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UBC Theses and Dissertations

Study of the quenching of O2 (1[Sigma]+g) Lakusta, Helen 1973

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STUDY OF THE QUENCHING OF O ^ E " ) 4  ^  8  BY  H. LAKUSTA B.Sc.  (Hons.). U n i v e r s i t y o f C a l g a r y , 1971  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE  REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of  CHEMISTRY  We a c c e p t t h i s t h e s i s as conforming required  THE  to the  standard  UNIVERSITY OF BRITISH COLUMBIA September, 1973  In p r e s e n t i n g an the  advanced degree at Library  I further for  this thesis  shall  the  of  this thesis  written  University  of B r i t i s h  permission for  s c h o l a r l y p u r p o s e s may his  f u l f i l m e n t of  make i t f r e e l y a v a i l a b l e  agree t h a t  by  in partial  representatives.  be  Department  of  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, C a n a d a  Columbia  Columbia,  for reference  the  It i s understood  permission.  requirements I agree and  shall  Head o f my  that  not  be  that  thesis  Department  copying or  for  study.  extensive copying of t h i s  g r a n t e d by  for f i n a n c i a l gain  the  or  publication  allowed without  my  - ii  -  ABSTRACT  A study of the quenching a b i l i t i e s o f a s e r i e s of compounds c o n t a i n i n g heavy atoms was  undertaken to a s c e r t a i n whether or not  0„("''E ) quenching i s s u b j e c t to heavy atom enhancement. +  heavy atom e f f e c t would  3 -  E ) transition.  8  Presence of a  suggest s i g n i f i c a n t quenching v i a the 0„(  The f a c t t h a t no such e f f e c t i s observed  the g e n e r a l l y assumed quenching mode: 0„( z  E  -> g  E  +  supports  A ). g  A s y s t e m a t i c i n v e s t i g a t i o n o f the use o f o v e r l a p of a b s o r p t i o n spectrum o f quencher w i t h e m i s s i o n spectrum of C^C^E*) to o b t a i n quenching r a t e c o n s t a n t s was  undertaken.  The c o r r e l a t i o n between  c a l c u l a t e d and e x p e r i m e n t a l r a t e c o n s t a n t s are examined and of the method are  presented.  calculated  shortcomings  - iii  -  TABLE OF CONTENTS Page ABSTRACT  1  1  LIST OF TABLES  i  v  LIST OF FIGURES ACKNOWLEDGEMENTS  v  v  i  INTRODUCTION  1  E l e c t r o n i c a l l y E x c i t e d Oxygen  ^  Sources o f S i n g l e t M o l e c u l a r Oxygen  ^  Methods f o r D e t e c t i o n o f S i n g l e t M o l e c u l a r Oxygen  1  0  P h y s i c a l Quenching EXPERIMENTAL Flow System  H 1  9  1  9  Materials  22  Production of C ^ E  22  Detection o f Excited Species  22  Determination of Experimental Values  24  RESULTS  2  5  Heavy Atom Study  25  Quenching Mechanism Study  26  DISCUSSION  5  3  Heavy Atom Study  5  3  Quenching Mechanism Study  5  9  SUMMARY AND CONCLUSIONS  68  BIBLIOGRAPHY  69  - iv -  LIST OF TABLES  Table  Page  I  ^"O^ T r a n s i t i o n s and Corresponding E n e r g i e s  II  Sources o f M a t e r i a l s  III IV  ^ 23  V a l u e s Obtained i n Heavy-Atom Study Overlap I n t e g r a l s , C a l c u l a t e d  ^9  Quenching Constants  and Corresponding E x p e r i m e n t a l Quenching C o n s t a n t s . . V  Most P r o b a b l e Mode of Quencher E x c i t a t i o n and 0 „ ( E ) Deactivation 1  VI VII  34-3  O^C^A  ) Quenching Rate Constants g 0 ( Z ) Quenching Rate Constants 1  ^9-5  +  +  (from r e f . 53) (from r e f . 53)  ....  ^7 5  8  - v -  LIST OF FIGURES Figure I  Page Oxygen P o t e n t i a l Energy Diagram as Compiled Gilmore  by  (6)  3  20  II  Schematic  III  R e p r e s e n t a t i v e Stern-Volmer  IV  Computed " i n d u c e d " E m i s s i o n Bands f o r the 0 ("*"£* -*  V a-d  o f Flow System Plot  27  (CH C l ) 2  2  "''A ) T r a n s i t i o n g Sample of Computer Drawn R e l a t i v e  3  Overlap  ^  Integral  (CHF )  3  "  6  3  VI  C o r r e l a t i o n Between k  q  e x p e r i m e n t a l and k  from IR a b s o r p t i o n s p e c t r a  o f quenchers  q  calculated  f o r 02( £ )  Quenching VII  Overlap I n t e g r a l Versus 0~(^L ) g +  Constants VIII  4  4  (from r e f . 53)  Quenching Rate 46  ....  