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Primary photochemical processes in hexafluorobiacetyl at 313 nm. Reid, William John 1969

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PRIMARY PHOTOCHEMICAL PROCESSES IN HEXAFLUOROBIACETYL AT 313 NM. by WILLIAM JOHN REID B . A . ( M o d e r a t o r s h i p ) , T r i n i t y C o l l e g e , U n i v e r s i t y o f D u b l i n , R e p u b l i c o f I r e l a n d , 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE e i n t h e Depar tmen t o f . C h e m i s t r y We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF B R I T I S H COLUMBIA December , 1969 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree tha 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. William J. Reicl Department of Chemistry The University of British Columbia Vancouver 8, Canada Date U t h December 1969 ( i i ) . ABSTRACT A b s o l u t e p h o t o c h e m i c a l quantum y i e l d s o f h e x a f l u o r o b i a c e t y l have been o b t a i n e d a t 313 nm. o v e r a range o f p r e s s u r e s . These were c o r r e l a t e d w i t h a b s o l u t e p h o s phorescence and f l u o r e s c e n c e quantum y i e l d s w h i c h had been p r e v i o u s l y o b t a i n e d i n the same p r e s s u r e range i n o r d e r t o f u r t h e r e l u c i d a t e t h e p r i m a r y p r o c e s s . A mechanism i s pr o p o s e d and t h e r e s u l t s a r e d i s c u s s e d i n r e l a t i o n t o i t . The r a t e c o n s t a n t s f o r s i n g l e t d i s s o c i a t i o n , f o r i n t e r s y s t e m c r o s s i n g from t h e l e v e l r e a c h e d on e x c i t a t i o n , and f o r t r i p l e t d i s s o c i a t i o n 8 -1 8 -1 were found t o be 0.7 x 10 sec , 0.6 x 10 sec and 7 -1 1.3 x 10 sec , r e s p e c t i v e l y . The f l u o r e s c e n c e l i f e t i m e i s e s t i m a t e d t o be o f t h e o r d e r o f 2 nse c . above 40 t o r r . ( i i i ) TABLE OF CONTENTS Page T i t l e Page ( i ) A b s t r a c t ( i i ) T a b l e o f C o n t e n t s . . . . ( i i i ) L i s t o f F i g u r e s (v) L i s t o f T a b l e s ( v i ) Acknowledgements ( v i i ) C h a p t e r I - I n t r o d u c t i o n 1 A. The P r i m a r y P r o c e s s 1 B. F l u o r i n a t e d Ketones 9 C. P r e v i o u s Work on H e x a f l u o r o a c e t o n e 11 D. Purpose o f t h i s I n v e s t i g a t i o n 17 C h a p t e r I I - E x p e r i m e n t a l Arrangement and P r o c e d u r e A. Vacuum System 19 B. P r e p a r a t i o n and P u r i f i c a t i o n o f C h e m i c a l s .... 21 C. O p t i c a l Arrangement 22 D. A c t i n o m e t r y 23 E. Measurement o f A b s o r p t i o n C o e f f i c i e n t 26 F. Gas A n a l y s i s 28 (iv) Page C h a p t e r I I I - R e s u l t s and D i s c u s s i o n 33 Comparison w i t h M c i n t o s h ' s Work . . 4 4 Complementary A s p e c t s o f t h i s and M c i n t o s h ' s Work 45 D i s c u s s i o n on t h e I n t e r c e p t 50 C o n c l u d i n g Remarks 51 S u g g e s t i o n s f o r F u r t h e r Work 54 B i b l i o g r a p h y 55 (v) LIST OF FIGURES F i g u r e Page 1. J a b l o n s k i d i a g r a m o f h e x a f l u o r o b i a c e t y l 4 2 . Schematic d i a g r a m o f vacuum system 20 3. Absorbance v e r s u s P r e s s u r e f o r h e x a l f u o r o -b i a c e t y l 27 , 4. A n a l y s i s System .29 5. Wheatstone b r i d g e c i r c u i t f o r d e t e c t o r system 51 6. I n v e r s e quantum y i e l d s v e r s u s p r e s s u r e u s i n g c a r b o n monoxide d a t a 56 7. I n v e r s e quantum y i e l d s v e r s u s p r e s s u r e u s i n g h e x a f l u o r o e t h a n e d a t a 57 8. P r i m a r y quantum y i e l d s v e r s u s p r e s s u r e - low p r e s s u r e r e g i o n . . . . . 58 9. R a t i o o f f l u o r e s c e n c e y i e l d t o p r i m a r y quantum y i e l d v e r s u s p r e s s u r e f o r HFB 49 1 0 . A f u n c t i o n o f t h e p r i m a r y quantum y i e l d v e r s u s i n v e r s e o f p r e s s u r e (High P r e s s u r e Region) 52 1 1 . Known quantum yields'" f o r HFB v e r s u s p r e s s u r e . 55 ( v i ) LIST OF TABLES Page Table 1 . . . . . . . . . . . . . 34 Table 2 35 ( v i i ) ACKNOWLEDGMENTS I w i s h t o e x p r e s s my s i n c e r e g r a t i t u d e t o Dr. G e r a l d B. P o r t e r , f o r h i s encouragement, a d v i c e and c r i t i c i s m t h r o u g h o u t t h i s i n v e s t i g a t i o n . I am i n d e b t e d t o Dr. J.S.E. M c i n t o s h and Dr. R.D. S u a r t f o r numerous f r u i t f u l d i s c u s s i o n s . F i n a l l y , I w i s h t o thank my w i f e whose p a t i e n c e , encouragement and t y p i n g made t h i s t h e s i s p o s s i b l e . CHAPTER ONE INTRODUCTION A. The P r i m a r y P r o c e s s C a r b o n y l compounds have a t t r a c t e d more a t t e n t i o n i n t h e p h o t o c h e m i c a l f i e l d t h a n any o t h e r c l a s s o f compound. The p r o d u c t i o n o f r a d i c a l s d u r i n g p h o t o l y s i s has been a major i n t e r e s t . A c c e s s i b i l i t y o f t h e l o n g - w a v e l e n g t h a b s o r p t i o n band i n the nea r UV r e g i o n o f t h e spectrum has e n a b l e d d e t a i l e d s t u d i e s t o be made. I n p a r t i c u l a r , t h e p h o t o c h e m i c a l and p h o t o p h y s i c a l b e h a v i o r o f a l i p h a t i c k e t o n e s i n t h e gas phase has been s t u d i e d e x t e n s i v e l y . I t has become o b v i o u s t h a t t h e systems have c o n s i d e r a b l e k i n e t i c c o m p l e x i t y . The P r i m a r y P r o c e s s c o m p r i s e s t h e i n i t i a l a c t o f a b s o r p t i o n o f a photon by a m o l e c u l e t o produce an e x c i t e d e l e c t r o n i c s t a t e , and a l l subsequent e v e n t s w h i c h l e a d e i t h e r t o d i s a p p e a r a n c e o f t h e m o l e c u l e ( p r i m a r y p h o t o c h e m i c a l p r o c e s s ) , o r t o t h e r e t u r n o f t h e m o l e c u l e t o i t s t h e r m a l l y e q u i l i b r a t e d ground e l e c -t r o n i c s t a t e ( p r i m a r y p h o t o p h y s i c a l p r o c e s s e s ) . - 2 -I n the p r i m a r y p h o t o c h e m i c a l p r o c e s s t h e r e i s u s u a l l y a v a r i e t y o f p a t h s f o r d e g r a d a t i o n o f the e l e c t r o n i c energy o f e x c i t a t i o n . C h e m i c a l p a t h s i n c l u d e i n t r a m o l e c u l a r r e a r r a n g e m e n t s (e.g. c i s - t r a n s i s o m e r i z a -t i o n o f s t i l b e n e ) and t h e f o r m a t i o n o f f r e e r a d i c a l s w h i c h combine w i t h each o t h e r , (e.g. 