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UBC Theses and Dissertations

Donor-acceptor properties of methylphosphonitriles Mah, Timothy W. J. 1974

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DONOR-ACCEPTOR PROPERTIES OF METHYLPHOSPHONITRILES by TIMOTHY W. J . MAH B.Sc. (Hons.) Unive r s i t y of B r i t i s h Columbia, 1972, • A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of Chemistry We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June, 1974 In presenting this thesis in partia l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of o ~~Jby,^ The University of B r i t i s h Columbia Vancouver 8, Canada ABSTRACT The m e t h y l p h o s p h o n i t r i l e s were shown by t h e method o f B e n e s i and H i l d e b r a n d t o form o u t e r complexes w i t h I 2 . The e l e c t r o n i c s p e c t r a o f t h e complexes as w e l l as t h e de t e r m i n e d e q u i l i b r i u m c o n s t a n t s i n d i c a t e t h e use o f t h e m e t h y l p h o s p h o n i t r i l e s as n-donors toward I 2 . T h i s i s c o n f i r m e d by t h e m o l e c u l a r s t r u c t u r e o f N 3 P 3 M e g - I 2 , i n w hich t h e N - - - I - I u n i t was found t o be l i n e a r . The i n t e r p r e t a t i o n o f b o t h t h e e l e c t r o n i c and p r o t o n s p e c t r a o f t h e s e complexes i n d i c a t e t h e r e l a t i v e base s t r e n g t h s o f t h e m e t h y l p h o s p h o n i t r i l e s as b e i n g i n t h e o r d e r N 4P 4Me g > N 5 P 5 M e 1 Q >N 3P 3Me 6. T h i s o r d e r i s e x p l i c a b l e i n terms o f t h e u - e l e c t r o n d e n s i t i e s a t t h e r i n g n i t r o g e n atoms, as a f f e c t e d by t h e homomorphic Tr-system, and t h e e f f e c t s o f c - h y b r i d i z a t i o n . The s y n t h e s i s o f i n n e r complexes formed by t h e i n t e r a c t i o n o f i o d i n e w i t h the m e t h y l p h o s p h o n i t r i l e s show f u r t h e r analogies o f p y r i d i n e w i t h t h e m e t h y l p h o s p h o n i t r i l e s . The s t r u c t u r e o f N ^ M e g l j was shown t o be ( N ^ M e g l ) + I 3 by t h e e l e c t r o n i c , p r o t o n , and v i b r a t i o n a l s p e c t r a as w e l l as by c o n d u c t i v i t y measurements. The e l e c t r o n i c s p e c t r a and p o l a r o g r a p h s o f t h e m e t h y l p h o s p h o n i t r i l i u m and d i m e t h y l p y r i d i n i u m i o d i d e s show t h a t t h e a c c e p t o r l e v e l s o f t h e p h o s p h o n i t r i l i c r i n g s l i e a t h i g h e r e n e r g i e s t h a n t h a t o f t h e p y r i d i n e r i n g . I n CHC13, t h e s i m i l a r i t i e s i n t h e s p e c t r a o f the m e t h y l p h o s p h o n i t r i l i u m , d i m e t h y l p y r i d i n i u m , and t e t r a -alkylammonium i o d i d e s i n d i c a t e t h a t a l l t h e ch a r g e -t r a n s f e r t o c a t i o n p r o c e s s e s a r e s i m i l a r , t h e r e l a t i v e e n e r g i e s o f t r a n s i t i o n b e i n g a f f e c t e d o n l y t o a s m a l l degree by t h e s p e c i f i c c a t i o n i n v o l v e d . TABLE OF CONTENTS Page ABSTRACT i TABLE OF CONTENTS , i i i LIST OF TABLES V LIST OF FIGURES v i i ACKNOWLEDGEMENTS i x CHAPTER I . INTRODUCTION 1 CHAPTER I I . CHARGE-TRANSFER SPECTRA OF THE IODIDE ION.. 18 I I . A . I n t r o d u c t i o n 18 I I . B . C h a r g e - T r a n s f e r - t o - S o l v e n t S p e c t r a 19 I I . C . C h a r g e - T r a n s f e r t o C a t i o n S p e c t r a 25 I I . D . C h a r g e - T r a n s f e r S p e c t r a o f t h e M e t h y l -p h o s p h o n i t r i l i u m I o d i d e s 32 I I . D . l . E x p e r i m e n t a l 34 a. P r e p a r a t i o n o f the D i m e t h y l p y r i d i n i u m I o d i d e s 34 I I . D . 2 . S p e c t r o s c o p i c R e s u l t s 37 I I . D . 3 . D i s c u s s i o n 41 I I . E. P o l a r o g r a p h i c R e d u c t i o n o f M e t h y l p h o s -p h o n i t r i l i u m and D i m e t h y l p y r i d i n i u m I o d i d e s 4 7 CHAPTER I I I . DONOR-ACCEPTOR CHARGE-TRANSFER COMPLEXES.. 51 I I I . A. I n t r o d u c t i o n 51 I I I . A . l . D e t e r m i n a t i o n o f t h e P o s i t i o n s o f E q u i l i b r i u m 57 I I I . A . 2 . C h a r g e - T r a n s f e r Complexes o f I o d i n e ... 62 I I I . B . C h a r g e - T r a n s f e r Complexes o f I o d i n e w i t h M e t h y l p h o s p h o n i t r i l e s 65 I I I . B . l . E x p e r i m e n t a l 69 Page A. D e t e r m i n a t i o n o f E q u i l i b r i u m C o n s t a n t s . . 70 B. Assignment o f A b s o r p t i o n Bands 72 C. N u c l e a r M a g n e t i c Resonance S p e c t r a o f Donor-Acceptor M o l e c u l a r Complexes 78 D. Temperature E f f e c t s on t h e Ch e m i c a l S h i f t o f N 3 P 3 M e 6 - I 2 79 E. S y n t h e s i s o f N ^ M e ^ ^ 80 I I I . B.2. D i s c u s s i o n 82 CHAPTER IV. INNER CHARGE-TRANSFER COMPLEXES 99 IV. A. I n t r o d u c t i o n 99 IV.B. E x p e r i m e n t a l and R e s u l t s 102 I V . B . l . S y n t h e s i s o f Complexes 102 IV.B.2. N u c l e a r M a g n e t i c Resonance S p e c t r a 103 IV.B.3. E l e c t r o n i c S p e c t r a 104 IV.B.4. V i b r a t i o n a l S p e c t r a o f N ^ M e ^ 104 IV.B.5. C o n d u c t i v i t y Measurements 106 IV.C. D i s c u s s i o n 107 REFERENCES 114 LIST OF TABLES Ta b l e Page 1 S t r u c t u r a l i n f o r m a t i o n o f p h o s p h o n i t r i l e s ... 4 2 I n f o r m a t i o n on the e f f e c t s o f t h e homomorphic and h e t e r o m o r p h i c i n t e r a c t i o n w i t h changes i n r i n g s i z e 10 3 X and dE /dT f o r KI i n v a r i o u s media ... 21 max max 4 P o s i t i o n s o f t h e c h a r g e - t r a n s f e r bands f o r some 1 - a l k y l - p y r i d i n i u m i o d i d e s 3 0 5 S p e c t r o s c o p i c R e s u l t s on the d i m e t h y l p y r i -d i n i u m , m e t h y l p h o s p h o n i t r i l i u m , and t e t r a -n-heptylammonium i o d i d e s . 38 6 X and e f o r t h e d i m e t h y l p y r i d i n i u m i o d i d e s m CH L l ^ 4 J-7 Half-wave p o t e n t i a l s o f d i m e t h y l p y r i d i n i u m i o d i d e s 50 pK a' v a l u e s f o r some p h o s p h o n i t r i l e s i n *>» "i A- V1* v\ rw "^v v\ 8 n i t r o b e n z e n e 66 9 Don o r - a c c e p t o r r a t i o s and o p t i c a l d e n s i t i e s f o r N-P-Me. and I 0 - 70 10 Donor-acceptor r a t i o s and o p t i c a l d e n s i t i e s f o r N 4 P 4 M e 8 and I 2 71 11 Do n o r - a c c e p t o r r a t i o s and o p t i c a l d e n s i t i e s f o r N 5 P 5 M e 1 Q and I 2 71 12 New a b s o r p t i o n bands of s o l u t i o n s o f I w i t h t h e m e t h y l p h o s p h o n i t r i l e s 77 13 Summary o f r e s u l t s f o r t h e complexes o f I w i t h t h e m e t h y l p h o s p h o n i t r i l e s .... 77 14 P r o t o n c h e m i c a l s h i f t s o f t h e d o n o r - a c c e p t o r complexes 79 T a b l e Page 15 Temperature e f f e c t s on t h e c h e m i c a l s h i f t s o f W ^ " ^ 8 0 16 X m a x o f t h e i o d i n e band f o r N- 3P 3Me 6'I 2 i n v a r i o u s s o l v e n t s 81 17 S t r u c t u r a l i n f o r m a t i o n o f some I 2 complexes .. 85 18 Magnitudes o f t h e b l u e - s h i f t f o r t h e complexes o f t h e m e t h y l p h o s p h o n i t r i l e s w i t h I 2 9 2 19 C a l c u l a t e d a s s o c i a t i o n s h i f t s f o r t h e o u t e r complexes 9 4 20 V a l u e s o f t h e i n t e r a c t i o n parameter f o r t h e o u t e r complexes 9 5 21 M i c r o a n a l y s i s r e s u l t s o f t h e i n n e r complexes . 102 22 P r o t o n c h e m i c a l s h i f t s o f t h e i n n e r complexes and N 0 P ^ M e c I 0 1 0 3 3 3 6 2 2 3 5nprEe? S p e C t r a l d a t a f ° r N 4 P 4 M e 8 a n d 1 0 5 24 M o l a r c o n d u c t i v i t i e s o f t h e complexes l i 0 7 LIST OF FIGURES F i g u r e Page 1 S t r u c t u r e s o f (NPF 2) 3. and ( N P C l 2 ) 4 2 2 Modes o f i n t e r a c t i o n i n t h e Tra and TVS systems 7 3 Schematic arrangement o f t h e TT-electron l e v e l s f o r homomorphic and h e t e r o m o r p h i c i n t e r a c t i o n s 9 4 V a l e n c e bond s t r u c t u r e s of CgH-N and 5 S t r u c t u r e s o f N ^ M e g and N ^ t t e g H 6 Thermodynamic c y c l e f o r t h e e v a l u a t i o n o f Emax 7 Models o f t h e c h a r g e - t r a n s f e r p r o c e s s 24 8 D e s c r i p t i o n o f t h e c h a r g e - t r a n s f e r t o c a t i o n p r o c e s s o f 1 - m e t h y l p y r i d i n i u m i o d i d e 29 8a) Spectrum o f 1 - m e t h y l p y r i d i n i u m i o d i d e 30 9 C h a r g e - t r a n s f e r spectrum o f ( N 3 P 3 M e 7 ) + I ~ 40 10. D e s c r i p t i o n of the ground and e x c i t e d s t a t e s o f ( N 3 P 3 M e 7 ) + l " i n CHC1 3 46 11 S t r u c t u r e s o f p h o s p h o n i t r i l e s c o n t a i n i n g 2,3-d i o x y n a p t h y l and 1,8 d i o x y n a p t h y l s i d e groups 47 12 P l o t o f t h e c o r r e l a t i o n between t h e i o n i z a t i o n p o t e n t i a l o f t h e donor and t h e energy o f c h a r g e - t r a n s f e r 55 13 P l o t o f t h e c o r r e l a t i o n between t h e o x i d a t i o n p o t e n t i a l o f t h e donor and t h e energy o f c h a r g e - t r a n s f e r 56 14 B e n e s i - H i l d e b r a n d p l o t f o r t h e system triethylamine«I 2 5 9 11 12 2 3 F i g u r e Page >15 V a r i a t i o n o f AS° w i t h AH° f o r i o d i n e complexes i n v a r i o u s s o l v e n t s 61 16 C o r r e l a t i o n between t h e b l u e - s h i f t o f t h e i o d i n e band and t h e e n t h a l p y o f f o r m a t i o n f o r some I 2 complexes 63 17 P l o t o f -S_L v e r s u s f o r N_P~Me, and I _ 73 Ac C A 3 3 6 2 18 P l o t o f P 1 v e r s u s f o r N.P.Meo and I 0 . . .. 74 A c C A 4 4 8 2 Q 0 1 1 "2 19 P l o t o f — v e r s u s (^-^ Tfor N 5 P 5 M e 1 Q and I 2 ... 75 C A 20 E l e c t r o n i c spectrum o f N c P c M e i n and I 0 i n C H 2 C 1 2 2 76 21 P l o t o f energy o f c h a r g e - t r a n s f e r v e r s u s i o n i z a t i o n p o t e n t i a l 83 22 S t r u c t u r e o f N 3 P 3 M e 6 ' I 2 84 23 Numbering scheme o f N ^ M e g 87 24 T T - l e v e l s o f N 3P 3Me6 and t h e 5-membered segment wh i c h r e s u l t s upon removal o f one n i t r o g e n from t h e r i n g TT-system 90 25 P l o t o f l o c a l i z a t i o n energy v e r s u s r i n g s i z e .. 91 26 P l o t o f A E t and 8 v e r s u s r i n g s i z e 97 27 S t r u c t u r e o f t h e P y 2 I + c a t i o n 100 28 P o t e n t i a l energy diagram o f complex f o r m a t i o n . . 108 29 S t r u c t u r e o f N 4 P 4 M e g I 4 110 30 Proposed s t r u c t u r e o f N ^ M e g l g 110 ACKNOWLEDGEMENTS The a u t h o r would l i k e t o thank P r o f e s s o r N. L. Paddock f o r h i s s u p e r v i s i o n and e n c o u r a g i n g i n t e r e s t t h r o u g h o u t t h e c o u r s e o f t h i s work. I am g r a t e f u l t o t h e o t h e r members o f t h e group f o r t h e i r comments and a d v i c e . A s p e c i a l t h a n k s goes t o Mr. M. LeGeyt f o r h e l p i n t h e i l l u s t r a t i o n s and t o Ms. L. Hon f o r t y p i n g t h e o r i g i n a l m a n u s c r i p t . F i n a n c i a l s u p p o r t from t h e U n i v e r s i t y o f B r i t i s h Columbia i s acknowledged. CHAPTER I  INTRODUCTION P h o s p h o n i t r i l i c d e r i v a t i v e s , c h a r a c t e r i z e d by the r e p e a t i n g u n i t NPX 2, e x i s t i n a s e r i e s o f c y c l i c and l i n e a r p o l y m e r s . The c y c l i c m o l e c u l e s (NPX 2). n, where X can be a v a r i e t y o f s u b s t i t u e n t s i n c l u d i n g F, C I , B r , NR 2, N 3 , OR, OAr, R and A r , o c c u r i n a l a r g e range o f r i n g s i z e s ( i n the ( N P F 2 ) n s e r i e s n ranges from 3 t o 1 7 ) . Though a l a r g e v a r i e t y o f p h o s p h o n i t r i l i c d e r i v a t i v e s a r e known ( f o r r e v i e w s see r e f e r e n c e s 1-5), f o r the homogeneously s u b s t i t u t e d d e r i v a t i v e s s t u d i e d i n t h e p r e p a r a t i o n o f t h i s t h e s i s t h e s i m p l e " p h o s p h o n i t r i l i c " n o t a t i o n can be used. F o r example, ( N P F 2 ) 3 and ( N P C 1 2 ) 4 w i l l be c a l l e d t r i m e r i c f l u o r o p h o s p h o n i t r i l e and t e t r a -m e r i c c h l o r o p h o s p h o n i t r i l e r e s p e c t i v e l y . These compounds are shown i n F i g u r e 1. c i F i g u r e 1. S t r u c t u r e s o f ( N P F 2 ) 3 and ( N P C 1 2 ) 4 . As seen from t h e i r s t r u c t u r e s , t h e c y c l i c p olymers a r e f o r m a l l y u n s a t u r a t e d . The f o r m a t i o n of TT-bonds i s b e l i e v e d t o be due t o o v e r l a p between t h e s i n g l y - o c c u p i e d 2p z o r b i t a l on n i t r o g e n w i t h a 3d o r b i t a l on t h e phosphorus. The o c c u r r e n c e o f s uch a h e t e r o m o r p h i c ,-system i n p h o s p h o n i t r i l i c d e r i v a t i v e s i s s u p p o r t e d by s t r u c t u r a l , t h e r m o c h e m i c a l , and s p e c t r o s c o p i c m e a s u r e m e n t s , 2 0 " 2 6 m a i n l y on homo-ge n e o u s l y s u b s t i t u t e d compounds. The P-N bonds i n homogeneously s u b s t i t u t e d p h o s p h o n i t r i l e s a r e n o r m a l l y e q u a l i n l e n g t h , t h e bond d i s t a n c e b e i n g dependent o n l y on t h e s u b s t i t u e n t and on t h e r i n g s i z e , and a r e i n t h e range 1.50 - 1.-60 A, c o n s i d e r a b l y s h o r t e r t h a n t h e s i n g l e P-N a bond d i s t a n c e o f 1.77 - 1.78 A / 2 7 - 2 9 Some s t r u c t u r a l d a t a o f p h o s p h o n i t r i l i c d e r i v a t i v e s a r e p r e s e n t e d i n Ta b l e I . L a t e r , i t was found t h a t some p r o p e r t i e s , , , 30-32 nj_ . , ... . such as base s t r e n g t h , a l t e r n a t e d w i t h i n c r e a s e i n r i n g s i z e , and t h e r i r - b o n d i n g t h e o r y was completed by the i n c l u s i o n o f an ( i n - p l a n e ) homomorphic 7r-system, u t i l i z i n g t he l o n e p a i r s o f e l e c t r o n s o f t h e n i t r o g e n s t o form a second ( i n - p l a n e ) c o o r d i n a t e ir-system. The use ,of 3d o r b i t a l s o f phosphorus i n f o r m i n g iT-bonds i s made p o s s i b l e by t h e e l e c t r o n e g a t i v e s u b s t i t u e n t s on t h e phosphorus w h i c h w i l l b r i n g about the c o n t r a c t i o n o f t h e 3d o r b i t a l s and lower t h e i r r- • • • 33-37 energy s u f f i c i e n t l y f o r bonding t o o c c u r . The e x p e r i m e n t a l evidence:'in f a v o u r o f t h i s t y p e o f i n t e r -a c t i o n i s v e r y s t r o n g . Those h i g h l y e l e c t r o n e g a t i v e s u b s t i t u e n t s w h i c h would be e x p e c t e d t o c o n t r a c t t h e most t h e d o r b i t a l s do i n d e e d s h o r t e n and s t r e n g t h e n the P-N bonds t h e most. Moreover, t h e b a s i c i t y o f t h e r i n g n i t r o g e n s d e c r e a s e s as the e l e c t r o n e g a t i v i t y o f the s u b s t i t u e n t s on t h e phosphorus i n c r e a s e , c o n s i s t e n t w i t h the i n c r e a s e d d r awing o f t h e l o n e p a i r e l e c t r o n s toward phosphorus. Measurements o f i o n i z a t i o n p o t e n t i a l s T a b l e I S t r u c t u r a l i n f o r m a t i o n Compound P-N(A) P-X(A) PNP(deg) NPN(deg)" XPX(deg) C o n f o r m a t i o n R e f . ( N P P h 2 ) 3 1.597 (.6) 1.804(7) 122.1(4) 117.8(3) 103.8(3) F l a t c h a i r , C 3 v ~ D 3 h 6 (NP (NMe 2) 2) 3 •1.588(3) 1.652 (4) 123.0(4) 116.7(4) 101.5(8) 7 ( N P C l 2 ) 3 1.581(3) .11993.(2) 121.4(4) 118.4(3) ' , 101.4(2) ~ D 3 h 8 ( N P ( O P h ) 2 ) 3 1.575(2) 1.582(2) 121.9(3) 117.3(3) . 98 ~ C 2 9 ( N P F 2 ) 3 1.560 (10) 1.521(10) 120.6(8) 119.4(9) 99.3(6) P l a n a r , D 3 h. 10 ( N P M e 2 ) 4 1.596(5) 1.805 (8) 132.0(3) 119.8(2) 104.1 (2) S a d d l e , S 4 ~ D 2 d 11 ( N P ( N M e 2 ) 2 ) 4 1.578(10) 1.678(10) 133.0(6) 120.0(5) 103.8 (5) S a d d l e , S 4 ~ D 2 d 12 ( N P C 1 2 ) 4 I K ) 1.570(9) 1.989(4) 131.3 (6) 121.2 (5) 102.8(2) Tub, S 4 13 ( N P C 1 2 ) 4 ( T ) 1.559(12) 1.989(4) 135.6(8) 120.5 (7) 103.1(2) C h a i r , C 2 h 14 ( N P F 2 ) 4 1.507(16) 1.515(15) 147.2(14) 122.8(10) 99.9(9) P l a n a r , D 4 h 15 ( N P C 1 2 ) 5 1.521(13) 1.961(8) 148.6(11) 118.4(8) 102.0 (4) P l a n a r , q 2 16 ( N P ( O M e ) 2 ) 6 1.567(8) 1.584(6) 134.4(5) 1113.6(4) 103.3 (3) Do u b l e Tub, C i 17 ( N P ( N M e 2 ) 2 ( ) 6 1.563.(10) 1.669(10) 147.5(7) 120.1(5) 102.9(5) R e l a t e d t o Tub, Sg 18 ( N P ( O M e ) 2 ) 8 1.561(14) 1.576(13) 136.7(10) 116.7(7) 101.3(7) C h a i r , C i - . 19 o f the ( N P F j s e r i e s i n d i c a t e t h a t t h e h i g h e s t ir-system 2 n i s of t h e homomorphic t y p e . 2 1 I t has a l s o been p o i n t e d out t h a t the 4s and p a r t i c u l a r l y t h e 4p o r b i t a l s would be lowered i n energy as w e l l , and t h a t t h e 4p o r b i t a l 38 3 9 c o u l d a l s o p a r t i c i p a t e i n -rr-bonding. ' I n f a c t , t h e use o f o n l y 3d o r b i t a l s i n ^-bonding i s p r o b a b l y an o v e r s i m p l i f i c a t i o n s i n c e 3d-4s m i x i n g i s l i k e l y , and the concept o f d i s c r e t e H i i c k e l - t y p e a - TT bonds may be i n v a l i d . N e v e r t h e l e s s , t h e use o f a 3dTr - 2pTT bonding model i n e x p l a i n i n g p h o s p h o n i t r i l i c c h e m i s t r y has been e x t r e m e l y s u c c e s s f u l and t h i s i s t h e model adopted f o r use i n t h i s t h e s i s . A b r i e f o u t l i n e o f the bonding t h e o r y i n v o l v i n g diT - p^ i n t e r a c t i o n f i r s t proposed by C r a i g and P a d d o c k 4 0 i s now p r e s e n t e d , w i t h t h e a s p e c t s r e l e v a n t t o t h i s t h e s i s b e i n g emphasized. F or a more comprehensive t r e a t m e n t o f the t h e o r y see r e f . 