4 n i t r o g e n , or e l s e the PC samples a l r e a d y contained s u f f i c i e n t water so t h a t the a d d i t i o n a l amount added i n these experiments would not have any i n c r e a s e d e f f e c t . Water would be expected to a c t as an e f f i c i e n t p o s i t i v e i o n scavenger, accepting a proton to form H_ 0 + . -126-In a d d i t i o n t o the independence of the hydrogen y i e l d on the presence of n i t r o u s o x i d e , the f a i l u r e of methanol at 0.2 M to a f f e c t the n i t r o g e n y i e l d f o r a n i t r o u s oxide c o n c e n t r a t i o n of 0.05 M, a l s o r u l e s out the p o s s i b i l i t y of H atoms being the reducing s p e c i e s . Methanol i s known to be a good hydrogen atom scavenger i n aqueous s o l u t i o n s , w i t h the r a t i o of the r a t e constants f o r H atom r e a c t i o n s w i t h methanol 22 and n i t r o u s oxide being 20. Thus the major reducing species i n i r r a d i a t e d PC would appear t o be a negative i o n , e i t h e r a s o l v a t e d e l e c t r o n or a molecular anion. This s p e c i e s , X~, which i s r e a d i l y scavenged by n i t r o u s oxide to g i v e n i t r o g e n , apparently has a y i e l d of G ( X " ) v 2 as i n d i c a t e d by the "plateau" value of G ( N 2 ) . I f the a l t e r n a t i v e f a t e of species X~ i s assumed to be r e a c t i o n w i t h u n s p e c i f i e d i m p u r i t y or s o l v e n t "S", then a n a l y s i s of the n i t r o g e n y i e l d dependence on n i t r o u s oxide c o n c e n t r a t i o n may be made on the b a s i s of simple c o m p e t i t i o n k i n e t i c s . The two r e a c t i o n s o c c u r i n g are (8) and (9). X~ + S — k s » products (8) X" + N 20 _kN2£L> N 2 + products (9) Steady s t a t e treatment of t h i s r e a c t i o n mechanism f o r species X~ gi v e s the k i n e t i c e x p r e s s i o n ( i x ) . G(N 2) G(X-) k s [s] K N 2 0 [ N2 ° 3 (ix) Thus a p l o t of 1/ G ( N 2 ) versus 1/ [N 2o3 should be a s t r a i g h t l i n e w i t h slope equal to k s L"s] / kjj^g G ( x ~ ) a n <3 the i n t e r c e p t -127-egual to 1/G(X~) i f the simple competition holds. Such a p l o t f o r the data of Fi g u r e 37 i s gi v e n i n Fi g u r e 38. The r e l a t i v e l y good l i n e a r i t y of the experimental data supports the simple competition mechanism and the i n t e r c e p t of 0.45 i m p l i e s t h a t G(X ) =2.2 , a value i n good agreement w i t h the val u e of G(N2) = 2.0 at 0.1 M n i t r o u s oxide. Another i m p l i -c a t i o n of the simple competition mechanism i s t h a t apparently o n l y one reducing species i s r e a c t i n g w i t h n i t r o u s oxide to give n i t r o g e n . The assignment of the reducing species to a negative i o n and the measured y i e l d of X~ are i n good agreement w i t h 14 the work r e c e n t l y p u b l i s h e d by Hayon on the f r e e negative i o n y i e l d i n propylene carbonate. He determined G(X~) = 2.25 by measuring the y i e l d of anthracene anions formed on pulse r a d i o l y s i s of the l i q u i d . The co m p e t i t i o n experiments between n i t r o u s oxide and i o d i n e f o r species X~ gave some i n t e r e s t i n g r e s u l t s . At the f i x e d n i t r o u s oxide c o n c e n t r a t i o n of 0.077 M, which was s u f f i c i e n t to scavenge at l e a s t 90% of the X~, i o d i n e at only -3 10 M reduced the n i t r o g e n y i e l d by almost a f a c t o r of two. However, as the i o d i n e c o n c e n t r a t i o n was increased to 0.068 M, the n i t r o g e n y i e l d d i d not decrease as r a p i d l y as would be expected on the b a s i s of a simple competition mechanism. Indeed i f the simple c o m p e t i t i o n k i n e t i c e x p r e s s i o n (ix) g i v e n above i s modified so th a t the a l t e r n a t e f a t e of species X~ i s r e a c t i o n w i t h I , then the expres s i o n (x) should h o l d . -128--129-G ( N 2 ) G ( X " ) k l 2 P 2 ] (x) K N 2 0 [N 20] A p l o t of 1/ G ( N 2 ) versus / L"N2O] should then be l i n e a r with slope equal to the r a t i o ^ j ^ / ^ N 20 G ( x ~ ) a n c * *~he i n t e r -cept equal to 1/G(X~) . This graph for the data i n Table 5 i s shown i n Figure 39. The f a c t that i t i s f a r from l i n e a r means that a secondary reaction must be occuring between the product of the iodine scavenging reaction and nitrous oxide to give nitrogen. This r e s u l t i s not p a r t i c u l a r l y suprising since the product of the reaction of X~ with iodine i s probably an iodine atom formed v i a the intermediate I 2 ~ . The iodine atom could then react with the nitrous oxide to give nitrogen and the following reaction scheme might p r e v a i l : X " + N 20 * N 2 + products (9) X - + I 2 »• I + I" + products (10) I + N 20 » N 2 + products (11) I + S • products (12) where reaction (12) represents the alternate fate of the iodine atoms. Unfortunately k i n e t i c analysis of reactions (9) - (12) gives a complex expression for the dependence of G ( N 2 ) on the £l 2 3 and C N 2 ° 3 a n <3 therefore the mechanism cannot be e a s i l y checked against the experimental data. However, the data shown i n Figure 39 do indicate that iodine reacts much more ra p i d l y with species X~ than does nitrous oxide. This i s evident from the i n i t i a l slope of the curve of about 40. At very low iodine concentrations, the simple competition mech-anism should hold at l e a s t approximately, and the slope would then mean that k j ^ / ^ JJ 2 O ^ S A P P r o x i m a t e l y equal to 80. This -130-Figu r e 39. P l o t of 1/G(N2) versus p J / ^ O ] f o r [T^O] = 0.077 M. -131-i s o n l y a f a c t o r of 9 g r e a t e r than t h a t measured f o r hydrated e l e c t r o n s i n water where k l 2 / k u 2 o = 9 * Assuming th a t r e a c t i o n (10) occurs at a r a t e s i m i l a r to the r a t e of r e a c t i o n of hydrated e l e c t r o n s w i t h i o d i n e i n water (k = a<3 + I 2 10 —1 —1 22 5 x 10 M s ) t h i s would imply t h a t species X r e a c t s w i t h n i t r o u s oxide w i t h a r a t e constant of about 5 x 10 M~ s~" . Since 0.02 M n i t r o u s oxide was r e q u i r e d to scavenge h a l f of , the s p e c i e s , the n a t u r a l l i f e t i m e of X can then be estimated t o be approximately 10 seconds (100 nsec). The f a c t t h a t a c i d a t 0.3 M H"*" was able t o reduce the n i t r o g e n y i e l d s i g n i f i c a n t l y i s supplementary evidence f o r the a n i o n i c c h a r a c t e r of the reducing species i n PC. The product of the H + scavenging r e a c t i o n w i t h species X~ does not appear to be molecular hydrogen as i n d i c a t e d by the l a c k of an i n c r e a s e i n the hydrogen y i e l d . However, i t i s conceivable t h a t hydrogen atoms (formed by r e a c t i o n of H+ w i t h a s o l v a t e d e l e c t r o n ) might r e a c t w i t h the ^C=0 system of PC p r e f e r e n t i a l l y t o g i v e a n o n - v o l a t i l e product r a t h e r than a b s t r a c t i n g a hydrogen atom to form molecular hydrogen. This argument i s p a r t i a l l y supported by the experiment i n which methanol was added and a s l i g h t i n c r e a s e i n the hydrogen y i e l d was observed. (Methanol i s known to be a good H atom scavenger i n aqueous systems, producing hydrogen i n the scavenging r e a c t i o n . ) On the other hand, i f a molecular anion i s the reducing species i n PC then an i n c r e a s e i n the H^ y i e l d would not be expected on a d d i t i o n of a c i d . In any event, s i n c e a c i d i s known to cause h y d r o l y s i s of PC these r e s u l t s are not p a r t i c u l a r l y r e l i a b l e . Carbon d i o x i d e and propionaldehyde are among the -132-h y d r o l y s i s products 40a and both are good H atom and e l e c t r o n scavengers. I t was mainly because of t h i s h y d r o l y s i s problem t h a t a sy s t e m a t i c examination of the competition between On the b a s i s of the r e s u l t s d i s c u s s e d above, i t appears t h a t ^ - r a d i o l y s i s of l i q u i d propylene carbonate produces the primary molecular products; hydrogen, carbon monoxide and carbon d i o x i d e w i t h y i e l d s : G = 0.75 - 0.05, G = 1.2 ± 0.1, i n v o l v i n g methyl r a d i c a l s w i t h a y i e l d G(CH^) = 0.20 ± 0.02 . In a d d i t i o n , an i o n i c reducing species i s formed w i t h a y i e l d G = 2.0 i 0.2 . This species i s most l i k e l y a s o l v a t e d e l e c t r o n although because of the l i m i t e d amount of data a v a i l -a b l e , a molecular anion cannot be r u l e d out. The best arguements i n favour of the s o l v a t e d e l e c t r o n hypothesis are: (a) the l a c k of r e a c t i v i t y of PC w i t h a l k a l i metals suggesting t h a t e l e c t r o n attachment t o PC i s not favoured, and (b) the l a r g e permanent d i p o l e moment of PC would supply ample s o l v -a t i o n energy t o a l l o w s t a b i l i z a t i o n of the thermal e l e c t r o n s i f they s u r v i v e d long enough t o a l l o w d i p o l e r e l a x a t i o n . The measured y i e l d of the reducing species i s i n good agreement 14 w i t h t h a t found by Hayon and w i t h t h a t expected on the b a s i s of the e m p i r i c a l r e l a t i o n s h i p between the y i e l d of f r e e ions and the s t a t i c d i e l e c t r i c constant of the l i q u i d (see F i g u r e 5 i n Chapter I ) . H + and N.O was not made. and G, CQ = 3.3 * 0.2, and methane v i a a secondary process -133-PART I I I - GENERAL CONCLUSION AND SUGGESTIONS FOR FURTHER STUDY  OF THE RADIATION CHEMISTRY OF PROPYLENE CARBONATE The p r e l i m i n a r y i n v e s t i g a t i o n s reported here f o r the e f f e c t s of r a d i a t i o n on the very p o l a r propylene carbonate system have provided some i n t e r e s t i n g r e s u l t s . Trapped e l e c -t r o n s were i d e n t i f i e d i n the low temperature g l a s s and i t i s very l i k e l y t h a t the i o n i c reducing species formed i n the l i q u i d phase were a l s o s t a b i l i z e d e l e c t r o n s . From the p h y s i c a l p r o p e r t i e s shown by the trapped e l e c t r o n i t i s p o s s i b l e to make a p r e d i c t i o n about the o p t i c a l a b s o r p t i o n maximum of a s o l v a t e d species i n the l i q u i d phase. In most systems where both trapped and s o l v a t e d e l e c t r o n s have been observed s p e c t r o s c o p i c a l l y , X m a x c o n s i s t e n t l y shows a blue s h i f t of between 50 and 200 nm on going from the s o l v a t e d to the trapped species at 77 °K. Thus on the b a s i s of the assigned a b s o r p t i o n band f o r trapped e l e c t r o n s i n PC, ^"max" ^ ® n m ' t n e s ° l v a t e < ^ e l e c t r o n would be expected to absorb p r e f e r e n t i a l l y i n the v i s i b l e r e g i o n w i t h a X i n max the r e g i o n of 400 - 600 nm . This p r e d i c t i o n i s c o n t r a r y t o t h a t expected from the proposed c o r r e l a t i o n between the i o d i d e i o n " c h a r g e - t r a n s f e r - t o - s o l v e n t " a b s o r p t i o n maximum and the \ 61 A of s o l v a t e d e l e c t r o n s p e c t r a . This c o r r e l a t i o n max places X f o r s o l v a t e d e l e c t r o n s i n PC at about 1200 nm, max a v a l u e d i f f i c u l t t o accept on the b a s i s of the p o l a r i t y of PC. I f 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 does represent a d i s t r i b u t i o n p r o f i l e of the d i f f e r e n t t r a p depths, then -134-e l e c t r o n s i n PC should be bound i n r e l a t i v e l y deep traps due t o i t s l a r g e d i p o l e moment and thus should absorb somewhere i n the v i s i b l e r e g i o n and not i n the i n f r a r e d . Hence an experiment of p a r t i c u l a r importance to i d e n t i f y i n g the reducing species i n PC i s an i n v e s t i g a t i o n of i t s pulse r a d -i o l y s i s t o determine i f any t r a n s i e n t o p t i c a l a b s o r p t i o n occurs which can be a t t r i b u t e d to e~. In l i g h t of the above remarks, some t r i a l experiments were performed on the pulse r a d i o l y s i s apparatus designed by a c o l l e a g u e , Dr. G.A. Kenney-Wallace^ 2. This system used the l i g h t from a helium-neon l a s e r t o monitor a b s o r p t i o n at 633 nm i n a sample pulse i r r a d i a t e d w i t h a 3 nsec pulse of 0.5 MeV e l e c t r o n s from a Febetron a c c e l e r a t o r . A s i g n i f i c a n t , s h o r t - l i v e d a b s o r p t i o n was detected at 633 nm f o r l i q u i d PC and a c i d i n c r e a s e d the r a t e of decay markedly. The species observed could c o n c e i v a b l y have been the s o l v a t e d e l e c t r o n , however, CO~ and HCO r a d i c a l s are both known to absorb i n t h i s r e g i o n and these species could not be excluded as the source of the t r a n s i e n t a b s o r p t i o n on the b a s i s of the data obtained. The author hopes to pursue these leads at The Ohio State U n i v e r s i t y where pulse r a d i o l y s i s equipment i s a v a i l a b l e to determine complete v i s i b l e and i n f r a r e d a b s o r p t i o n s p e c t r a . An i n t e r e s t i n g o p p o r t u n i t y a l s o e x i s t s w i t h propylene carbonate to i n v e s t i g a t e the e f f e c t s of changes i n d i e l e c t r i c constant on the y i e l d (and spectra) of s o l v a t e d e l e c t r o n s . Since PC shows such a l a r g e change i n d i e l e c t r i c constant over a comparatively s m a l