R e l a t i v e P o s i t i o n of the 0 ( Z  ) Transitions g and V i b r a t i o n a l Bands o f Diatomic M o l e c u l e s 1  o  -> A 1  2  (from r e f . 53)  5  5  4  3  - vi -  ACKNOWLEDGEMENTS  The author would l i k e t o express h e r g r a d i t u d e to Dr. E.A. O g r y z l o f o r h i s guidance  through t h i s study; l a b o r a t o r y c o l l e a g u e ,  f o r h e l p f u l d i s c u s s i o n ; Dr. J.A. Davidson f o r i n v a l u a b l e  R.D. A s h f o r d assistance  b o t h t e c h n i c a l l y and i n t h e form o f d i s c u s s i o n and s p e c i a l thanks t o K.B.  S t o r e y f o r encouragement and moral support throughout h e r s t u d i e s .  - 1 -  INTRODUCTION  E x c i t e d s i n g l e t oxygen molecules were f i r s t d e s c r i b e d by Mulliken"'' and were subsequently d e t e c t e d 2 by Herzberg ; l i q u i d oxygen by E l l i s gaseous d i s c h a r g e the p o s s i b i l i t y  4 by Van V l e c k .  in:  i n 1928  the upper atmosphere  3 and Kneser ; and i n e m i s s i o n from  As e a r l y as 1939 Kautsky  5  suggested  t h a t an e l e c t r o n i c a l l y e x c i t e d , m e t a s t a b l e s i n g l e t  oxygen molecule a c t e d as the r e a c t i v e i n t e r m e d i a t e photo-oxygenation r e a c t i o n s .  i n dye-sensitized  However, u n t i l 1964 i n t e r e s t i n the  p r o p e r t i e s of s i n g l e t oxygen was p r i m a r i l y the concern o f a s t r o p h y s i c i s t s , s m a l l molecule s p e c t r o s c o p i s t s and some gas phase k i n e t i c i s t s . prominance o f s i n g l e t m o l e c u l a r oxygen as a c h e m i c a l s p e c i e s the f o l l o w i n g :  Recent  i s due t o  i t i s e a s i l y and abundantly generated; a v a r i e t y o f  d e t e c t i o n techniques e x i s t ; and i t has s p e c i f i c c h e m i c a l r e a c t i v i t y at  ordinary  temperatures.  E l e c t r o n i c a l l y E x c i t e d Oxygen The  lowest e l e c t r o n i c c o n f i g u r a t i o n o f oxygen  2  2  KK(o- 2s) (a 2s) (ir 2p) g  u  g  can g i v e r i s e t o t h r e e s t a t e s : is  t h e ground s t a t e w i t h "'"A  2  (TT^P)  4  (  y  p)  2  3 - 1 1 + E , A , E . g g g  and ^T/~ b e i n g  Of these s t a t e s  3 E g  e x c i t e d s t a t e s a t 22.5 k c a l  - 2 -  and  38.5  k c a l r e s p e c t i v e l y .above the  ground s t a t e .  the p e r t i n e n t p o t e n t i a l energy c u r v e s . a p a i r of A states  These low  lying excited  radiative  l i f e t i m e s , can  A l t h o u g h the are  axis.  The  o r b i t a l a n g u l a r momentum of the  e n e r g y , and  &  state  two  singlet states  is  actually angular  i s diamagnetic, since  upper e l e c t r o n s e x a c t l y have e x t r a o r d i n a r i l y  the phenomenon of  ->- 3 £ ~ 12,690 1 and g g  1  £  +  cancel.  long  s u f f e r many c o l l i s i o n s without l o s i n g  as a r e s u l t e x h i b i t 1  state  degenerate i n energy having components o f  momentum + 2h about the 0-0 the  The  F i g u r e 1 ^ shows  their  'energy p o o l i n g ' .  -»• E ~ 7,619 g g  A  3  transitions  s p i n f o r b i d d e n both have been observed as weak a b s o r p t i o n bands i n  the near i n f r a r e d . radiative  From a b s o r p t i o n i n t e n s i t i e s the  l i f e t i m e s of 45 min^'^ f o r ^"A  and  7-12  extremely l o n g  sec^'^  for  g have been c a l c u l a t e d .  g  These t r a n s i t i o n s have been i d e n t i f i e d as  ' m a g n e t i c - d i p o l e ' t r a n s i t i o n s which have t r a n s i t i o n p r o b a b i l i t i e s many orders of magnitude s m a l l e r than those f o r a l l o w e d e l e c t r i c d i p o l e o 11 o 9 12 t r a n s i t i o n s . Bands found at 6340 A and 7030 A and 4800 A have been a s c r i b e d to the phenomenon o f whereby two the  excited  'energy-pooling', a cooperative process  m o l e c u l e s p o o l t h e i r energy to produce a photon w i t h  combined energy of both m o l e c u l e s .  a r i s e from combination of two arises  02(  1  The  6340 A and  A ) m o l e c u l e s and  1 1 from an 0 n (s Z ) 0 „ V( A ) p a i r . 2 g' 2 g'  A summary of  13 f i r s t observed at h i g h oxygen p r e s s u r e s  low  sun  ? 