5. Bonding i n P h o s p h o n i t r i l e s In p h o s p h o n i t r i l i c d e r i v a t i v e s , phosphorus uses 3 s p 3 - h y b r i d o r b i t a l s t o form a-bonds t o t h e n e i g h b o u r i n g r i n g n i t r o g e n s and the two s u b s t i t u e n t s , t h e 3d o r b i t a l s b e i n g u t i l i z e d f o r Tr-bonding. The r i n g n i t r o g e n uses 2 s p 2 - h y b r i d o r b i t a l s t o form a-bonds mm mm t o t h e a d j a c e n t phosphorus atoms and t o house the l o n e p a i r o f e l e c t r o n s , t h e r e m a i n i n g 2-p o r b i t a l b e i n g used f o r ir-bonding. The h y b r i d i z a t i o n a t phosphorus and n i t r o g e n i s o n l y approximate s i n c e t h e a n g l e s d i f f e r a p p r e c i a b l y from 109°28' and 120° a t t h e r e s p e c t i v e atoms. There are two t y p e s o f d e r e a l i z a t i o n p o s s i b l e : i n one t h e p a r t i c i p a t i n g o r b i t a l s a r e a n t i s y m m e t r i c w i t h r e s p e c t t o r e f l e c t i o n i n the m o l e c u l a r p l a n e (TT s y s t e m ) , and i n t h e o t h e r t h e y a r e symmetric (TT a * 1 ' 2 s s y s t e m ) . The phosphorus and n i t r o g e n a tomic o r b i t a l s a r e c l a s s i f i e d a c c o r d i n g t o t h e r e p r e s e n t a t i o n s o f t h e p o i n t group C^, t h i s b e i n g t h e l o c a l symmetry o f t h e N-P^N u n i t , r e g a r d l e s s o f the p l a n a r i t y o f t h e r i n g . The p a r t i c i p a t i n g a t o m i c o r b i t a l s i n t h e ^ system b e l o n g t o the o r B 2 r e p r e s e n t a t i o n s , whereas t h o s e i n t h e TT system b e l o n g t o t h e A, o r B, . (The a x i s 1 1 system used i n c l a s s i f y i n g t h e o r b i t a l s i s shown i n F i g u r e 2.) O v e r l a p c a l c u l a t i o n s suggest t h a t t h e two d - o r b i t a l s m a i n l y i n v o l v e d i n t h e PTT - dTr i n t e r a c t i o n a r e do o (TT ) and d (TT ) . The TT system i s o f t h e x'-y' s xz a s J homomorphic t y p e , as i n e.g. benzene, where PTT - pir i n t e r a c t i o n o c c u r s , s i n c e i n t e r a c t i o n o f t h e 3 d x 2 _ y 2 o r b i t a l w i t h t h e 2 s p 2 - h y b r i d o r b i t a l s c o n t a i n i n g t h e n i t r o g e n l o n e p a i r s a r e o f the same s i g n . On t h e o t h e r (b) F i g u r e 2. ( r e f . 5) ._ (a) i n t e r a c t i o n o f t h e d o r b i t a l a t phosphorus w i t h t h e p„ o r b i t a l a t n i t r o g e n . A p * a o r b i t a l o f an e x o c y c l i c g r o u p i s a l s o shown. (b) i n t e r a c t i o n o f t h e y 2 o r b i t a l a t phosphorus w i t h t h e s-p o r b i t a l a t n i t r o g e n . A P 7 T S •r o r b i t a l o f an e x o c y c l i c g r o u p i s a l s o shown. hand, t h e 7 i a system i s o f t h e h e t e r o m o r p h i c t y p e , s i n c e i n t e r a c t i o n s o f t h e 3d o r b i t a l w i t h the 2p x z z o r b i t a l s o f t h e a d j a c e n t n i t r o g e n s a r e o f t h e o p p o s i t e s i g n . U s i n g t h e H i i c k e l a p p r o x i m a t i o n , s e c u l a r e q u a t i o n s f o r t h e two ,-systerns can be formed, s o l u t i o n s o f w h i c h g i v e s t h e energy l e v e l s o f t h e two ,-systems. I f t h e e l e c t r o n e g a t i v i t i e s o f phosphorus and n i t r o g e n a r e r e l a t e d by = ap + 6, where, f o r example, i s the Coulomb i n t e g r a l f o r n i t r o g e n and B i s t h e resonance i n t e g r a l f o r phosphorus i n t e r a c t i n g w i t h n i t r o g e n , the s c h e m a t i c arrangement o f t h e , - e l e c t r o n energy l e v e l s f o r v a r i o u s r i n g s i z e s i s as shown i n F i g u r e 3. 01 he i * d i f f e r e n c e s ID3twesn t h e two TT Sys16ms 3.ire i l l u s t r a t e d i n T a b l e 2. Because o f t h e s i m i l a r v a l e n c e bond s t r u c t u r e s (see F i g u r e 4 ) , many comparisons have been made between the m e t h y l p h o s p h o n i t r i l e s and t h e m e t h y l p y r i d i n e s . Due t o the e l e c t r o n - r e l e a s i n g e f f e c t s o f t h e m e t h y l sub-s t i t u e n t s on t h e phosphorus,, e l e c t r o n d e n s i t y i s c o n c e n t r a t e d enough on t h e r i n g n i t r o g e n s o f t h e m e t h y l p h o s p h o n i t r i l e s so t h a t t h e y a.ct a.s s t r o n g e l e c t r o n clox^ors • a Av -G—O -@—G-3 -6<—©-(o) 6 • 2/5 + 2 / 3 - 2 / 5 °5 Av n = 3 -C—O-6 2 / 5 4 5 (b) F i g u r e 3. S c h e m a t i c arrangement o f t h e T T - e l e c t r o n l e v e l s f o r (a) homomorphic and (b) h e t e r o m o r p h i c i n t e r -a c t i o n s . H. M. 0. c a l c u l a t i o n s , oC = CC~ + 5 N P ( a f t e r C r a i g and P a d d o c k 5 ) . Table 2 Symmetric Interactions 3 Antisymmetric Interactions 3 n Energy of highest occ. orbital Delocalisation energy/electron TT-charge on N Energy of highest occ. orbital Delocalisation energy/electron TT-charge on N 3 1.117 0.314 1.379 0.500 0.250 1.518 4 0.500 0.272 1.477 0.500 0.272 1.477 5 0.795 0.289 1.418 0.500 0.280 .1.458 6 0.500,: 0.282 1.449 0.500 0.282 1.449 7 0.669 0.286 1.431 | 0.500 0.284 1.444 8 0.500 0.284 1.440 i 0.500 0.284 1.440 '. a) The delocalisation energies are in units of S; the orbital energies are in units of 6, relative to the average Coulomb integral a. C H 3 / H 3 F i g u r e 4. V a l e n c e bond s t r u c t u r e s . .. i n . f a c t , t h e P K a v a l u e s o f p y r i d i n e (5.21) 42 41 and N 3 P 3 M e g (5.03) a r e comparable i n aqueous s o l u t i o n . On t h e o t h e r hand, t h e m e t h y l p y r i d m e s and m e t h y l -p h o s p h o n i t r i l e s a r e e x p e c t e d t o d i f f e r a t l e a s t i n some r e s p e c t s s i n c e t h e n i t r o g e n l o n e p a i r i n t h e p y r i d i n e i s n ot f o r m a l l y i n v o l v e d i n t h e Tr-system whereas t h o s e i n t h e m e t h y l p h o s p h o n i t r i l e s a r e i n v o l v e d i n t h e homomorphic T r - s y s t e m . 2 1 ' 4 0 T h i s i s i l l u s t r a t e d by t h e changes i n t h e r i n g s t r u c t u r e o f N ^ t t e g upon p r o t o n a t i o n , 4 4 whereas such changes a r e n e g l i g i b l e i n t h e case o f p y r i d i n e . CHg-N 1.60A CHo CH, \ / \\ / CHo CH, CH, CH< 132 N N // I P — C H 3 C H 3 ^ P •CH, CH< CH, 1.70A \l-61A N /l.54A 7 \ •CH, F i g u r e 5. S t r u c t u r e s o f N,P.Me R and N 4 P 4 M e R H + . 4 4 8 P h o s p h o n i t r i l e s a r e known t o form a d d u c t s w i t h hydrogen h a l i d e s i n much t h e same way as t h e d e r i v a t i v e s o f p y r i d i n e , w i t h p r o t o n a t i o n o c c u r r i n g a t th e r i n g n i t r o g e n s i n s t e a d o f a t t h e l i g a n d s , even f o r a m i n o p h o s p h o n i t r i l e s . 4 5 M e t h y l p h o s p h o n i t r i l e s and m e t h y l p y r i d i n e s form q u a t e r n a r y s a l t s w i t h a l k y l i o d i d e s . 4 2 , 4 6 , 4 7 These a l k y l i o d i d e s 4 7 , 5 0 r e a c t i n a 48 s i m i l a r manner as t h e i o d i d e s o f t h e p y r i d i n i u m and 49 . t e t r a a l k y l a m m o n i u m i o n s w i t h t r a n s i t i o n m e t a l c a r b o n y l s , r e s u l t i n g i n d i s p l a c e m e n t o f c a r b o n monoxide. B + I + M(CO) 6 > B + [ M ( C O ) 5 I ] + CO M = C r , Mo P h o s p h o n i t r i l i c d e r i v a t i v e s have been shown t o a c t as l i g a n d s i n m e t a l complexes, m a i n l y as a-donors t o the m e t a l t h r o u g h d o n a t i o n o f t h e l o n e p a i r e l e c t r o n s o f the r i n g n i t r o g e n s . For example, ( N P C 1 2 ) 3 and ( N P B r 2 ) 3 form 1:1 adducts w i t h A l B r ^ i n CS,,.5"^ (NPMe 2) 4 r e a c t s w i t h C u C l 2 i n m e t h y l e t h y l k e t o n e 5 2 t o 53 form a complex o f s t r u c t u r e (N^P^MegH)CuCl^, i n w h i c h c o o r d i n a t i o n t o the m e t a l o c c u r s by a - d o n a t i o n t h r o u g h a r i n g n i t r o g e n , w i t h a p r o t o n c o v a l e n t l y bonded t o t h e o p p o s i t e r i n g n i t r o g e n . ( NPMe 2) 4 ,- and ( N P ( N M e 2 ) 2 ) 4 r e a c t w i t h molybdenum and t u n g s t e n hexa-c a r b o n y l t o y i e l d complexes o f the t y p e (NPMe 2) -Mo(CO) 3 and N 4P 4(NMe 2)g-W(CO) 4, i n f r a r e d s p e c t r a s u g g e s t i n g a g a i n t h a t c o o r d i n a t i o n t o t h e m e t a l o c c u r s by a-d o n a t i o n t h r o u g h the r i n g n i t r o g e n s r a t h e r t h a n t h r o u g h i r - d o n a t i o n . 5 ^ There has o n l y been one b r i e f r e p o r t o f 54 a 7r-complex, a compound r e p o r t e d t o be T T -(NPC1 2) 3Mo (CO) 3 but d i r e c t s t r u c t u r a l e v i d e n c e i s l a c k i n g . C h a r g e - t r a n s f e r - t o - c a t i o n s p e c t r a o f t h e m e t h y l p y r i d i n i u m i o d i d e s has been r e p o r t e d by Kosower e t . a l . 5 5 S i m i l a r s t u d i e s were u n d e r t a k e n w i t h the m e t h y l p h o s p h o n i t r i l i u m i o d i d e s i n o r d e r t o a s c e r t a i n t h e d i f f e r e n c e s i n the i r - l e v e l s o f the two d e l o c a l i z e d systems. I n a d d i t i o n , s i n c e no a b s o r p t i o n s due t o the p h o s p h o n i t r i l i c r i n g a r e o b s e r v a b l e i n t h e v i s i b l e -u l t r a v i o l e t s p e c t r a o f t h e m e t h y l p h o s p h o n i t r i l e s , i t was hoped t h a t t r a n s i t i o n s due not o n l y t o c h a r g e -t r a n s f e r - t o - c a t i o n b u t a l s o c h a r g e - t r a n s f e r - t o - s o l v e n t may be o b s e r v a b l e i n an a p p r o p r i a t e s o l v e n t . No s p e c t r a have thus f a r been o b t a i n e d w i t h any compound where t h e s e two p r o c e s s e s o c c u r s i m u l t a n e o u s l y . P o l a r o g r a p h i c r e d u c t i o n e x p e r i m e n t s were performed on t h e m e t h y l -p y r i d i n i u m as w e l l as the m e t h y l p h o s p h o n i t r i l i u m i o d i d e s i n o r d e r t o c o n f i r m t h e r e s u l t s o b t a i n e d s p e c t r o s c o p i c a l l y i n r e g a r d s t o t h e a c c e p t o r l e v e l s i n the two d e l o c a l i z e d systems. P y r i d i n e i s known as w e l l t o form b o t h o u t e r and i n n e r d o n o r - a c c e p t o r complexes w i t h 1" 2- 5 6 A l t h o u g h phase diagrams suggest t h a t s o l i d s t a t e i n t e r a c t i o n s o c c u r between a v a r i e t y o f p h o s p h o n i t r i l i c d e r i v a t i v e s J x. a. 4-u -i u 4.1. - i w 57-59 , and t e t r a c y a n o e t h y l e n e o r hexamethylbenzene, and t h a t the more b a s i c a m i n o p h o s p h o n i t r i l e s i n t e r a c t w i t h 60,61 xodine i n n o n p o l a r s o l v e n t s , t h e e x a c t n a t u r e o f t h e i r i n t e r a c t i o n s i s not f u l l y c l e a r . From t h e i r s i m i l a r i t i e s w i t h t h e d e r i v a t i v e s o f p y r i d i n e , the m e t h y l p h o s p h o n i t r i l e s a r e e x p e c t e d t o form c h a r g e -t r a n s f e r complexes w i t h a c c e p t o r s . W i t h the use o f t h e m e t h y l p h o s p h o n i t r i l e s as d o n o r s , i n t e r a c t i o n s can be d e t e c t e d w i t h s t a n d a r d c h e m i c a l t e c h n i q u e s such as p r o t o n n u c l e a r magnetic resonance as w e l l as s p e c t r o -s c o p i c a l l y . F u r t h e r , from the well-known p r o p e r t i e s o f complexes i n v o l v i n g i o d i n e as a c c e p t o r , t h e i n t e r -a c t i o n s of t h e m e t h y l p h o s p h o n i t r i l e s , ( N P M e 2 ) n , n = 3,4,5, w i t h i o d i n e a l l o w an e l u c i d a t i o n o f t h e r e l a t i v e base s t r e n g t h s o f t h e s e r i e s o f m e t h y l p h o s -p h o n i t r i l e s , l e a d i n g t o a c l e a r e r v i e w o f t h e e f f e c t s on base s t r e n g t h o f a - h y b r i d i z a t i o n as w e l l as TT-e l e c t r o n d e n s i t y . T h i s method has i t s advantages i n t h a t t h e s p e c i f i c e f f e c t o f s o l v a t i o n e n e r g i e s a r e v e r y s m a l l i f n o t n e g l i g i b l e due t o t h e s m a l l amounts o f charge t r a n s f e r r e d i n the ground s t a t e o f t h e complexes. I n t h e d e t e r m i n a t i o n o f pK v a l u e s , p r o t o n a t i o n l e a d s t o l a r g e s o l v a t i o n e f f e c t s t h e magni-tudes o f w h i c h cannot be d e t e r m i n e d q u a n t i t a t i v e l y T h i s work i s t h e r e f o r e concerned w i t h t h e complexes o f t h e m e t h y l p y r i d i n e s and t h e methylphos-p h o n i t r i l e s . I t s o b j e c t i s t o compare th e c h a r a c t e r i s t i c s o f t h e b e t t e r known d e l o c a l i z e d system w i t h t h a t o f t h e p h o s p h o n i t r i l e s t o see how f a r t h e same q u a l i t a t i v e c o n c e p t s a r e a p p l i c a b l e , and t o draw q u a n t i t a t i v e c o n c l u s i o n s where p o s s i b l e . The r e s u l t s , as o b t a i n e d , show t h e s i m i l a r i t y between the m e t h y l p h o s p h o n i t r i l e s and t h e d e r i v a t i v e s o f p y r i d i n e i n t h a t b o t h form o u t e r as w e l l as i n n e r complexes w i t h i o d i n e , though t h e s t r u c t u r e s o f t h e i n n e r complexes a r e d i f f e r e n t . R e s u l t s from t h e o u t e r complexes show t h e r e l a t i v e base s t r e n g t h s o f t h e m e t h y l p h o s p h o n i t r i l e s as N ^ M e g > N ^ M e ^ > N ^ M e ^ e x p l i c a b l e i n terms o f t h e combined e f f e c t s o f a-h y b r i d i z a t i o n and T r - e l e c t r o n d e n s i t y a t t h e n i t r o g e n s due t o the homomorphic T r-system. The c h a r g e - t r a n s f e r s p e c t r a and p o l a r o g r a p h i c e x p e r i m e n t s show t h a t t h e a c c e p t o r l e v e l s o f the m e t h y l p h o s p h o n i t r i l i u m i o n s l i e a t h i g h e r e n e r g i e s than t h o s e o f t h e p y r i d i n i u m i o n . C a r e f u l i n t e r p r e t a t i o n o f t h e c h a r g e - t r a n s f e r -t o - c a t i o n s p e c t r a o f t h e m e t h y l p h o s p h o n i t r i l i u m and tetra-n-heptylammonium i o d i d e s , i n which c h a r g e -t r a n s f e r — t o - s o l v e n t s p e c t r a were s i m u l t a n e o u s l y o b t a i n e d , show i n t h i s i n s t a n c e a g r e a t e r s i m i l a r i t y between t h e m e t h y l p h o s p h o n i t r i l i u m and t h e t e t r a a l k y l — ammonium i o d i d e s The s i m i l a r i t i e s i n t h e e n e r g i e s o f c h a r g e - t r a n s f e r - t o - c a t i o n i n t h e t h r e e t y p e s o f a c c e p t o r s i n d i c a t e t h a t a l l t h e c h a r g e - t r a n s f e r p r o c e s s e s a r e s i m i l a r , the r e l a t i v e e n e r g i e s o f t r a n -s i t i o n b e i n g a f f e c t e d o n l y t o a s m a l l degree by the c a t i o n . CHAPTER I I CHARGE-TRANSFER SPECTRA OF THE IODIDE ION I I . A. I n t r o d u c t i o n I n s t u d i e s o f t h e e l e c t r o n i c s p e c t r a o f t h e i o d i d e i o n i n s o l u t i o n , two p r i m a r y t y p e s o f t r a n s i t i o n s have thus f a r been d e t e r m i n e d , t h a t o f (1) c h a r g e ¬* *. o . u i ..62-67 , t r a n s f e r t o t h e s o l v e n t and (2) c h a r g e - t r a n s f e r t o t h e c a t i o n . f b H These t r a n s i t i o n s i n v o l v e t h e s e p a r a t i o n o f an e l e c t r o n from t h e a n i o n , t h e e x c i t e d s t a t e s c o n s i s t i n g o f an i o d i n e atom and an e l e c t r o n w hich i s t r a n s f e r r e d r e s p e c t i v e l y t o a c a v i t y surrounded by s o l v e n t m o l e c u l e s o r t o t h e l o w e s t u n o c c u p i e d o r b i t a l o f t h e c a t i o n . E v i d e n c e t h a t t h e t r a n s i t i o n s r e s u l t i n an i o d i n e atom i s found i n t h e o b s e r v a t i o n o f two s i m i l a r bands s e p a r a t e d by an energy d i f f e r e n c e o f a p p r o x i m a t e l y 21.8 k c a l m o l e " 1 , t h a t c a l c u l a t e d f o r t h e energy d i f f e r e n c e between t h e 2 P and 2 P * 4-v, • 4. 69 ^ s t a t e s or trie i o d i n e atom. For t h e p r o c e s s ..fry. „ i - + e " t h e i o d i n e atom produced may be e i t h e r i n t h e 2 P ^ o r 2 P s t a t e s . The p o s s i b i l i t y o f o b s e r v i n g any i n t e r n a l e l e c t r o n i c t r a n s i t i o n s can be r u l e d out s i n c e t h e energy a a s s o c i a t e d w i t h 5 p 6 — > 5 p 5 6 s x t r a n s i t i o n i s g r e a t e r t han th e i o n i z a t i o n p o t e n t i a l o f the i o d i d e i o n . The c h a r a c t e r i s t i c s o f t h e two t y p e s o f t r a n s i t i o n s are p r e s e n t e d b e f o r e d i s c u s s i o n o f t h e p r e s e n t work. I I . B. C h a r g e - T r a n s f e r - t o - S o l v e n t S p e c t r a C h a r g e - t r a n s f e r - t o - s o l v e n t s p e c t r a ( a b b r e v i a t e d as CTTS) are smooth, s t r u c t u r e l e s s , i n t e n s e a b s o r p t i o n bands i n t h e u l t r a v i o l e t r e g i o n . S o l v a t e d i o d i d e i o n s i n aqueous s o l u t i o n , f o r example, absorb i n t h e 200 my r e g i o n o f t h e spectrum. These t r a n s i t i o n s a r e independent o f c a t i o n and obey Be e r ' s Law. The CTTS s p e c t r a o f t h e i o d i d e i o n , i n p a r t i c u l a r , have r e c e i v e d d e t a i l e d a t t e n t i o n because o f t h e good r e s o l u t i o n o f t h e i o d i d e d o u b l e t (work p r i o r t o 1942 i s summarized i n r e f . 70). I n g e n e r a l , CTTS s p e c t r a a r e c h a r a c t e r i z e d by t h e i r s e n s i t i v i t y t o changes i n s o l v e n t , t e m p e r a t u r e , p r e s s u r e , and t h e c o n c e n t r a t i o n o f added s a l t s . (For a r e c e n t r e v i e w see r e f . 71.) The e n e r g i e s o f a b s o r p t i o n maxima, E ^ ^ ' max f o r i o d i d e i n a range o f s o l v e n t s show extreme s o l v e n t s e n s i t i v i t y . T h i s i s t o be e x p e c t e d , s i n c e t h e e x c i t e d s t a t e o f t h e t r a n s i t i o n i s l e s s p o l a r t h a n t h e ground s t a t e . The obser v e d e f f e c t , however, i s v a r i a b l e i n magnitude and d i r e c t i o n , u n d e r g o i n g s m a l l b l u e - s h i f t s i n h y d r o x y l i c s o l v e n t s , but l a r g e r r e d s h i f t s i n non-h y d r o x y l i c s o l v e n t s , as t h e p o l a r i t y o f t h e medium i s reduced 6 3 ' 6 4 E f o r a CTTS a b s o r p t i o n d e c r e a s e s w i t h max c an i n c r e a s e i n t e m p e r a t u r e . 6 3 " 6 6 ' 7 2 ~ 7 5 The l i n e a r dependence o f E m a x on temp e r a t u r e i s such t h a t t h e CTTS s p e c t r a f o r a g i v e n i o n i n a p a r t i c u l a r s o l v e n t can be c h a r a c t e r i z e d by t h e s l o p e , d E m a x . 