l temperature range (90 at - 60 °C -135-to 60 a t +30 °C), t h i s e f f e c t may be observable i n v a r i a b l e temperature s t u d i e s . Some e f f e c t s are c e r t a i n l y evident i n o the steady s t a t e r a d i o l y s i s where t r i a l experiments at -40 C showed t h a t the y i e l d s of gaseous products decreased sub-s t a n t i a l l y w i t h G(CH 4) = 0.10 - 0.02, = 0.9 - 0.1 and G „ = 2.6 ± 0.3 as compared w i t h G(CH.) = 0.20 i 0.02, c o 2 * G c 0 = 1.2 t 0.1 and GQQ2 = 3.3 1 0.3 at 25 °C . In a d d i t i o n , the n i t r o g e n y i e l d f o r a sample c o n t a i n i n g 0.08 M n i t r o u s oxide decreased to G(N 2) = 1.6 as compared w i t h G(N 2) = 2.0 at 25 °C . Some of these changes may of course be due t o v i s c o s i t y e f f e c t s s i n c e the v i s c o s i t y i n c r e a s e s s u b s t a n t i a l l y on c o o l i n g . However, one would have expected the d i e l e c t r i c constant i n c r e a s e to cause an inc r e a s e i n G(N 2) and the obser-ved decrease must have r e s u l t e d from a combination of d i f f e r e n t temperature e f f e c t s . F u r t h e r s t u d i e s of the r a d i a t i o n chemistry of PC i n the s o l i d s t a t e are c l e a r l y warranted to u n r a v e l the mystery of the abnormal spontaneous decay of e^ r . O p t i c a l k i n e t i c s t u d i e s of the i r r a d i a t e d g l a s s y m a t e r i a l and c o r r e l a t i o n of these data w i t h the ESR measurements would be h e l p f u l , p a r t -i c u l a r l y i n con f i r m i n g the assignment of the v i o l e t a b s o r p t i o n band to e . Experiments w i t h deuterated propylene carbonate c o u l d a l s o h e l p i n the i d e n t i f i c a t i o n of the primary r a d i c a l s p e c i e s , as would s t u d i e s of the ESR of other o r g a n i c carbon-ates such as ethylene carbonate and t e t r a m e t h y l ethylene carbonate. - 1 3 6 -BIBLIOGRAPHY 1. J . H. O ' D c n n e l l a n d D. F. S a n g s t e r , " P r i n c i p l e s o f R a d i a t i o n C h e m i s t r y " , E l s e v i e r , New Y o r k , N.Y., 1970. 2. E. 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THEORY F r i c k e f e r r o u s s u l f a t e dosimetry was used to determine 60 the dose r a t e d e l i v e r e d by the Co Gammacell. The F r i c k e dosimeter u t i l i z e s the o x i d a t i o n of fer r o u s i r o n to f e r r i c i r o n i n a c i d i c , oxygenated aqueous s o l u t i o n to measure the dose absorbed. In 0.8N a c i d s o l u t i o n c o n t a i n i n g atmospheric oxygen, the two r a d i o l y t i c a l l y generated primary reducing s p e c i e s , e~g and H atoms, are converted to o x i d i z i n g species by r e a c t i o n s (A-1) and ( A - 2 ) . e~ + H+ H + H o O (A-1) k = 2 x l 0 1 0 M~ 1s- 1 aq aq * H + ° 2 *" H 0 2 ( A _ 2 ) k = l x l ° 1 0 M'-'-s"1 The H O 2 r a d i c a l s formed v i a r e a c t i o n (A-2) and the primary OH r a d i c a l s and molecular hydrogen peroxide then o x i d i z e the fe r r o u s i r o n to f e r r i c v i a r e a c t i o n s (A-3), (A-4) and (A-5). H+ + H 0 o + F e + 2 »- H o 0 , + F e + 3 (A-3) X 2 W 2 , + 3 +2 _ „ + 3 H + + H 2 0 2 + F e + 2 • F e 3 + OH + H 2 0 (A-4) H + + OH + Fe Fe J + H 2 0 (A-5) Thus each primary reducing species leads to the o x i d a t i o n of +2 ,-5 three Fe and each primary OH r a d i c a l o x i d i z e s one F e + Z . For Co *y~rays the t o t a l y i e l d of Fe has been determined a c c u r a t e l y using c a l o r i m e t r y as the standard, and G(Fe ) = 15.5 . 3 The f e r r o u s s u l f a t e dosimeter i s ve r y s e n s i t i v e to tr a c e s of orga n i c i m p u r i t i e s . O x i d a t i o n of the organic m a t e r i a l -142-by OH r a d i c a l s and subsequent r e a c t i o n of the organic r a d i c a l w i t h oxygen v i a r e a c t i o n s (A-6) and (A-7) produces an organic OH + RH • R« + H 20 (A-6) R* + 0 2 * R0 2- (A-7) peroxide. The organic peroxide then r e a c t s w i t h the Fe+Z i n +3 . an analagous way t o HO^ to produce three Fe i n s t e a d of the normal one f o r each OH r a d i c a l . Thus i n the presence of +3 o r g a n i c substances G(Fe ) > 15.5 . To suppress r e a c t i o n (A-6) c h l o r i d e i o n i s normally added to the dosimeter s o l u t i o n . OH r a d i c a l s r e a c t very e f f i c i e n t l y w i t h c h l o r i d e ions v i a r e a c t i o n (A-8). OH + C l " • OH" + C l - (A-8) k = 4 x l 0 9 M~ 1s~ 1 +2 The r e s u l t i n g c h l o r i n e atom o x i d i z e s o n l y one Fe v i a r e a c t i o n (A-9) C l * + F e + 2 *• F e + 3 + C l " (A-9) B. PROCEDURE A stock s o l u t i o n of fe r r o u s s u l f a t e was made by d i s -s o l v i n g 0.4 g of a n a l y t i c a l grade f e r r o u s ammonium s u l f a t e , 0.065 g of a n a l y t i c a l grade sodium c h l o r i d e and 22.0 ml of a n a l y t i c a l grade s u l f u r i c a c i d i n t r i p l y d i s t i l l e d water to a t o t a l volume of 1.00 l i t r e . A l l apparatus and c e l l s used i n the dosimetry work were s c r u p u l o u s l y cleaned using the permanganic a c i d - hydrogen peroxide - d i s t i l l e d water r o u t i n e . The f e r r o u s s u l f a t e s o l u t i o n was i r r a d i a t e d i n the sample c e l l s using the app r o p r i a t e volumes and p o s i t i o n s i n the Gammacell c a v i t y . The i r r a d i a t i o n s were a u t o m a t i c a l l y -143-timed w i t h the b u i l t - i n timer on the Gammacell. The times ranged from 0.1 second t o 10 minutes w i t h the corresponding doses of about 1000 t o 50,000 rads. Longer i r r a d i a t i o n times were avoided because above 50,000 rads oxygen d e p l e t i o n i n the s o l u t i o n causes G ( F e + 3 ) t o decrease. A f t e r i r r a d i a t i o n , the f e r r i c i o n c o n c e n t r a t i o n was determined s p e c t r o p h o t o m e t r i c a l l y by measuring the absorbance a t 304 nm usi n g e i t h e r 1 or 5 cm c e l l s and a Cary 14 s p e c t r o -photometer. The i r r a d i a t e d samples were measured w i t h an u n i r r a d i a t e d "blank" i n the reference beam of the spectrometer. The absorbed dose was then c a l c u l a t e d using the e q u a t i o n : 3 Dose = 0.965 x 10 9 x ( A 3°t ) r a d s  € 3 0 4 P 1 G<Fe+3> 304 where A n e t i s the net absorbance of the i r r a d i a t e d sample a t 304 nm, € i s the molar a b s o r p t i v i t y of Fe. at 304 nm, 1 i s the o p t i c a l path l e n g t h and pis the d e n s i t y of the f e r r o u s s u l f a t e s o l u t i o n (1.024 ± 0.001 between 15 and 25 °C). G ( F e + 3 ) = 15.5 was used i n a l l c a l c u l a t i o n s . € w a s taken t o be 2174 M^cm""! i n one s e r i e s of experiments, w h i l e i n the other s e r i e s ( r a d i o l y s i s of l i q u i d PC) ^-^04 