4800 A  bands 12  band  8  c o r r e s p o n d i n g e n e r g i e s i s g i v e n i n T a b l e 1.  15  the  7030 A  transitions  and  These t r a n s i t i o n s were  14 '  and  i n the  atmosphere at  4 '  and  have subsequently been observed i n gas  s t u d i e s u s i n g v a r i o u s methods of "HD  formation.H>16,17  phase l a b o r a t o r y  - 3 -  F i g u r e I.  Oxygen p o t e n t i a l energy diagram as compiled by G i l m o r e .  - 5 -  T a b l e I.  Transition oA 2  -» g  Energy (A)  0. V 2 g  12690  O.V"-* 0 V 2 g 2 g  7620  0  O-^A 2 g  ->0.(V) 2 g  +  (\)-»0(V)  +  ) + 0A\) 2 g  0,(V)  + 0  0,(V) 2 g  +  0 (V)->0 9  2  g  2  (V)g  0 ( Z ) 2 g  6340  0(h)  4760  3  + 0  (V)g  2  3800  1 6  - 6 -  E i n s t e i n A f a c t o r s of A = 0.145  1 + have been r e p o r t e d the p r e f e r r e d of A = 2.58 10  —3  sec  E  g i n most s t u d i e s .  sec  —1 20  f o r the  -1  f o r the  A  sec  A = 0.085 sec  ^  ^®>^  3 -*  E  t r a n s i t i o n s , the f i r s t v a l u e b e i n g g Badger et a l . ^ have determined a v a l u e  1 3 -»•  g  and  have been r e p o r t e d  E  transition.  g  f o r the  A v a l u e of A = 2.5  forbidden  ''Noxon'  1 + E  g  •+  1  A  x g  transition.  Sources of S i n g l e t M o l e c u l a r Oxygen Study of any  e x c i t e d molecule i s r e s t r i c t e d by  generation a v a i l a b l e . has  facilitated  The  studies  the methods o f  v a r i e t y of a v a i l a b l e methods f o r p r o d u c t i o n  of s i n g l e t m o l e c u l a r oxygen.  Thus t h e r e  follows  a summary o f methods of f o r m a t i o n of s i n g l e t oxygen. S i n g l e t oxygen can be  formed u s i n g  chemical reactions  r e a c t i o n o f hydrogen p e r o x i d e w i t h hypobromite or  such  as:  hypochlorite;  decomposition of o z o n i d e s ; decomposition of s u p e r p e r o x i d e i o n s ; t i o n o f endoperoxides; and (one  the  d e c o m p o s i t i o n of p e r o x y a c e t y l n i t r a t e  of the components of smog).  reactions  20  and  a d e s c r i p t i o n of these  various  review a r t i c l e s  21.  S i n g l e t oxygen molecules may by  For  p r o d u c i n g s i n g l e t m o l e c u l a r oxygen see  references  decomposi-  a l s o be  energy t r a n s f e r from the e x c i t e d  generated w i t h h i g h e f f i c i e n c y  triplet  s t a t e o f some s e n s i t i z e r  22-25 molecules. the e x c i t e d  The  key  step i s the  s e n s i t i z e r to the ground s t a t e oxygen molecule which r e s u l t s  i n 0;, b e i n g r a i s e d to an e x c i t e d triplet  t r a n s f e r of e l e c t r o n i c energy from  excited  s t a t e of the  spectroscopically,  to be  singlet state.  s e n s i t i z e r has  Transfer  from  the  been shown, k i n e t i c a l l y  and  the predominant mode of oxygen e x c i t a t i o n i n  - 7 -  all  cases.  26  '  27  The  s i n g l e t s t a t e generated  upon the t r i p l e t energy of the s e n s i t i z e r  28  i s suggested thus:  g r e a t e r than 50 k c a l w i l l y i e l d predominantly l e s s than 38 k c a l can r e s u l t o n l y i n  O^C^E ^  A g ) ; and  to depend  s e n s i t i z e r s of ) ; s e n s i t i z e r s of E~ 8 s e n s i t i z e r s of  1 1 29 i n t e r m e d i a r y energy would produce r e l a t i v e amounts of 0_( E ) and 0 „ ( A ) . 2 g 2 g C e r t a i n r e a c t i o n s of ozone r e s u l t i n f o r m a t i o n of e x c i t e d m o l e c u l a r oxygen. of e i t h e r O-C^A ^ 8 processes  30  Use  electronically  o  of X = 2537 A r e s u l t s i n p r o d u c t i o n  ) o r O-C^E ) from primary p r o c e s s e s . ^ 8  Secondary  i n v o l v i n g energy t r a n s f e r from 0(^"D) r e s u l t i n f o r m a t i o n of  0„("'"E ) . Ozone p h o t o l y s i s i s not g e n e r a l l y c o n s i d e r e d u s e f u l f o r ^* § g e n e r a t i n g l a r g e amounts of s i n g l e t oxygen. 31 Vacuum UV  f l a s h p h o t o l y s i s has been used  produce 0„ (''"E ) .  