'CF3 +-CF^ -> C2Fg i n h e x a f l u o r o a c e t o n e ) o r w h i c h combine w i t h o t h e r m o l e c u l e s i n s econdary p r o c e s s e s (e.g. acetone) t o form new p r o d u c t s . F r e q u e n t l y t h e unknown n a t u r e and magnitude o f t h e s e l a t t e r p r o c e s s e s t e n d t o o b s c u r e the p r i m a r y p r o c e s s . The f i r s t a b s o r p t i o n band o f k e t o n e s , w h i c h g e n e r a l l y has a maximum around 2 80 nm., r e s u l t s from e x c i t a t i o n o f one o f t h e non-bonding e l e c t r o n s o f t h e c a r b o n y l oxygen i n t o an a n t i b o n d i n g o r b i t a l i . e . an TT* «- n t r a n s i t i o n . The e x c i t e d s i n g l e t s t a t e ' so p r o -duced i s d e s i g n a t e d S - ^ . Other e l e c t r o n i c s t a t e s are a b b r e v i a t e d t h u s : SQ (ground); , S ^ ... ( e x c i t e d s i n g -l e t s ) ; T^, T 2 ... ( e x c i t e d t r i p l e t s ) . A b s o r p t i o n o f l i g h t produces a s p e c i e s con-t a i n i n g p a r t o f t h e e x c i t a t i o n energy as e x c e s s v i b r a t i o n -a l energy. The s t a t i o n a r y s t a t e c o n c e n t r a t i o n o f e x c i t e d s p e c i e s i s , under normal c i r c u m s t a n c e s , so s m a l l t h a t - 3 -t h e y a r e e s s e n t i a l l y s u r r o u n d e d by a h e a t b a t h o f u n e x c i t e d m o l e c u l e s . I n p r i n c i p l e , t h e v i b r a t i o n a l l y e x c i t e d m o l e c u l e i n an upper e l e c t r o n i c s t a t e can a t t a i n v i b r a t i o n a l e q u i l i b r i u m by two mechanisms. E i t h e r t h i s d e g r a d a t i o n o f v i b r a t i o n a l energy i s a c c o m p l i s h e d by a s i n g l e - s t e p d e a c t i v a t i o n from h i g h t o low v i b r a t i o n a l l e v e l s o r by a m u l t i s t e p cascade from one group o f v i b r a t i o n a l l e v e l s . t o the n e x t l o w e r group."'" ^ Recent s t u d i e s on h e x a f l u o r o a c e t o n e and a l s o 7 on t h e i s o m e r i z a t i o n o f 1 , 3 , 5 - c y c l o h e p t a t r i e n e s u p p o r t a m u l t i s t a g e p r o c e s s . I n t h e l a t t e r s t u d y t h e p h o t o -c h e m i c a l i s o m e r i z a t i o n t o t o l u e n e o c c u r s v i a t h e v i b r a -t i o n a l l y e x c i t e d ground s t a t e m o l e c u l e . A t k i n s o n and Thrush o b t a i n e d t h e v i b r a t i o n a l energy removed p e r c o l l i s i o n when qu e n c h i n g o f i s o m e r i z a t i o n by v a r i o u s added gases was s t u d i e d a t a v a r i e t y o f w a v e l e n g t h s . N e v e r t h e l e s s , much o f t h e contemporary k i n e t i c d a t a l e n d s s u p p o r t t o t h e c o n c e p t o f s t r o n g c o l l i s i o n a l t r a n s f e r . The p a t h s t h a t e x i s t f o r energy d i s s i p a t i o n from t h e p h o t o e x c i t e d m o l e c u l e a r e i l l u s t r a t e d s c h e m a t i -c a l l y by t h e J a b l o n s k i d i a g r a m f o r h e x a f l u o r o b i a c e t y l ( F i g u r e 1 ) . - 4 -2 CO FIGURE 1. J a b l o n s k i d i a g r a m o f H e x a f l u o r o b i a c e t y 1 a t 313 nm. P r o c e s s e s : R a d i a t i v e n o n - r a d i a t i v e p h o t o c h e m i c a l Processes (1) and (8), photodissociation, w i l l both occur i n the general case. They must take place rapi d l y to compete with i n t e r n a l conversion and loss of v i b r a t i o n a l energy by c o l l i s i o n s . D issociation from the t r i p l e t w i l l occur from the v i b r a t i o n a l l e v e l reached from intersystem crossing. Although i t seems that T° (lowest v i b r a t i o n a l l e v e l of f i r s t excited t r i p -l e t state) may be long-lived enough to be re-energized by c o l l i s i o n and di s s o c i a t e i n c e r t a i n molecules (e.g. hexafluoroacetone), t h i s thermal d i s s o c i a t i o n should be absent i n HFB as the phosphorescence l i f e t i m e i s both temperature independent and pressure independent near room temperature. Process (1). w i l l have a very small temperature dependence due to the d i s t r i b u t i o n of thermal energy i n the ground state. Both (1) and (8) should be wavelength dependent. ..•>.!.•.• - From•level 0 of S^, the. molecule can return to any one of the v i b r a t i o n - r o t a t i o n l e v e l s of the ground state with the emission of fluorescence. Fluorescence i s a r a d i a t i v e t r a n s i t i o n between states of l i k e m u l t i p l i c i t y With some molecules i t seems that fluorescence i s coming also from non-equibrated l e v e l s of S, (e.g.. HFB)"*". - 6 -I n t h e absence o f quenchers th e f l u o r e s c e n c e e f f i c i e n c y depends on t h e r e l a t i v e r a t e s o f t h e r a d i a t i v e p r o c e s s on t h e one hand, and t h e r a d i a t i o n l e s s p r o c e s s e s o f i n t e r s y s t e m c r o s s i n g (k. ) and i n t e r n a l c o n v e r s i o n (k,) J i s c 6 on t h e o t h e r . 8 I n a r e c e n t s t u d y by Parmenter and White on t h e f l u o r e s c e n c e o f benzene vapour a t p r e s s u r e s below 0.01 t o r r i t was found t h a t (J>^  was c o n s t a n t a t 0.4. T h i s i n d i c a t e d t h a t s i g n i f i c a n t n o n - r a d i a t i v e r e l a x a t i o n . . . was o c c u r r i n g even though th e p r e s s u r e s used were such t h a t h a r d - s p h e r e c o l l i s i o n s c o u l d n o t r e l a x t h e ''"B- s t a t e o f ^ 2u benzene. A f i r m c h a r a c t e r i z a t i o n o f t h e n o n - r a d i a t i v e r e l a x a t i o n can o n l y be a c c o m p l i s h e d by o b s e r v a t i o n o f t h e s t a t e t h a t t r a p s the r a d i a t i o n . A t p r e s e n t t h i s i s g i m p o s s i b l e t o do f o r benzene but t h e s e a u t h o r s f e e l t h a t t h e n o n - r a d i a t i v e decay i s p r i m a r i l y m a n i f e s t as t r i p l e t s t a t e f o r m a t i o n o r c h e m i c a l i s o m e r i z a t i o n o r b o t h . Parmen-9 t e r e t a l have r e c e n t l y extended t h i s v e r y low p r e s s u r e work t o b i a c e t y l . Whereas w i t h benzene the f i n a l s t a t e i n v o l v e d i n t h e n o n - r a d i a t i v e t r a n s i t i o n was n o t d i r e c t l y o b s e r v e d under t h e i r i s o l a t e d m o l e c u l e c o n d i t i o n s , w i t h b i a c e t y l i t was o b s e r v e d . Both t h e i n i t i a l s t a t e (the e x c i t e d s i n g l e t ) and t h e f i n a l s t a t e (the t r i p l e t ) i n v o l v e d i n i n t e r s y s t e m c r o s s i n g can be o b s e r v e d d i r e c t l y by t h e i r e m i s s i o n . They d i d t h e i r e x p e r i m e n t s below 10 t o r r where th e y e x p e c t e d t o see r e l a x a t i o n o f e x c i t e d b i a c e t y l under i s o l a t e d m o l e c u l e c o n d i t i o n s . They found t h a t t h e s i n g l e t -e m i s s i o n y