w a s measured e x p e r i m e n t a l l y . C. RESULTS 1. N i t r o g e n F i x a t i o n Experiments The dose r a t e f o r the syringes i n s i d e the s t a i n l e s s s t e e l h i g h pressure c e l l was determined using the above pro-cedure w i t h 15 ml of the fe r r o u s s u l f a t e s o l u t i o n i n the syringe. -144-On December 3, 1969 the dose r a t e was found t o be 2500 rads min" ,o 304 assuming € = 2174 and G(Fe ) = 15.5 . A__o. was d e t e r -304 net-mined as a f u n c t i o n of time by i r r a d i a t i n g samples f o r v a r i o u s 304 time i n t e r v a l s and the slope of the p l o t of versus time was used i n the dose r a t e c a l c u l a t i o n . The i n t e r c e p t of t h i s l i n e was p o s i t i v e r a t h e r than zero because the samples r e c e i v e d a dose e q u i v a l e n t t o about 1/10 minute during the lowering and r a i s i n g of the Gammacell c a v i t y i n t o the r a d i a t i o n f i e l d . (The mic r o - s w i t c h which a c t i v a t e d the automatic timer was not engaged u n t i l the Gammacell drawer was f u l l y lowered.) This s m a l l c o r r e c t i o n f a c t o r was taken i n t o account i n sub-sequent dose c a l c u l a t i o n s . Since the dosimetry f o r t h i s e x p e r i m e n t a l setup was done a t ambient temperature and without checking the value of ^ ^ 0 4 ' W a S P r°k akly n o t accurate to more than * 5%. This u n c e r t a i n t y was however t o l e r a b l e s i n c e p r e c i s e G value determinations were not i n v o l v e d . 2. R a d i o l y s i s of L i q u i d PC The d e t e r m i n a t i o n of the dose r a t e f o r t h i s experimental setup was done more p r e c i s e l y t o minimize the u n c e r t a i n t y . The r a d i o l y s i s c e l l c o n t a i n i n g 17.9 ml of the f e r r o u s s u l f a t e s o l u t i o n was thermostated a t 25 * 0.1 °C durin g the i r r a d i a t i o n s and absorbance measurements f o r F e + 3 were a l s o made at t h i s temperature. The l a t t e r was r e q u i r e d s i n c e the va l u e of € 3 0 4 +3 f o r Fe has a l a r g e temperature c o e f f i c i e n t and u n c e r t a i n t y i n the temperature can introduce s i g n i f i c a n t e r r o r i n t o the dose c a l c u l a t i o n s . In a d d i t i o n the value of € 3 0 4 w a s exper-i m e n t a l l y determined us i n g the same spectrometer and c e l l s . S o l u t i o n s of known Fe c o n c e n t r a t i o n were made from f r e s h -145-a n a l y t i c a l grade (> 99% pure) f e r r i c ammonium s u l f a t e . The data obtained are p l o t t e d i n Figu r e A l - 1 and from the slope of the l i n e € was determined to be 2184 i 8 a t 25 °C . 304 This i n i n good agreement w i t h the value of 2194 c a l c u l a t e d from data g i v e n i n reference 3 f o r 25 °C. The dosimetry r e s u l t s f o r i r r a d i a t i o n s of the sample c e l l are p l o t t e d i n Fi g u r e A l - 2 . From the slope of the l i n e , the e x p e r i m e n t a l l y determined value of ^^04 a n d t a ^ i n g G(Fe ) = 15.5, the dose r a t e was c a l c u l a t e d to be 4545 rads m i n " 1 f o r the f e r r o u s s u l f a t e s o l u t i o n on August 1, 1970. The i n t e r c e p t was found to be e q u i v a l e n t to 725 rads or about 10 seconds i r r a d i a t i o n time. 3 . Dose C o r r e c t i o n Procedures - Computer Program C o r r e c t i o n s f o r " e l e c t r o n d e n s i t y " d i f f e r e n c e s between the f e r r o u s s u l f a t e s o l u t i o n and the experimental l i q u i d were i n c o r p o r a t e d i n a computer program which a u t o m a t i c a l l y c o r r e c t e d f o r the n a t u r a l decay of the source a c t i v i t y . This program * was devised by a colleag u e and w i l l be pu b l i s h e d elsewhere. I t c o r r e c t e d f o r the e l e c t r o n d e n s i t y d i f f e r e n c e by t a k i n g the average Z/A value f o r the fe r r o u s s u l f a t e s o l u t i o n (0.5533), water (0.5551) and propylene carbonate (0.5289) and m u l t i p l y i n g the dose c a l c u l a t e d f o r the fe r r o u s s u l f a t e s o l u t i o n by the r a t i o of the Z/A values of the experimental l i q u i d and the fe r r o u s s u l f a t e s o l u t i o n . C o r r e c t i o n f o r the n a t u r a l r a d i o -a c t i v e decay of the ^ C o w a s made from the known h a l f - l i f e * f o o t n o t e 3. G„ J . F l y n n , t h i s l a b o r a t o r y , communicated. -146--147-100 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 IRRADIATION TIME (sec) Figure A l - 2 . Ferrous s u l f a t e dosimetry r e s u l t s f o r the "bubbler" c e l l used i n the l i q u i d phase r a d i o l y s i s of PC. using the formula: corr o r i g - days x 0.693 Dose = Dose • e 1924.9 where "days" i s the elapsed time i n days from the date of the 60 o r i g i n a l dosimetry, 1924.9 i s the h a l f - l i f e of Co i n days and 0.693 i s a constant equal to l n 2 . - 1 4 9 -APPENDIX 2  INDOPHENOL BLUE AMMONIA ANALYSIS A. THEORY The method used t o produce the indophenol blue dye i n v o l v e s r e a c t i o n of ammonia w i t h sodium h y p o c h l o r i t e and phenol i n a v e r y a l k a l i n e s o l u t i o n as d e s c r i b e d by Tetlow 37 and W i l s o n . The r e a c t i o n mechanism i s b e l i e v e d t o i n v o l v e the f o l l o w i n g s e r i e s of r e a c t i o n s (A-10), ( A - l l ) and (A-12). NH 3 + OCl" • NH 2C1 + OH" (f a s t ) (A-10) NH 2C1 + 3^"0" + 2 0 C 1~"~* CI-N=<^)rO + 2 C l " ( A - l l ) + 2 OH" (slow) £ J - 0 ' + Cl-N=<^>0 - "°-<C^N=0=0 + C 1 ~ (A-12) • * v T ' (f a s t ) indophenol blue The indophenol blue dye has a broad asymmetric o p t i c a l a b s o r p t i o n band w i t h a maximum at 630 nm as i l l u s t r a t e d i n F i g u r e A2-1. B. EXPERIMENTAL PROCEDURE 1. Reagents A s t o c k s o l u t i o n of sodium h y p o c h l o r i t e w i t h 1.00% a v a i l a b l e c h l o r i n e was prepared from F i s h e r reagent grade NaOCl s o l u t i o n and doubly d i s t i l l e d water. The a v a i l a b l e c h l o r i n e i n the u n d i l u t e d reagent (4 - 6 %) was determined 6 0 0 6 5 0 7 0 0 WAVELENGTH (nm) Figure A2-1. Absorption spectrum of the indophenol blue dye i n a l k a l i n e aqueous so l u t i o n . -151-p r e c i s e l y by r e a c t i o n w i t h i o d i d e and t i t r a t i o n of the f r e e i o d i n e w i t h standardized sodium t h i o s u l f a t e i n a c i d s o l u t i o n 36 u s i n g a s t a r c h i n d i c a t o r . The sodium t h i o s u l f a t e s o l u t i o n was s t a n d a r d i z e d u s i n g s o l i d potassium i o d a t e as the primary 36 standard as de s c r i b e d i n V o g e l . The 1.00% (weight/volume) a v a i l a b l e c h l o r i n e stock s o l u t i o n was s t o r e d i n a darkened v o l u m e t r i c f l a s k i n a r e f r i g e r a t o r u n t i l immediately before use. I t was s t a b l e under these c o n d i t i o n s f o r a t l e a s t 4 months. Stock s o l u t i o n s of sodium phenate were made f r e s h before each run by d i s s o l v i n g 62.5 i 0.1 g of reagent grade phenol i n 135 ml of 5N sodium hydroxide s o l u t i o n and then d i l u t i n g t h i s w i t h doubly d i s t i l l e d water t o a t o t a l volume of 500 ml. This s o l u t i o n was a l s o p r o t e c t e d from l i g h t and was s t a b l e o n l y f o r about 6 hours. Acetone used as a " c a t a l y s t " i n the procedure was F i s h e r spectrograde. Doubly d i s t i l l e d water was used t o make up a l l s o l u t i o n s . The f i r s t d i s t i l l a t i o n was from tap water and the second from a c i d i f i e d dichromate s o l u t i o n . Standard ammonia s o l u t i o n s were prepared from reagent grade ammonium s u l f a t e . 