to  photochemically  T h i s method i s h i g h l y dependent upon a h i g h degree of  s e n s i t i v i t y o f the d e t e c t i o n method, however, i t o f f e r s the advantage t h a t the secondary r e a c t i o n s w i t h ^ ( ^ E g ) ° ^ encountered i n microwave d i s c h a r g e may  atomic oxygen or ozone o f t e n  be n e g l e c t e d as a r e s u l t of  the  m i n i s c u l e concentration of 0 produced. The method of g e n e r a t i o n used i n t h i s study was  electric  ( s p e c i f i c a l l y microwave) d i s c h a r g e through a stream of oxygen. d i s c h a r g e through a stream of m o l e c u l a r  Electric  oxygen produces a p p r e c i a b l e  q u a n t i t i e s of e l e c t r o n i c a l l y e x c i t e d s i n g l e t oxygen as w e l l as atomic 1 32 oxygen. The presence of 02( A g ) i n a d i s c h a r g e was shown by Noxon 33 34 i n 1961  and has been confirmed by mass s p e c t r a l s t u d i e s .  Approximately  10%  '  of the oxygen i n a f l o w system i s e x c i t e d to the "*"A  s t a t e i n r a d i o f r e q u e n c y d i s c h a r g e t u b e s , some however, t h i s i s r a p i d l y r e l a x e d to  8  .  1  E  35  8  i s a l s o produced, 8 Most "*"E i s formed downstream §  - 8 -  from the d i s c h a r g e by the energy p o o l i n g p r o c e s s :  0  2  (\)  02(1Ag)  +  ->  +  O ^ )  (i)  (to be d e a l t w i t h i n g r e a t e r d e t a i l i n the f o l l o w i n g s e c t i o n ) .  Although  t h e r e seems to be no g r e a t d i f f e r e n c e between the e x c i t a t i o n p r o p e r t i e s of r a d i o f r e q u e n c y and microwave d i s c h a r g e s , i t i s e a s i e r to h i g h e r power i n t o d i s c h a r g e w i t h a microwave source and  i s e a s i e r to To study ^0^  m a i n t a i n a s t a b l e d i s c h a r g e over a wider p r e s s u r e r a n g e . it  i s necessary  couple  to f r e e the system o f oxygen atoms produced i n the  d i s c h a r g e , t h i s can be accomplished  by i n t e r p o s i n g a f i l m of  mercuric  36 oxide  between the d i s c h a r g e and o b s e r v a t i o n p o i n t and/or by adding an 37 excess o f N0 2 to the oxygen f l o w . In the absence of f o r e i g n gases added to the f l o w b o t h "*"A  and  ^E  w i l l decay p r i m a r i l y by w a l l  „ . . 38 collisions. The b u i l d u p to a steady s t a t e c o n c e n t r a t i o n of ° 2 ^ ^ g ^  ^  n  a  s t r e a m  a f t e r a d i s c h a r g e can be accounted f o r by the energy p o o l i n g p r o c e s s :  °2(1v  +  °2  ( 1  y  1  ~ ^  v ^  +  °2  ( 3  y  Three e n e r g y - p o o l i n g p r o c e s s e s have been proposed:  ( i ) 3 9  (3) energy d i s p r o p o r t i o n a t i o n .  * t r a n s f e r processes  are:  12,16  4 0  (1) c o o p e r a t i v e  e m i s s i o n or r a d i a t i v e p o o l i n g ; (2) c o o p e r a t i v e t r a n s f e r o r t r a n s f e r ; and  '  ,termolecular  Examples of c o o p e r a t i v e  - 9 -  0 ( A  )  X  9  0  +  g  2  ( V )  2 g 9  0„( A  )  1  >  OA l ) 3  2 g +  0 (E  1  CT  — >  )  3  2 g  0.( A )  2 g  +  +  +  hv  (ii)  +  h v( i i i )  2 g  0,( E")  +  3  2 g  O . ( V )  2 g  Process (ii) results in emissions at 6340 and 7030 A.  The 4800 and 5200 A  bands resulting from process ( i i i ) appear with significant intensities 41 only at low temperatures.  Cooperative or termolecular transfer  involves simultaneous transfer of excitation energy of two oxygen molecules to a third molecule as seen in the excitation of violanthrone by the process: 20 ( A ) + 2 g V = violanthrone 1  9  V  >  20, ( V ) 2 g  +  V*  (iv)  The process whereby two excited molecules pool their energy to raise one to a higher state while the other relaxes to ground state is referred 42 to as energy disproportionation which is exemplified by process ( i ) . This process i s the method whereby a steady state concentration of 0 (^E ) is built up after a discharge flow system. +  9  From the buildup  of [O^C^E )] under nonstationary conditions in a system containing known X 3 1 ' 1 4- 3 [O^i Ag)] a rate constant of 1.3 x 10 l.mole sec has been obtained for the process shown above. 37 43 time  '  The invariance of [0.(^£ )] with 2 g +  can be explained on the basis of steady state equilibrium by  (i) and deactivation reactions.  - 10 -  Methods f o r D e t e c t i o n o f S i n g l e t M o l e c u l a r Oxygen The presence of s i n g l e t m o l e c u l a r  oxygen may  be d e t e c t e d by  direct  o b s e r v a t i o n i f p o s s i b l e or i n d i r e c t l y as a r e s u l t of i t s chemical reactions. EPR  A review of gas phase d e t e c t i o n techniques  44 has been used by F a l i c k e t a l . to observe the AM = 1  f o r the J = 2 s t a t e of O-C^A 2  ).  The  transition  r e s u l t s o b t a i n e d were used to: (1)  g  10% of d i s c h a r g e d gases i s 0^~k  show t h a t approximately  r e l a t i v e r e a c t i o n r a t e s of e t h y l e n e w i t h O-C^A oxygen.  follows.  The paramagnetic p r o p e r t y of the "'"A  and  (2)  study  ) and w i t h atomic s t a t e of oxygen m o l e c u l e  i s a p r o d u c t of i t s o r b i t a l angular momentum. 33 Foner and Hudson have used mass s p e c t r a l d e t e c t i o n methods to study 0 „ ( ^ A ) c o n c e n t r a t i o n s i n an oxygen f l o w system. ^ 8 Isothermal  c a l o r i m e t r y can a l s o be used to d e t e c t 02("'~A ) and  to 45  determine c o n c e n t r a t i o n s of e x c i t e d oxygen i n f l o w systems. by use o f two of the 1.27  isothermal calorimeters i n conjunction with  u band has  present.  Luminescence s p e c t r o s c o p y  i s the d e t e c t i o n method used i n t h i s  C h a r a c t e r i s t i c s i n g l e and  double molecule simultaneous  may  be employed f o r i d e n t i f i c a t i o n and  The  s i n g l e molecule t r a n s i t i o n s r e s  02(^2^) and ®2^^g  J  observation  shown t h a t a c o b a l t coated d e t e c t o r d e s t r o y s  g r e a t e r than 95% of °2^^g'  study.  Arnold  P  e c t  l  v e  ly  d e t e c t i o n o f the e x c i t e d s p e c i e s .  [(0-0) bands] a t 7620 1 and a r e  transitions  1.27  expected to be the s t r o n g e s t .  1 O h i g h 0 „ ( A ) c o n c e n t r a t i o n the dimol e m i s s i o n bands at 6340 A and ^ 8 41 43 may  be d e t e c t e d .  '  u for  U n c e r t a i n t y over a b s o l u t e r a t e constant  At  o  7030 A  f o r the  reaction O.^A 2  ) g  +  0o(1A 2  ) g  >  20 (V) 0  2  g  +  hv  6340 A  - 11 -  l i m i t s the use of d i m o l e m i s s i o n s p e c t r o s c o p y to the study o f  relative  46 47 A ) concentration. ' Use o f e m i s s i o n s p e c t r o s c o p y p e r m i t s 2- 8 1 1 d e t e c t i o n o f both £ and A over wide p r e s s u r e r a n g e s .  0„(  1  .  8  8  D e a c t i v a t i o n o f S i n g l e t M o l e c u l a r Oxygen E l e c t r o n i c a l l y e x c i t e d molecules  can be d e a c t i v a t e d e i t h e r  r a d i a t i v e e m i s s i o n o r v i a v a r i o u s n o n - r a d i a t i v e pathways.  The  by low  A  f a c t o r s and l o n g r a d i a t i v e l i f e t i m e s o f e x c i t e d s i n g l e t m o l e c u l a r oxygen make r a d i a t i v e e m i s s i o n a n e g l i g i b l e p r o c e s s compared to r a d i a t i v e processes of d e a c t i v a t i o n . can o c c u r v i a : (3) p h y s i c a l  non-  Non-radiative deactivation  (1) e l e c t r o n i c energy t r a n s f e r ;  (2) c h e m i c a l  reactions;  quenching.  E l e c t r o n i c energy t r a n s f e r from e x c i t e d s i n g l e t oxygen s t a t e s o f ^E"*" and ^"A i s seldom observed because few m o l e c u l e s have e l e c t r o n i c a l l y 8 g e x c i t e d s t a t e s below the e x c i t a t i o n e n e r g i e s o f 22.5 38.5  kcal  (^E+)  o f these two l o w - l y i n g s t a t e s .  e x c i t a t i o n v i a dimol p r o c e s s e s has been observed  kcal  ( A ) and  However, e l e c t r o n i c i n a number of  12,41,48 systems. The o n l y c h e m i c a l r e a c t i o n s o f s i n g l e t m o l e c u l a r oxygen which have been w e l l s t u d i e d a r e those w i t h ozone  11 31 37 49 50 » » > »  a n (  j  those w i t h  •» 44,45,50,52 olefins.  P h y s i c a l Quenching P h y s i c a l quenching  i s a p r o c e s s whereby e l e c t r o n i c e x c i t a t i o n  be degraded i n t o n u c l e a r motion w i t h i n the system.  