6 3 - 6 5 A dT marked s h i f t o f E w i t h t e m p e r a t u r e i s d i a g n o s t i c max o f t h i s t y p e o f t r a n s i t i o n s i n c e , i n systems where i o d i d e i s p a r t o f a complex, t h e s h i f t o f the: low-i -i -., , 65 energy band i s v e r y s m a l l and sometimes z e r o . The dependence o f E m a x on p r e s s u r e f o r i o d i d e i n w ater i s such t h a t E i n c r e a s e s w i t h p r e s s u r e , max T a b l e 3 A m a x ( A ) ' AND d E m a x / d T (Cal./deg.) FOR KI IN VARIOUS MEDIA s o l v e n t H ?0 added s a l t (M) temp. 20 X . max 2261 d E m a x / d T 32 r e f . 65 D 2 ° 20 2251 74 H 2 ° NaCl(0.67) 20 2254 30 65 N H 3 2240 132 MeOH 20 2198 11 65 Me OH 2196 133 MeOH 2210 131 MeOH L i C l ( O . l ) 20 2190 65 EtOH 2170 133 EtOH 20 2185 10 65 EtOH 2192 131 n-PrOH 2156 133 n-BuOH 2180 131 iso-PrOH 2 0 2190 65 iso-PrOH L i C l ( 0 . 2 4 ) 20 2185 65 iso-PrOH L i C l ( l . O ) 20 2180 65 MeCN 20 2458 57 65 MeCN 2444 133 MeCN 2461 131 EtCN 20 2494 47-57 65 H 2 ° + MeCN(55%) 20 2308 50 65 H 2 ° + MeCN(84.2%) 20 2372 60 65 H 2 ° + EtOH(39%) 20 2212 43 65 H 2 ° + EtOH(70%) 20 2194 27 65 H 2 ° + EtOH(83.5%) 20 2192 26 65 t h e p r e s s u r e s e n s i t i v i t y b e i n g g e n e r a l l y g r e a t e r t h a n t h a t f o r i n t r a m o l e c u l a r t r a n s i t i o n s . 7 6 , 7 7 A s i m i l a r e f f e c t i s o b s e r v e d w i t h t h e a d d i t i o n o f a l k a l i h a l i d e s and o t h e r m e t a l s a l t s t o aqueous s o l u t i o n s o f i o d i d e , w i t h an i n c r e a s e i n E upon i n c r e a s i n g t h e s a l t , ,. 63,64,70*73,78,79 ^ c o n c e n t r a t i o n . ' ' ' ' ' These s h i f t s a r e a l s o 63 64 d i a g n o s t i c o f CTTS t r a n s i t i o n s . ' T h e o r e t i c a l Treatment Q u a n t i t a t i v e t r e a t m e n t s o f t h e p r o c e s s o f charge t r a n s f e r t o s o l v e n t have attempted m a i n l y t o c o r r e l a t e t h e v a r i a t i o n s o f E w i t h changes i n max ^ s o l v e n t and temperature. E a r l i e r t h e o r i e s had a ttempted t o c o r r e l a t e dE /dT w i t h e n t r o p i e s o f max c e l e c t r o d e r e a c t i o n s 8 0 and o f s o l v a t i o n . 7 5 A l l c u r r e n t t h e o r i e s i n v o l v e t h e l o s s o f a p e l e c t r o n by t h e a n i o n but they d i f f e r i n t h e i r d e s c r i p t i o n o f t h e d e s t i n a t i o n o f the e l e c t r o n upon p h o t o e x c i t a t i o n (see r e f s . 64 and 71 f o r d e t a i l e d d i s c u s s i o n ) . The c o n c e p t t h a t t h e e j e c t e d e l e c t r o n becomes s o l v a t e d 8 1 i n t h e 8 2 same sense as f o r e l e c t r o n s i n l i q u i d ammonia has been r e j e c t e d , however, s i n c e major r e o r g a n i z a t i o n o f t h e s o l v e n t m o l e c u l e s would be r e q u i r e d d u r i n g the a c t o f l i g h t a b s o r p t i o n . By f a r t h e most i m p o r t a n t t h e o r e t i c a l model was t h a t f i r s t proposed by P l a t z m a n and F r a n c k , 6 7 w i t h a l l subsequent models based on i t s main f e a t u r e s . The e x c i t e d e l e c t r o n was d e s c r i b e d as moving i n a d i s c r e t e , c e n t r o s y m m e t r i c o r b i t a l d e f i n e d l a r g e l y by the p o t e n t i a l f i e l d o f t h o s e p o l a r i z e d s o l v e n t mole-c u l e s which a r e o r i e n t e d around t h e a n i o n . The e x p r e s s i o n f o r E m a x was d e r i v e d u s i n g t h e f o l l o w i n g c y c l e . X + aq B X aq(aq) Emax X • + e + aq -> X'- (aq) F i g u r e 6. On the b a s i s o f t h i s model, w h i c h c o u l d n o t however p r e d i c t a t e m p e r a t u r e dependent E , Smith c c c max and Symons proposed a model which (1) p r e d i c t e d q u a l i t a t i v e l y t h e t e m p e r a t u r e dependence o f E m a x and (2) c o r r e l a t e d t h e energy o f t r a n s i t i o n w i t h t h e r a d i u s o f t h e p r i m a r y s o l v e n t s h e l l around t h e a n i o n , t h e m o d i f i c a t i o n made b e i n g based on t a k i n g i n t o a c c o u n t the e f f e c t o f the f i r s t s o l v e n t l a y e r . I n t h e f o l l o w i n g f i g u r e t h i s p r o c e s s i s r e p r e s e n t e d by (a) whereas (b) en t s th e model o f Con n i c k e t . a l . r e p r e s e n t s the 81 8 G3 i-G3 -ft-lot txrt cawrry 8 Q Iodine otoat In oriqioal ecitj \~"/ i n f a n t cgvlt] .Itctron Elflctron inn** cavity. F i g u r e 7. P h o t o c h e m i c a l s t u d i e s have p l a y e d an impor-t a n t r o l e i n s u p p l y i n g e v i d e n c e f o r t h e o v e r a l l c h a r g e -t r a n s f e r p r o c e s s . F l a s h p h o t o l y s i s s t u d i e s 8 3 on aqueous s o l u t i o n s o f h a l i d e i o n s have l e d t o t e n t a t i v e i d e n t i f i c a t i o n o f a t r a n s i e n t s p e c i e s as th e d i h a l i d e • . i o n . No a b s o r p t i o n s i n t h e 900 my r e g i o n were o b s e r v e d , however, w h i c h m i g h t be expected i f s o l v a t e d e l e c t r o n s were formed. L a t e r work on th e p h o t o l y s i s o f aqueous i o d i d e i n n e u t r a l and a l k a l i n e s o l u t i o n s y i e l d e d i o d i n e i n t h e p r e s e n c e o f s o l u t e s w h i c h scavenged hydrogen o r • s o l v a t e d e l e c t r o n s . 8 5 ' 8 6 The l a t t e r s p e c i e s was c o n s i d e r e d f o l l o w i n g advances made i n t h e i n t e r p r e ¬t a t i o n o f t h e r a d i o l y s i s o f aqueous s o l u t i o n s . ' y J • A s e r i e s o f s t u d i e s 9 4 " 1 0 0 have e s t a b l i s h e d t h a t the i n i t i a l a c t i n v o l v e s f o r m a t i o n o f an e x c i t e d s t a t e w h i c h then decays e i t h e r v i a a r a d i a t i o n l e s s t r a n s i t i o n t o the ground s t a t e o r i n t o a r a d i c a l and s o l v a t e d e l e c t r o n i n c l o s e p r o x i m i t y . — X > (X ' • + e ) aq aq aq aq ground e x c i t e d s t a t e s t a t e T h i s d e s c r i p t i o n i s i n g e n e r a l agreement w i t h t h e model o f t r a n s i t i o n f i r s t proposed by P l a t z m a n and F r a n c k . 6 7 I I . C. C h a r g e - T r a n s f e r t o C a t i o n S p e c t r a I n a d d i t i o n t o CTTS s p e c t r a due t o s o l v a t e d h a l i d e i o n s , i n p a r t i c u l a r i o d i d e , s p e c t r a due t o charge t r a n s f e r t o the c a t i o n may be o b t a i n e d i n s o l v e n t s o f low p o l a r i t y where the a n i o n and the c a t i o n i n t e r a c t w i t h one a n o t h e r , r e s u l t i n g i n the f o r m a t i o n o f i o n - p a i r s . The a b s o r p t i o n s p e c t r a i n the two cases d i f f e r s i n c e t h e environment around t h e h a l i d e i o n s a r e d i f f e r e n t . The c oncept o f i o n - p a i r i n g i s not s t r a i g h t -f o r w a r d . The a n a l y s i s o f c o n d u c t i v i t y d a t a , 1 0 1 , 1 0 2 . . , 103,104 . k i n e t i c d a t a , e l e c t r o n s p i n resonance s p e c t r a o f r a d i c a l a n i o n s , 1 0 5 and u l t r a s o n i c r e l a x a t i o n d a t a 1 0 6 - 1 1 4 made i t n e c e s s a r y t o d e f i n e t h r e e c l a s s e s o f i o n - p a i r s i n s o l u t i o n . The f o l l o w i n g d e f i n i t i o n s a r e t h o s e o f G r i f f i t h s ana Symons : (1) C o n t a c t i o n - p a i r s - I o n s , i n c o n t a c t , w i t h no i n t e r v e n i n g s o l v e n t m o l e c u l e s . (2) S o l v e n t - s h a r e d i o n - p a i r s - P a i r s o f i o n s l i n k e d e l e c t r o s t a t i c a l l y t h r o u g h a s o l v e n t m o l e c u l e , t h i s m o l e c u l e b e i n g p a r t o f t h e p r i m a r y s o l v a t i o n s h e l l o f b o t h i o n s . (3) S o l v e n t - s e p a r a t e d i o n - p a i r s - P a i r s o f i o n s , l i n k e d e l e c t r o s t a t i c a l l y but s e p a r a t e d by more th a n one s o l v e n t m o l e c u l e . The p o s s i b i l i t y o f the f o r m a t i o n o f d i f f e r e n t t y p e s o f i o n - p a i r s a r e n e c e s s a r y t o r e c o n c i l e e v i d e n c e , e.g., c o n d u c t i v i t y and s p e c t r o s c o p y , w h i c h g i v e d i f f e r e n t X.- 4. 4= 4-U J 4T • • 4-' 1 1 5 T e s t i m a t e s o f the degree o f i o n a s s o c i a t i o n . I o n - p a i r s c h a r a c t e r i z e d by c o n d u c t i v i t y may be s e p a r a t e d by one or more s o l v e n t m o l e c u l e s , and, w h i l e t h e r e may be a consequent change i n i o n i c m o b i l i t y , one may ob s e r v e no changes i n the a b s o r p t i o n s p e c t r a . I t i s o n l y t h r o u g h t h e f o r m a t i o n o f c o n t a c t o r p o s s i b l y s o l v e n t - s h a r e d i o n -p a i r s t h a t one i s a b l e t o o b s e r v e changes i n t h e a b s o r p -t i o n s p e c t r a o t h e r t h a n s m a l l s h i f t s . I n a s o l u t i o n , t h e r e i s a p o s s i b i l i t y o f the f o u r s p e c i e s b e i n g p r e s e n t s i m u l t a n e o u s l y , w i t h e q u i l i b r i a e x i s t i n g between f r e e i o n s and c o n t a c t i o n - p a i r s as w e l l as t h e i n t e r m e d i a t e t y p e s o f i o n - p a i r s . I t i s f o r t h i s r e a s o n t h a t a t t e m p t s a t l i n k i n g changes i n t h e a b s o r p t i o n s p e c t r a o f a s o l u t e w i t h t h e changes i n a s s o c i a t i o n c o n s t a n t d e r i v e d from c o n d u c t i v i t y d a t a have sometimes been d i s a p p o i n t i n g . 1 1 6 " 1 2 0 For i o d i d e s i n l o w - p o l a r i t y s o l v e n t s , where c o n t a c t i o n - p a i r s a r e l i k e l y t o form, one might e x p e c t t o o b s e r ve a s p e c i f i c c h a r g e - t r a n s f e r t o t h e c a t i o n , g i v i n g s p e c t r a comparable t o t h o s e o f a l k a l i h a l i d e s i n t h e gas phase. However, such s p e c t r a have n o t been obser v e d f o r alkylammonium i o d i d e s , and c l a i m s t h a t an a b s o r p t i o n band a t 290 my i n b o t h carbon t e t r a c h l o r i d e and benzene a r e due t o such t r a n s i t i o n s 6 5 have been withdrawn, th e t r a n s i t i o n i n carbon t e t r a c h l o r i d e b e i n g r e a s s i g n e d as a c h a r g e - t r a n s f e r p r o c e s s i n v o l v i n g one s o l v e n t m o l e c u l e as a c c e p t o r . 1 2 2 i n s p i t e o f t h e s e l i m i t a t i o n s , t h e s p e c t r a o f i o d i d e s i n l o w - p o l a r i t y s o l v e n t s i • 121,123 u . show good e v i d e n c e f o r i o n - p a i r s , ' though n o t n e c e s s a r i l y c o n t a c t i o n - p a i r s . E v i d e n c e f o r i o n -p a i r i n g i n t h e s e s o l v e n t s i s based on the o b s e r v a t i o n t h a t E i s s o l v e n t and c a t i o n d e p e n d e n t , 1 2 1 E i n max ^ ' max a g i v e n s o l v e n t g e n e r a l l y d e c r e a s i n g w i t h i n c r e a s e i n c a t i o n s i z e , b e i n g s m a l l e r f o r s u b s t i t u t e d alkylammonium s a l t s than a l k a l i - m e t a l i o d i d e s . T h e r e f o r e , i n g e n e r a l , the g r e a t e r t h e s t a b i l i z a t i o n o f the i o n - p a i r due t o e l e c t r o s t a t i c i n t e r a c t i o n , the g r e a t e r i s E m . For max alkylammonium i o n s c o n t a i n i n g a m e t h y l group, e.g., R-jN+—Me, E i s not dependent on R, which i n d i c a t e s 3 max c t h a t t h e s e i o n - p a i r s have s t r u c t u r e s where the c h a r g e -c e n t e r s a r e c l o s e s t , e.g. R 3 N + - M e I - . 1 2 4 ' 1 2 5 Th i n f l u e n c e o f s i z e and shape o f t h e c a t i o n on E and max the r e l a t i v e unimportance o f the n a t u r e o f t h e c a t i o n shows t h a t the c a t i o n i s not a c t i n g as an e l e c t r o n a c c e p t o r . The good c o r r e l a t i o n 1 2 1 between E f o r max i o d i d e i n t e t r a h y d r o f u r a n and t h e sum o f i o n i c r a d i i o p l u s 3 A p o i n t s t o t h e pr e s e n c e o f s o l v e n t - s h a r e d i o n -p a i r s . The o n l y e v i d e n c e p r e s e n t e d t h u s f a r o f d i r e c t c h a r g e - t r a n s f e r t o t h e c a t i o n i s t h a t o f t h e s p e c t r a o f p y r i d i n i u m i o d i d e s , t h e t r a n s f e r r e d e l e c t r o n e n t e r i n g the l o w e s t u n o c c u p i e d i r - l e v e l . ' T h i s p r o c e s s s h o u l d y i e l d E v a l u e s s e n s i t i v e t o t h e s u b s t i t u t i o n on the J max r i n g , s i n c e s u b s t i t u t i o n would a f f e c t m a r k e d l y t h e a c c e p t o r a b i l i t i e s o f t h e r i n g . The h i g h s e n s i t i v i t y of the band positions to the nature of the substituent on the rin g implies strongly that an electron transfer process i s responsible for the absorption bands. As expected for a process whereby an electron i s transferred from an iodide ion, two s i m i l a r bands of 21.8 Kcal m o l e - 1 separation were observed for 1-methylpyridinium iodide i n c h loroform. 5 5 The spectrum of 1-methylpyridinium i o d i d e 5 5 and the positions of the charge-transfer bands 126 for some 1-alkyl-pyridinium iodides are shown i n Table 4 and Figure 8a. The t r a n s i t i o n for 1-methylpyri CH 3 Figure 8. Table 4 Substituent x m a x a ( e m a x ) E T b 4 - C H 3 3590 (1230) 79.64 3 - C H 2 3700 (1310) 77.27 H 3738 (1200) 76.49 3 - C O O C H 2 4070 (1850) 70.25 4-COOCH3 4489 (1230) 63.69 4-CN 4912 (922) 58.20 Figure 8a. E v i d e n c e f o r t h i s t y p e o f p r o c e s s was found i n t h e f l a s h p h o t o l y s i s o f l - e t h y l - 4 - c a r b o m e t h o x y p y r i d i n i u m 2t i o d i d e , 1 2 7 i n which t h e s o l u t i o n was found t o c o n t a i n the l - e h y l - 4 - c a r b o m e t h o x y p y r i d i n y l r a d i c a l . S i n c e c h a r g e - t r a n s f e r r e s u l t s i n a l e s s p o l a r e x c i t e d s t a t e , b l u e s h i f t s o f t h e c h a r g e - t r a n s f e r a b s o r p t i o n o c c u r on c h a n g i n g from a l e s s t o a more p o l a r s o l v e n t . F o r example, a s h i f t from 4489 A i n c h l o r o f o r m 12 8 t o 3311 A i n 7:3 e t h a n o l - w a t e r i s r e p o r t e d f o r t h e c h a r g e - t r a n s f e r band o f 4 - m e t h o x y c a r b o n y l p y r i d i n e e t h i o d i d e . A d i r e c t r e l a t i o n s h i p between E and t h e max W i n s t e i n - G r u n w a l d Y v a l u e s (a k i n e t i c measure o f s o l v e n t i o n i z i n g p o w e r ) 1 2 9 seems t o e x i s t f o r a l k y l p y r i d i n i u m 128 i o d i d e s , whereas t h e e f f e c t o f change o f s o l v e n t i n 63 64 the case o f CTTS i s v a r i a b l e i n magnitude and d i r e c t i o n . ' I n g e n e r a l , i t i s a f a i r l y s i m p l e p r o c e d u r e t o d i s t i n g u i s h between the p r o c e s s e s o f charge t r a n s f e r t o the c a t i o n and CTTS, s i n c e CTTS s p e c t r a a r e found u s u a l l y a t h i g h e r e n e r g i e s , and a l s o have l a r g e r molar e x t i n c t i o n c o e f f i c i e n t s . A comparison can be made o f 1 - m e t h y l p y r i d -i n i u m i o d i d e i n CHC1-. 5 5 (X : 3796, 2945 A; e : 1210, 3 max max 1550-) and t h e i o d i d e i o n i n aqueous o r a l c o h o l i c s o l u t i o n s 1 3 0 (X : 2260, 1940 A; e : 12,600, 12,600), x max ' max though i t s h o u l d be noted t h a t the E v a l u e s i n the max case o f c h a r g e - t r a n s f e r t o t h e c a t i o n a r e o n l y a p parent v a l u e s s i n c e t h e s e a b s o r p t i o n s do not obey Beer's Law. I n h i g h l y p o l a r s o l v e n t s , where i o n - p a i r i n g o f any form i s not f a v o u r e d , one would e x p e c t t o observe o n l y CTTS s p e c t r a . I n l e s s p o l a r s o l v e n t s , where i o n - p a i r s a r e formed i n s o l u t i o n , one would e x p e c t t o observe a b s o r p t i o n s due t o c h a r g e - t r a n s f e r t o t h e c a t i o n as w e l l as CTTS. Thus f a r , however, i n no pure s o l v e n t have s i m u l t a n e o u s bands been o b s e r v e d w h i c h can be s e p a r a t e l y i d e n t i f i e d w i t h i o d i d e i n an i o n - p a i r and i n the f r e e s t a t e . I I . D. C h a r g e - T r a n s f e r S p e c t r a o f t h e M e t h y l p h o s p h o n i - t r i l i u m I o d i d e s . M e t h y l p h o s p h o n i t r i l e s a r e s u f f i c i e n t l y s t r o n g bases t o a l l o w q u a t e r n i z a t i o n by m e t h y l i o d i d e o r o t h e r 11 i • x-j, 42,46 _ . _ . ^ _ a l k y l i o d i d e s , f o r m i n g s a l t s analgous t o tho s e o f 55 the p y r i d i n i u m i o d i d e s . Kosower e t . a l . o b s e r v e d a b s o r p t i o n bands due t o c h a r g e - t r a n s f e r from t h e i o d i d e t o the p y r i d i n i u m c a t i o n . E x c e p t f o r 1 - m e t h y l p y r i d i n i u m i o d i d e i n c h l o r o f o r m , t h e second band e x p e c t e d a t s h o r t e r wave-l e n g t h s f o r t r a n s i t i o n s from the i o d i d e was o n l y p a r t i a l l y v i s i b l e f o r t h e o t h e r a l k y l p y r i d i n i u m i o d i d e s s t u d i e d , owing t o o c c u l t a t i o n by t h e a b s o r p t i o n s o f t h e p y r i d i n i u m r i n g . I n t h e case of t h e m e t h y l p h o s p h o n i t r i l e s , ( N P M e 2 ) n , n = 3,4,5, no a b s o r p t i o n s due t o the p a r e n t compound a r e o b s e r v a b l e down t o 190 my, a l l o w i n g f o r t h e o b s e r v a t i o n o f charge t r a n s f e r a b s o r p t i o n bands, i f p r e s e n t , a t the s h o r t e r w a v e l e n g t h s . T h e r e f o r e , i f c h a r g e - t r a n s f e r t o t h e m e t h y l p h o s p h o n i t r i l i u m c a t i o n s o c c u r , t h e t r a n s i t i o n s would be e x p e c t e d a t s h o r t e r wavelengths t h a n t h o s e f o r the pyridinium c a t i o n s , assuming t h a t t h e t r a n s f e r r e d charge would e n t e r the l o w e s t u n o c c u p i e d l e v e l o f t h e p h o s p h o n i t r i l i u m r i n g . F u r t h e r , i f the charge was t r a n s f e r r e d t o a homomorphic i r - l e v e l , one would e x p e c t an a l t e r n a t i o n i n t h e t r a n s i t i o n e n e r g i e s on g o i n g from + - + -(N-.P3Me_) I t o (Nj.Pj.Me,,) I . V i e w i n g t h e s o l v e n t m o l e c u l e s as c o m p e t i t i v e a c c e p t o r s o f t h e t r a n s f e r r e d • charge, more f r e q u e n t o c c u r r e n c e o f CTTS tha n c h a r g e -t r a n s f e r t o t h e c a t i o n would be e x p e c t e d f o r t h e m e t h y l -p h o s p h o n i t r i l i u m i o d i d e s compared t o t h e p y r i d i n i u m i o d i d e s , as t h e a c c e p t o r - l e v e l e n e r g i e s a r e more comparable i n the former c a s e . Owing t o the t r a n s p a r e n c y o f t h e methylphos-p h o n i t r i l e s a t the s h o r t e r w a v e l e n g t h s , i t was hoped t h a t t h e e l e c t r o n i c s p e c t r a o f t h e m e t h y l p h o s p h o n i t r i l i u m i o d i d e s d e t e r m i n e d i n v a r i o u s s o l v e n t s would y i e l d s p e c t r a due t o b o t h CTTS and c h a r g e - t r a n s f e r t o t h e c a t i o n . The r e l a t i v e t r a n s i t i o n e n e r g i e s f o r t h e s e r i e s ( N ^ M e ^ I t o ( N 5 P 5 M e i ; L ) + I ~ may y i e l d i n f o r m a t i o n c o n c e r n i n g the a c c e p t o r l e v e l s i n the p h o s p h o n i t r i l i c r i n g s . T h i s s t u d y i n c l u d e d 1,2-, 1,3-, and 1 , 4 - d i m e t h y l p y r i d i n i u m i o d i d e s as w e l l as tetra-n-heptylammonium i o d i d e , p r o v i d i n g a b a s i s f o r comparison w i t h t h e s p e c t r a o f the m e t h y l p h o s p h o n i t r i l i u m i o d i d e s as w e l l as e x t e n d i n g ! v. v 4 - T 55,134 p r e v i o u s work by Kosower e t . a l . I I . D . l . E x p e r i m e n t a l The m e t h y l p h o s p h o n i t r i l i u m i o d i d e s can be p r e p a r e d by a p r e v i o u s l y r e p o r t e d m e t h o d . 