2. A n a l y s i s Procedure A l l g l a s s apparatus used i n the procedure ( i . e . f l a s k s , p i p e t t e s , b u r e t t e s etc.) were i n i t i a l l y cleaned w i t h the -152-normal permanganic a c i d - hydrogen peroxide - d i s t i l l e d water r o u t i n e . Subsequently the apparatus was washed w e l l w i t h doubly d i s t i l l e d water a f t e r each use and allowed to a i r dry. The v o l u m e t r i c f l a s k s were s t o r e d f i l l e d w i t h doubly d i s t i l l e d water. The reagent s o l u t i o n s were d e l i v e r e d i n t o 50 ml v o l u m e t r i c f l a s k s u s i n g f a s t f l o w b u r e t t e s . The ammonia standard (or unknown sample) was f i r s t added to the f l a s k and d i l u t e d a p p r o p r i a t e l y so t h a t the t o t a l sample volume was 25 ml. I f the unknown s o l u t i o n was s t r o n g l y a c i d i c or b a s i c , i t was f i r s t n e u t r a l i z e d using standard I^SO^ or NaOH. To the 25 ml of sample 0.30 ± 0.05 ml of acetone was added and the s o l u t i o n mixed w e l l . Then 10.0 i 0.2 ml of the sodium phenate s o l u t i o n was added, immediately f o l l o w e d by 5.0 ±0.2 ml of the standard h y p o c h l o r i t e s o l u t i o n . The sample was q u i c k l y mixed and d i l u t e d "to the mark" w i t h doubly d i s t i l l e d water. The v o l u m e t r i c f l a s k was then stoppered, shaken v i g o r o u s l y and p l a c e d i n a darkened constant temperature bath a t 25 * 0.5 °C f o r 6 0 - 5 minutes during which time the indophenol dye developed. The absorbance was then measured ( w i t h i n - 5 minutes of 60 minutes developement time) i n a 5 cm c e l l versus d i s t -i l l e d water a t 630 nm using a Cary 14 r e c o r d i n g spectrometer. The absorbance values obtained above were c o r r e c t e d f o r the absorbance of the reagent s o l u t i o n s by determining the absorbance of a "reagent blank". Tetlow and Wilson found t h a t i f the reagents were mixed together f i r s t and allowed t o stand f o r s e v e r a l minutes before adding an ammonia s o l u t i o n , -153-then no indophenol blue was developed except f o r t h a t pro-duced by any t r a c e s of ammonia i n the reagents. This reagent blank procedure a l s o allowed f o r the determination of the ammonia c o n c e n t r a t i o n i n the "pure" doubly and t r i p l y d i s t i l l e d water samples. C. RESULTS The c o r r e c t e d absorbance at 630 nm was found to be l i n e a r w i t h ammonia c o n c e n t r a t i o n up to at l e a s t 10~ 4 M NH^ + as shown i n F i g u r e A2-2 where the r e s u l t s of a t y p i c a l c a l i b r a t i o n run are p l o t t e d . The con c e n t r a t i o n s quoted here are the o r i g i n a l u n d i l u t e d c o n c e n t r a t i o n of i n the 25 ml of sample before adding the reagents. (This i s twice the e f f e c t i v e c o n c e n t r a t i o n of NH^ + i n the f i n a l 50 ml of s o l u t i o n . ) From the sl o p e of the c a l i b r a t i o n l i n e the response f a c t o r was c a l c u l a t e d t o be: —5 + Response = 0.156 absorbance u n i t s / 10 M NH^ f o r a 5 cm pa t h l e n g t h c e l l . Since the minimum p r a c t i c a l absorbance which could be measured was 0.005 u n i t s , the _7 a b s o l u t e s e n s i t i v i t y of the technique was about 5 x 10 M The pure water samples always gave i d e n t i c a l absorbance readings t o those of the reagent blank s o l u t i o n s i n d i c a t i n g -7 + t h a t the water contained l e s s than 5 x 10 M NH^ . Since the s e n s i t i v i t y of the method depended on the age o f the phenate reagent and the p a r t i c u l a r batch of 37 acetone used , the c a l i b r a t i o n curves were rechecked f r e q -u e n t l y . Each time an unknown sample was analysed, an i n t e r n a l Figure A2-2. T y p i c a l c a l i b r a t i o n graph f o r the indophenol blue ammonia analysis procedure. -155-standard as w e l l as a normal standard were run t o be sure t h a t the procedure and reagents were a l r i g h t . I t was found on a n a l y s i s of i r r a d i a t e d s o l u t i o n s which contained oxygen during the i r r a d i a t i o n , t h a t hydrogen peroxide formed dur i n g the r a d i o l y s i s i n t e r f e r e d q u i t e markedly w i t h the a n a l y s i s . In f a c t a n a l y s i s of a sample c o n t a i n i n g o n l y -4 6 x 10 M hydrogen peroxide gave an orange coloured s o l u t i o n which had an absorbance of 0.8 a t 630 nm i n a 5 cm c e l l . T his absorbance was due t o the t a i l of an i n t e n s e UV band. O x i d a t i o n of the phenate reagent to g i v e a quinone was thought to be r e s p o n s i b l e f o r t h i s UV a b s o r p t i o n . Because of t h i s i n t e r f e r e n c e , the working s e n s i t i v i t y of the a n a l y s i s pro-cedure was about 5 x 10"^ M NH^+ when a d e f i n i t e peak could be observed a t 630 nm i n d i c a t i n g the absorbance was due to indophenol b l u e and not caused by hydrogen peroxide i n t e r -ference. r -156-APPENDIX 3  ELECTRON SPIN RESONANCE* A. BASIC THEORY By v i r t u e of i t s i n t r i n s i c angular momentum and i t s charge, an e l e c t r o n has a magnetic moment a s s o c i a t e d w i t h i t . In the presence of an e x t e r n a l l y a p p l i e d magnetic f i e l d , H, the alignment of the e l e c t r o n ' s magnetic moment has a p r e f e r r e d d i r e c t i o n , i . e . p a r a l l e l to the e x t e r n a l f i e l d . From quantum mechanics i t i s known t h a t the e l e c t r o n has a s p i n quantum number of % which means t h a t i t s s p i n angular momentum can have o n l y two o r i e n t a t i o n s w i t h respect t o a g i v e n a x i s . Thus the magnetic moment a s s o c i a t e d w i t h the e