may  Quenching s t u d i e s  of 0 o (^A ) have shown i t to be somewhat immune t o d e a c t i v a t i o n , most 2V g' ^"A  quenching  constants are approximately  10^ times s m a l l e r than  - 12 -  c o r r e s p o n d i n g "*"E+ c o n s t a n t s . " * " E + , more e a s i l y d e a c t i v a t e d and thus somewhat e a s i e r t o study was the s p e c i e s examined i n t h i s work. In the absence o f a quenching gas t h e steady s t a t e c o n c e n t r a t i o n o f O^C^E*) m a i n t a i n e d  i n a f l o w system i s g i v e n by the f o l l o w i n g  reactions:  0,(V) + O.^A ) 2 g 2 g 1 + 0 ( Eg)  2  and  (i)  k  +  2  [° (V  029<V) + 02(V) g g ——V  wall  to V (1  =^  ) ]  products  (v)  (vi)  ]2  2  the e m i s s i o n i n t e n s i t y from "^E i s g i v e n by:  I  =  o  k[02  (V)] g  - K ^ I O , ^ ) ] S w ^  2  (vii)  Upon a d d i t i o n o f a quencher gas the f o l l o w i n g r e a c t i o n must be added:  1 + 0 ( E ) ^  and  +  8  Q  k"s0  products  (viii)  t h e steady s t a t e c o n c e n t r a t i o n i s now g i v e n by:  [0 ( A ) ]  k [  °2<X  ) ]  =  k  X  +  W  and  >  the ^Z  kn[g] Ij  e m i s s i o n i n t e n s i t y by:  2  ( 1 X )  - 13 -  kk.[09(1A)]2 d 2 R k w + kQ[Q]  Q  (x)  The r a t i o of e m i s s i o n w i t h and without quencher i s :  =  1  +  [ Q ]  (  x  i  )  w From the Stern-Volmer p l o t s o f  I 0  /Iq  v s  «  a  [Q]  s t r a i g h t l i n e i s obtained  h a v i n g a s l o p e o f k^/k^, w h i c h , used i n c o n j u n c t i o n w i t h k w w i l l g i v e k_. Q  k  w  determinations  can be c a l c u l a t e d from a b s o l u t e measurements of  [CLC^A ) ] i n the absence of ^ 8  quencher.  R e s t r i c t i n g study to p h y s i c a l quenchers w i t h ground s t a t e 1 + t h e r e a r e then two p o s s i b l e mechanisms whereby 0^(. £ ) may  (1)  O.cV)  2  (2)  +  2  M  >  °O(  A  I  g  0 ( V ) +  1  hi.  >  J  +  singlets  be quenched:  ^  g  °2 h (3  )  1 + P r o c e s s (2) i n v o l v e s the t r a n s i t i o n o f 0_( E ) ^ g  +  3 0~( E ) which i s z g  —>  s p i n f o r b i d d e n u n l e s s the p e r t u r b e r i s paramagnetic  and i s thus  expected to be i n e f f e c t i v e f o r diamagnetic quenchers.  Although i t has  g e n e r a l l y been assumed t h a t quenching by diamagnetic s p e c i e s w i l l i n ^"E  g  —>  ^"A  g  t r a n s i t i o n , ^ ' ^ ^ t h e r e has been no d i r e c t evidence  r e p o r t e d t o support t h i s .  The b e s t i n d i r e c t evidence comes from  d i r e c t p h o t o - e x c i t a t i o n experiments  of  the  Evans.  I t has p r e v i o u s l y been mentioned t h a t ^"A to d e a c t i v a t i o n .  result  i s p a r t i c u l a r l y immune  T h i s i s p r o b a b l y a r e s u l t of two f a c t o r s :  (1) the  - 14 -  energy gap f o r the  1 A  3 £  t r a n s i t i o n i s considerably greater  t h a t o f 1 Z + '-»- "'"A t r a n s i t i o n ; g g  (2) the p r o c e s s  i s f o r b i d d e n by v a r i o u s s e l e c t i o n r u l e s , i . e .  O-^A ) 2 g  than  0„(3E~) 2 g  S = 0, i t i s s p i n  f o r b i d d e n , ft = 0 + 1, £ «-> A t r a n s i t i o n s a r e f o r b i d d e n , u <-> g o r g odd  g  terms combine o n l y w i t h even and v i c e v e r s a , i . e . a d i p o l e must be  created or destroyed  i n the t r a n s i t i o n .  However, o n l y the s p i n  s e l e c t i o n r u l e w i l l n e c e s s a r i l y h o l d t r u e f o r m o l e c u l e s undergoing collisions.  Thus ^A^ can be expected to be much l e s s s u s c e p t i b l e t o  quenching than . the ^"E  state.  r a t e s have been observed species  N0 2  , NO.  53  However, unexpectedly h i g h quenching  1 f o r quenching o f 0_( A ) by the paramagnetic 2 g  These anomalously h i g h v a l u e s a r e a t t r i b u t e d to  a paramagnetic e f f e c t , whereby the presence o f a paramagnetic s p e c i e s enhances s p i n o r b i t a l c o u p l i n g thus making the f o r m e r l y s p i n f o r b i d d e n 1  3  A —>-  E t r a n s i t i o n formally  allowed.  