4 6 ( N 3 P 3 M e 7 ) + I ~ and ( N 5 P 5 M e i : L ) + I ~ were p r e v i o u s l y p r e p a r e d by o t h e r workers i n the l a b o r a t o r y . The compounds 1,2-, 1,3-, and 1 , 4 - d i m e t h y l p y r i d i n i u m i o d i d e s were s y n t h e s i z e d by 4-u * n • 4-w A 47 the f o l l o w i n g method. I I . D . l . a . P r e p a r a t i o n o f the D i m e t h y l p y r i d i n i u m I o d i d e s The m e t h y l p y r i d i n e s used i n t h e p r e p a r a t i o n s were pur c h a s e d from B r i t i s h Drug House L t d . and t h e m e t h y l i o d i d e from F i s h e r S c i e n t i f i c Co. B e f o r e use, the m e t h y l p y r i d i n e s were p u r i f i e d by d i s t i l l i n g from c a l c i u m h y d r i d e w h i l e under an i n e r t atmosphere o f d r y n i t r o g e n . The m e t h y l i o d i d e was d r i e d w i t h m o l e c u l a r s i e v e s . Dry d i e t h y l e t h e r was o b t a i n e d by d i s t i l l i n g i t from l i t h i u m aluminum h y d r i d e under an atmosphere o f d r y n i t r o g e n . 1. 1 , 4 - D i m e t h y l p y r i d i n i u m I o d i d e E xcess m e t h y l i o d i d e (20 ml.) was added t o 4 - m e t h y l p y r i d i n e (1.1755 g) t h r o u g h th e t o p o f a condenser i n s m a l l p o r t i o n s , a l l a i r h a v i n g been f l u s h e d o u t o f the r e a c t i o n f l a s k w i t h d r y n i t r o g e n ; A f l o w o f n i t r o g e n t h r o u g h the system was m a i n t a i n e d d u r i n g t h e r e a c t i o n . The f l a s k was shaken a f t e r each a d d i t i o n o f M e l . A s l i g h t l y e x o t h e r m i c r e a c t i o n o c c u r r e d a l m o s t i n s t a n t a n -e o u s l y , a w h i t e c r y s t a l l i n e compound b e i n g formed. The f l a s k was then wrapped i n t i n - f o i l , the s a l t formed b e i n g l i g h t - s e n s i t i v e , decomposing t o an o r a n g e - c o l o u r e d s o l i d . The e x c e s s Mel was removed by Vacuum and the p r o d u c t washed c o p i o u s l y w i t h d r y d i e t h y l e t h e r under a p r e s s u r e o f d r y n i t r o g e n . A f t e r d r y i n g , t h e c o l o u r l e s s c r y s t a l s o f 1 , 4 - d i m e t h y l p y r i d i n i u m i o d i d e were c o l l e c t e d and s t o r e d i n a v i a l wrapped i n t i n f o i l . The r e s u l t s o f the m i c r o a n a l y s i s were as f o l l o w s : Element C H N Exp e c t e d (%) 35.74 4.25 5.96 " Found (%) 35.51 4.08 5.73 Determined m.p.: 155-157.5°C L i t . v a l u e s : 153.3-154.3°C 1 3 5 157-158°C 1 3 6 2. 1 , 2 - D i m e t h y l p y r i d i n i u m I o d i d e The same p r o c e d u r e as t h a t used i n the p r e p a r a -t i o n o f the 1,4-compound was u t i l i z e d . The p r o d u c t o b t a i n e d was a l s o c o l o u r l e s s but l e s s c r y s t a l l i n e . The r e s u l t s o f t h e m i c r o a n a l y s i s were as f o l l o w s : Element C H N Expected (%) 35.74 4.25 5.96 Found (%) 35.84 4.45 5.77 Determined m.p.: 228-232°C L i t . v a l u e : 229-231°C 1 3 5 3. . 1 , 3 - D i m e t h y l p y r i d i n i u m I o d i d e The same p r o c e d u r e as t h a t used i n t h e p r e v i o u s c a s e s was found t o be - i n a p p l i c a b l e t o 3 - m e t h y l p y r i d i n e . The r e a c t i o n was r e p e a t e d , t h i s t i m e w i t h the 3 - m e t h y l p y r i d i n e d i s s o l v e d i n 20 ml o f d r y d i e t h y l e t h e r b e f o r e t h e a d d i t i o n o f M e l . The p r o d u c t o b t a i n e d was c o l o u r l e s s and resembled t h e o t h e r i o d i d e s . The r e s u l t s o f t h e m i c r o a n a l y s i s were as f o l l o w s : Element C H N Exp e c t e d (%) 35.74 4.25 5.96 Found (%•) 35.53 4.09 5.7 9 Determined m.p.: 97-99°C I I . D.2. S p e c t r o s c o p i c R e s u l t s A l l s o l v e n t s used i n o b t a i n i n g t h e e l e c t r o n i c s p e c t r a were o f S p e c t r o g r a d e q u a l i t y ( M a l l i n c k r o d t ) e x c e p t f o r w a t e r , which was p u r i f i e d by a s p e c i a l p r o c e d u r e d e v i s e d by Dr. J.B. Farmer o f t h i s Department. The s p e c t r a were r e c o r d e d on a Cary 14 Spe c t r o p h o t o m e t e r u s i n g a p a i r o f matched 1-cm q u a r t z c e l l s . T e t r a - n - h e p t y l -ammonium I o d i d e was purchased from Eastman O r g a n i c C h e m i c a l s and used w i t h o u t f u r t h e r p u r i f i c a t i o n . The r e s u l t s o f the s p e c t r o s c o p i c measurements ar e t a b u l a t e d i n T a b l e 5, a l o n g w i t h the assignment o f the o b s e r v e d a b s o r p t i o n bands. T a b l e 5. Compound S o l v e n t x a max ( £max» Assignment 1 , 2 - d i m e t h y l p y r i d i n i u m i o d i d e C H 2 C 1 2 363.5 292.5 (990) (1530) CT t o c a t i o n 1 , 3 - d i m e t h y l p y r i d i n i u m i o d i d e CH2 C12 367. 0 (1735) CT t o c a t i o n 1 , 4 - d i m e t h y l p y r i d i n i u m i o d i d e C H 2 C 1 2 359.5 304. 0 (1469) (1670) CT t o c a t i o n ( N 3 P 3 M e ? ) + I ~ H 2 ° 226.0 194.0 CTTS C H 2 C 1 2 246.0 CTTS CHgCN 246 .0 CTTS CHC1 3 245.0 292.0 363 .0 CTTS CT t o c a t i o n ( N 4 P 4 M e 9 ) + I - H 2 ° 226.0 193.0 CTTS C H 2 C 1 2 246.0 CTTS CHC1 3 243. 0 361. 0 s h o u l d e r a t 290-300 CTTS CT t o c a t i o n T a b l e 5 (cont'd) Compound S o l v e n t Amax a ( e m a x ) b Assignment ( N 5 P 5 M e i : L ) + l " [ C H 3 ( C H 2 ) g ] 4 N + I ~ CHC1 3 C H 3 OH CHC1 3 242.0 370.0 s h o u l d e r a t 290-300 220.5 244.0 364. 0 CTTS CT t o c a t i o n CTTS CTTS CT t o c a t i o n a I n my. (±5) b M o l a r e x t i n c t i o n c o e f f i c i e n t s . The a b s o r p t i o n spectrum o f ( N 3 P 3 M e 7 ) + I ~ i n CHC1 3 i s shown i n F i g u r e 9. FIG.9: Charge-transfor spectrum of ( N 3 P 3 M e ? ) I Concentration- saturated Solvent: C H C l 3 O I I . D.3. D i s c u s s i o n The c h a r g e - t r a n s f e r a b s o r p t i o n s o f t h e d i m e t h y l p y r i d i n i u m i o d i d e s i n C H 2 C 1 2 • a r e comparable t o t h o s e o f Kosower and S k o r c z measured i n CHCl^. T h e i r r e s u l t s a r e g i v e n i n T a b l e 6. T a b l e 6 D i m e t h y l p y r i d i n i u m I o d i d e s S u b s t i t u e n t X a ; b A e max max 1-Me 3738 1200 1.2- Me 2 3640 860 1.3- Me 2 3700 1310 1.4- Me 2 3590 1230 a Second maxima not g i v e n . b Determined i n CHC1 3. The r e p l a c e m e n t o f a n u c l e a r hydrogen atom by a m e t h y l group s h i f t s t h e maximum t o s h o r t e r w a velengths from t h a t o f 1 - m e t h y l p y r i d i n i u m i o d i d e , but the e x t e n t o f the change depends markedly upon the p o s i t i o n o f the a d d i t i o n a l m e t h y l group. The same o r d e r o f t r a n s i t i o n e n e r g i e s f o r t h e d i m e t h y l p y r i d i n i u m i o d i d e s was found i n C H 2 C 1 2 as i n C H C 1 3 . 1 3 4 1,4-dimethyl > 1 , 2 - d i m e t h y l > 1, 3 - d i m e t h y l The g e n e r a l i n c r e a s e i n t r a n s i t i o n e n e r g i e s upon a d d i n g a m e t h y l group t o t h e r i n g i s e x p l i c a b l e i n terms o f s t a b i l i z a t i o n o f t h e c a t i o n by t h e e l e c t r o n -r e l e a s i n g e f f e c t s o f an a l k y l group. The r e l a t i v e e n e r g i e s o f t r a n s i t i o n i n t h e d i m e t h y l p y r i d i n i u m i o d i d e s a r e a p p r o x i m a t e l y e x p l i c a b l e i n terms o f c a l c u l a t e d r r - e l e c t r o n d e n s i t i e s a t t h e 2-, 3-, and 4 - p o s i t i o n s i n p y r i d i n e (0.943, 0.991, and 0.950 r e s p e c t i v e l y ) 1 3 7 , t h e i n c r e a s e d p o s i t i v e charge a t t h e 2- and 4- p o s i t i o n s r e s u l t i n g i n g r e a t e r i n t e r a c t i o n w i t h t h e m e t h y l s u b s t i t u e n t , (though the s i m p l e r r - e l e c t r o n e f f e c t would r e v e r s e the o r d e r o f the 1,2- and 1 , 4 - d i m e t h y l p y r i d i n i u m i o d i d e s ) . A l i k e l y a d d i t i o n a l e f f e c t i s t h a t s t e r i c s t r a i n g e n e r a t e d by t h e i n t e r a c t i o n o f t h e 2 - a l k y l group w i t h t h e i o d i d e i o n d e s t a b i l i z e s t h e c a t i o n and hence reduces t h e t r a n s i t i o n energy f o r 1 , 2 - d i m e t h y l -p y r i d i n i u m i o d i d e . The combined e f f e c t s o f T r - e l e c t r o d e n s i t i e s and o f s t e r i c s t r a i n c o u l d t h e r e f o r e e x p l a i n q u a l i t a t i v e l y t h e o b s e r v e d r e l a t i v e t r a n s i t i o n e n e r g i e s o f t h e d i m e t h y l p y r i d i n i u m i o d i d e s . The r e l a t i v e t r a n s i t i o n e n e r g i e s o f t h e d i m e t h y l -p y r i d i n i u m i o d i d e s a r e i n t h e same o r d e r as t h e i r r e d u c t i o n p o t e n t i a l s i n water (see p a r t I I . E ) . The f a c t o r o f s t e r i c s t r a i n due t o i n t e r a c t i o n o f t h e 2 - a l k y l group w i t h the i o d i d e i o n cannot be c o n s i d e r e d i n the case o f t h e p o l a r o g r a p h i c e x p e r i m e n t s , however, s i n c e t h e i o n s a r e e x p e c t e d t o be f u l l y s o l v a t e d i n water. The c o r r e l a t i o n between t h e e n e r g i e s o f t r a n s i t i o n and r e d u c t i o n p o t e n t i a l s p r o v i d e e v i d e n c e f o r t h e t r a n s f e r o f charge t o t h e l o w e s t u n o c c u p i e d - r r-levels o f t h e p y r i d i n i u m r i n g s . The r e l a t i v e e n e r g i e s o f t h e a c c e p t o r l e v e l s i n the t h r e e d i m e t h y l p y r i d i n i u m i o n s a re t h e r e f o r e w e l l e s t a b l i s h e d , even though q u a n t i t a t i v e agreement i s s t i l l i n c o m p l e t e . None o f t h e i o d i d e s s t u d i e d were s o l u b l e enough i n CC1 4 f o r t h e i r s p e c t r a t o be d e t e r m i n e d i n t h i s s o l v e n t . I n the more p o l a r s o l v e n t s , H 20, CH 3OH, and CH^CN, o n l y CTTS s p e c t r a were o b t a i n a b l e , t h e A J max v a l u e s d e t e r m i n e d b e i n g i n agreement w i t h p r e v i o u s r e s u l t s found f o r s o l v a t e d i o d i d e 1 n s (s e e T a ID 1 e 3 ) I n C H ^ C l o n l y one a b s o r p t i o n a t 246 my was o b s e r v e d — f o r (N^P^Me^) I and (N^P^Me^) I , d e f i n i t e l y due t o CTTS by c o n s i d e r a t i o n o f t h e energy o f t r a n s i t i o n , t h u s i n d i c a t i n g l i t t l e o r no i o n — p a i r i n g i n t h i s s o l v e n t . I n the s l i g h t l y p o l a r s o l v e n t CHC1 3 and u s i n g more concen-t r a t e d s o l u t i o n s , however, weak a b s o r p t i o n s a t l o n g e r w avelengths i n a d d i t i o n t o t h o s e due t o CTTS appear i n the s p e c t r a o f t h e m e t h y l p h o s p h o n i t r i l i u m and t e t r a - n -heptylammonium i o d i d e s , t h e s e new a b s o r p t i o n s b e i n g i n the same r e g i o n as t h o s e due t o c h a r g e - t r a n s f e r i n . t h e case o f t h e p y r i d i n i u m i o d i d e s . The spectrum o f t e t r a -n-butylammonium i o d i d e i n CHCl^ as r e p o r t e d by G r i f f i t h s 65 and Symons showed o n l y t h e 245 my band due t o CTTS, no a b s o r p t i o n s b e i n g r e p o r t e d i n the l o n g e r w a v e l e n g t h r e g i o n s . They r e p o r t e d , however, a b s o r p t i o n a t 290 my i n the s o l v e n t s C rH,, E t o 0 , and t e t r a - h y d r o f u r a n , a s c r i b e d b o A t o the f o r m a t i o n o f i o n - p a i r s . The band a t 364 my ob s e r v e d i n the spectrum o f tetra-n-heptylammonium i o d i d e i n CHC1 3 t h e r e f o r e appears t o be t h e o t h e r h a l f o f t h e ex p e c t e d d o u b l e t f o r i o d i d e i n an i o n - p a i r , t h e 290 my band b e i n g o b s c u r e d by t h e i n t e n s e CTTS a b s o r p t i o n . The p a r t i a l f o r m a t i o n o f i o n - p a i r s i n t h e s o l v e n t CHC1 3 f o r te t r a - a l k y l a m m o n i u m i o d i d e s i s t h e r e f o r e p o s t u l a t e d , a new i n t e r p r e t a t i o n b e i n g g i v e n t o t h e r e s u l t s o f G r i f f i t h s and Symons. 6 5 For ( N 3 P 3 M e 7 ) + I ~ , t h e two a b s o r p t i o n s a t 292 and 363 my g i v e an energy d i f f e r e n c e o f 19.2 K c a l mole 1 , i d e n t i f y i n g p o s i t i v e l y t h e i o d i d e i o n as t h e o r i g i n o f t h e t r a n s i t i o n . F or ( N 4 P 4 M e 9 ) + I _ and ( N ^ M e ^ ) + I ~ , the second band e x p e c t e d i n the 2 90-3 00 my r e g i o n a r e seen o n l y as s h o u l d e r s on the CTTS band. S i n c e t h e e n e r g i e s d e r i v e d from t h e p o s i t i o n s o f t h e f i r s t A do not show max any c o r r e l a t i o n w i t h t h e i r-systems o f t h e methylphospho-n i t r i l e s f charcje tr.ansf e r t o the u n o c c u p i e d TT l e \ r e l s cannot be p o s t u l a t e d . The s i m i l a r e n e r g i e s o f t r a n s i t i o n i n t h e case of t e t r a n hepty1 amnionium i o d i d e / i n which no TT l e v e l s e x i s t s a l s o sucjc^est t h e f o r m a t i o n o f s o l v e n t s h a r ed i o n - p a i r s , i n which th e e n e r g i e s o f t r a n s i t i o n a r e o n l y s l i g h t l y a f f e c t e d by t h e c a t i o n . S i n c e a l l t h e bands a s c r i b e d t o c h a r g e - t r a n s f e r w i t h t h e i o d i d e i n a s o l v e n t s h a r e d i o n p a i r a r e i n t h e saine r e c j i o n ^ one may expanded o r b i t a l on t h e c a t i o n s which i s c e n t e r e d on t h e n i t r o g e n atom. The c o r r e s p o n d i n g a b s o r p t i o n s f o r 1—methyl— p y r i d i n i u m i o d i d e i n C H C l ^ 5 5 o c c u r a t 379.6 and 294.5 my, b e i n g a t l o n g e r w a velengths t h a n t h o s e o f t h e m e t h y l -p h o s p h o n i t r i l i u m or tetra-n-heptylammonium i o d i d e s , g i v e c o n f i r m a t i o n o f the b e t t e r a c c e p t o r p r o p e r t i e s o f t h e p y r i d i n i u m c a t i o n s . The p o s s i b i l i t y i s r a i s e d o f t h e p y r i d i n i u m i o d i d e s i n CHCl^ and C B ^ C ^ e x i s t i n g as s o l v e n t -s h a r ed i o n — p a i r s . As shown by t h e s i m u l t a n e o u s measurement o f s p e c t r a due t o b o t h CTTS and c h a r g e - t r a n s f e r t o the c a t i o n f o r t e t r a a l k y l a m m o n i u m as w e l l as t h e m e t h y l -p h o s p h o n i t r i l i u m i o d i d e s , i o n - p a i r i n g i s not complete i n CHC1 3, as p r e v i o u s l y thought f o r t h e p y r i d i n i u m i o d i d e s . 