l e c t r o n ' s s p i n angular momentum may e i t h e r be a l i g n e d w i t h the e x t e r n a l f i e l d o r a g a i n s t i t . Since the p a r a l l e l c o n f i g u r a t i o n i s p r e f e r r e d , an energy d i f f e r e n c e e x i s t s between the two s p i n o r i e n t a t i o n s . T r a n s i t i o n s i n which the e l e c t r o n changes i t s s p i n o r i e n t a t i o n may be induced by su p p l y i n g the app r o p r i a t e amount of e l e c t r o -magnetic energy. This i s b a s i c a l l y what ESR measures; the energy r e q u i r e d t o reverse the s p i n of an unpaired e l e c t r o n . Normally i n ESR a constant energy source i n the form of microwave r a d i a t i o n i s used and the e x t e r n a l magnetic f i e l d i s v a r i e d u n t i l the unpaired e l e c t r o n "resonates". When the d i f f e r e n c e i n energy between the two s p i n s t a t e s equals the energy of the microwave r a d i a t i o n , there i s an exchange of *fo o t n o t e 4. Prepared from references 48, 53 and 63. -157-energy between the two energy systems which i s e f f e c t i v e l y a "resonance" process. This resonance c o n d i t i o n i s normally w r i t t e n as: h V = g/?H (A3.1) where hV i s the microwave energy, H i s the e x t e r n a l f i e l d and /3is the Bohr magneton, g i s c a l l e d the s p e c t r o s c o p i c s p l i t t i n g f a c t o r and i t i s the parameter which d e s c r i b e s the p o s i t i o n of the resonance a b s o r p t i o n . The value of g f o r an e l e c t r o n w i t h o n l y s p i n angular momentum, i . e . the " f r e e - s p i n " g - f a c t o r , i s 2.0023 . (The d e v i a t i o n from the i n t e g r a l number i s due to r e l a t i v i s t i c v e l o c i t y c o r r e c t i o n s . ) For the X-band spectrometer used i n t h i s r e s e a r c h , the micro-wave frequency was about 9.3 GHz f o r which the f i e l d c o r r e s -ponding t o the resonance of a f r e e e l e c t r o n i s about 3300 G . In most r a d i c a l s unpaired e l e c t r o n s have o r b i t a l a n g u l a r momentum i n a d d i t i o n to s p i n angular momentum. The e f f e c t of the o r b i t a l angular momentum i s to s h i f t the g-value of the resonance from the f r e e - s p i n v a l u e . The degree of s p i n - o r b i t c o u p l i n g determines the magnitude of the s h i f t . For most organic r a d i c a l s the d e v i a t i o n s are s m a l l and may be thought t o a r i s e through an a d d i t i o n a l s m a l l p e r t u r b i n g magnetic f i e l d caused by a very s l i g h t o r b i t a l motion. This m o d i f i e s the e f f e c t i v e f i e l d t h a t the unpaired e l e c t r o n experiences by v e c t o r i a l l y adding to the e x t e r n a l l y a p p l i e d f i e l d and thus causes p o s i t i v e or negative g - f a c t o r s h i f t s . Therefore equation (A3.1) may be r e w r i t t e n as: h^= g j2 ( H e + H.) (A3.2) where H and H- are the e x t e r n a l and i n t e r n a l f i e l d s r e s p e c t i v e l y . -158-From equation (A3.2) i t i s evident t h a t the g - f a c t o r depends both on the magnitude of the e x t e r n a l f i e l d and i t s o r i e n t a t i o n w i t h respect t o the l o c a l i n t e r n a l f i e l d s . This a n i s o t r o p y of the g - f a c t o r i s the reason t h a t i t i s c u s t o m a r i l y expressed as a tensor w i t h p r i n c i p a l v a l u e s : g„ . g„ and g ; where c c 3 xx yy zz the axes are g e n e r a l l y chosen to correspond to the symetry axes of the r a d i c a l . H y perfine s p l i t t i n g of the e l e c t r o n ' s resonance l i n e i n t o m u l t i p l e t s i s the r e s u l t of i n t e r a c t i o n s of the e l e c t r o n w i t h the magnetic moments a s s o c i a t e d w i t h those n u c l e i having n u c l e a r s p i n . Since the o r i e n t a t i o n of nuclear s p i n i s qu a n t i z e d , the nuclear f i e l d s do not cause a displacement of the resonance but r a t h e r s p l i t i t i n t o a number of components corresponding to the d i f f e r e n t o r i e n t a t i o n s of the nuclear moment w i t h respect t o the e x t e r n a l f i e l d . In the s i m p l e s t case of the i n t e r a c t i o n of an unpaired e l e c t r o n w i t h a nucleus w i t h s p i n % (e.g. a proton) there are two o r i e n t a t i o n s of the n u c l e a r magnetic moment, one opposing and the other adding to the e x t e r n a l f i e l d . The unpaired e l e c t r o n then experiences a f i e l d of H - § H where § H i s the f i e l d due to the nucleus. Thus two l i n e s are observed e q u a l l y spaced about the p o s i t i o n where the resonance would have occured i f there were no hyper-f i n e i n t e r a c t i o n . In g e n e r a l f o r a nucleus of s p i n I , the spectrum w i l l c o n s i s t of 21+1 l i n e s of equal i n t e n s i t y . A much more complex s i t u a t i o n e x i s t s when the unpaired e l e c t r o n i n t e r a c t s w i t h more than one magnetic nucleus, p a r t i c u l a r l y when they are not e q u i v a l e n t . -159-Since h y p e r f i n e s p l i t t i n g r e s u l t s from the i n t e r a c t i o n of two d i p o l e s , t h i s e f f e c t depends on t h e i r mutual o r i e n t -a t i o n . Thus f o r e l e c t r o n s i n o r b i t a l s which are not s p h e r i c a l l y symmetric, e.g. p - o r b i t a l s , the h y p e r f i n e i n t e r a c t i o n i s a n i s o t r o p i c and t h i s g i v e s r i s e t o a h y p e r f i n e t e n s o r , A, where the p r i n c i p a l values are u s u a l l y c a l c u l a t e d f o r the same a x i s system used f o r the g-tensor, i . e . A , A and A . X X jf jf zz Because of the s p h e r i c a l symmetry of e l e c t r o n s i n s - o r b i t a l s , a l l the d i p o l a r i n t e r a c t i o n w i t h the nucleus average to zero. However, s i n c e s - o r b i t a l s have a non-zero wavefunction at the nucl e u s , t h i s permits a h y p e r f i n e i n t e r a c t i o n which i s known as the Fermi contact i n t e r a c t i o n and i t i s i s o t r o p i c . Consequently i f an e l e c t r o n i s i n a sp - h y b r i d o r b i t a l on an atomic nucleus there w i l l be both an i s o t r o p i c term w i t h an a n i s o t r o p i c component superimposed on i t . B. APPLICATION OF ESR TO AMORPHOUS SYSTEMS For r a d i c a l s formed i n amorphous or p o l y c r y s t a l l i n e m a t r i c e s th e r e i s a complete randomization of the r a d i c a l o r i e n t a t i o n s w i t h respect to the e x t e r n a l magnetic f i e l d ; a lthough i n the p o l y c r y s t a l l i n e system there i s an ordered environment on the mi c r o s c o p i c s c a l e . In these systems, the ESR spectrum obtained i s an