There a r e now b e l i e v e d to be t h r e e major modes o f s i n g l e t oxygen deactivation:  (1) quenching by amines and s u l f i d e s has been proposed 56—58 to o c c u r v i a f o r m a t i o n o f a c h a r g e - t r a n s f e r complex ; (2) quenching by atomic s p e c i e s has been suggested t o i n v o l v e t r a n s f o r m a t i o n o f e l e c t r o n i c e x c i t a t i o n energy to t r a n s l a t i o n a l energy  53  ; (3) quenching  by m o l e c u l e s has been suggested t o i n v o l v e t r a n s f o r m a t i o n o f e l e c t r o n i c > e n e r g y .53,59 e x c i t a t i o n t o v i-u b r a t«.. i o n a l57 Ackerman e t a l . and O g r y z l o  and Tang  58  i n their respective  s t u d i e s on s u l f i d e s and amines have both p o s t u l a t e d f o r m a t i o n  of a  c h a r g e - t r a n s f e r complex between 0„("'"A ) and t h e quencher, a m i n e / s u l f i d e . 2- g I t was suggested t h a t f o r m a t i o n amine/sulfide  and O-C^A  2  o f a c o n t a c t p a i r occurs between the  ) and f u r t h e r t h a t the r e s u l t i n g zero o r d e r s t a t e g  - 15 -  then mixes w i t h the s i n g l e t c h a r g e - t r a n s f e r s t a t e  ('''CT).  A  similar  3_ triplet  zero o r d e r s t a t e of C^C  £ ) and quencher would then mix w i t h  3 the t r i p l e t c h a r g e - t r a n s f e r s t a t e  ( CT).  Because these c h a r g e - t r a n s f e r  s t a t e s a r e v e r y c l o s e i n energy, a s m a l l s p i n - o r b i t i n t e r a c t i o n due to the n i t r o g e n / s u l f u r would then a l l o w the n e c e s s a r y m u l t i p l i c i t y  change.  The p r o c e s s can be w r i t t e n a s :  1  [0/V  where:  I I + X < 1 C I > ]  +  S^ctL ' W *  "  <3CI>)  X r e p r e s e n t s the s u l f i d e o r amine A,u  r e p r e s e n t the amount of c h a r g e - t r a n s f e r c h a r a c t e r mixed  i n t o the zero o r d e r s t a t e s ; a r e dependent upon the energy o f the charge t r a n s f e r s t a t e s and t h e r e f o r e upon i o n i z a t i o n p o t e n t i a l o f the Thus s i n c e l o g  quencher.  a I P ( q ) , a c o r r e l a t i o n between l o g k^ and IP i s  expected i f the above mechanism h o l d s ; such c o r r e l a t i o n s have been observed. In  t h e i r study of quenching  o f s i n g l e t m o l e c u l a r oxygen Davidson  53 and O g r y z l o  suggested t h a t the observed quenching  r a t e s of the atomic  s p e c i e s He and Ar r e p r e s e n t the b a s i c r a t e o f a p r o c e s s whereby the e l e c t r o n i c e x c i t a t i o n energy from the p r o c e s s  -*•  S  i s transformed  8  i n t o t r a n s l a t i o n a l motion o f both quencher and oxygen m o l e c u l e . have f u r t h e r suggested t h a t h i g h e r quenching  r a t e s observed f o r m o l e c u l e s  can be a s s o c i a t e d w i t h e x c i t a t i o n of v i b r a t i o n a l motion i n the Kearns^ O-C^A  r e c e n t l y completed  They  quencher.  a study o f the r a d i a t i o n l e s s decay o f  ) i n s o l u t i o n , and p r e s e n t e d a theory f o r the quenching  o f 0„ (^"A  by s o l v e n t i n terms o f i n t e r m o l e c u l a r e l e c t r o n i c to v i b r a t i o n a l  energy  )  - 16 -  transfer.  Quenching e f f i c i e n c i e s c o u l d , a c c o r d i n g to h i s t h e o r y , be  q u a n t i t a t i v e l y r e l a t e d to i n t e n s i t i e s o f IR o v e r t o n e and  combination  a b s o r p t i o n bands o f s o l v e n t s a t 7880 cm ^ and 6280 cm ^ which correspond to the (0-0) and  (0-1)  1  A  3 -»• E t r a n s i t i o n . g g  I t i s assumed t h a t the  3 ^ 1 E and A 8 8 1 + 3 1 + s t a t e s through the E s t a t e by s o l v e n t i n t e r a c t i o n where E and E g g g are mixed v i a i n t r a m o l e c u l a r s p i n o r b i t c o u p l i n g . Because E and A 8 8 quenching p r o c e s s i n v o l v e s second o r d e r i n d i r e c t m i x i n g of  m i x i n g i s a l s o important i n ®2^^g  J  quenching  the m a t r i x elements  used  to account f o r observed 0_(^A ) quenching are a l s o a p p l i c a b l e to 0~(^~Z ) ^ g ^ g quenching. The quenching r a t e c o n s t a n t can be expressed a s :  2ir x  Bcn  2  h  where:  2 mn  i and f r e f e r to i n i t i a l and f i n a l T  v i b  states.  i s the v i b r a t i o n a l r e l a x a t i o n time o f the s o l v e n t . e coso. cosf . "I 1 i • = < * ( A)! 