i e l d and t h e t r i p l e t - e m i s s i o n y i e l d were c o n s t a n t o v e r th e p r e s s u r e range 40 t o 0.1 t o r r . As one o f t h e e x c i t e d - s i n g l e t decay p a t h s i s i n t e r s y s t e m c r o s s i n g the r a t e o f t r i p l e t f o r m a t i o n s h o u l d be p r e s s u r e i n d e p e n d e n t . The c o n s t a n t t r i p l e t e m i s s i o n y i e l d i s c o n s i s t e n t w i t h t h i s r e -q u i r e m e n t . T h e r e f o r e i t i s a p p a r e n t t h a t c o l l i s i o n r a t e has no o b s e r v a b l e e f f e c t on t h e r a t e o f T i n t e r s y s t e m c r o s s i n g r a t e a t p r e s s u r e s as low as 0.1 t o r r . A l a r g e •volume o f energy t r a n s f e r " ^ and p h o t o - o x i d a t i o n s t u d i e s " ^ have l e d t o t h e q u a n t i t a t i v e c o n c l u s i o n t h a t i n t e r s y s t e m c r o s s i n g i n b i a c e t y l a c c o u n t s f o r a l l n o n - r a d i a t i v e decay o f the e x c i t e d s i n g l e t s t a t e . The s i n g l e t - t r i p l e t i n t e r s y s t e m c r o s s i n g p r o -c e s s e s , (3) and ( 7 ) , a l t h o u g h s p i n f o r b i d d e n o c c u r w i t h r a t e s comparable t o t h a t o f a s p i n a l l o w e d r a d i a t i v e t r a n s -8 - 1 9 i t i o n ( k ^ s c = 1 x 10 sec i n b i a c e t y l ). As w i t h i n t e r n a l c o n v e r s i o n t o t h e ground s t a t e , t h e r a t e o f i n t e r s y s t e m - 8 -c r o s s i n g i n c r e a s e s r a p i d l y as the s e p a r a t i o n o f t h e l e v e l s d e c r e a s e s and, i n a d d i t i o n i t depends on the degree o f m i x i n g o f t h e s t a t e s i . e . t h e degree of t r i p l e t c h a r a c t e r 13 i n t h e s i n g l e t s t a t e and v i c e - v e r s a . In i t * <• n s t a t e s , i n t e r s y s t e m c r o s s i n g i s r a p i d , t h e energy s e p a r a t i o n o f S-^  and b e i n g s m a l l owing t o t h e s m a l l o v e r l a p o f t h e o r b i t a l s . S i n c e t h e competing r a d i a t i v e t r a n s i t i o n from upper s i n g l e t t o ground s t a t e i s symmetry f o r b i d d e n , t h e quantum e f f i c i e n c y o f t r i p l e t f o r m a t i o n o f t e n approaches u n i t y (e.g. b i a c e t y l e x c i t e d by 436 nm). R a d i a t i v e t r a n s i t i o n s between s t a t e s o f d i f f -e r e n t m u l t i p l i c i t y (e.g. t r i p l e t - s i n g l e t ) can t a k e p l a c e . T h i s l u m i n e s c e n c e i s c a l l e d p h osphorescence. The i m p o r t a n c e o f f l u o r e s c e n c e and phosphores-cence l i e s n o t i n t h e i r a b s o l u t e magnitude ( u s u a l l y l e s s t h a n 20% o f e x c i t e d m o l e c u l e s l o s e energy v i a e m i s s i o n i n c a r b o n y l compounds) but i n t h e i r d i a g n o s t i c v a l u e . F o r example, where the v a r i a t i o n o f p r e s s u r e does n o t a f f e c t t h e y i e l d r a t i o o f f l u o r e s c e n c e t o p h o s p h o r e s c e n c e , i t i s r e a s o n a b l e t o c o n c l u d e t h a t i n t e r s y s t e m c r o s s i n g t o t h e t r i p l e t s t a t e and f l u o r e s c e n c e b o t h o c c u r from th e same v i b r a t i o n a l l e v e l o f t h e s i n g l e t s t a t e . T h i s was found t o - 9 -be t h e case i n a c e t a l d e h y d e . An i n c r e a s e i n f l u o r e s c e n c e y i e l d as p r e s s u r e i s r a i s e d i n d i c a t e s t h a t p h o t o c h e m i s t r y i s quenched from a h i g h e r v i b r a t i o n a l l e v e l . A l t h o u g h i t i s a s p i n f o r b i d d e n t r a n s i t i o n , c o n v e r s i o n from t h e l o w e s t t r i p l e t t o ground s t a t e p r o c e s s (11) seems t o be more i m p o r t a n t t h a n c o n v e r s i o n from the l o w e s t s i n g l e t l e v e l . There a r e two re a s o n s f o r t h i s . F i r s t l y , t h e energy s e p a r a t i o n o f t h e t r i p l e t and the ground s t a t e i s always l e s s t h a n t h a t o f t h e s i n g l e t . S e c o n d l y , t h e competing r a d i a t i v e p r o c e s s i s v e r y much s l o w e r . I n ke t o n e s t h i s p r o c e s s appears t o be a major pathway f o r energy d i s s i p a t i o n u n l e s s t h e p r i m a r y p h o t o c h e m i c a l y i e l d . , 1 3 i s l a r g e . B. F l u o r i n a t e d Ketones P h y s i c a l p h o t o c h e m i s t s a r e c o n t i n u a l l y s e a r c h i n g f o r t h e p e r f e c t m o l e c u l e w i t h w h i c h t o s t u d y t h e p r i m a r y p r o c e s s . T h e i r f i r s t c r i t e r i o n i s f o r t h e p h o t o c h e m i s t r y t o be " c l e a n " , i . e . l i m i t e d p r e d i c t a b l e p r o d u c t s o v e r a wide range o f t e m p e r a t u r e s and p r e s s u r e s . S e c o n d l y , i t i s des-i r a b l e f o r e m i s s i o n t o t a k e p l a c e c o n c u r r e n t l y i n o r d e r t o - 9 -be t h e c a s e i n a c e t a l d e h y d e . An i n c r e a s e i n f l u o r e s c e n c e y i e l d as p r e s s u r e i s r a i s e d i n d i c a t e s t h a t p h o t o c h e m i s t r y i s quenched from a h i g h e r v i b r a t i o n a l l e v e l . A l t h o u g h i t i s a s p i n f o r b i d d e n t r a n s i t i o n , c o n v e r s i o n from t h e l o w e s t t r i p l e t t o ground s t a t e P r o c e s s (11) seems t o be more i m p o r t a n t t h a n c o n v e r s i o n from t h e l o w e s t s i n g l e t l e v e l . There a r e two r e a s o n s f o r t h i s . F i r s t l y , t h e energy s e p a r a t i o n o f t h e t r i p l e t and t h e ground s t a t e i s always l e s s t h a n t h a t o f t h e s i n g l e t . S e c o n d l y , t h e competing r a d i a t i v e p r o c e s s i s v e r y much s l o w e r . I n k e t o n e s t h i s p r o c e s s appears t o be a major pathway f o r energy d i s s i p a t i o n u n l e s s t h e p r i m a r y p h o t o c h e m i c a l y i e l d . , 1 3 i s l a r g e B. F l u o r i n a t e d Ketones P h y s i c a l p h o t o c h e m i s t s a r e c o n t i n u a l l y s e a r c h i n g f o r t h e p e r f e c t m o l e c u l e w i t h w h i c h t o s t u d y t h e p r i m a r y p r o c e s s . T h e i r f i r s t c r i t e r i o n i s f o r t h e p h o t o c h e m i s t r y t o be " c l e a n " , i . e . l i m i t e d p r e d i c t a b l e p r o d u c t s o v e r a wide range o f t e m p e r a t u r e s and p r e s s u r e s . S e c o n d l y , i t i s des-i r a b l e f o r e m i s s i o n t o t a k e p l a c e c o n c u r r e n t l y i n o r d e r t o — 10 -f u r t h e r e l u c i d a t e t h e p r i m a r y p r o c e s s . A l i p h a t i c k e t o n e s q u a l i f y under t h e second c r i t e r i o n b u t u n f o r t u n a t