envelope c o n t a i n i n g a l l the fea t u r e s f o r a l l p o s s i b l e o r i e n t a t i o n s of the r a d i c a l . The type of l i n e shape t h a t one observes w i l l depend on the magnitudes of the a n i s o t r o p i c s of the g - f a c t o r and h y p e r f i n e i n t e r a c t i o n s . I t w i l l a l s o depend on c o n t r i b u t i o n s to l i n e w idth from d i p o l a r i n t e r a c t i o n s . F i g u r e A3-1 shows some of the t h e o r e t i c a l -160-(a) (b) yA r jj yspectrum in the absence of splitting A 9l • •V '/ T 911 f 1 r A|| (C) F i g u r e A3-1. T h e o r e t i c a l ESR d e r i v a t i v e l i n e s h a p e s f o r amorphous s a m p l e s when t h e r a d i c a l i s c h a r a c t e r -i z e d by: (a) an a x i a l l y s y m m e t r i c g - t e n s o r and no h y p e r f i n e s t r u c t u r e , (b) a n a s y m m e t r i c g - t e n s o r and no h y p e r f i n e s t r u c t u r e , and (c) a x i a l l y s y m m e t r i c g - a n d A - t e n s o r s w i t h t h e same symmetry ax e s and a l a r g e h y p e r f i n e s p l i t t i n g o f a s p i n % n u c l e u s . ( a f t e r F i g u r e s 9.3, 9.4, 9.7 i n A y s c o u g h , r e f e r e n c e 63, pages 324, 325, and 327) -161-l i n e shapes f o r the the more simple cases: (a) an a x i a l l y symmetric g-tensor ( i . e . g x x=g y y=g|| and g z 2=gjL w i t h g^ / g±) and no h y p e r f i n e s t r u c t u r e , (b) an asymmetric g-tensor and no h y p e r f i n e s t r u c t u r e , a n d (c) a x i a l l y symmetric g- and A-tensors w i t h the same symmetry axes and w i t h the h y p e r f i n e s p l i t t i n g of a s p i n % nucleus being much l a r g e r than the q u a n t i t y y$H(gj|-g^ ). A n a l y s i s of the l i n e shapes f o r t o t a l l y asymmetric g- and A-tensors w i t h more than one i n t e r a c t i n g nucleus i s not e a s i l y accomplished. When the magnitude of the d i p o l a r broadening of the ' l i n e s i s g r e a t e r than the a n i s o t r o p i c s of the g- and A-tensors then the l i n e broadening r e s u l t s i n l o s s of the i n d i v i d u a l f e a t u r e s corresponding t o g^ , g^ e t c . and the formation of 6 3 a l i n e of approximately a Gaussian shape. In t h i s case, the peak-to-peak s e p a r a t i o n corresponds f a i r l y c l o s e l y to the i s o t r o p i c component of the h y p e r f i n e s p l i t t i n g . This i s the s i t u a t i o n most o f t e n encountered f o r organic r a d i c a l s trapped i n low temperature m a t r i c e s . The observed l i n e widths are o g e n e r a l l y 10 -15 G at 77 K, although tumbling or p a r t i a l r o t a t i o n of the r a d i c a l s can reduce the l i n e widths to some extent by time averaging the d i p o l a r i n t e r a c t i o n s . -162-APPENDIX 4 PURIFICATION AND ANALYSIS OF PROPYLENE CARBONATE Propylene carbonate was p u r i f i e d using the f o l l o w i n g procedure: 7 l i t r e s of Eastman Kodak p r a c t i c a l grade propylene carbonate was d r i e d f o r s e v e r a l weeks over Linde 4A molecular s i e v e s . (The molecular s i e v e s were p r e v i o u s l y d r i e d by h e a t i n g t o 300 °C under a dynamic vacuum overnight.) The d r i e d s o l v e n t was vacuum f r a c t i o n a t e d i n two 3.5 l i t r e p o r t i o n s u s i n g a c l e a n , dry, s t i l l w i t h a 5 l i t r e s t i l l - p o t and a 4 f o o t by 1 i n c h column packed w i t h 3/16 i n c h g l a s s h e l i c e s . The d i s t i l l a t i o n s were c a r r i e d out at l e s s than 1 mm pressure • o w i t h a column head temperature of 86 1 1 C and a d i s t i l l a t i o n r a t e of about 0.4 l i t r e s per hour. From each of the two 3.5 l i t r e batches, about 0.7 l i t r e f o r e - r u n was r e j e c t e d and about 1.8 l i t r e s of the middle f r a c t i o n was c o l l e c t e d , l e a v i n g about 1 l i t r e i n the s t i l l - p o t . The two 1.8 l i t r e middle cuts were combined and d r i e d overnight w i t h molecular s i e v e s . The 3.6 l i t r e f i r s t d i s t i l l a t i o n f r a c t i o n was then r e d i s t i l l e d , r e j e c t i n g 0.8 l i t r e s of the f o r e - r u n and c o l l e c t i n g about 1.9 l i t r e s of the middle f r a c t i o n i n a thoroughly cleaned 2 l i t r e round bottom f l a s k . This doubly d i s t i l l e d PC was immediately deoxygenated and f l u s h e d f o r s e v e r a l hours w i t h d r y helium ( d r i e d v i a a l i q u i d n i t r o g e n trap) and then sealed i n the 2 l i t r e storage -163-f l a s k . A s i n t e r e d g l a s s bubbler - dispenser u n i t as shown i n F i g u r e A4-1 was used i n the 2 l i t r e f l a s k t o f l u s h and dispense the PC. T h i s s i n g l e sample of p u r i f i e d PC was then used f o r a l l experiments r e p o r t e d i n t h i s t h e s i s . The r e q u i r e d q u a n t i t y was dispensed by a p p l y i n g helium pressure above the l i q u i d . The f l a s k was then w e l l f l u s h e d w i t h d r y helium v i a the s i n t e r e d g l a s s bubbler and se a l e d under the helium atmosphere w i t h the t e f l o n stopcocks. The d i s t i l l a t i o n and d r y i n g procedure most probably reduced the water content t o l e s s than 1 ppm (6 x 10 M) s i n c e i t was more e l a b o r a t e than t h a t used by J a s i n s k i and 42a K i r k l a n d who found t h a t double vacuum f r a c t i o n a t i o n lowered the water content to l e s s than 2 ppm. In a d d i t i o n , the continued f l u s h i n g procedure w i t h d r y helium helped t o remove 42a t r a c e s of low b o i l i n g o rganic i m p u r i t i e s as w e l l as water. The p h y s i c a l analyses made on the p u r i f i e d PC i n c l u d e d a mass spectrum, u l t r a v i o l e t spectrum, n u c l e a r magnetic resonance spectrum and i n f r a r e d spectrum as shown i n Figures A4-2, A4-3, A4-4 and A4-5 r e s p e c t i v e l y . The r e f r a c t i v e index o f the PC was found t o be n 2 0 = 1.4213 as compared w i t h the l i t e r a t u r e v a l u e s of 1.4214 4 0 b and 1.4212 4 0 d. -164-Fi g u r e A4-1. Storage - dispensing f l a s k used t o keep the p u r i f i e d PC under a helium atmosphere. 90 UJ U z < a z z> DO < 80 70 60 50 UJ > .— 40 UJ 30| ° 201 10 H 2 0 * 10 { P C - C O 3 - C H 3 ) * : (CH 2=CH)* O , (PC - C 0 2 H)* CO* ( C H 3 - C ^ ) 4 C O , u ( P C - C H 3 ) * 20 30 40 50 60 70 80 m/e PC* 1. 90 100 cn l F i g u r e A4-2. Mass spectrum of doubly d i s t i l l e d propylene carbonate (PC) 220 240 260 280 300 320 340 WAVELENGTH ( n m ) F i g u r e A4-3. U l t r a v i o l e t a b sorption s p e c t r a of Eastman Kodak p r a c t i c a l grade PC and i t s s i n g l y and doubly d i s t i l l e d f r a c t i o n s . CH* y c x.3 / I I C O b C ^ 4.0 30 2.0 PPM(S) 1.0 F i g u r e A4-4. 