1 2 8 T h i s i s t h e f i r s t r e p o r t o f c h a r g e - t r a n s f e r a b s o r p t i o n s i n a pure s o l v e n t which can be s e p a r a t e l y i d e n t i f i e d w i t h i o d i d e i n an i o n - p a i r and i n the f r e e s t a t e . I t i s u n c e r t a i n why no a b s o r p t i o n s were ob s e r v e d by G r i f f i t h s and Symons 6 5 f o r (n-Bu).NI i n such s o l v e n t s as CC1 4 and t e t r a h y d r o f u r a n i n t h e 360 my r e g i o n , t h e p o s i t i o n o f the o t h e r h a l f o f t h e e x p e c t e d d o u b l e t , when a b s o r p t i o n s were r e c o r d e d i n t h e 290 my r e g i o n . The f o l l o w i n g t y p e o f e q u i l i b r i u m p r o c e s s would seem b e s t t o d e s c r i b e t h e ground and e x c i t e d s t a t e s o f , f o r example, ( N 3 P 3 M e y ) + l i n CHC1 3, based on models ™ „ , ™ - , „ 63,64,128 + I s o l v s o l v e n t - s h a r e d i o n p a i r f r e e i o n s [ ( N 3 P 3 M e 7 - ) C H C l 3 ( I . ) ] s o l v ( I - e ) s o l v I I . E. P o l a r o g r a p h i c R e d u c t i o n o f M e t h y l p h o s p h o n i t r i l i u m  and D i m e t h y l p y r i d i n i u m I o d i d e s A t t e m p t s have been made t o reduce p h o s p h o n i t r i l i c compounds p o l a r o g r a p h i c a l l y i n nonaqueous media t o y i e l d A- i 138,139 , • . , . ' ' ' r a d i c a l a n i o n s , w h i c h can t h e n be s t u d i e d by e l e c t r o n s p i n r e s o n a n c e t e c h n i q u e s . I t has been found t h a t t r i m e r i c and t e t r a m e r i c p h o s p h o n i t r i l e s w h i c h c o n t a i n o n l y f l u o r o , c h l o r o , bromo, t r i f l u o r o e t h o x y , methoxy, phenoxy, g - d i o x y p h e n y l , o r phenylamino s u b s t i t u e n t s cannot be r e d u c e d a t p o t e n t i a l s more p o s i t i v e t h a n -3V. However, s p e c i e s c o n t a i n i n g p h e n y l , . p - n i t r o p h e n o x y , 2 , 3 - d i o x y n a p t h y l ( 1 ) , and 1 , 8 - d i o x y n a p t h y l (2) s i d e groups can be r e d u c e d i n t h e -1 t o -3V r a n g e , and e s r s p e c t r a can be d e t e c t e d . J 3 I n t h e l a t t e r t h r e e s p e c i e s , however, i t i s t h e s u b s t i t u e n t s w h i c h become re d u c e d , w i t h t h e e s r h y p e r f i n e s p l i t t i n g s o b s e r v e d b e i n g c h a r a c t e r i s t i c o f the o r g a n i c r a d i c a l a n i o n . I n t h e case o f ( N P P h 2 ) 3 and ( N P P h 2 ) 4 , r e d u c t i o n o c c u r s a t -2.65 V (even though benzene and t o l u e n e cannot be reduced a t t h i s p o t e n t i a l ) , and an u n r e s o l v e d s i n g l e t e s r spectrum i s o b s e r v e d . I t i s assumed t h a t e x t e n s i v e e l e c t r o n s p i n d e r e a l i z a t i o n o c c u r s o r t h a t r a p i d e l e c t r o n exchange t a k e s p l a c e . The e q u i v a l e n t r e d u c t i o n p o t e n t i a l o f b o t h compounds s u g g e s t s t h a t the l o w e s t a n t i b o n d i n g o r b i t a l l e v e l s i n t h e p h o s p h o n i t r i l i c r i n g a r e u n a f f e c t e d by r i n g s i z e . The c o n j u g a t i o n o f t h e i r-system o f t h e p e n t a f l u o r o p h e n y l group w i t h a homomorphic Tr-system i n t h e p h o s p h o n i t r i l i c irincf has tooen shown by F ninr s p e c t r a , 1 4 0 t h e s e i n t e r -a c t i o n s b e i n g p o s s i b l y r e s p o n s i b l e f o r a l t e r i n g t h e l e v e l s i n t h e p h o s p h o n i t r i l i c r i n g , r e s u l t i n g t h e r e f o r e i n a l ower r e d u c t i o n p o t e n t i a l i n t h e p h e n y l p h o s p h o n i t r i l e s . S i n c e a p o s i t i v e charge on t h e r i n g i s e x p e c t e d t o lower t h e r e d u c t i o n p o t e n t i a l s r e q u i r e d , t h e attempt was made t o reduce p o l a r o g r a p h i c a l l y t h e s e r i e s o f m e t h y l p h o s p h o n i t r i l i u m i o d i d e s i n aqueous s o l u t i o n . M a i n l y f o r comparison w i t h t h e p h o s p h o n i t r i l i u m i o d i d e s , the r e d u c t i o n p o t e n t i a l s o f t h e s e r i e s o f d i m e t h y l p y r i d i n i u m i o d i d e s were a l s o d e t e r m i n e d . E x p e r i m e n t a l R e d u c t i o n p o t e n t i a l s were d e t e r m i n e d on a Heath Dropping Mercury E l e c t r o d e Model EU A-19-6 i n water w h i c h was s p e c i a l l y p u r i f i e d . D i s s o l v e d oxygen w h i c h may be reduced was removed by b u b b l i n g d r y n i t r o g e n t h r o u g h t h e s o l u t i o n f o r a t l e a s t o n e - h a l f hour p r i o r t o e x p e r i m e n t a t i o n . A c o n s t a n t stream o f d r y n i t r o g e n was m a i n t a i n e d o v e r t h e s o l u t i o n t h r o u g h o u t t h e e n t i r e s t a g e s o f t h e e x p e r i m e n t . The s u p p o r t i n g e l e c t r o l y t e a t a c o n c e n t r a t i o n o f 1 0 _ 1 M was KC1, w i t h t h e i o d i d e s a t an approximate c o n c e n t r a t i o n o f 10 3M. R e s u l t s and D i s c u s s i o n I t was found t h a t none o f t h e methylphospho-n i t r i l i u m i o d i d e s were r e d u c i b l e w i t h i n t h e c a p a b i l i t i e s • o f t h i s p o l a r o g r a p h (^-2.3 V) whereas the d i m e t h y l -p y r i d i n i u m i o d i d e s were found t o reduce i r r e v e r s i b l y . The h a l f - w a v e p o t e n t i a l s a r e as f o l l o w s : T a b l e 7 Half-Wave P o t e n t i a l s Compound 1.2- d i m e t h y l p y r i d i n i u m 1.3- d i m e t h y l p y r i d i n i u m 1.4- d i m e t h y l p y r i d i n i u m i o d i d e -1.96 V i o d i d e -1.93 V i o d i d e -2.13 V From t h e s e r e s u l t s , i t may be c o n c l u d e d t h a t t h e empty a n t i b o n d i n g l e v e l s i n t h e p h o s p h o n i t r i l i c r i n g s l i e a t h i g h e r e n e r g i e s t h a n t h o s e i n t h e p y r i d i n e r i n g . The s l i g h t v a r i a t i o n i n v a l u e s i n the t h r e e d i m e t h y l -p y r i d i n i u m i o d i d e s can be q u a l i t a t i v e l y r a t i o n a l i z e d i n terms o f the i n d u c t i v e e f f e c t s o f the m e t h y l group a t the 2-, 3-, and 4 - p o s i t i o n s r e s p e c t i v e l y , the same arguments as were used i n the d i s c u s s i o n o f t h e r e l a t i v e e n e r g i e s o f c h a r g e - t r a n s f e r t o t h e c a t i o n (see I I . D) b e i n g s t i l l v a l i d . The c o r r e l a t i o n between t h e e n e r g i e s o f c h a r g e - t r a n s f e r and t h e r e d u c t i o n p o t e n t i a l s o f the d i m e t h y l p y r i d i n i u m i o d i d e s , b e i n g i n t h e o r d e r 1,4-d i m e t h y l > 1 , 2 - d i m e t h y l > 1 , 3 - d i m e t h y l , shows t h a t , r e g a r d l e s s o f the e x a c t d e s c r i p t i o n , t h e charge i s t r a n s f e r r e d e i t h e r e l e c t r o l y t i c a l l y o r p h o t o l y t i c a l l y t o the same ty p e o f v a c a n t o r b i t a l . CHAPTER I I I DONOR-ACCEPTOR CHARGE-TRANSFER COMPLEXES I I I . A. I n t r o d u c t i o n In 1949, Benesi and Hildebrand 1 4''" r e p o r t e d t h a t s o l u t i o n s c o n t a i n i n g an aromatic hydrocarbon and i o d i n e had an e l e c t r o n i c a b s o r p t i o n band not presen t i n e i t h e r component 142 alone. M u l l i k e n was the f i r s t to r e c o g n i z e t h a t the c h a r a c t e r i s t i c a b s o r p t i o n of such s o l u t i o n s c o u l d a r i s e through an i n t e r m o l e c u l a r c h a r g e - t r a n s f e r t r a n s i t i o n . The complexes were d e s c r i b e d i n g e n e r a l as being formed by the weak i n t e r a c t i o n between e l e c t r o n donors and e l e c t r o n a c c e p t o r s , i n v o l v i n g u s u a l l y simple i n t e g r a l r a t i o s of the components. The term complex i n t h i s case i s taken to mean a substance formed by the i n t e r a c t i o n of two or more components which can r e v e r s i b l y d i s s o c i a t e i n t o i t s compo-nents under such c o n d i t i o n s as being i n the vapor phase or on d i s s o l u t i o n . A ( g ) + n B ( g ) A B r i ( g ) A ( s o l v ) + n B ( s o l v ) ' ^ ^ ' " ( s o l v ) T h i s d e f i n i t i o n s u g g e s t s t h a t AH o f complex f o r m a t i o n i s v e r y s m a l l and t h a t c o v a l e n t bonding does not p l a y a s t r o n g r o l e i n t h e ground s t a t e of t h e complex. The t y p e s o f donors used i n t h e s t u d i e s o f t h e s e complexes can be p l a c e i n two g e n e r a l c a t e g o r i e s : (1) TT donors a r e compounds i n which th e e l e c t r o n s donated are t h o s e c o n t a i n e d i n TT m o l e c u l a r o r b i t a l s and t h e adducts t h e y form a r e c a l l e d T r - c o m p l e x e s . 1 4 3 They i n c l u d e u n s a t u r a t e d compounds such as a l k e n e s , a l k y n e s , and a r o m a t i c r i n g systems. (2) n donors a r e compounds i n w h i c h non-bonded e l e c t r o n s a r e a v a i l a b l e f o r d o n a t i o n . They i n c l u d e such compounds as a l c o h o l s , s u l f i d e s , i o d i d e s , n i t r o g e n bases, and h e t e r o c y c l i c r i n g systems, where i f t h e y a r e a l s o a r o m a t i c the l o n e p a i r s a r e donated i n p r e f e r e n c e t o t h e i T - e l e c t r o n s . ^ T h e a c c e p t o r s used i n the s t u d i e s a r e m a i n l y the h a l o g e n s , e s p e c i a l l y i o d i n e , b u t a l s o i n c l u d e a g r e a t number o f more uncommon compounds such as t e t r a c y a n o e t h y l e n e . 1 4 5 I n the s t u d y of t h e s e complexes, the donor and a c c e p t o r a r e u s u a l l y s i m p l y mixed i n s o l u t i o n , t h e r e s u l t a n t c o l o u r change b e i n g u s u a l l y i n d i c a t i v e o f complex f o r m a t i o n . I t i s now g e n e r a l l y a c c e p t e d t h a t t h e i n t e n s e c o l o u r o f t h e s e complexes i s t o be a s s o c i a t e d w i t h t h e t r a n s f e r o f an e l e c t r o n from th e donor t o t h e a c c e p t o r . M , , 142,146 . . ^ t M u l l i k e n was t h e f i r s t t o p u t t h i s i n t o quantum m e c h a n i c a l language. I n h i s v a l e n c e bond t r e a t m e n t o f complex f o r m a t i o n , he c o n s i d e r e d t h e i n t e r a c t i o n of a no-bond ground s t a t e T(D,A) and a p o l a r e x c i t e d s t a t e + -m(D -A ), where D i s the donor and A the a c c e p t o r , t o produce a s t a b i l i z e d ground s t a t e h a v i n g a wave f u n c t i o n ¥ Q = f(D,A) + AY(D +-A ) (1) where X and y w i l l m most cases be s m a l l compared w i t h u n i t y , The no-bond ground s t a t e wave f u n c t i o n d e s c r i b e s . a s t r u c t u r e i n w h i c h b i n d i n g r e s u l t s o n l y from such • ' p h y s i c a l ' f o r c e s as van der Waals i n t e r a c t i o n s . The e x c i t e d s t a t e wave f u n c t i o n d e s c r i b e s a s t r u c t u r e i n w h i c h one e l e c t r o n has been t r a n s f e r r e d from th e donor t o t h e a c c e p t o r . I n the case where D and A a r e b o t h n e u t r a l , t h i s w i l l i n v o l v e c o v a l e n t bonding between the odd e l e c t r o n s i n A~ and D +. T h i s bonding i n g e n e r a l w i l l be weak however because o f t h e l a r g e i n t e r n u c l e a r d i s t a n c e . The c h a r g e -t r a n s f e r band i s . a s s o c i a t e d w i t h t h e e l e c t r o n i c t r a n s i t i o n ^0 ~* f l ' o c c u r r i n g a t t h e f r e q u e n c y v = ( E 1 - E Q ) / h , where h i s P l a n c k ' s c o n s t a n t . As a f i r s t a p p r o x i m a t i o n , the energy of c h a r g e -t r a n s f e r , h v C T , from a donor w i t h an i o n i z a t i o n p o t e n t i a l , I D , t o an a c c e p t o r w i t h an e l e c t r o n a f f i n i t y , E^, i s • • • • where A, assumed t o be c o n s t a n t i n a s e r i e s of r e l a t e d compounds, i s the d i f f e r e n c e between the b i n d i n g e n e r g i e s o f the components i n t h e ground and t h e e x c i t e d s t a t e s . For a s e r i e s o f m o l e c u l a r complexes i n v o l v i n g e l e c t r o n donors and the same e l e c t r o n a c c e p t o r , such as I 2 , i t has been shown t h a t t h e r e i s a r e a s o n a b l e l i n e a r c o r r e l a t i o n between the i o n i z a t i o n p o t e n t i a l of t h e donor and t h e f r e q u e n c y of the c h a r g e - t r a n s f e r b a n d . 1 4 7 " 1 5 0 T h i s can be seen i n t h e f o l l o w i n g f i g u r e f o r complexes o f I 0 . 2.51 : I I I 1 — i i i i 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 115 /D(eV) Figure 12 (From r e f . 154(a)). For compounds whose d i r e c t i o n i z a t i o n p o t e n t i a l s are not yet av a i l a b l e , polarographic oxidation may provide a measure of the i o n i z a t i o n p o t e n t i a l of a compound. For a series of chemically r e l a t e d compounds, values of e ° X for donor molecules, obtained from polarographic measure¬ments of a c e t o n i t r i l e solutions, when plotted against v_,m CT values for complexes of a common acceptor, y i e l d s t r a i g h t l i n e s . 2-8 2-4 2 0 0-8 10 13 !-1 I S £ ? X / V (donor) 5 Figure 13 (From r e f . 151). 68 S i m i l a r l y , for a p a r t i c u l a r donor, the frequency of the charge-transfer band i s proportional to the electron a f f i n i t y of the acceptor i n a re l a t e d s e r i e s , but the p r o p o r t i o n a l i t y constants, however, do not always have the expected value of unity. For example, Av/AI D for the case of iodine and a series of saturated, o l e f m i c , A 148 . n c n ' , and aromatic donors i s 0.67, and for tetracyanoethylene with a series of methylbenzenes, 1 4 9 the value i s 0.49. Apart from the uncertainties i n the values of the i o n i z a t i o n p o t e n t i a l s , s t e r i c factors seem to be important. In the series of unsubstituted p o l y c y c l i c aromatic hydrocarbons, where s t e r i c factors are less important, slopes close to unity are obtained with a number of acceptors. I I I . A . l . D e t e r m i n a t i o n o f the P o s i t i o n s o f E q u i l i b r i u m Many s t u d i e s have been made o f t h e e q u i l i b r i a between c h a r g e - t r a n s f e r complexes and t h e i r f r e e component m o l e c u l e s i n s o l u t i o n . A v a r i e t y o f methods such as u l t r a v i o l e t and v i s i b l e s p e c t r o p h o t o m e t r y , i n f r a r e d s p e c t r o -scopy, and n u c l e a r magnetic resonance has been used. The r e s u l t s o b t a i n e d a r e then used t o d e t e r m i n e th e c o r r e s p o n d i n g e n t h a l p y and e n t r o p y changes o f complex f o r m a t i o n . By f a r t h e most o f t e n u t i l i z e d method of e q u i l i -b r ium c o n s t a n t d e t e r m i n a t i o n r e l i e s upon the c h a n g i n g i n i n t e n s i t y o f t h e c h a r g e - t r a n s f e r band w i t h c o n c e n t r a t i o n o f the donor o r a c c e p t o r . A b r i e f o u t l i n e o f t h i s method i s g i v e n . Method of B e n e s i a n d ' H i l d e b r a n d 1 4 1 For t h e r e a c t i o n D + A r ^ C , K = —=r-= (3) C D C A where C c f o r example i s the c o n c e n t r a t i o n o f the complex a t e q u i l i b r i u m . S i n c e t h e absorbance A due t o C i n a r e g i o n where c th e complex absorbs i s g i v e n by Beer's Law, A c i c c c (4) _ A c must a l s o i n c r e a s e as C q i n c r e a s e s . From the absorbance A c a t a s e t wa v e l e n g t h f o r a s e r i e s o f c o n c e n t r a t i o n s C , w i t h C A k e p t f i x e d and s m a l l r e l a t i v e t o C N , K and e, can A D A be o b t a i n e d f o r the e q u i l i b r i u m p r o c e s s by the method o f B e n e s i and H i l d e b r a n d . The B e n e s i - H i l d e b r a n d a n a l y s i s 1 4 1 s t a r t s from the assumption t h a t o n l y one e q u i l i b r i u m e x i s t s i n the s o l u t i o n . Then, u s i n g a zer o s u b s c r i p t t o denote t h e t o t a l c o n c e n t r a t i o n , we have c \ c / K C ~^2 J -D " ( CA°- Cc» ™ c \ c / As s t a t e d , C D° i s much g r e a t e r t h a n C A ° , f o r ' s m a l l K, t h e amount o f complex f o r m e d , i s a l s o s m a l l ( i . e . C D° >> C A° > c c ) . The term (C A°-C c) can t h e r e f o r e be n e g l e c t e d . U s i n g t h e e x p r e s s i o n f o r CQ from Beer's Law, the e q u a t i o n then becomes 1 EA 1 K = "AT" C D ° C A ° " CD° • ( 6 ) D i v i d i n g t h r o u g h by e and r e a r r a n g i n g , one o b t a i n s t h e B e n e s i - H i l d e b r a n d e q u a t i o n I C ° Ke. V + (7) I n a C D° a r e known; A q i s measured f o r g i v e n e x p e r i m e n t , t h e v a l u e s o f l , C A ° , and a s e r i e s o f s o l u t i o n s w i t v a r y i n g C ° , E^ L. i s t h e n p l o t t e d a g a i n s t ^ t o o b t a i n A C D v a l u e s o f K and e A . An example o f a p l o t f o r t h e complex t r i e t h y l a m i n e - I 2 i s g i v e n . 1 5 2 0 4 8 12 16 20 24 28 32 1 0 _ , / C D . F i g u r e 14. I l l u s t r a t i o n o f t h e use o f t h e B e n e s i - H i l d e b r a n d e q u a t i o n t o o b t a i n K and e a t f o u r w a v e l e n g t h s f o r t h e t r i e t h y l a m i n e - I 2 complex. (From r e f . 152) K c and e m a x v a l u e s d e t e r m i n e d f o r n-donors w i t h th e a c c e p t o r i 2 range from 0.5 - 7,500 and 3000 - 30,000 r e s p e c t i v e l y , whereas complexes o f .-donors w i t h l 2 have K Q a n d £max v a l u e s o f o n l Y 0- 1 " 2.0 and 5000 - 15,000 r e s p e c t i v e l y . I f K can be measured, t h e o t h e r thermodynamic p r o p e r t i e s t h a t a r e a s s o c i a t e d w i t h complex f o r m a t i o n can be o b t a i n e d by u s i n g well-known thermodynamic r e l a t i o n s h i p s . I f t h e a c t i v i t i e s a r e approximated as c o n c e n t r a t i o n s , then AG° = -RT £n K (8) c and AH° and AS° can be o b t a i n e d by t h e use of t h e e q u a t i o n Assuming t h a t AH° w i l l be constant over the temperature range s t u d i e d , a p l o t of Jin K versus ijj- w i l 1 — AH ° y i e l d a s t r a i g h t l i n e of slope R and whose i n t e r c e p t i s AS° R. * When one r e f e r s t o t h e " s t a b i l i t y " o f t h e complex, what i s meant i s the magnitude o f AH°. From r e s u l t s o b t a i n e d i t i s e v i d e n t t h a t as t h e complex becomes more s t a b l e , t h e d e c r e a s e i n e n t r o p y due t o t h e l o s s o f freedom as D and A combine to form C also tends to become larger. I t has been found empirically that -AS° i s a l i n e a r function of -AH° for most complexes. The two functions AH° and AS° have opposing e f f e c t s on the value of AG0 so that as AH0 becomes more negative the corresponding decrease i n entropy prevents AG° from becoming negative as r a p i d l y as does AH0 expected mu * 11 • 1 4-154b . • , 1 4.1, 4. A 1 4 - -The following p l o t shows c l e a r l y the expected r e l a t i o n -ship between the two functions. 20 -19 i—i—I—I—I—I—I—I—I—I—t~jr, . • Figure 15. V a r i a t i o n of AS° with AH0 for iodine complexes i n various solvents. (From r e f . 154b). I I I . A.2. C h a r g e - T r a n s f e r Complexes o f I o d i n e I n a d d i t i o n t o the appearance o f a c h a r g e -t r a n s f e r band upon complex f o r m a t i o n , the e l e c t r o n i c s p e c t r a of complexes w i t h i o d i n e as a c c e p t o r show a s h i f t o f the v i s i b l e a b s o r p t i o n band o f i o d i n e t o h i g h e r e n e r g i e s 1 5 5 , 1 5 6 accompanied by an i n c r e a s e i n i t s i n t e n s i t y . T h i s s h i f t o f the v i s i b l e band i s o f t e n l a r g e enough t o make p o s s i b l e s p e c t r o p h o t o m e t r i c d e t e r m i n a t i o n s o f thermodynamic c o n s t a n t s o f complexes f o r w h i c h t h e c h a r g e - t r a n s f e r band i s not o b s e r v e d e i t h e r because i t i s too f a r i n t h e u l t r a v i o l e t o r i t i s o v e r l a p p e d by a donor band. The same t e c h n i q u e s as a r e used f o r the c h a r g e - t r a n s f e r band can be a p p l i e d t o the s h i f t e d band. The s h i f t i s due t o the r a i s i n g o f t h e energy o f the l o w e s t u n o c c u p i e d o r b i t a l of i o d i n e on complex f o r m a t i o n , t h i s o r b i t a l s e r v i n g as b o t h the upper l e v e l o f the i o d i n e a b s o r p t i o n i n t h e v i s i b l e and t h e a c c e p t o r l e v e l o f t h e c h a r g e - t r a n s f e r a b s o r p t i o n i n t h e u l t r a v i o l e t r e g i o n . I t seems e v i d e n t t h a t t h e r e i s a c o r r e l a t i o n between the s t a b i l i t y o f the complex, as d e f i n e d by the e n t h a l p y o f f o r m a t i o n , and the b l u e - s h i f t o f t h e i o d i n e band. The c o r r e l a t i o n between the magnitude o f t h e b l u e s h i f t and the e n t h a l p y o f f o r m a t i o n i s shown i n t h e f o l l o w i n g g r a p h . 