e l y , t h e i r p h o t o c h e m i s t r y can be 14 c o m p l i c a t e d by t h e l a r g e number o f p r o d u c t s formed . The s i m p l e s t member o f t h e s e r i e s , a c e t o n e , can g i v e as p r o d u c t s c a r b o n monoxide, e t h a n e , methane, b i a c e t y l , m e t h y l e t h y l k e t o n e , k e t e n e o r a c e t a l d e h y d e depending on t h e c o n d i t i o n s 12 of p h o t o l y s i s . B i a c e t y l (butane - 2 , 3 - d i o n e ) , t h e f i r s t member o f t h e homologous s e r i e s o f t h e d i k e t o n e s , i s e q u a l l y complex. These compounds f a i l t o be p h o t o c h e m i c a l l y s i m p l e due t o t h e r e l a t i v e weakness o f t h e C-H bond (82 k c a l / m o l e ) . R a d i c a l s formed i n t h e p r i m a r y p h o t o c h e m i c a l a c t r e a d i l y a b s t r a c t hydrogen atoms from t h e p a r e n t k e t o n e , p r o d u c i n g many f i n a l p r o d u c t s . Acetone has t h e added d i s a d v a n t a g e t h a t one o f i t s p r o d u c t s , b i a c e t y l , quenches i t s e m i s s i o n . O t h e r members o f t h e a c e t o n e and b i a c e t y l homologous s e r i e s a r e p l a g u e d by s i m i l a r c o m p l i c a t i o n s . The s i t u a t i o n i s n o t much improved i n f u l l y 12 15 c h l o r i n a t e d o r p a r t i a l l y c h l o r i n a t e d k e t o n e s . F o r f u l l y f l u o r i n a t e d k e t o n e s , however, th e s t r o n g C-F bond (114 k c a l / mole) r e s u l t s i n a s u p p r e s s i o n o f secondary r e a c t i o n s o v e r a wide range o f c o n d i t i o n s . F o r example, t h e p r i m a r y p h o t o -- 11 -c h e m i c a l a c t i n h e x a f l u o r o a c e t o n e r e s u l t s s o l e l y i n the f o r m a t i o n o f CF^ and CO i n a 2:1 m o l a r r a t i o a t normal -t e m p e r a t u r e s ( i . e . < 370° C ) 1 ^ . A g r e a t d e a l o f p h o t o c h e m i c a l as w e l l as p h o t o -2 4 p h y s i c a l s t u d i e s have been done on HFA ' . I t b o t h f l u o r -e s c e s and p h o s p h o r e s c e s , so i t would seem t o be an i d e a l m o l e c u l e i n w h i c h t o o b s e r v e d e t a i l s o f t h e p r i m a r y p r o c e s s . However i t has one drawback: i t s e q u i l i b r a t e d t r i p l e t s t a t e i s l o n g - l i v e d enough t o d i s s o c i a t e t h e r m a l l y a t room t e m p e r a t u r e . T h i s c o m p l i c a t e s the d i a g n o s i s o f t h e p r i m a r y p r o c e s s . I t appears t h a t i n HFB, t h i s pathway (analagous t o t h e r m a l d i s s o c i a t i o n ) i s a b s e n t a t a s i m i l a r temperature''". C. P r e v i o u s Work on H e x a f l u o r o b i a c e t y 1 17 Whittemore and Szwarc i n 1963 p u b l i s h e d a s h o r t n o t e on t h e gas phase p h o t o l y s i s o f HFB a t 25° C and 150° C. They f o u n d . c a r b o n monoxide and h e x a f l u o r o e t h a n e produced i n a 2:1 m o l a r r a t i o . Mcintosh^" s t u d i e d t h e phosphorescence and f l u o r e s c e n c e quantum y i e l d s o f HFB vapour a t v a r i o u s e x c i t i n g w a v e l e n g t h s between 290 and 440 nm, t h e r e b y showing, i n a q u a n t i t a t i v e manner, t h e .importance o f e m i s s i o n p r o c e s s e - 12 -i n energy d i s s i p a t i o n . Furthermore, he showed the impor-tance of v i b r a t i o n a l relaxation processes and how they a f f e c t other parameters. He was also able to make assign-ments regarding the observed el e c t r o n i c t r a n s i t i o n s from the absorption and emission spectra. Interestingly, he observed that the phosphorescence l i f e t i m e of HFB vapour was independent of temperature from 27° to -57° C. Deta i l s of Mcintosh's work: to explain his r e s u l t s quanti-t a t i v e l y he assumed only three species were of importance, namely, the v i b r a t i o n a l l y hot s i n g l e t state reached on ex c i t a t i o n and the thermally e q u i l i b r a t e d excited s i n g l e t e and t r i p l e t states which r e s u l t a f t e r v i b r a t i o n a l relaxation. Such a "strong" c o l l i s i o n a l mechanism i s not altogether 4 r e a l i s t i c i n view of recent work by Kutschke and co-workers and also of his own r e s u l t s . A k i n e t i c d e s c r i p t i o n of the primary process, which includes a complete multistage v i b r a t i o n a l degradation, becomes a l g e b r a i c a l l y unmanageable i n the sense of an actual evaluation of rate-constants, or even i n terms of a quantitative t e s t of the mechanism v i a various graphical p l o t s . A c t u a l l y , the strong c o l l i s i o n approach i s already complicated even though i t i s a gross - 13 -s i m p l i f i c a t i o n . The processes necessary to account for the observed r e s u l t s are as follows: B + hv -*• 1B* a $d 2C0 + C 2 F g (1) XB* £f hv f .. + B (2) 1B* k | S C 3B* • (3) 1 B*' .+ M <$. 1 B ° + M * ( 4) ^•B0 £f hv f + B (5) 1 B ° $6 B (6) 1 B ° k | s c 3B* • (7) 3B* k d 2C0 + C 2 F 6 ( g ) 3B* + M + 3 B ° + M* (9) 3 B ° Jp hV + B (10) 3 B ° *11 B (11) 3B o + 3 B o ^12 ? (12) - 14 -with B a ground state hexafluorobiacetyl molecule, and M, any molecule which causes v i b r a t i o n a l e q u i l i b r a t i o n . From a steady-state treatment of the mechanism, the emission y i e l d s were given by the expressions k f uM k* (k^+ k. + k^) (k* + k* + k*+ UM) (k*+ k* + k*+ UM) f xsc 6 f i s c d w f i s c d <{>p = 3 coM . i s c CQM (a)M+kd) (kf+ k. s c+ k 6) (k*+ k j s c + k* +_UM) k* + 3 coM , i s c (coM + k,) (k* + k* " + k* + uM) d f i s c d with 3 = k p k + k,, p 11 when M = I t f o l l o w s from e q u a t i o n s (13) and (14) t h a t 0, ° k f * f * f • (15) a c o n s t a n t , (k* + k* + k* ) f i s c d ' and d>° = 0, A t i n f i n i t e p r e s s u r e the l i m i t i n g y i e l d s become: k f (16) and = B k i s c (k~ + k. +. kc) (k-+ k. + kc) f i s c 6 f i s c 6 i . e . <J>^  and <(> a r e c o n s t a n t , i n d e p e n d e n t o f p r e s s u r e and w a v e l e n g t h . I n t h e i n t e r m e d i a t e p r e s s u r e range t h e p a t t e r n i s more complex. However, t h e shape t h e e m i s s i o n y i e l d s s h o u l d t a k e a r e shown below. M c i n t o s h ' s r e s u l t s showed t h e s e g e n e r a l t r e n d s . - 16 -Predicted form of the emisson y i e l d s . Because Mcintosh found that at low gas pressures c()p decreases much fast e r than o)^  i t - was necessary to consider fluorescence from non-equilibrated "*"B* l e v e l s . The value of (J>£ extrapolates smoothly to a f i n i t e l i m i t at zero pressure. This residual fluorescence y i e l d decreases- with increasing e x c i t a t i o n energy as process (1) presumably becomes dominant. 