60 MHz nuclear magnetic resonance spectrum of doubly d i s t i l l e d PC. WAVENUMBER (cm -1 ) 3000 2000 1500 1000 900 800 700 I I 11 I I I I I I I I I I I I l I l I I I l I I | I I I | L _ _ J | I . . . . 1 . . I . I . . I . I . . . . I . . . . 1 . . . . t . J 1 1— 1 1 L_ I J I • i 4 5 6 7 8 9 10 11 12 13 14 WAVELENGTH ( nmx10"3 = microns) Figure A4-5. Infrared absorption spectrum of doubly d i s t i l l e d PC. -169-APPENDIX 5 GAS CHROMATOGRAPHIC ANALYSIS SYSTEM A V a r i a n Aerograph A-90-P2 gas chromatograph was s p e c i a l l y m o dified to measure the gaseous product y i e l d s from the r a d i o l y s i s of l i q u i d PC. The system was designed to separate and measure q u a n t i t a t i v e l y n i t r o g e n , methane, carbon monoxide and carbon d i o x i d e . Hydrogen could a l s o be detected by t h i s technique but w i t h very poor s e n s i t i v i t y . Figure A5-1 i s a schematic drawing of the a n a l y s i s system. An 8 f o o t by 1/8 i n c h Porapak Q column maintained at 0 °C ou t s i d e the chromatograph was connected between the input of the sample s i d e of the de t e c t o r and the i n j e c t i o n system v i a 1/8 i n c h copper tub i n g . The e x i t of the sample s i d e of the d e t e c t o r was connected, through a flo w r e v e r s i n g v a l v e , to a 20 f o o t by % i n c h 13X molecular s i e v e column th a t was o maintained at 100 C i n the chromatograph oven. E f f l u e n t from t h i s column then passed through the reference s i d e of the de t e c t o r block and was vented i n t o the atmosphere. The thermal c o n d u c t i v i t y d e t e c t o r s used WX f i l a m e n t s o p e r a t i n g at 180 ma and 140 - 150 °C . The output of the de t e c t o r bridge was connected v i a a p o l a r i t y r e v e r s i n g s w i t c h to a Leeds and Northrup - Speedomax W 1 mV f a s t reponse chart r e c o r d e r . (The p o l a r i t y r e v e r s i n g s w i t c h allowed s i g n a l s from the reference d e t e c t o r to be d i s p l a y e d i n the normal " p o s i t i v e " manner.) 8' Porapak Q column at 0 t EXTERNAL SAMPLE LOOP gas standards Porapak Q 'pre-column connectors for sample cell y _ standards ^ loop sample loop valve liquid n i t rogen vapour trap flow •Q-rl fl° I x I me I i flow meter outlet for vdeoxygenat ing samples regulator u flow reversing valve to recorder polarity reversing switch septum 20' 13X molecular sieve column at 100% F i g u r e A5-1. Schematic diagram o f the m o d i f i e d V a r i a n Aerograph gas chromatography system. -171-P r o i r to e n t e r i n g the 8 f o o t Porapak Q column, the c a r r i e r gas passed through a 1 f o o t s e c t i o n of % i n c h copper t u b i n g , mounted e x t e r n a l l y , which could be cooled to l i q u i d n i t r o g e n temperature to t r a p out C0 2 and N 20. The t r a p a l s o had a heater s e c t i o n on the 1/8 i n c h tubing i n p u t and output connections to prevent blockage of t h i s narrow bore tubing caused by conduction c o o l i n g and subsequent condensation of CO2 and N 20. The e x t e r n a l sample loop system was connected i n t o the i n j e c t o r p o r t i o n of the chromatograph v i a a s t a i n l e s s s t e e l bypass v a l v e and 1/8 i n c h copper tub i n g . I t c o n s i s t e d of S13 socket connections f o r the sample c e l l , a 2 f o o t by 1/8 i n c h Porapak Q "pre-column" vapour t r a p and a g l a s s " i n - l i n e " standards gas sample loop of c a l i b r a t e d volume. Helium,used as the c a r r i e r gas,was fed at 65 p s i g i n t o the chromatograph v i a a l i q u i d n i t r o g e n t r a p which removed water vapour and t r a c e s of organic contaminants. Normal f l o w r a t e was 60 ml/min. With the columns at 0 and 100 °C, n i t r o g e n e l u t e d i n about 10 minutes, methane i n about 15 minutes and carbon monoxide i n about 20 minutes. Carbon d i o x i d e was sub-sequently measured by warming the e x t e r n a l t r a p q u i c k l y to room temperature. I t e l u t e d from the Porapak Q column about 4 minutes l a t e r to be detected by the sample s i d e of the d e t e c t o r as a sharp peak. I t was then "trapped" i n the molec-u l a r s i e v e column. N i t r o u s oxide had v i r t u a l l y the same r e t e n t i o n time as C0 2 and thus prevented measurement of C0 2 i n i t s presence. The f l o w r e v e r s i n g v a l v e on the molecular s i e v e column allowed the C0 n and N o0 to be "back-flushed" from i t . -172-A t y p i c a l chromatogram i s shown i n Fi g u r e A5-2. A n i t r o g e n standard was r o u t i n e l y i n j e c t e d from the " i n - l i n e " sample loop before each sample to monitor any changes i n d e t e c t o r s e n s i t i v i t y . Peak areas were measured by manual t r i a n g u l a t i o n . The l i n e a r i t y of the d e t e c t o r response and absolute s e n s i t i v i t y was determined by i n j e c t i n g known q u a n t i t i e s of the sample gases u s i n g a second sample loop of c a l i b r a t e d volume which was f i l l e d on a vacuum l i n e to known pressure. I t was connected to the e x t e r n a l sample loop system v i a the S13 sockets used f o r the r a d i o l y s i s c e l l . The graphs of d e t e c t o r response (peak area) versus sample s i z e (molecules) were a l l l i n e a r over a range of at l e a s t 100 from the l i m i t of d e t e c t i o n as shown i n Fi g u r e A5-3. The response f a c t o r s as determined from the slopes of these p l o t s were c a l c u l a t e d to be: N 2 = 3.2 ± 0.1 X i o 1 4 molecules mm CH4= 3.6 + 0.1 X 1 0 1 4 •i M CO = 3.1 + 0.1 X i o 1 4 II •I co 2= 2.7 + 0.1 X 1 0 1 4 n II (H, ~ 220 X i o 1 4 II Since the minimum area which could be measured reasonably 2 2 a c c u r a t e l y was about 100 mm f o r C0 2 and about 500 mm f o r the other gases, the d e t e c t i o n l i m i t s were thus approximately: 19 (H^ ~ 1 x 10 molecules (20 micro-moles)) N 2 /~ 1.6 x 1 0 1 7 " (0.3 " " ) CH. ^ 1.8 x 1 0 1 7 " (0.3 " " ) DETECTOR RESPONSE -ZL1-F i g u r e A5-3. Detector response to N , CH 4, CO and CO f o r the GC system shown i n Fig u r e A5-1. -17 5-17 CO ~ 1.6 x 10 molecules (0.3 micro-moles) C0 2~0.28 x 1 0 1 7 " (0.04 " " ) The v a r i a t i o n of detector s e n s i t i v i t y from day to day as monitored by the nitrogen standards was found to be less than - 5% from the mean. Since the majority of th i s deviation could be accounted for on the basis of uncertainties introduced by the manual inte g r a t i o n technique and by atmospheric pressure and temperature changes, no corrections were applied to the experimental data for t h i s apparent s e n s i t i v i t y v a r i a t i o n . 

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