1 5 4 c 16.0 112.13 2.0 4.0 6.0 8.0 10.0 12.0 14.0 -Atf°(kcal/mo!e) F i g u r e 16. C o r r e l a t i o n between t h e b l u e s h i f t o f t h e v i s i b l e I 2 band ( A W v i s = h v c o m p - h v f r e e i 2 ) and t h e e n t h a l p y o f f o r m a t i o n , -AH°, f o r some I 2 complexes. (From r e f . 1 5 4 c ) . M u l l i k e n 1 5 7 has p o i n t e d o u t t h a t t h e o u a n t i -b o n d i n g M.O., w h i c h c o n t a i n s t h e e l e c t r o n e x c i t e d by t h e a b s o r p t i o n o f l i g h t i n t h e v i s i b l e band, must be l a r g e r t h a n t h e o u t e r o c c u p i e d M.O.'s i n t h e normal s t a t e o f 1^- When the i o d i n e m o l e c u l e i s t h e n p a i r e d up w i t h a p a r t n e r m a complex, and t h e n i s e x c i t e d b y ' a b s o r p t i o n o f l i g h t ( a u - , g ) , i t s s u d d e n l y s w o l l e n s i z e i n c r e a s e s t h e r e p u l s i o n energy between i t and t h e donor. T h i s r e p u l s i o n e nergy, w h i c h s h o u l d be g r e a t e r t h e more i n t i m a t e t h e complex, i s added t o t h e u s u a l energy o f t h e e x c i t e d i o d i n e m o l e c u l e , g i v i n g a b l u e s h i f t i n t h e a b s o r p t i o n f r e q u e n c y . I f the 0 s t a b i l i t y o f a complex can be i n t e r p r e t e d i n terms o f t h e donor s t r e n g t h o f the donor m o l e c u l e , t h e n , from t h e magnitudes o f the b l u e - s h i f t , one may deduce t h e r e l a t i v e donor s t r e n g t h s f o r a s e r i e s of compounds. Simple m o l e c u l a r o r b i t a l t r e a t m e n t o f t h e s e complexes i n v o l v i n g i o d i n e as a c c e p t o r has been done w i t h t h e complex P y r i d i n e • I 2 . 1 5 8 Q u a l i t a t i v e l y t h e same i n t e r p r e t a t i o n s can be p l a c e d on the f r e q u e n c i e s o f t h e s h i f t e d band and the c h a r g e - t r a n s f e r band as was deduced by M u l l i k e n . The r e s u l t s o b t a i n e d show t h a t , f o r i n c r e a s i n g i n t e r a c t i o n between the n i t r o g e n o f the p y r i d i n e r i n g and t h e i o d i n e m o l e c u l e , t h e f r e q u e n c y of t h e s h i f t e d band i n c r e a s e s . A l s o , f o r a l l degrees o f i n t e r a c t i o n between t h e donor and a c c e p t o r , t h e t r e a t m e n t p r e d i c t s an i n c r e a s e i n the f r e q u e n c y of the charge t r a n s f e r band, a s m a l l d e c r e a s e i n t h e f r e q u e n c y o f t h e v i s i b l e band, and a d e c r e a s e i n the e n t h a l p y o f f o r m a t i o n w i t h i n c r e a s i n g base e l e c t r o n e g a t i v i t y . The g e n e r a l r e s u l t s quoted above o u t l i n e s i m p l y some o f t h e f e a t u r e s o f d o n o r - a c c e p t o r complexes r e l e v a n t t o t h e work u n d e r t a k e n i n t h i s t h e s i s . For a g e n e r a l complete r e v i e w of t h e work i n the f i e l d o f c h a r g e - t r a n s f e r complexes, the books l i s t e d i n r e f . 159 a r e a v a i l a b l e a l o n g w i t h some r e v i e w s l i s t e d i n r e f . 160. I I I . B. C h a r g e - T r a n s f e r Complexes o f I o d i n e w i t h  M e t h y l p h o s p h o n i t r i l e s The r e l a t i v e base s t r e n g t h s o f h e t e r o a r o m a t i c m o l e c u l e s a r e u s u a l l y c o n s i d e r e d i n terms of ir-charge d e n s i t y a t the n i t r o g e n , as a f f e c t e d by s u b s t i t u e n t s . The p h o s p h o n i t r i l i c r i n g i s c h a r a c t e r i z e d by t h e pr e s e n c e o f a l o n e p a i r o f e l e c t r o n s on each n i t r o g e n , though f o r m a l l y t h e s e a r e i n v o l v e d i n the homomorphic r r-system. Owing t o t h e g r e a t d i f f e r e n c e s i n e l e c t r o n e g a t i v i t y between phosphorus and n i t r o g e n (2.1 and 3.0 r e s p e c t i v e l y on t h e _ , 161. . , _ ^ u' ^ , P a u l i n g S c a l e ), however, much o f t h e e l e c t r o n d e n s i t y i s c o n c e n t r a t e d on the n i t r o g e n atoms and base s t r e n g t h s o f n i u IA u • A i 162,169 t h e s e m o l e c u l e s s h o u l d be s i g n i f i c a n t . Shaw and coworkers have measured the P-Ka v a l u e s o f a l a r g e number o f d i f f e r e n t p h o s p h o n i t r i l e s i n n i t r o b e n z e n e w i t h the use of p e r c h l o r i c a c i d as a r e a g e n t . I n some c a s e s , b o t h P Ka-^ a n < ^ P K a 2 v a l u e s c o u l d be d e t e r m i n e d f o r t h e i n t r o d u c t i o n of. t h e f i r s t and second p r o t o n s . Some o f t h e r e s u l t s a r e p r e s e n t e d i n T a b l e 8. T h e i r r e s u l t s i l l u s t r a t e how base s t r e n g t h s v a r y w i t h the e l e c t r o n - w i t h d r a w i n g o r - s u p p l y i n g c h a r a c t e r i s t i c s o f t h e s u b s t i t u e n t s . Some c y c l i c t e t r a m e r s a r e s l i g h t l y s t r o n g e r bases t h a n the analogous t r i m e r s , but t h i s i s not a g e n e r a l r u l e , t h e r e v e r s e b e i n g o f t e n t r u e when s t r o n g l y e l e c t r o n - s u p p l y i n g s u b s t i t u e n t s a re p r e s e n t . T a b l e 8' R e p r e s e n t a t i v e pK ' V a l u e s f o r P h o s p h o n i t r i l e s i n N i t r o b e n z e n e a t 25°. Compound P K a / : Ref ( N P C 1 2 ) 3 o r 4 -6.0 169 (NPPh 2) 1.50 164 ( N P P h 2 ) 4 2.20 -5.80 164 ( N P E t 2 ) 3 . 6.40 164 ( N P E t 2 ) 4 7.60 0.20 164 [N P ( O M e ) 2 ] 3 -1.9 164 [N P ( O M e ) 2 ] 4 -1.0 164 [NP(NHMe) ] 8.8 ± 0.6 -2.0 ±0.4 163 [NP(NHMe) ] 8.2 3.4 163 [ N P ( N H E t ) 2 ] 3 8.2 -1.3 163 [ N P ( N H E t ) 2 ] 4 8.1 3.8 163 [ N P ( N M e 2 ) 2 ] 3 7.6 -3.3 163 [NP(NMe 2) ] 8.3 0.6 163 When c o n s i d e r i n g t h e r e l a t i o n s h i p between base s t r e n g t h s and u - e l e c t r o n d e n s i t i e s , t h e homomorphic and h e t e r o m o r p h i c ir-systems a r e e x p e c t e d t o have d i f f e r e n t e f f e c t s as a f u n c t i o n o f r i n g s i z e . The h e t e r o m o r p h i c system d e c r e a s e s charge d e n s i t y a t the r i n g n i t r o g e n atoms as r i n g s i z e i n c r e a s e s , an e f f e c t w h i c h would l e a d t o d e c r e a s i n g base s t r e n g t h w i t h i n c r e a s i n g r i n g s i z e . For the homomorphic system, however, an a l t e r n a t i o n i n the charge d e n s i t i e s a t n i t r o g e n i s c a l c u l a t e d (see Chapter I ) , r e s u l t i n g i n t h e e x p e c t e d o r d e r o f base s t r e n g t h s t e t r a m e r > pentamer > t r i m e r . A l o n g w i t h t h e c o n s i d e r a t i o n o f t h e charge d e n s i t i e s , t h e e f f e c t s of h y b r i d i z a t i o n must a l s o be c o n s i d e r e d as r i n g s i z e s are v a r i e d . A c o mparison of t h e mean a n g l e s a t n i t r o g e n i n the c h l o r o p h o s p h o n i t r i l e s ( N 3 P 3 C 1 6 8 121.4°; N 4 P 4 C l g 1 3 131.3°; N ^ C l ^ 1 6 148.6°) sug g e s t s t h a t t h e a n g l e s a t n i t r o g e n i n c r e a s e w i t h r i n g s i z e . I f t h e l o n e p a i r o f e l e c t r o n s a t t h e n i t r o g e n atom ar e t a k e n .to occupy an sp h y b r i d o r b i t a l , i n c r e a s i n g the a n g l e a t n i t r o g e n would r e s u l t i n i n c r e a s e d p c h a r a c t e r , and hence making the e l e c t r o n s , more r e a d i l y a v a i l a b l e f o r d o n a t i o n . The d i f f e r e n c e i n b a s i c i t y between p y r i d i n e and a t y p i c a l a l i p h a t i c t e r t i a r y amine (e.g. t r i m e t h y l a m i n e ) had been a t t r i b u t e d by I n q o l d 4 1 t o t h e d i f f e r e n c e s i n h v b r i d i -2 3 nyiorrax z a t i o n , sp and sp r e s p e c t i v e l y , o f t h e l o n e p a i r o r b i t a l , i n t h e two c a s e s . A d i f f e r e n c e o f 4.5 i n t h e pK 's were a. o b s e r v e d . B a s i c i t i e s of the m e t h y l p h o s p h o n i t r i l e s determined by Ranganathan 4 2 showed a steady i n c r e a s e i n base s t r e n g t h w i t h i n c r e a s i n g r i n g s i z e (N 3P 3Me 6 p'K = 5.03 ± .01); N 4P 4Me g pK a = 5.72 ± .01; N 5 P 5 M e 1 Q pK a = 6.69 ± .01, pK = 3.97 ± .01). T h i s was i n t e r p r e t e d as being due to * a2 the dominating e f f e c t s of a - h y b r i d i z a t i o n over t h a t of e l e c t r o n d e n s i t y . In the measurement of base s t r e n g t h s by the det e r m i n a t i o n of pK v a l u e s , i t seems t h e r e f o r e t h a t no a b s o l u t e l y c l e a r trends can be observed. The i n h e r e n t « • i 1 ' _ I 1 "1 • 1 1 1 ^ "1 i 1 rf— a y— problem i n t h i s method i s t h a t , as w e l l as the e f f e c t s of ^ I "1 B I 1 1 1 ' T • I 1 T t ' e l e c t r o n d e n s i t y and a - h y b r i d i z a t i o n , s o l v a t i o n e f f e c t s a l s o p l a y a major r o l e i n determining the r e l a t i v e base s t r e n g t h s . I f t h i s s o l v a t i o n e f f e c t can be e l i m i n a t e d , • 1 T I • I ' l l 1 i I 1 T i l i — • /~ the r e l a t i o n s h i p between base s t r e n g t h and the e f f e c t s of T I *1 1 I " 1 1 1 - l - i i - i e l e c t r o n d e n s i t y and h y b r i d i z a t i o n would be more c l e a r l y e v i d e n t . The use of donor-acceptor complexes, with the 61 hi^ ^ 3_ tio s 1"T^ ) n I t !C i 1 s s SL S clo noi* s s SL t i s f t ti 1 s GCJ U. i r cm on t • f "1 "I 1 T • i — *1 "1 T I • s i n c e no f o r m a l l y charged s p e c i e s are formed and s o l v a t i o n e f f e c t s are t h e r e f o r e n e g l i g i b l e . In u s i n g the m e t h y l p h o s p h o n i t r i l e s as donors i n an attempt to d e t e c t c h a r g e - t r a n s f e r complex formation i n v o l v i n g i o d i n e as acceptor, the methyl s u b s t i t u e n t s on the phosphorus atoms are expected to be e l e c t r o n r e l e a s i n g , r e s u l t i n g i n g r e a t e r charge d e n s i t i e s at the r i n g n i t r o g e n atoms and t h e r e f o r e s i g n i f i c a n t donor p r o p e r t i e s . The e x i s t e n c e o f t h e complex P y r i d i n e • I 2 1 5 6 would l e a d one t o e x p e c t a s i m i l a r t y p e o f i n t e r a c t i o n t o o c c u r between t h e m e t h y l p h o s p h o n i t r i l e s and i o d i n e . The methods of p r o t o n n u c l e a r m agnetic resonance and v i s i b l e - u l t r a v i o l e t s p e c t r o s c o p y a r e used t o d e t e r m i n e th e e x t e n t o f t h e i n t e r -a c t i o n s . I I I . B . l . E x p e r i m e n t a l The m e t h y l c y c l o p h o s p h o n i t r i l e s s y n t h e s i z e d p r e v i o u s l y and commercial i o d i n e used were s u b l i m e d i n vacuo i m m e d i a t e l y p r i o r t o use. E l e c t r o n i c s p e c t r a o f t h e complexes ( N P M e 2 ) n - x I 2 were o b t a i n e d i n d i c h l o r o m e t h a n e ( M a l l i n c k r o d t S p e c t r o g r a d e ' d r i e d o ver s i l i c a g e l ) by m i x i n g s o l u t i o n s o f i o d i n e and the p h o s p h o n i t r i l e s . S p e c t r a of t h e complexes were r e c o r d e d on a Cary 14 s p e c t r o p h o t o m e t e r u s i n g matched q u a r t z c e l l s . The d e t e c t i o n o f new a b s o r p t i o n s not due t o e i t h e r o f the components a l o n e l e d t o t h e use o f t h e B e n e s i -H i l d e b r a n d method t o d e t e r m i n e the e q u i l i b r i u m c o n s t a n t o f complex f o r m a t i o n . S o l u t i o n s c o n t a i n i n g v a r y i n g r a t i o s of m e t h y l p h o s p h o n i t r i l e t o i o d i n e were p r e p a r e d and t h e absorbance v a l u e s a t s p e c i f i c w avelengths d e t e r m i n e d on a Turner S p e c t r o m e t e r . S o l u t i o n s o f t h e p e n t a m e r i c m e t h y l -p h o s p h o n i t r i l e were p r e p a r e d under an atmosphere o f d r y n i t r o g e n due t o i t s h y d r o s c o p i c p r o p e r t y . The r e s u l t s a re as f o l l o w s : A. D e t e r m i n a t i o n o f E q u i l i b r i u m C o n s t a n t s (1) N 3 P 3 M e 6 + I 2 [N 3P 3Me 6] = 5.44 x 1 0 ~ 4 M Temp. room temp. T a b l e 9 M o l a r i t y o f I x 1 0 3 6.76 5. 64 4. 51 3.38 2.25 1.13 O p t i c a l D e n s i t y a t 360 my 1.130 ± 0.050 1.040 ± 0.050 0.960 + 0.025 0.880 ± 0.025 0.725 ± 0.010 0.500 ± 0.005 _ (2)' N 4 P 4 M e g +. I 2 [N 4P 4Me g] = 4.27 x 1 0 ~ 4 M Temp. = room temp. T a b l e 10 M o l a r i t y o f I 2 x 1 0 3 O p t i c a l D e n s i t y a t 380 my 6.12 5.10 4. 08 3. 06 2. 04 1. 02 1.190 ± 0.050 1.060 ± 0.050 0.900 ± 0.025 0.755 ± 0.010 0.570 ± 0.005 0.321 ± 0.005 ;(3) N 5 P 5 M e 1 Q + 1"2 [N P Me L 5 5 10 4.02 x 1 0 "4 M Temp. room temp. Ta b l e IT M o l a r i t y o f I 2 x 10- O p t i c a l D e n s i t y a t 420 my 7.13 5.94 4.75 3. 56 2.38 1.370 ± 0.050 0.970 ± 0.025 0.672 ± 0.010 0.396 ± 0.0050 0.125 ± 0.0025 _ C °'l P l o t s o f — v e r s u s y i e l d s t r a i g h t l i n e s f o r t h e i n t e r a c t i o n of N ^ M e g and N ^ M e g w i t h i o d i n e , i n d i c a t i v e o f the f o r m a t i o n i n s o l u t i o n of 1:1 d o n o r - a c c e p t o r complexes. c D°t 1 P l o t s o f ~ — v e r s u s -^r y i e l d a s t r a i g h t l i n e f o r the A c C A i n t e r a c t i o n o f N 5 P 5 M e 1 Q and I 2 . A m o d i f i e d B e n e s i - H i l d e b r a n d e q u a t i o n C n°l , ~~A = K7- ( C T ° - 2 ) + do) c X A x can be o b t a i n e d f o r t h e e q u i l i b r i u m p r o c e s s N 5 P 5 M e 1 0 + 2 I 2 v — N 5 P 5 M e 1 0 - 2 l 2 by making a f u r t h e r assumption o f C D ° b e i n g e q u a l t o C . T e n t a t i v e l y , t h e n , t h i s i n d i c a t e s t h e f o r m a t i o n o f a 1:2 d o n o r - a c c e p t o r complex. B. Assignment o f A b s o r p t i o n Bands For d o n o r - a c c e p t o r complexes i n v o l v i n g l 2 , two a b s o r p t i o n s n o t due t o e i t h e r o f t h e components a l o n e a re ex p e c t e d . The i o d i n e v i s i b l e a b s o r p t i o n i s e x p e c t e d t o be s h i f t e d t o s h o r t e r w a v e l e n g t h s , t h i s a b s o r p t i o n b e i n g due to i o d i n e i n a complex. An i n t e n s e band, a t s h o r t e r wave-l e n g t h s t h e n the i o d i n e band, i s due t o t h e i n t e r m o l e c u l a r charge-transfer t r a n s i t i o n . The e l e c t r o n i c spectra of (NPMe 2) n n = 3,4,5, i n solution with I show the following new absorption bands as well as the band at 503 my due to uncomplexed I : Table 12 Compound New Absorption Band Maxima (my) N 3P 3Me 6 N 4P 4Me g N 5P 5Me 1 ( ) 390,257.5 360,285.0 363,292.0 Analogous to the spectra of complexes involving iodine and other o-donors, the longer wavelength absorption i n each case i s assigned as the iodine absorption and the shorter wavelength absorption as that due t o c h a r g e - t r a n s f e r from the donor to iodine. The r e s u l t s are summarized i n the following table: Donor CH2C1 N 3P 3Me 6 N 4 P 4 M e 8 • K c 477 Sim'1 i n „ -1 14 j Jim Table 13 A I2' max emax 503 897 360 ^CT max _CT "max N 5 P 5 M e l O 5 . 3 9 x l o V m " ^  363 3.85 x l 0 4 292.0 390 3.74 x l 0 3 257.5 10.3 x l 0 4 7 . 2 3 x l 0 3 285.0 3.90 x l 0 4 2.20 x ,10 5 C. N u c l e a r Magnetic Resonance S p e c t r a o f Donor-Acceptor  Mo:lecular Complexes The f o r m a t i o n o f c h a r g e - t r a n s f e r complexes h a v i n g been e s t a b l i s h e d by t h e e l e c t r o n i c s p e c t r a , t h e c h e m i c a l s h i f t s o f the p r o t o n s on t h e p h o s p h o n i t r i l i c r i n g s r e l a t i v e t o i n t e r n a l t e t r a m e t h y l s i l a n e were d e t e r m i n e d t o a s c e r t a i n t h e magnitude of charge d o n a t i o n t o t h e i o d i n e m o l e c u l e . Upon complex f o r m a t i o n , i t i s e x p e c t e d t h a t t h e p r o t o n s become more d e s h i e l d e d as e l e c t r o n d e n s i t y i s removed from t h e p h o s p h o n i t r i l i c r i n g . P r o t o n c h e m i c a l s h i f t s o f v a r y i n g r a t i o s o f p h o s p h o n i t r i l e t o i 2 were o b t a i n e d i n C H 2 C 1 2 on a V a r i a n HA-60 s p e c t r o m e t e r a t a temperature o f 35°C. A l l s p e c t r a o b t a i n e d showed a d o u b l e t due t o c o u p l i n g w i t h t h e phosphorus n u c l e i , t h e c o u p l i n g c o n s t a n t b e i n g 12 c.p.s. Though the c h e m i c a l s h i f t s move i n c r e a s i n g l y d o w n f i e l d - w i t h an i n c r e a s e i n the r a t i o o f l 2 , t h e e q u i v a l e n c e o f t h e p r o t o n s i n d i c a t e a r a p i d e q u i l i b r i u m p r o c e s s whereby l 2 m o l e c u l e s a r e exchanged among t h e donor s i t e s . The r e s u l t s a r e summarized i n t h e f o l l o w i n g t a b l e : A =•• 6 , , - 6 obsd p a r e n t T a b l e 14 Compound 6 o b s d a A N-.P-.Me, 3 3 6 1.40 0 N 4 P 4 M e g 1.41 0 N 5 P 5 M e 1 Q 1.40 0 N 3 P 3 M e 6 + X2 1. 56 0.16 NgPgMe + 2 I 2 1. 63 0.23 N 3 P 3 M e 6 + 3 I 2 1.67 0.27 N 4 P 4 M e 8 + I 2 1.56 0.15 N.P.Me_ + 4 4 8 2 I 2 1. 63 0.22 N 5 P 5 M e 1 Q + X2 1.52 0.12 N 5 P 5 M e 1 Q + 2 I 2 1. 