4 Kutschke and co-workers have observed t h i s e f f e c t i n HFA . - 17 -Mcintosh postulated that process (8) becomes important at low pressures (around 1 torr) where process (9) cannot compete with i t . This i s indicated by the constant Q)^ while <f) begins to decrease rapi d l y as the pressure i s reduced below 5 t o r r . The i n c l u s i o n of process (3) ( i . e . intersystem crossing to the t r i p l e t manifold from excited s i n g l e t levels) i s necessitated by the observation that while o> i s pressure independent above 20 t o r r , o)^  i s s t i l l increasing. If intersystem crossing to the t r i p l e t i s only v i a process (7), which competes with fluorescence from ^B^, then pressure dependencies within the s i n g l e t manifold should be r e f l e c t e d by the phosphorescence as well. This i s experimentally not the case. D. Purpose of t h i s Investigation 17 Although Whittemore and Szwarc have reported i n a short note that the gas phase photolysis of HFB at both 25° and 150° C produced only CO and C 2 F g i n a 2:1 molar r a t i o , they gave no quantitative photochemical y i e l d s . Mcintosh, on the other hand, has r e c e n t l y 1 determined the - 18 -absolute emission quantum y i e l d at 250 t o r r of HFB and has put his yields,' obtained as a function of pressure and e x c i t a t i o n energy, on an absolute basis. I t i s now necessary to assess the ro l e of d i s s o c i a t i o n i n energy d i s s i p a t i o n , to determine the rate of s i n g l e t d i s s o c i a t i o n k* and the rate constants for other d i s s o c i a t i o n s , d Absolute photochemical quantum y i e l d s of HFB at 313 nm at 23° C over a range of pressures are therefore required to a r r i v e at values for k* and k* . These can ^ d i s c then be compared with Mcintosh's re s u l t s which were obtained from fluorescence y i e l d measurements alone. CHAPTER II EXPERIMENTAL ARRANGEMENT AND PROCEDURE A. Vacuum System In view of the fac t that mercury i s an e f f i c i e n t 4 quencher of hexafluoroacetone t r i p l e t state i t was thought a d v i s i b l e to b u i l d a mercury-free vacuum system to handle hexafluorobiacetyl. The vacuum system was of standard, a l l glass construction (Figure 2) consisting of a ga l l e r y of storage globes, a section leading to the photolysis c e l l and a manifold with a series of traps for p u r i f i c a t i o n together with a Le Roy-Ward s t i l l . The temperature of the s t i l l was measured with Cu-constantan thermocouples placed at the top and bottom and two intermediate pos i t i o n s . Apiezon N grease was used on the stopcocks. The pumping system consisted of a standard rotary o i l pump and a metal two-stage d i f f u s i o n pump (Edwards E01) operated with S i l i c o n e 704 o i l (Dow Corning). The system could be evacuated to 3 x 10 to r r -5 with a r e l i a b l e working vacuum of 4 x 10 to r r a f t e r i s o l a -t i o n from the pump. Pressures were measured with an NRC S T O R A G E GLOBES DIFFUSI0N4 PUMP T R A P S u S T I L L C E L L FIGURE 2. Schematic diagram of vacuum system. - 21 -thermocouple, an NRC 538P i o n i z a t i o n gauge, and a pyrex s p i r a l gauge accurate to ±0.5 t o r r . B. Preparation and P u r i f i c a t i o n of Chemicals (i) Hexafluorobiacetyl (HFB) HFB was prepared by the chromic acid oxidation of 2,3-dichloro-l,1,1,4,4,4-hexafluoro-2-butene following 1 18 the modification by Mcintosh of Moore and Clark's method. The crude condensate containing the HFB was p a r t i a l l y ' p u r i f i e d by trap-to-trap d i s t i l l a t i o n i n vacuo from -78° C (methanol/CC>2) 'to' -96° C (toluene/liquid N^) slush baths. This procedure was repeated several times. The majority of t h i s semi-pure material was stored i n break-seals at -78° C. The portion of HFB to be used i n the series of experiments was further p u r i f i e d by d i s t i l l a t i o n through a Le Roy-Ward s t i l l set at -65° C and then stored i n the side arm of a blackened 1 - l i t r e globe at -78° C. Immediately p r i o r to use the sample was degassed at -196° C by trap-to-trap d i s t i l l a t i o n and again put through the Le Roy-Ward s t i l l . ( i i ) Hexafluoroethane (HFE) HFE, supplied by Matheson Co. (Freon -116) was degassed at -196° C by trap-to-trap d i s t i l l a t i o n . - 22 -( i i i ) Carbon Monoxide Carbon monoxide, supplied by Matheson Co. (C.P. Grade) was used without further p u r i f i c a t i o n . C. Optical Arrangement The e x c i t a t i o n source for the experiments consis-ted of a PEK 110 mercury arc lamp operated at 100 watts from a s t a b i l i z e d DC power supply (PEK model 401) . A Bausch and Lomb Grating Monochrometer (33-86-25) was used for i s o l a t i o n of the e x c i t i n g wavelength. A UV grating (33-86-01) 2700 grooves/mm. , blazed at 250 -mm. , r e c i p r o c a l l i n e a r dispersion of 3.2 nm/mm was used. The e x i t s l i t was set at 3.4 mm. The divergent beam was passed through a combina-ti o n of two quartz plano-convex lenses and a Corning 7-54 v i s i b l e absorbing f i l t e r . This produced a p a r a l l e l beam of UV r a d i a t i o n which then entered the reaction c e l l . The reaction vessel made of Pyrex, had p a r a l l e l quartz windows attached to i t by "Ar a l d i t e " epoxy r e s i n . It was 10.5 ± 0.05 cm. i n length and 8.0 ± 0.5 mm. i n diameter. It was placed as accurately p a r a l l e l as possible i n the path of the beam. The Pyrex body of the c e l l was blackened to - 23 -exclude external r a d i a t i o n . The entire o p t i c a l t r a i n was enclosed by aluminum shielding to exclude stray external l i g h t . D. Actinometry The i n t e n s i t y of the absorbed r a d i a t i o n at the various pressures was monitored by means of a phototube (RCA 935 vacuum photodiode, S-5 response) operated at 90 v o l t s . I t was c a l i b r a t e d against the potassium f e r r i o x a l a t e 3 actinometer of Hatchard and Parker . The quartz actinometer c e l l , diameter 1.6 cm., depth 1.0 cm., was placed immediately behind the reaction vessel. O p t i c a l densities of exposed and developed solutions were determined on a Cary 14 spectro-photometer at 510 nm. The usual blank correction was made with an unexposed solu t i o n . 1 Because of the large transmission losses from the s i l i c a - a i r interfaces, of the reaction vessel and the actinometer c e l l , i t was necessary to apply a correction factor to obtain the absolute i n t e n s i t y of the absorbed ' 2 0,21 r a d i a t i o n . "For convenience a l l windows were assumed to have i d e n t i c a l losses from r e f l e c t i o n . - 24 -Let a be the f r a c t i o n of l i g h t l o s t in.lpassirig through a window. Evacuated c e l l : -^actinometer measured I - 2al + a I o o o - a (I - 2al + a I } o o o 1W3 \ \ W2 I - a l o o ,K , - a (I - a I ) h hJ. o 0& I - a l y,o o I - a l o o WI \ Actinometer C e l l Photolysis C e l l ^actinometer measured (no gas) I o ~ 2 o t I o + ° - 2 ' L r , " a l + 2a 2I o o o o o I - 3al + 3a 2I o o o I Q (1 - 3a + 3a 2) I Q (1 - 3 [a - a 2 ] ) a I o ^actinometer measured (no gas) 1 - 3 (a - a ) However, we want to measure the i n t e n s i t y of r a d i a t i o n just inside the front window (W^). i . e . I - a l = I o o I = I Q - a I Q = I Q (1 - a) ^.actinometer measured (no gas) ., . . . I = x (1 - a) 1 - 3(a - a 2) Mcintosh"*" had found that the calculated decrease i n r e f l e c t i o n using Fresnel's law was i n excellent agreement with the experimentally determined value. Therefore, the calculated value of 0.115 was used and only the f i r s t pass was considered to be important.-The phototube, i n conjunction with a Leeds and Northrup 10 mv. s t r i p chart recorder, was found to have a l i n e a r response over the range of i n t e n s i t i e s required for the series of experiments. This was done using a set of neutral density f i l t e r s which had been previously c a l i b r a t e d on a Cary 14 spectrophotometer. - 2 6 -The following procedure was used to r e l a t e the recorder reading from the phototube to the i n t e n s i t y of r a d i a t i o n monitored by the chemical actinometer: the phototube reading was taken before and a f t e r each actino-meter run. Three of the l a t t e r were done at 313 nm. It was found that the actinometer measurements were within 1% of each other while the phototube reading, i . e . the lamp, changed by less than ±2% over 3 hours. E. Measurement of Absorption C o e f f i c i e n t The absorption of HFB was determined over the range of pressures to be studied using the photocell and recorder. 1° was the measured transmitted i n t e n s i t y with the c e l l empty while l9<j*s was the measured transmitted i n t e n s i t y with gas i n the c e l l . If only the f i r s t pass i s considered i n the c e l l we f i n d : 1° A = l o g ^ Q = ecd j 9 a s m By p l o t t i n g A against c (d constant) we get (Figure 3): e = 5.8 M. ^ cm. ^ FIGURE 3: A b s o r b a n c e v e r s u s P r e s s u r e f o r h e x a f l u o r o b i a c e t y l - 28 -This p l o t was only v a l i d where the absorption of the gas i n the c e l l was greater than 10%. For smaller absorptions the value of a was assumed to be the same as obtained at higher pressures. As the absorption spectrum i n the region of e x c i t a t i o n i s not l e v e l and as a was not found i n a longer c e l l at the same concentration a small unknown experi-mental uncertainty i s introduced i n i t s value. — Ecd Since 10 i s the f r a c t i o n of the incident rad-20 l a t i o n which i s transmitted, the f r a c t i o n which i s absorbed must be: 1 - 1 0 _ C C d o and the amount of radiant energy absorbed i s 1° (1 - 10 _ £ c d) m ' F. Gas Analysis The analysis system i s shown schematically i n Figure (4). A f t e r photolysis the contents of the c e l l were expanded into the evacuated a l l metal tubing bounded by valves 1, 2, 4 and 5. Then valve 3 was closed and the portion H O K E G R E A S E L E S S V A L V E S FIGURE 4. A n a l y s i s S y s t e m . i I - 30 -of gas bounded by valves 4, 3 and 5 was q u a n t i t a t i v e l y estimated. Normally, valves 4 and 5 were closed while 6 was open allowing helium gas to by-pass the sampling tube. When the analysis was to be performed valve 6 was closed and valves 4 and 5 were opened sequencially and almost immediately. This hopefully had the e f f e c t of pushing the sample into a "small packet" of gas near valve 5 which was then released onto the column. The l a t t e r was a 10 f t . 1/8" column operated at 23° C , packed with s i l i c a g e l . The detector was a Varian Aerograph Thermal Conductivity Detector (No. 01-000334-00) with two matched pairs of 30 ohm. tungsten rhenium (WX) f i l a -ments. They were operated at 150 mA and at ambient tempera-ture. The power supply was a Kepco regulated DC supply (Model PAT 21-1T). Great care was taken to s h i e l d the various components from external noise. The detector block was housed i n a copper box which was then f i l l e d up with mica chips to keep temperature fluctuations to a minimum. The c i r c u i t diagram i s given i n Figure (5). The signal was fed into a Leeds and Northrup microvolt amplifier coupled to a 1 mV recorder. The chromatograph was c a l i b r a t e d using known / R E F E R E N C E - 31 -150 MA 10 VOLTS AMP .: -LlFIEft 2 0 n oo a s = s =3on I 2 R=R =30H I 2 fcECQRPER] FIGURE 5. Wheatstone bridge c i r c u i t for detector system. - 32 -samples of C 2Fg and CO. These samples were expanded from the c e l l as described above. Plots of peak height against concentration were found to be l i n e a r . Measurement of peak height as opposed to peak area 22 23 was preferred for a number of reasons: ' (i) Non-Gaussian shaped CO chromatograms. This was probably due to some sort of non-ideal desorption behav-i o r on the column. Operating the column at a higher tempera-ture might have eliminated t h i s problem. ( i i ) The resolution between the two components was much greater than unity (achieved by having the helium flow at 23 mis./minute). Thus the p a r t i t i o n c o e f f i c i e n t of one component was not thought to be altered by the p a r t i a l pressure of the other component. ( i i i ) Neither the detector or column were over-loaded during a run as the HFB, which was flushed onto the column with the photolysis products, had a retention time of hours. The HFB was eluted afterwards with the column temperature at 150° C. The sample size of the photolysis -7 products was of the order of 10 moles. - 33 -CHAPTER III RESULTS AND DISCUSSION The quantum y i e l d s of carbon monoxide and of ' hexafluoroethane obtained by photolysing HFB at 313 nm. and 23° C. are given i n Tables (1) and (2). These r e s u l t s are shown i n Figures (6) to (11) i n which various functions r e l a t i n g to <j> are plotted i n order to extract pertinent information r e l a t i n g to the primary process. It i s seen that the quantum y i e l d generally decreases with increasing pressure. As i n other s i m i l a r systems, t h i s e f f e c t r e f l e c t s the competition between d i s s o c i a t i o n and de-a c t i v a t i o n by c o l l i s i o n of excited molecules. The various p l o t s , Figures 6, 7 and 8, show that extrapolation to zero pressure gives 0.55 ± 0.10 as the l i m i t i n g d i s s o c i a t i o n quantum y i e l d i n the absence of perturbing c o l l i s i o n s . Theo-r e t i c a l l y t h i s l i m i t i n g y i e l d should be near unity (one less the small zero pressure y i e l d of fluorescence). This discrep-ancy, although large, i s not unexpected, because of the complications i n the extrapolation. - 34 -TABLE I PHOTOLYSIS OF HFB AT 313 nm. CELL LENGTH: 1 0 . 