62 0.22 I n p.p.m. D. Temperature E f f e c t s on t h e Chemical S h i f t o f N - ^ M e ^ ^ P r o t o n c h e m i c a l s h i f t s o f N-P-Me c.I„ were o b t a i n e d a t a s e r i e s o f low te m p e r a t u r e s i n an e f f o r t t o slow the exchange o f I 2 among t h e donor s i t e s and hence a l l o w a d i f f e r e n t i a t i o n o f the p r o t o n s c l o s e s t t o t h e donor s i t e . However, the d e c r e a s e i n the temp e r a t u r e r e s u l t e d o n l y i n the b r o a d e n i n g o f t h e resonances b u t t h e s p e c t r a s t i l l showed 31 a s i n g l e d o u b l e t . D e c o u p l i n g o f t h e P n u c l e i r e s u l t e d i n t h e o b s e r v a t i o n o f a s i n g l e oeak w h i c h became b r o a d e r as the t e m p e r a t u r e was d e c r e a s e d . A t t h e e x t r e m e l y low tempe r a t u r e s the chemical s h i f t s appear to move increasingly downfield. The r e s u l t s determined i n CDC13 r e l a t i v e to i n t e r n a l tetramethylsilane on a Varian HA-100 spectrometer are as follows: Compound Table 15 Temperature obsd N 3 P 3 M e 6 35°C 1.50 N 3P 3Me g-I 2 35°C 1.55 -N0P->Me, • I» 0°C 1.55 -30°C 1.60 N 3P 3Me g-I 2 -80°C 1.65 a In p.p.m. E. Synthesis of N-^Me^]^ ':' . N 3P 3Me g-I 2 can be prepared by mixing together solutions of I 2 and N^Meg i n a nonpolar solvent such as heptane, the quantitative formation of an orange p r e c i p i t a t e occurring almost immediately upon mixing. The s o l i d was found to be soluble in the more polar solvents such as CHC13 and CH 2C1 2 but i t i s insoluble i n nonpolar solvents. The s o l i d can a l s o be s u b l i m e d i n vacuo w i t h s l i g h t h e a t i n g w i t h o u t d e c o m p o s i t i o n , i n d i c a t i v e o f i t s m o l e c u l a r n a t u r e . C r y s t a l s o f t h e compound s u i t a b l e f o r c r y s t a l and m o l e c u l a r s t r u c t u r a l d e t e r m i n a t i o n were o b t a i n e d by d i s s o l v i n g t h e powder i n a m i x t u r e o f CHC1 3 and CC1 4 and the n a l l o w i n g t h e s o l v e n t t o e v a p o r a t e s l o w l y . The m i c r o a n a l y s i s r e s u l t s a r e as f o l l o w s : Element C H N Ex p e c t e d (%) 15.12 3.81 8.82 Found (%) 15.33 3.91 8.58 E l e c t r o n i c s p e c t r a o f the compound i n CH 2C1 2 show the same a b s o r p t i o n s as were obser v e d f o r .mixtures o f N 3 P 3 M e 6 and I 2 , c o n f i r m i n g t h e r e s u l t s o f a 1:1 d o n o r - a c c e p t o r complex as d e t e r m i n e d by t h e method o f B e n e s i and H i l d e b r a n d . The s p e c t r a o f the s o l i d i n v a r i o u s s o l v e n t s show s m a l l v a r i a t i o n s i n a b s o r p t i o n maxima. The p o s i t i o n o f t h e s h i f t e d i o d i n e band i n v a r i o u s s o l v e n t s a re as f o l l o w s : T a b l e 16 S o l v e n t A 1 2 ' (my) max c c l4 CHC1 3 C H 2 C 1 2 405 395 390 I I I . B.2. D i s c u s s i o n From t h e v a l u e s o f K and t h e p o s i t i o n s o f t h e c c h a r g e - t r a n s f e r and b l u e - s h i f t e d i o d i n e bands, i t i s c e r t a i n t h a t the m e t h y l p h o s p h o n i t n l e s a c t as n-donors toward i o d i n e , u t i l i z i n q the n i t r o q e n l o n e p a i r s i n much the same way as do amines i n t h e i r i n t e r a c t i o n w i t h Q1G C110 O IT 9. C C 6]3 t o 10 S • The dependence o f t h e energy o f c h a r g e - t r a n s f e r upon the i o n i z a t i o n P ° ^ i a l o f a s e r i e s o f a m i n e s h a s been p r e v i o u s l y shown. The i o n i z a t i o n p o t e n t i a l s o f N 3 P 3 M e 6 and N ^ M e have been d e t e r m i n e d as 8.35 and 7.99 eV r e s p e c t i v e l y . Though the v a l u e f o r Nj-Pj-Me^Q i s not known, i t can be appr o x i m a t e d as 8 eV by comparison w i t h the obse r v e d t r e n d o f i o n i z a t i o n p o t e n t i a l s f o r the s e r i e s o f c h l o r o p h o s p h o n i t r i l e s . 2 1 P l a c i n g t h e c o r r e s p o n d i n g p o i n t s on the p l o t o f energy of c h a r q e - t r a n s f e r v e r s u s i o n i z a t i o n p o t e n t i a l f o r some t y p i c a l amines ( F i q . 2 1 ) , g i t i s e v i d e n t t h a t o n l y N_,P.,Me, a c t s as a t y p i c a l amine, whereas t h e p o i n t s f o r N.P.Me,, and N rP_Me,_ f a l l w e l l o f f c 4 4 8 5 5 10 the l i n e . The o b s e r v e d c o r r e l a t i o n between t h e e n e r g i e s o f c h a r g e - t r a n s f e r and t h e i o n i z a t i o n p o t e n t i a l s f o r the s e r i e s o f m e t h y l p h o s p h o n i t r i l e s does c o n f i r m , however, t h a t i o n i z a t i o n i n v o l v e s the l o s s o f an e l e c t r o n from t h e homomorphic - rr-system, of w h i c h t h e n i t r o g e n l o n e - p a i r s T ZD/eV F i g . 21. Plot of energy of charge-transfer versus i o n i z a t i o n p o t e n t i a l from r e f . 160(e). Added points are 11, 12, 13. The points correspond to (1) ammonia; (2) pyridine; (3) methylamine; (4) ethylamine; (5) n-butylamine; (6) dimethy1-amine; (7) diethylamine; (8) trimethylamine; (lD^P^Meg ^ l I ) N 4P 4Megt U3) N^Me^.' are an i n t e g r a l part. The assignment of the methylphosphonitriles as n-donors i s confirmed by cr y s t a l l o g r a p h i c studies of the complex N 3 P 3 M e 6 • I 2 . 1 7 0 The structure was found to be simi l a r to that of a l l the amine-iodine complexes deter¬mined thus f a r , with the N••••I-I group found to be linear, C H 3 ^ , C H 3 C H -N / N \ •CH, C H 3 Figure 22. Structure of N^P^Me c'I 0. The N-I distance (2.42 A) i s shorter than the sum of the Van der Waals r a d i i and the I-I distance (2.82 A) i s increased over that of uncomplexed I 2 (2.67 A), being consistent with the donation of charge into the o u antibonding o r b i t a l on iodine. The s i m i l a r i t i e s between N 3P 3Me 6*I 2 and the amine-iodine complexes are summarized i n Table 17. T a b l e "IT S t r u c t u r a l I n f o r m a t i o n Compound 4 - m e t h y l p y r i d i n e • I N-I(A) I - I ( A ) 171 ( C H 3 ) 3 N ' I 2 N 3 P 3 M e 6 - I 2 1 7 0 2.31 2.27 2.42 2.83 2.83 2 . 82 C o n f i g u r a t i o n • •«N•••I-I l i n e a r N• • • I - I l i n e a r N• • - I - I l i n e a r The two P-N bonds i n v o l v i n g the n i t r o g e n atom bonded t o i o d i n e a r e s l i g h t l y l o n g e r , mean 1.64 A, t h a n the o t h e r f o u r P-N bonds, mean 1.60 A, b e i n g i n d i c a t i v e o f o n l y s l i g h t e l e c t r o n w i t h d r a w a l from t h e r i n g . The s m a l l e r a n g l e a t the i n t e r a c t i n g n i t r o g e n atom, 123.4°, as compared t o t h a t a t t h e o t h e r two r i n g n i t r o g e n atoms, mean 124.4°, i s c o n s i s t e n t w i t h l e s s e l e c t r o n d e n s i t y i n the P-N bonds meeting a t t h e i n t e r a c t i n g n i t r o g e n atom, r e s u l t i n g i n weaker i n t e r b o n d r e p u l s i o n s . The ob s e r v e d e f f e c t i n the s t r u c t u r e s o f [ (NPMe_) .H] C u C l / 7 4 and 175 2 4 3 [ ( N P M e 2 ) 4 H ] 2 C o C l 4 a r e much l a r g e r , as e x p e c t e d , s i n c e more e l e c t r o n d e n s i t y i s removed from the r i n g upon p r o t o n -a t i o n . The f o r m a t i o n o f a d o n o r - a c c e p t o r complex can t h e r e f o r e be viewed as an i n t e r m e d i a t e s t a g e i n t h e f o r m a t i o n o f a a-bond between the n i t r o g e n atom o f the r i n g and the n e a r e s t i o d i n e atom o f t h e l 2 m o l e c u l e . C o n t i n u i n g w i t h t h i s t r a i n of t h o u g h t , t h e e n e r g i e s o f c h a r g e - t r a n s f e r might be e x p e c t e d t o c o r r e -l a t e w i t h t h e l o c a l i z a t i o n e n e r g i e s r e q u i r e d f o r t h e removal o f a p a i r of e l e c t r o n s from th e homomorphic TT-system o f t h e r i n g , such as o c c u r s upon p r o t o n a t i o n o r a l k y l a t i o n of a r i n g n i t r o g e n atom. Though i n the case of d o n o r - a c c e p t o r complex f o r m a t i o n , o n l y a s m a l l amount o f charge i s removed from the r i n g , t h e q u a l i t a t i v e r e s u l t s e x p e c t e d s h o u l d s t i l l h o l d . The l o c a l i z a t i o n e n e r g i e s f o r the s e r i e s o f m e t h y l p h o s p h o n i t r i l e s can be c a l c u l a t e d as f o l l o w s . L o c a l i z a t i o n Energy C a l c u l a t i o n s Based on t h e s i m p l e H i i c k e l M o l e c u l a r O r b i t a l t r e a t m e n t o f t h e i r - l e v e l s i n p h o s p h o n i t r i l i c r i n g s , t h e T r-energy l e v e l s o f any s e r i e s of p h o s p h o n i t r i l e s can be r e a d i l y o b t a i n e d . U s i n g the numbering scheme shown i n F i g u r e 23 and l e t t i n g f o r example N x r e p r e s e n t the a p p r o p r i a t e o r b i t a l on the same atom, the a t o m i c o r b i t a l s , w i t h o u t b e i n g s p e c i f i e d as t o t h e i r e x a c t n a t u r e , are combined t o form the m o l e c u l a r o r b i t a l s . The symmetry of the N 3 P 3 M e g m o l e c u l e i s used i n t h i s case i n f o r m i n g i n t e r m e d i a t e o r b i t a l s such t h a t t h e s o l v i n g o f the s e c u l a r d e t e r m i n a n t s w i l l become s i m p l i f i e d . For t h e CH CH o CH Q 3 |2 r 6 CH 3 N N, x C H 3 C H 3 Figure 23. Numbering scheme of rin g atoms. homomorphic system of N^Meg, the symmetric and a n t i -symmetric combinations of atomic o r b i t a l s are as follows (symmetric) A ' : N l ' / f ( P 2 + P 6 ) ' / f ^ N 3 + N 5 ) ' P< (antisymmetric) A": (P 2-Pg) , J=- (N 3"N 5) . Setting the r e l a t i v e e l e c t r o n e g a t i v i t i e s by using a N = a p + 23 and using a = ^ ( a N + a p ) , the secular deter-minants are as follows: a + 3 - E /2~B ... 0 0 /2 e a - 3 3 0 E 0 3 a + 3 - E /2 3 • 0 0 a - 3 A" a - 3 - E 3 6 a + 3 - E = 0 S u b s t i t u t i n g x - a g E and s o l v i n g , t h e e i g e n v a l u e s o b t a i n e d a r e as f o l l o w s : A': E = a ±2.2360 3, a ± 1.4142 3 A" : E = a ± 1.4142 3 F i l l i n g t h e t h r e e l o w e s t l e v e l s w i t h the l o n e p a i r s o f e l e c t r o n s on the n i t r o g e n atoms, t h e energy o f the Tr-system i s found t o be 10 .1288 3 r e l a t i v e t o a. L o c a l i z a t i o n o f a p a i r of e l e c t r o n s on one of the n i t r o g e n atoms r e s u l t s i n a ir-system c o n s i s t i n g o f the r e m a i n i n g f i v e atoms, t h e energy o f wh i c h can be s i m i l a r l y c a l c u l a t e d . (symmetric) A': ( P ^ P g ) , ^ ( N 2 + N 4 ) ' P 3 ( a n t i s y m m e t r i c ) A": i ( P - P ) , i (N„-N„) v2 /T  z 4 The c a l c u l a t e d e i g e n v a l u e s a re as f o l l o w s : A': E = a ± 2 3 , E = a - 3 A" : E = a ± vT 3 The two l o w e s t l e v e l s a r e o c c u p i e d by t h e f o u r l o n e - p a i r e l e c t r o n s o f t h e n i t r o g e n atoms i n t h e segment, w i t h the l o c a l i z e d p a i r o f e l e c t r o n s now o c c u p y i n g t h e l e v e l o f energy a + 3, e q u i v a l e n t t o t h e Coulomb i n t e g r a l as d e f i n e d f o r n i t r o g e n atoms. The energy o f t h e s i x -e l e c t r o n system i s t h e r e f o r e found t o be 8.8284 3 r e l a t i v e t o a. The l o c a l i z a t i o n energy r e q u i r e d f o r t h e removal of a p a i r o f e l e c t r o n s from t h e N 3 P 3 M e 6 .-system i s t h e r e f o r e (10.1288 - 8.8284)3 = 1.3004 3. S i m i l a r c a l c u l a t i o n s c a r r i e d t h r o u g h f o r N ^ M e g and N 5 P 5 M e 1 Q y i e l d - l o c a l i z a t i o n e n e r g i e s o f 1.2157 3 and 1.2429 3 r e s p e c t i v e l y . F i g u r e 24 shows t h e changes i n t h e o r b i t a l l e v e l s o f N 3 P 3 M e 6 upon l o c a l i z a t i o n of a l o n e - p a i r o f e l e c t r o n s . F i g u r e 2 5 shows t h e r e l a t i o n s h i p between r i n g s i z e and l o c a l i z a t i o n e n e r g i e s . The c o n s i d e r a t i o n o f l o c a l i z a t i o n e n e r g i e s t h e r e f o r e p r e d i c t a l a r g e r energy o f c h a r g e - t r a n s f e r f o r N 3 p 3 M e 6 t h a n N 4 P 4 M e 8 o r N 5 P 5 M e 1 0 ' a s f o u n d ' but does n o t g i v e t h e c o r r e c t o r d e r f o r t h e l a t t e r two c a s e s . The l o w e r i n g of. t h e energy r e q u i r e d f o r N 5 P 5 M e 1 Q may be due t o r e a s o n s o f geometry, whereby t h e I m o l e c u l e i s a b l e t o approach more c l o s e l y t h e i n t e r a c t i n g n i t r o g e n atom because o f the l a r g e r a n g l e s i n t h e ten-membered r i n g and g r e a t e r m o l e c u l a r f l e x i b i l i t y . 6-membered r i n g 5-raerabered segment F i g . 24 . T r - l e v e l s o f N 3 P 3 M e 6 and t h e 5-membered segment w h i c h r e s u l t s upon l o c a l i z a t i o n o f one l o n e -p a i r o f e l e c t r o n s . F i g . 25. A p l o t o f l o c a l i z a t i o n energy v e r s u s r i n g s i z e f o r p h o s p h o n i t r i l i c r i n g s . The magnitude o f the b l u e - s h i f t o f t h e i o d i n e v i s i b l e band, as i n t e r p r e t e d by M u l l i k e n 1 5 7 , g i v e s an i n d i c a t i o n of t h e r e l a t i v e base s t r e n g t h s of t h e m e t h y l -p h o s p h o n i t r i l e s , i f the mode o f i n t e r a c t i o n i s the same i n each c a s e . The c o r r e l a t i o n o f t h e b l u e - s h i f t w i t h the h e * ^ o f f o r m a t i o n o f the complexes has a l r e a d y been shown. The magnitude o f the s h i f t s f o r t h e s e r i e s o f m e t h y l p h o s p h o n i t r i l e s , e x p r e s s e d as energy d i f f e r e n c e s , are as shown i n T a b l e 18. Ta b l e 18 • Magnitudes o f t h e B l u e - s h i f t s Compound N 3P 3Me g•! N 4 P 4 M e g • I 2 N 5 P 5 M e 1 ( ) - I 2 A E — hv comp h v f f r e e —2 0.712 eV 0.980 eV 0.94 9 eV From t h e v a l u e s o f AE t, i t seems t h e r e f o r e t h a t t h e o r d e r of base s t r e n g t h s i s N„P,.Me0 > N c P c M e n n > N 0P 0Me,. 4 4 o o b 1 (J 3 3 6 The t r a n s f e r o f charge from the donor t o t h e a c c e p t o r even i n the ground s t a t e o f t h e complex s h o u l d have a measurable e f f e c t on t h e e l e c t r o n d e n s i t i e s o f t h e donor m o l e c u l e s . I n t h e i n t e r a c t i o n s o f i o d i n e and o f i o d i n e m o n o c h l o r i d e w i t h v a r i o u s m e t h y l p y r i d i n e s , d o w n f i e l d s h i f t s o f t h e m e t h y l p r o t o n s i n the donor are o b s e r v e d when the a c c e p t o r i s added. ' Such s h i f t s a r e e x p l i c a b l e i n terms o f a t r a n s f e r - o f - c h a r g e l o w e r i n g t h e m e t h y l - p r o t o n s h i e l d i n g c o n s t a n t s . S i m i l a r d o w n f i e l d s h i f t s o f t h e m e t h y l protons are d e t e c t e d i n t h e m e t h y l -p h o s p h o n i t r i l e s when i o d i n e i s added. From t h e d i f f e r e n t i a l s h i f t s measured, i t i s p o s s i b l e t o c a l c u l a t e A n, the a s s o c i a t i o n s h i f t , f o r each m e t h y l p h o s p h o n i t r i l e i n t e r a c t i n g w i t h i o d i n e . For P ^ num. p r o c s s s the f o l l o w i n g ecjuation i s a p p l i c a b l e . A = A Q P c p _ no. o f moles o f complexed base c t o t a l no. of moles o f base A - d i f f e r e n t i a l s h i f t =• 6 . , - 6 £ obsd f r e e A Q = a s s o c i a t i o n s h i f t = 6 q - 6 f r e e 6 f r e e = c h e m i c a l s h i f t of t h e f r e e donor i n t h e absence of exchange ^complex = c h e m i c a l s h i f t of the complexed donor i n t h e absence o f exchange S i n c e the c h e m i c a l s h i f t s o b s e r v e d are not e x t r e m e l y s e n s i t i v e t o t e m p e r a t u r e o v e r a s m a l l r a n g e , t h e K v a l u e s d e t e r m i n e d a t room t e m p e r a t u r e by t h e method of B e n e s i and H i l d e b r a n d are used t o c a l c u l a t e P , hence c e n a b l i n g the e v a l u a t i o n o f A Q. The r e s u l t s a r e as f o l l o w s : T a b l e 19.--C a l c u l a t e d A s s o c i a t i o n S h i f t s Compound -0-3. f 3D N 3 P 3 M e 6 - I 2 N 4 P 4 M e g - I 2 N 5 p 5 M e i o , : c : 0.96 0. 92 0.986 0.17 0.16 0.12 a I n CH 2C1 2 b I n u n i t s o f p.p.m. Assuming t h a t t h e d o w n f i e l d s h i f t s o f t h e p r o t o n r esonances a r e an i n d i c a t i o n o f t h e degrees o f i n t e r a c t i o n between t h e donor and t h e a c c e p t o r , t h e n i n o r d e r t o compare t h e r e l a t i v e degrees o f c h a r g e - t r a n s f e r , i t i s n e c e s s a r y t o t a k e i n t o a c c o u n t t h e number o f donor s i t e s a v a i l a b l e p e r m e t h y l p h o s p h o n i t r i l i c molecule.. . The c a l c u l a t e d a s s o c i a t i o n s h i f t s a r e f o r t h e e f f e c t s of one I 2 m o l e c u l e i n t e r a c t i n g w i t h one donor m o l e c u l e . S i n c e a l l the p r o t o n s o f one donor m o l e c u l e have the same c h e m i c a l s h i f t , each n i t r o g e n atom e f f e c t i v e l y i n t e r a c t s w i t h ^ m o l e c u l e of l 2 , where n i s t h e number o f n i t r o g e n atoms p e r donor m o l e c u l e . As a measure o f t h e r e l a t i v e degrees of i n t e r a c t i o n , an i n t e r a c t i o n parameter 6 i s d e f i n e d as 9 = nA Q. C a l c u l a t e d v a l u e s o f 0 f o r the s e r i e s of m e t h y l -p h o s p h o n i t r i l e s i s l i s t e d i n T a b l e 20. Ta b l e 20 V a l u e s o f t h e I n t e r a c t i o n Parameter Compound e(p.p.m.) N 3 P 3 M e 6 - I 2 0.51 N 4 P 4 M e 8 - I 2 0.64 N 5 P 5 M e 1 ( ) . I 2 0.60 Based on t h e c a l c u l a t e d v a l u e s o f 9, t h e o r d e r o f base s t r e n g t h s i s once more N 4 P 4 M e g > N 5 P 5 M e 1 Q > N ^ M e , , b e i n g i n q u a l i t a t i v e agreement w i t h t h e c o n c l u s i o n s reached by a c o n s i d e r a t i o n o f t h e b l u e - s h i f t s o f t h e i o d i n e band. F i g u r e 26 shows the r e a s o n a b l e q u a n t i t a t i v e agreement between the r e l a t i v e magnitudes o f A E t and e f o r the s e r i e s o f m e t h y l p h o s p h o n i t r i l e s . The o r d e r i n g o f the m e t h y l p h o s p h o n i t r i l e s i n r e g a r d s t o donor s t r e n g t h s as deduced from t h e c h a r g e -t r a n s f e r complexes formed w i t h i o d i n e seems t o be w e l l e s t a b l i s h e d . The p r i m a r y e f f e c t o f i - e l e c t r o n d e n s i t i e s a t t h e r i n g n i t r o g e n atoms due t o t h e homomorphic system o f ^ - i n t e r a c t i o n would g i v e the same o r d e r i n g o f base s t r e n g t h s as was found b u t w i t h a g r e a t e r d i f f e r e n c e between t h e t e t r a m e r and t h e pentamer. The i n c l u s i o n of a secondary e f f e c t due t o the d i f f e r e n c e s i n t h e s t a t e s o f h y b r i d i z a t i o n i n the two c a s e s would r e s u l t i n a g r e a t e r s i m i l a r i t y i n t h e donor s t r e n g t h s . A g e n e r a l upward t r e n d o f base s t r e n g t h s as a f f e c t e d by h y b r i d i z a t i o n was assumed on g o i n g t o l a r g e r r i n g s because o f t h e i n c r e a s e d p - o r b i t a l c h a r a c t e r i n t h e h y b r i d o r b i t a l s . R e i d and M u l l i k e n 1 5 6 had found t h a t , f o r the s h i f t e d i o d i n e band, a change i n p o s i t i o n t o s h o r t e r wave-l e n g t h s and an i n c r e a s e i n i n t e n s i t y o c c u r on a d d i n g more p y r i d i n e t o the s o l u t i o n s o f P y l 2 i n n-heptane. A s i m i l a r s o l v e n t e f f e c t was o b s e r v e d f o r t h e complex N^P ?Me,-I 9. The d e c r e a s e i n w a v e l e n g t h o f t h e s h i f t e d i o d i n e band on g o i n g from CC1 4 t o t h e more p o l a r s o l v e n t C H 2C1 2, i n d i c a t i v e F i g . 