5 cm. TEMPERATURE: 23° C . Run No . P r e s s u r e [HFB]mm. <t> CO M o l a r R a t i o <TVP. (CO da t a ) ^P.P. ( C 2 F 6 d a t 24 0 .63 1.070 0 .550 1.93 0 .535 0 .550 23 1.3 1.035 0. 522 2 .00 0. 518 0 .522 21 3.4 0 . 840 0 .480 1.75 0 .410 0 .480 22 5.4 0. 743 0 .478 1. 54 0 . 372 0 .478 19 6.6 0 .655 0 . 314 2 .06 0. 328 0 . 314 25 7.9 0 .680 0 .259 2 .62 0. 340 0 .259 18 12 . 0 0. 538 0 .179 3 .00 0 .269 0 .179 16 1 1 . 3 0 .538 0. 358 1. 50 0 .269 0 . 358 15 1 2 . 1 0 .530 0 . 393 1. 37 0 .265 0 . 393 14 1 2 . 0 0 .545 0 .335 1.62 0 .273 0 . 335 13 2 4 . 0 0 . 481 0 .250 1.92 0 .241 0 .250 17 33 . 9 0. 379 0 .176 2 .10 0 .190 0 .176 12 50 .4 0 .342 0 .181 1. 89 0 .171 0 .181 10 57 . 6 0 . 309 0 .134 2 . 39 0 .155 0 .134 26 100 . 0 0 .228 0 .089 2 .56 0 .114 0. 089 11 154 .0 0 .174 0 .062 2 . 84 0 .087 0. 062 *28 2 1 1 . 0 0 .139 0 .042 3.54 0 .070 0 .042 29 335 . 0 0 .119 0. 069 1. 72 0 .060 0. 069 *30 463 . 4 0 .152 0 .030 5 .00 0 .076 0 .030 *32 521 . 0 0 .118 0 .020 6 .00 0 .059 0. 020 • E x p e r i m e n t a l l y u n r e l i a b l e CO d a t a - 35 -TABLE 2 PHOTOLYSIS OF HFB AT 313 nm. C E L L LENGTH: 10.5 cm. TEMPERATURE: 2 3° C . Run No,. P r e s s u r e [HFB] mn i 1 1 1 4> P.P. fi P . P . <£p.P. (CO d a t a ) (C^F^ d a t a ) 2. 6 [HFB] mm i (CO -<t> d a t a ? ' P : 1 " P P . P . ( C 2 F g d a t a ) 24 0.63 1.587 1.86 1.82 1.15 1.22 23 1.3 0. 769 1.92 1.92 1. 07 1.09 21 3.4 0.294 2.44 2.08 0.695 0.925 22 5.4 0.185 2.70 2.09 0.592 0.915 19 6.6 0.152 3.06 3.18 0.488 0.457 25 7.9 0.127 2.94 3.86 0.515 0. 350 18 12.0 0.0833 3. 72 5.59 0.368 0.218 16 11.3 0.0885 3.72 2.79 0. 368 0.558 15 12.1 0.0826 3.78 2.54 0. 361 0. 647 14 12. 0 0.0833 3.66 2.99 0.376 0. 505 13 24.0 0.0417 4.16 4.00 0.318 0. 333 17 33.9 0.0295 5.28 5.68 0. 234 0.214 12 50. 4 0.0198 5.84 5.50 0.206 0.221 10 57. 6 0.0174 6.48 7.46 0.184 0.155 26 100.0 0.0100 8 . 78 11.2 0.129 0.098 11 154.0 0.0065 11.50 16.1 0.0956 0.0662 *28 211.0 0.0047 14.38 23.6 0. 0753 0.044 29 335. 0 0.0030 16.8 14.5 0. 064 0.074 *30 463. 4 0.0022 13. 2 33. 9 0.082 0.031 I *32 521. 0 0.0019 17.1 50.0 0.063 0.020 * CO d a t a e x p e r i m e n t a l l y u n r e l i a b l e _ HFB] m.m. FIGURE 6: I n v e r s e quantum y i e l d s v e r s u s p r e s s u r e u s i n g c a r b o n monoxide d a t a . FIGURE 7: Inverse quantum yields versus pressure using, hexafluoroethane data. FIGURE 8. P r i m a r y quantum y i e l d s v e r s u s p r e s s u r e low p r e s s u r e r e g i o n . - 3 9 -The processes necessary to account for the obser-ved r e s u l t s are as follows:-B + hv ->• 1B* k* 2 C 0 + C F 2 6 1 1B* 'I* h v f + B 2 1B* k* + 1 S C 3B ** 3 1B* + M to V + M* 4 1 B ° hv f + B 5 ' 1 B ° B 6 k. i s c 3. B* 7 3B* 2 C 0 + C F ^ 2 6 8 3 * * 2 C 0 + C F U 2 6 8 a 3B* + M 3 B ° + M* 9 3B** + M 3 B ° + M* 9 a 3 B ° k hv P + B 1 0 3 B ° ^ 1 B 1 1 This i s e s s e n t i a l l y the same mechanism proposed by Mcintosh 1. Here, however, we di s t i n g u i s h two d i f f e r e n t -vibrational l e v e l s of the t r i p l e t state. The l e v e l reached on intersystem crossing from a high v i b r a t i o n a l - 40 -3 3 l e v e l of the s i n g l e t state i s referred to as B** while B* refers to the l e v e l reached on intersystem crossing from the e q u i l i b r a t e d s i n g l e t . From a steady-state treatment of the mechanism, the primary quantum y i e l d for d i s s o c i a t i o n i s given by: 4> k* ' primary _ d process (k* + k* + coM) MOLECULES DECOMPOSING FROM NON-EQUILIBRATED SINGLET STATE k, k. coM d x i s c x (k, + coM) (k. + k £ + k,) (COM + k* + k* + k* d i s c f 6 d f i s c MOLECULES DECOMPOSING FROM TRIPLET STATE HAVING ISC FROM VIBRATIONALLY EQUILIBRATED SINGLET STATE k,, k* d x i s c (k d,+ coM) ( k | s c + k* + k* + 0)M) MOLECULES DECOMPOSING FROM TRIPLET STATE HAVING ISC FROM NON-VIBRATIONALLY EQUILIBRATED SINGLET STATE - 41 -Substituting:-k* + k* + k* , k* f d i s c b = d co c = d d = i s c s = d 1 to CO CO k. + k^ + k^ g = i s c f 6 k. I S C gives: *P.P. 77T + ~ ( c ~ l ~ M ) (a~T-M) + (s~T-M) ( i _ f l 4 ) """ ( l i a + M g We s h a l l ignore the second term because i t s maxi-mum contribution to the sum i n Equation 18 i s 0.05, as can be deduced drom the l i m i t i n g value at high pressure. Further-more we s h a l l at f i r s t assume that intersystem crossing from a high v i b r a t i o n a l l e v e l leads only to deactivation and not to decomposition at the pressures used i n t h i s work. This l a t t e r statement i s supported by the fac t that at 2 0 t o r r the - 42 -ratio of phosphorescence yield to fluorescence yield has reached i t s limiting value. Therefore we find: b k* *P.P. = = ^ — (19) a + M k* + k* + o)M d isc = 1 + k i s c + a) M (20) *P.P. k* k* d d Equation (20) gives us k* from the slope using a co l l i s i o n number (w) of 1.2 x IO 1 1 1/mole. s e c , while from the intercept, 1 + k* , we obtain k* ^ isc isc d Figure (6) in the region of 20 to 200 torr gives 3 a slope .of 1.7 x 10 1/mole using the CO data. k* = 7 x 10 7 sec 1 d - 43 -Using the low pressure CO data (Figure 8), we obtain from the intercept: Ir* 1 + i s c = 1.8 k3 k* i s c = 0.8 k* K d 7 -1 k* = 6 x 10 sec i s c k* + k* = 1.3 x 10 8 s e c " 1 d i s c Although the stoichiometry <f>0/_ = 2u>_ _ was ' CO C 2 F g generally obeyed, the analysis of C^F^, p a r t i c u l a r l y at high HFB pressures was not as r e l i a b l e as those of CO. A p l o t such as Figure 7 i s strongly influenced by high pressure data, hence the slope i n Figure 7 i s appreciably larger than i n Figure 6, for CO data. - 44 -C o m p a r i s o n w i t h M c i n t o s h ' s Work By a p p l y i n g t h e same mechanism t o M c i n t o s h ' s f l u o r e s c e n c e y i e l d s as we have j u s t used f o r t h e decompo-s i t i o n r e s u l t s a t modera te t o h i g h p r e s s u r e s we f i n d : , . . k* + k* 1 ( i + _ d i s c ) cj)^  a coM , k . where a = f k £ + k . + k^ f i s c 6 From F i g u r e (33)"^ 1 * 180 d i s c = 1.2 x 10 t o r r a co k* + k* = 4.2 x 1 0 8 sec 1 d i s c T h i s r e s u l t compares r e a s o n a b l y w i t h t h e v a l u e o b t a i n e d i n t h i s w o r k . - 45 -Complementary A s p e c t s o f t h i s and M c i n t o s h ' s Work I t has been n e c e s s a r y , i n o r d e r t o e x p l a i n our r e s u l t s , t o assume t h a t no d e c o m p o s i t i o n o c c u r s from t h e t r i p l e t m a n i f o l d a t t h e p r e s s u r e s used i n t h i s work. A t l o w e r p r e s s u r e s ( < 2 0 t o r r ) we might n o t expect..:this a s