26. A p l o t o f A E t and 6 v e r s u s r i n g s i z e t o show the agreement i n t h e two p a r a m e t e r s . o f i n c r e a s e d s t a b i l i t y o f the complexes formed, i s a t t r i b u t a b l e t o t h e i n c r e a s e d s t a b i l i z a t i o n by t h e s o l v e n t o f t h e d i p o l a r complex formed. The e f f e c t i s s m a l l s i n c e t h e amount of charge t r a n s f e r r e d i n t h e ground s t a t e o f t h e complex i s s m a l l . INNER-CHAPTER IV CHARGE-TRANSFER COMPLEXES IV. A. I n t r o d u c t i o n I t i s w e l l known t h a t a l o n g w i t h f o r m i n g donor-a c c e p t o r complexes, where th e t r a n s f e r r e d charge goes from the donor t o the a u m o l e c u l a r o r b i t a l of I , i o d i n e i n t e r a c t s s t r o n g l y w i t h c e r t a i n b a s e s , r e s u l t i n g i n the b r e a k i n g o f t h e i o d i n e - i o d i n e bond and the f o r m a t i o n o f i o n i c compounds. These a r e termed i n n e r c h a r g e - t r a n s f e r complexes as opposed t o t h e o u t e r c h a r g e - t r a n s f e r complexes d e s c r i b e d i n Chapter I I I . G l u s k e r and M i l l e r 1 7 8 have examined two d i f f e r e n t 1 * n - i t - - i - . - , - . . . i d compounds formed by 4 - m e t h y l p y r i d x n e and i o d i n e . Compound I , ( C 6 H 7 N ) 2 I 2 i s water s o l u b l e , a l c o h o l i n s o l u b l e , and has a m.p. of 244°C. Compound I I , C g H 7 N I 2 , i s water i n s o l u b l e and i t s m.p. i s 83°C. R a d i a l d i s t r i b u t i o n f u n c t i o n s worked o u t based on i n t e n s i t i e s c o l l e c t e d from X-ray powder e x p e r i m e n t s l e d t o t h e c o n c l u s i o n t h a t compound I has an i o n i c s t r u c t u r e w h i l e t h e second compound i s an o u t e r complex. H a s s e l and H o p e 1 7 9 by c a r r y i n g o u t a s t r u c t u r a l d e t e r m i n a t i o n o f a r e a c t i o n p r o d u c t o f p y r i d i n e and i o d i n e d e m o n s t r a t e d th e e x i s t e n c e + . . o f a c e n t r o s y m m e t r i c (C J - H J - N ) 2 I c a t i o n . The c a t i o n of compound I mentioned above p r o b a b l y has a s i m i l a r s t r u c t u r e . N — I- N F i g u r e 27. The P y 2 I + c a t i o n . The v a r i a t i o n o f t h e c h a r g e - t r a n s f e r band i n t e n s i t y w i t h t i m e o f t h e system P h 3 A s and I 2 has been a t t r i b u t e d by Bhasker e t . a l . t o t h e t r a n s f o r m a t i o n o f t h e o u t e r complex t o t h e i n n e r complex, where t h e i n n e r complex i s a p p a r e n t l y e n e r g e t i c a l l y s t a b l e i n t h i s . . . . c a s e . They proposed th e mechanism o f t r a n s f o r m a t i o n as P h 3As + I 2 i x ' [Ph 3As • I 2 ] o u t e r complex [Ph 3As • I 2 ] — > [ P h 3 A s I ] + l " i n n e r complex The c h a r g e - t r a n s f e r band o f P h 3 S b - I 2 d i d n o t show much v a r i a t i o n w i t h time as i n the case o f P h 3 A s - I 2 , i n d i c a t i v e o f l i t t l e i n n e r complex f o r m a t i o n . The i n n e r complex i n the P h 3 P - I 2 system i s so much more s t a b l e t h a t the o u t e r complex i s n o t even o b s e r v e d i n the spectrum, t h e h i g h b a s i c i t y o f t r i p h e n y l p h o s p h i n e b e i n g the cause o f t h e g r e a t e r s t a b i l i t y o f t h e i n n e r complex as compared t o t h e o u t e r complex. The e x i s t e n c e of b o t h t y p e s o f complexes has l e d to a g r e a t d e a l o f i n t e r e s t i n t h i s a r e a o f s t u d y , t h e main problem h a v i n g been t o d e t e r m i n e whether i n t e r a c t i o n s between the base and f o r example i o d i n e r e s u l t i n i n n e r complexes b e i n g formed i n t h e case o f the use o f s t r o n g donors. I n the l i g h t o f the h i g h b a s i c i t i e s o f t h e m e t h y l -p h o s p h o n i t r i l e s , i t would not be s u r p r i s i n g t h a t they s h o u l d form h i g h l y s t a b l e i n n e r complexes w i t h i o d i n e . However, i t was t h e a c c i d e n t a l s y n t h e s i s o f t h e compound N 4 P 4 M e g I 4 , l a t e r i d e n t i f i e d as t h e i o n i c compound ( N 4 P 4 M e g I ) + I 3 ~ , t h a t l e d t o the study of i n n e r complexes formed by i o d i n e w i t h the m e t h y l p h o s p h o n i t r i l e s . I t was i n t h e attempt o f s y n t h e s i z i n g a d i q u a t e r n a r y p h o s p h o n i t r i l e by r e f l u x i n g N 4P 4Me g i n neat 1 , 4 - d i i o d o b u t a n e t h a t t h e compound N P Me I was formed. IV. B. Experimental and Results IV. B . l . Synthesis of Complexes Compounds of formula N ^ M e ^ , N ^ M e ^ , N 4P 4Me 8I 6, and NgPgMe^Ig can be prepared by heating together the appropriate methylphosphonitrile with an excess of iodine i n a sealed glass tube at 140°C for several hours. The s o l i d s were extracted with either CHC13 or CH 2C1 2 a f t e r the removal of excess iodine by washing with CC1 4 and sublimation i n vacuo, the exception being N 4P 4Me 8I 6 which required extraction with a c e t o n i t r i l e . These compounds are soluble only i n the more polar solvents such as CH 2C1 2 and C H 3 C N and do not sublime. Microanalysis r e s u l t s of the compounds are as Table 21 Req'd (%) Found (%) Compound C H N P I C H N P I N 3 P 3 M e 6 I 4 9 , 8 4 2 , 4 8 5 - 7 3 1 2 - ' 6 8 69.27 9.65 2.40 5. 50 69.00 N 4 P 4 M e 8 I 4 1 1 > 8 8 2 - 9 9 6 - 9 3 1 5 - 3 4 6 2 8 4 H ' 6 9 3.15 6.79 15.07 62.81 N 4P 4Me 8I g 9.05 2.28 5.28 11.67 71.72 9.16 2.35 4.95 71.58 N 5 P 5 M e 1 0 I 6 1 0 - 5 7 2 - 6 6 6 - 1 6 13.62 66.99. 10.80 2.68 6.20 66.67 IV. B.2. N u c l e a r M a g n e t i c Resonance S p e c t r a The p r o t o n magnetic resonance s p e c t r a o f t h e i n n e r complexes are o b t a i n e d f o r comparison w i t h t h o s e of the o u t e r complexes. R e s u l t s Temperature: 35°C S o l v e n t : CDC1 3 (+ few drops of DMSO-dg) S t a n d a r d : i n t e r n a l TMS T a b l e 22 Compound 6 o b s ( P - P - m - > A = 6 o b s ~ 6 p a r e n t donor N 3 P 3 M e 6 1.50 0 N 4P 4Me g 1.50 0 N 5 P 5 M e 1 Q 1.51 0 N 3 P 3 M e 6 I 2 1 , 5 5 ° - 0 5 N 3 P 3 M e 6 I 4 1 - 7 3 ° - 2 3 N 4 P 4 M e 8 I 4 1 ' 8 0 ° - 3 0 N 4 P 4 M e g I 6 1.96 0.46 N 5 P 5 M e 1 Q I 6 1.73 0.22 A l l s p e c t r a o b t a i n e d show a d o u b l e t o f a p p r o x i -m a t e l y t h e same c o u p l i n g c o n s t a n t s (12 Hz) as t h e p a r e n t m e t h y l p h o s p h o n i t r i l e s , w i t h no d i f f e r e n t i a t i o n of t h e m e t h y l groups b e i n g shown upon complex f o r m a t i o n . IV. B.3. E l e c t r o n i c S p e c t r a UV-VIS s p e c t r a o f a l l t h e i n n e r complexes show two i n t e n s e a b s o r p t i o n s a t 293 and 362 my, w i t h the h i g h e r energy band a p p r o x i m a t e l y t w i c e as i n t e n s e as the o t h e r . IV. B.4. V i b r a t i o n a l S p e c t r a o f N 4 P 4 M e g I 4 Raman s p e c t r a o f t h e s o l i d N 4 P 4 M e g I 4 showed two bands, one a t 104 and the o t h e r a t 143 cm - 1. I n C H 2C1 2, the two bands appear a t 109 and 144 cm" 1, w i t h a r e l a t i v e i n t e n s t i y o f 10:2, and t h e 109 cm" 1 band was found t o be p o l a r i z e d . These s p e c t r a were o b t a i n e d on a Cary 81 s p e c t r o m e t e r equipped w i t h a S p e c t r a - P h y s i c s helium-neon l a s e r s o u r c e . I n f r a r e d s p e c t r a o f N 4 P 4 M e g I 4 o b t a i n e d as a N u j o l m u l l between C s l p l a t e s on a P e r k i n - E l m e r 457 s p e c t r o m e t e r showed s i g n i f i c a n t changes from t h a t o f N 4P 4Me g. S h i f t s i n band p o s i t i o n s as w e l l as the appear-ance o f new bands a r e shown i n T a b l e 23. I n f r a r e d S p e c t r a T a b l e 23  of N 4P 4Me g N 4 P 4 M e 8 3160 cm" 1 m 1306 s 1287 1220 868 860 754 733 647 629 430 384 s s 1180 s 920 m s s m m m m and N 4 P 4 M e 3 I 4 a, b N 4 P 4 M e g I 4 3160 m 1314 s 1301 s 1290 s 1266 s 1240 s 1197 s 922 s 915 s 890 m 880 s 870 m 853 m 775 w 765 w 753 m 739 m 648 m 612 m 502 w 433 m 395 m 380 w s s a A b s o r p t i o n s due t o the N u j o l a r e not i n c l u d e d . b T -1 I n cm IV. B.5. Conductivity Measurements Molar co n d u c t i v i t i e s of both the inner and the outer complexes were determined i n spectrograde a c e t o n i t r i l e which was further p u r i f i e d by d i s t i l l a t i o n from calcium hydride while under an atmosphere of dry nitrogen. Measurements were obtained on a Wayne Kerr Universal Bridge, the temperature being regulated by a Sargent Model ST Thermoniter. The c e l l constant of the conductivity c e l l employing platinum electrodes was 0.176 + 0.001 cm"1, as determined with a standard s o l u t i o n of KC1. Conductivity measurements i n each case were corrected f o r that due to the solvent. For each compound either s p e c i f i c r a t i o s of methylphosphonitrile to iodine or the s o l i d complexes were weighed out and the solutions prepared i n volumetric f l a s k s . Since the values of Ag, the molar conductivity at i n f i n i t e d i l u t i o n , could not be determined by extrapolation with a Kohlrausch p l o t , molar c o n d u c t i v i t i e s at the concentration of 10 _ 3M were determined for a basis of comparison. T a b l e 24  Mo l a r C o n d u c t i v i t i e s Compound A N P Me T "3*3 6 4 ^150 N 4 P 4 M e g I 4 136 N 4 P 4 M e g I 6 337 N^P.Me, + 3 3 6 J2 2 9 N 3 P 3 M e g + 44 N 4P 4Me g + "^ 2 38 N 4P 4Me g + 2 I 2 . 57 a A t 10 3 M c o n c e n t r a t i o n , k I n l i t e r mole 1 ohm 1 cm 1 . IV. C. D i s c u s s i o n The o b s e r v a t i o n t h a t t h e f o r m a t i o n o f N„P„Me - I 3 3 6 2 o c c u r s almost i m m e d i a t e l y upon m i x i n g the r e a c t a n t s whereas the i n n e r complexes a re formed s l o w l y a t h i g h e r t e m p e r a t u r e s c o n f i r m s i n g e n e r a l the p o t e n t i a l energy diagram f o r complex forma-t i o n proposed by Bhaskar e t . a l . 1 8 0 , a g r e a t e r energy o f a c t i v a t i o n b e i n g r e q u i r e d f o r i n n e r complex f o r m a t i o n , i n e s t a b i l i t y or an o u t e r complex d e c r e a s e s t h e tendency toward f o r m a t i o n o f t h e i n n e r complex by i n c r e a s i n g t h e v a l u e o f Ea". c D,A Reaction Coordinate nner Complex F i g u r e 28. P o t e n t i a l energy d i a g r a m o f complex f o r m a t i o n . Compared t o t h e o u t e r complexes, t h e f o r m a t i o n o f t h e i n n e r complexes s h o u l d r e s u l t i n g r e a t e r d e s h i e l d i n g o f t h e m e t h y l p r o t o n s . I n comparing t h e c h e m i c a l s h i f t s , t h e d i f f e r e n t i a l s h i f t s o f t h e i n n e r complexes a r e i n g e n e r a l about f o u r t o f i v e t i m e s as g r e a t as t h o s e o f the o u t e r complexes. F o r t h e system t r i m e t h y l p y r i d i n e , new peaks o b s e r v e d i n t h e p r o t o n s p e c t r a upon a d d i n g I 2 had been a t t r i b u t e d 1 ^ 1 " t o t h e i o n i c s p e c i e s o f f o r m u l a (TMP) 2 1 i whereas t h e s l i g h t s h i f t o f t h e p a r e n t peak t o low e r f i e l d had been a t t r i b u t e d t o f o r m a t i o n of t h e o u t e r complex TMP-I 2. D i f f e r e n t i a l s h i f t s o b s e r v e d o f t h e a-me t h y l groups were a p p r o x i m a t e l y 0.1 and 0.33 p.p.m. r e s p e c t i v e l y f o r the o u t e r and i n n e r complexes o f t r i m e t h y l -p y r i d i n e and i o d i n e . The d i f f e r e n t i a l s h i f t s o f t h e complexes o f t h e m e t h y l p h o s p h o n i t r i l e s w i t h i o d i n e a r e of the same magnitude as t h o s e o b s e r v e d f o r t r i m e t h y l p y r i d i n e . I t i s s u r p r i s i n g t h a t , even w i t h i n n e r complex f o r m a t i o n , no d i f f e r e n t i a t i o n o f t h e m e t h y l groups as t o t h e i r p o s i t i o n s r e l a t i v e t o t h e i n t e r a c t i n g n i t r o g e n atom can be d e t e c t e d . I t i s d i f f i c u l t , i n v i e w o f the h i g h a c t i v a t i o n energy r e q u i r e d , t o c o n c e i v e o f t h e r a p i d b r e a k i n g and r e f o r m i n g o f t h e N-I bond, t h e i o d i n e a l t e r n a t i v e l y i n t e r a c t i n g w i t h each n i t r o g e n atom, r e s u l t i n g i n the m e t h y l groups b e i n g e q u i v a l e n t l y d e s h i e l d e d . The p r o t o n s p e c t r a o f t h e i n n e r complexes a t low tempera-t u r e s show b r o a d e n i n g o f the resonances as w e l l as a s l i g h t s h i f t t o lower f i e l d . T h i s may be i n d i c a t i v e o f a s l o w i n g 'of t h e exchange p r o c e s s . N 4 P 4 M e g I 4 i s a s s i g n e d the s t r u c t u r e ( N ^ M e g l ) + I 3 ~ i n t h e s o l i d s t a t e as w e l l as i n s o l u t i o n based on e v i d e n c e from many s o u r c e s . The Raman bands o b s e r v e d can be a s s i g n e d as v± (109 cm" 1) and v 3 (144 cm - 1) of t h e t r i -• • 182 , ^ ^ , , i o d i d e i o n . The e l e c t r o n i c spectrum shows bands and r e l a t i v e i n t e n s i t i e s c h a r a c t e r i s t i c o f t h e t r i i o d i d e i o n , 1 1 8 confirming the assignments of the vi b r a t i o n s . The proton spectra indicates a p o s i t i v e charge on the r i n g , the molar conductivity of 136 being i n the range of 120-160 expected for 1:1 e l e c t r o l y t e s i n a c e t o n i t r i l e . 1 8 3 CH CH. 3' 3' \ CH, ,| \ / V ^ p = N . CHg ^ \ H \ Figure 29. Structure of N.P,Me QI,. 4 4 8 4 By analogy, along with the consideration of the A value of ^150, the structure of N ^ M e ^ can be assigned as (N3P3Me 6I) +I 3-. The A value of N ^ M e ^ seems to indicate t h i s as b e i n g , a 2:1 e l e c t r o l y t e . A values for 2:1 e l e c t r o l y t e s i n a c e t o n i t r i l e are i n the range 220-280. 1 8 3 A possible structure for the compound as a diquaternary i s postulated. ,CH 3 r 1 CH 3  C H 3 " " ~ i II II « . - - - C H , \ = = = N C \ H 3 CH3 \ \ CH 3 I I; 3 Figure 30. structure of N ^ M e I The s t r u c t u r e of N ^ M e ^ I g i s expected to be s i m i l a r to t h a t of N.P.Me-l^. In g e n e r a l , whereas the i n f r a r e d spectrum 4 4 o b of N 3 P 3 M e 6 - I 2 showed o n l y s m a l l changes from t h a t of N 3P 3Me 6, the spectrum of N 4 P 4 M e g I 4 showed l a r g e changes from t h a t of N 4P 4Me 8. The 1220 c m - 1 band of the t e t r a m e r i c methyl-p h o s p h o n i t r i l e has been assigned to the P=N s t r e t c h i n g of the r i n g . 1 8 4 The replacement of t h i s band by two bands at 12 6 6 and 12 4 0 cm 1 show a change s i m i l a r to t h a t found i n the s p e c t r a of a s e r i e s of hexa-n-alkylamino t r i p h o s -p h o n i t n l i c h y d r o c h l o r i d e s . The same v a g P=N v i b r a t i o n i s s h i f t e d to 1170 cm" 1 from 1180 c m - 1 i n N ^ M e ' g 1 8 4 upon formation of the outer complex N 3P 3Me 6-I 2- I t i s e v i d e n t t h a t the l o c a l i z a t i o n of the " l o n e - p a i r " a t a r i n g n i t r o g e n produces s i m i l a r e f f e c t s i n the v i b r a t i o n a l s p e c t r a of the p h o s p h o n i t r i l e s . Though the outer complexes show f a r lower con-d u c t i v i t i e s than the i o n i c i n n e r complexes, i n d i c a t i v e of t h e i r molecular s t r u c t u r e , the f a c t t h a t t h e i r conduc-t i v i t i e s are s t i l l a p p r e c i a b l e i s s u r p r i s i n g . T h i s anoma-lous behaviour had a l s o been observed f o r s o l u t i o n s of I 2 i n p y r i d i n e . 1 8 5 A u d r i e t h and B i r r a t t r i b u t e d the c o n d u c t i v i t y to the f o l l o w i n g e q u i l i b r i u m : Py + l 2 * - * P y l 2 P y l + + i " P y + + + 2 l " (1) 2 [ P y l 2 ] v = * P y l + + I - P y l 2 " (2) ,_ _ I n l i g h t o f t h e known s t r u c t u r e [ P y 2 I ] + I 3 " , i t seems more l i k e l y t h a t t h e f o l l o w i n g e q u i l i b r i u m r e p r e s e n t s a t r u e r v i e w . r i 2 (3) 2 P y l 2 ^ [ P y 2 I ] + I 3 " . (4) A n a l o g o u s l y , t h e c o n d u c t i v i t y o f t h e o u t e r complexes o f th e m e t h y l p h o s p h o n i t r i l e s w i t h . I • i s p o s t u l a t e d as b e i n g due t o t h e f o l l o w i n g e q u i l i b r i u m , i n v i e w o f t h e e v i d e n c e p r e s e n t e d f o r t h e s t r u c t u r e [ N 4 P 4 M e 8 I ] + I 3 - . N 4 P 4 M e g + l 2 . N 4 P 4 M e 8 - I 2 (5) N 4 P 4 M e 8 , i ; 2 + X2 [ N 4 P 4 M e 8 I ] + I 3 ~ ; F = = ^ t N 4 P 4 M e 8 I ] + * • I3" ( 6 ) Popov and D e s k i n 1 8 ^ had found e v i d e n c e f o r t h e s i t i o n o f t h e o u t e r complex CH^CN-I. t r a n s i t i - t o t h e i n n e r complex [CH 3CNI] I from c o n d u c t i v i t y measurements. I n t h i s work, however, l i t t l e e v i d e n c e f o r t h i s e q u i l i b r i u m u i l i b r i u m was found i n t h e d e t e r m i n a t i o n o f t h e c o n d u c t i v i t y o f I ~ i n a c e t o n i t r i l e . I t seems e v i d e n t t h a t t h e m e t h y l p h o s p h o n i t r i l e s form b o t h i n n e r and o u t e r complexes w i t h I _ , much i n t h e same way as does p y r i d i n e . The d i f f e r e n c e between t h e two t y p e s o f donors l i e s i n t h e s t r u c t u r e s o f t h e c a t i o n s t h a t a r e formed. The e x i s t e n c e o f t h e c a t i o n (Py-I-Py) has 158 been r a t i o n a l i z e d by a 3 - c e n t e r M.O. t r e a t m e n t w h i c h • . i n d i c a t e s a s u b s t a n t i a l s t a b i l i t y f o r t h i s c a t i o n r e l a t i v e t o the s e p a r a t e d ( P y l ) + and Py. The same p r o b a b l y h o l d s t r u e f o r the m e t h y l p h o s p h o n i t r i l e s as w e l l , w i t h the f a c t o r o f s t e r i c h i n d r a n c e due t o the a d j a c e n t m e t h y l groups on th e phosphorus p l a y i n g a r o l e i n f a v o u r i n g t h e f o r m a t i o n o f c a t i o n s such as ( N 4 P 4 M e g I ) + . 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