s u m p t i o n t o h o l d . The f u l l mechanism g i v e s t h e f l u o r e s c e n c e y i e l d , 4>f' OJM . k* a + f k* + k* + k* + WM k* + k* + k* + OJM f i s c d f i s c d and t h e decompositon y i e l d , cf> , • k* • • k,-, k* <Pp p = d + d' . . i s c  k* + k* + k* + OJM k-,. k* + k*+ wM + k* f i s c - d d'+ OJM i s c d f The r a t i o of t h e f l u o r e s c e n c e y i e l d t o t h e d e c o m p o s i t i o n y i e l d i s : - 46 -a>£ _ aioM + k* {21) *P.P. k* + ( k d ' ) k* a r ISC d' +OJM auM + k f (22) where 3 = k d ' (23) k* + 3k* k,, + OJM d i s c d 1 If we assume that intersystem crossing leads only to deactivation and not to d i s s o c i a t i o n (as before) i . e . k d 1 < < ^ ' W e ^^ n <^ that -*f '• _ a m (3 = 0) Therefore-from high-pressure data we fi n d - 1 :•: I" rs-k* = 1.04 x 10 8 sec" 1 Figure (9) shows that at high pressures the r a t i o i s a l i n e a r function of HFB pressure but that below 30 t o r r the r a t i o decreases more rapid l y . This r e s u l t s from 3" changing from 0 to 1 as the pressure drops to zero. If we extrapolate the l i n e a r portion of the plot we fi n d that i t goes through the o r i g i n as Equation 22 predicts. Although the difference between the extrapolated value - 47 -4>s and the experimentally determined value of -r is s t i l l 9 p.p. increasing below 10 torr we would expect them to coverage at zero pressure. There is however, no experimental evidence available below 10 torr. At intermediate pressures we find: < , ) ' *P.P. •P.P. 3 = zero experimental acoM _ otioM = acoM 3k| s c k* k* + 3k* k* (k*+ 3k* ) d d isc d d isc lc* k * k * 3 = d ( K d + 3 isc) 24 atoM k* isc Using values k* = 1.04 x 10 8 sec" 1 (this work) k* + k* = 4.2 x 10 8 s e c - 1 (Mcintosh) isc d . . k* = 3.16 x 10 8 sec 1 ISC - 48 -We f i n d a t 20 t o r r : 3 = 0.094 S u b s t i t u t i n g i n t o E q u a t i o n 2 3 g i v e s : k d , = 1.25 x 1 0 7 s e c " 1 •f F i n a l l y , we c a l c u l a t e — ^ — by E q u a t i o n 21 u s i n g t h e f o l l o w i n g r a t e c o n s t a n t s : k d , = 1.25 x 1 0 7 s e c - 1 k* = 3.16 x 1 0 8 s e c - 1 . ISC k* = 1.04 x 1 0 8 s e c " 1 to = 1.2 x 1 0 1 1 l i t r e / m o l e , sec, a = i 180 I t i s seen t h a t t h e e x p e r i m e n t a l and c a l c u l a t e d p l o t s , F i g u r e ( 9 ) , show t h e same g e n e r a l t r e n d s a l t h o u g h t h e y do n o t e x a c t l y c o i n c i d e - e s p e c i a l l y a t h i g h e r p r e s s u r e s . T h i s i s because • f i s v e r y s e n s i t i v e t o t h e v a l u e o f 3 w h i c h i s * P P c a l c u l a t e d " ' from E q u a t i o n 24 u s i n g r a t e c o n s t a n t s d e r i v e d from two d i f f e r e n t s t u d i e s . F i g u r e 9. R a t i o o f f l u o r e s c e n c e y i e l d t o p r i m a r y quantum y i e l d v e r s u s p r e s s u r e f o r HFB. - 50 -I t should be stressed that t h i s value for k,. i s d' very approximate as Mcintosh has no fluorescence y i e l d s a v a i l a b l e at 313 nm. below 20 t o r r for HFB. Extrapolated values for down to 10 t o r r were therefore used i n calcu-l a t i n g the r a t i o s . Figure (9) shows that decomposition can compete with deactivation i n the t r i p l e t manifold below 30 t o r r . Our intercept of 1.8 (Figure 8) i s therefore not too s u r p r i -sing i n view of i t s having been obtained by extrapolation from above 5 t o r r . Discussion on the Intercept Therefore there are at . least two p o s s i b i l i t i e s for the magnitude of the intercept. (1) The ^P.P. increases r a p i d l y below 10 torr and so even from 10 t o r r to 1 t o r r we are not on a l i n e a r portion of the p l o t . The dotted curve on Figure (8) shows t h i s to be a d i s t i n c t p o s s i b i l i t y . The l i n e a r portion would then l i e below one t o r r . - 51 -(2) The quantum y i e l d s are not accurate enough i n -sofar as there may have been some quenching impurity present. This seems u n l i k e l y as many d i f f e r e n t p u r i f i e d batches were used. On a compressed plot i t i s possible to extrapolate (Figures 6 and 7) to zero pressure i n order to obtain a ^P.P. 4 of unity. This i s what Whytock and Kutschke have done for HFA. In t h e i r case, although order of magnitude r e s u l t s could be obtained, precise estimates of rate constants would be impossible. Apparently, they found i t impossible to make quantitative measurements below one t o r r . Although the mechanism predicts that ^P.P. goes to zero as the pressure goes to i n f i n i t y , we f i n d that (Figure oo , 10) <j> has a f i n i t e value (extrapolated) . Whether t h i s i n -dicates a thermal decomposition of the equ i l i b r a t e d 3 B ° state s i m i l a r to that i n HFA or whether i t i s an a r t i f i c i a l e f f e c t due to i n s u f f i c i e n t data at high pressure remains unknown. Concluding Remarks .The known emission and d i s s o c i a t i o n quantum y i e l d s at 313 nm. are plotted i n Figure .(11) . F i g u r e 10 : A f u n c t i o n o f t h e p r i m a r y quantum y i e l d v e r s u s i n v e r s e o f p r e s s u r e (High P r e s s u r e R e g i o n ) . CO d a t a Q C F d a t a I • 1 * ' * 1 ' 1 • 1 • 1 100 2 0 0 300 -400 5 0 0 6 0 0 [ H F B ] mm F i g u r e 1 1 : Known quantum y i e l d s f o r HFB v e r s u s p r e s s u r e . E m i s s i o n d a t a a f t e r M n T n f n c h l . - 54 -The fluorescence l i f e t i m e (T) would seem to have a value of 2.5 ± 1.0 from 40 t o r r upwards. Suggestions for Further Work Future experiments w i l l have to proceed along three d i r e c t i o n s . F i r s t l y , photochemical quantum yi e l d s w i l l be determined over a range of wavelengths ( e.g. 270 nm -> 436 nm) . Hopefully i n the high wavelength region some of the processes w i l l be eliminated (e.g. k* s c, k* = k^) thus providing easier evaluation of the data. Secondly, the photophysical data w i l l have to be extended into the pressure region where no c o l l i s i o n induced relaxation i s operative. T h i r d l y , s i n g l e t l i f e t i m e s over a range of pressures w i l l be attempted using a pulsed laser of very short duration i n order to f i n a l l y elucidate the primary process. - 55 -BIBLIOGRAPHY 1. J . S . E . M c i n t o s h , P r i m a r y P h o t o p h y s i c a l P r o c e s s e s i n H e x a f l u o r o b i a c e t y l P h . D . T h e s i s , U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1969 2 . P . G . B o w e r s , The P r i m a r y P h o t o c h e m i c a l P r o c e s s i n H e x a f l u o r o -a c e t o n e V a p o u r . P h . D. T h e s i s , t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1964 3. G . B . P o r t e r and B . T . 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