1 £ 5—1- | > R±2 e  B  6 ±  R. i s the d i s t a n c e from the c e n t r e of the s o l v e n t to the l oxygen n u c l e u s 6 . i s the angle between the two -1 1 1 3 BgQ = 140 cm (matrix element c o u p l i n g E and E) F  m  i s the Franck-Condon f a c t o r f o r t r a n s i t i o n from the z e r o t h  v i b r a t i o n a l l e v e l of ^"L M  electronic  A  to the mth v i b r a t i o n a l l e v e l of ground  state.  = t h a t f a c t o r which determines  n t r a n s i t i o n s i n the IR r e g i o n .  the i n t e n s i t y o f 0-n  A n a l y s i s of gas and s o l u t i o n phase quenching f o r both  8  constants i n d i c a t e s  and ^"A , r a t e c o n s t a n t s o b t a i n e d i n one phase may 8  to c a l c u l a t e quenching  solvent  c o n s t a n t s i n the o t h e r phase.  be  that used  Davidson and O g r y z l o have r e c e n t l y p r e s e n t e d for  a similar  theory  53  gas phase quenching of C- (^l"1") which i n v o l v e s the c o n v e r s i o n of  e l e c t r o n i c e x c i t a t i o n energy of the e x c i t e d  species into  e x c i t a t i o n o f the quencher.  would be e x p e c t e d ,  a f i r s t approximation,  k  =  c  1  J o  If this i s so,  to obey the  vibrational to  relationship:  e „ ( v ) f _ (v)dv q °2  (xiii)  where f  (v) i s the r e l a t i v e e m i s s i o n i n t e n s i t y of transition 2 £v;(v) i s quencher e x t i n c t i o n c o e f f i c i e n t ; c i s the p r o p o r t i o n a l i t y c o n s t a n t ; and  integration  i s over  the l i m i t s of O.^E*) 2 g  O.^A 2V  ) g'  progression.  which can be s e p a r a t e d  k  q  =  F C  e  l/ q  as:  ( v ) f  ( O  v)dv  FC  e  v f (v)dv+  (xiv)  + 2J q( > 0  where FC i s the Franck-Condon f a c t o r f o r the t r a n s i t i o n and integration f  i s over a p a r t i c u l a r band i n the  the  p r o g r e s s i o n and  (v) i s the r e l a t i v e CL e m i s s i o n i n t e n s i t y w i t h i n the band. °2  In t h i s p r e l i m i n a r y r e p o r t they s t u d i e d o n l y the o v e r l a p i n the 5240 the  (0-0) band r e g i o n .  cm  In c o n t r a s t t o the homonuclear d i a t o m i c s , the  i n f r a r e d a b s o r p t i o n s of o t h e r molecules r a t h e r poor c o r r e l a t i o n o b t a i n e d was  may  be more i m p o r t a n t .  p a r t i a l l y attributed  modes of v i b r a t i o n and consequent a b s o r p t i o n s due  to  The  additional  to combination  bands.  - 18 -  As d i d M e r k e l and K e a r n s , ^ Davidson and O g r y z l o " ^ u t i l i z e d molecule Franck-Condon f a c t o r s i n t h e i r i n i t i a l s t u d i e s .  free  However,  Franck-Condon v a l u e s f o r a molecule i n v o l v e d i n the quenching  process  v e r y l i k e l y d i f f e r from those f o r a f r e e m o l e c u l e , t h i s has been observed i n a subsequent  study by Davidson and Ogryzlo.^"''  B e f o r e a d e t a i l e d q u a n t i t a t i v e theory c o u l d be d e v e l o p e d , c o n s i d e r a b l y more d a t a i s r e q u i r e d p a r t i c u l a r l y a d e t e r m i n a t i o n of r e l i a b l e Franck-Condon f a c t o r s .  Thus the o b j e c t o f the p r e s e n t study was  (1) the s y s t e m a t i c study of quenching  twofold:  a b i l i t i e s of a s e r i e s o f molecules  c o n t a i n i n g heavy atoms to unambiguously show the presence or absence of a heavy atom e f f e c t ;  (2) the d e t e r m i n t i o n of the complete  IR s p e c t r a  over the energy r e g i o n s o f the ( 0 - 0 ) , ( 0 - 1 ) , ( 0 - 2 ) , (0-3) bands of ^"Z -*• +  t r a n s i t i o n f o r a v a r i e d s e r i e s o f quenchers, the m a j o r i t y 53  of whose k^ have been p r e v i o u s l y r e p o r t e d ,  i n an e f f o r t to e m p i r i c a l l y  determine a s e t of r e l i a b l e Franck-Condon f a c t o r s . the study was  The major aim o f  to f u r t h e r c l a r i f y the above proposed mechanism f o r p h y s i c a l  quenching o f 0~(^~T, ) and to d e t e r m i n e , i f p o s s i b l e , r e l i a b l e 8 1 Condon f a c t o r s f o r the ( 0 - 0 ) , ( 0 - 1 ) , (0-2) and (0-3) 1 E + A 8 +  Franck8  transitions.  -  19 -  EXPERIMENTAL  Flow System F i g u r e I I shows a schematic Oxygen was  admitted  drop of mercury was  o f the f l o w system used i n t h i s  study.  to the system v i a an Edwards needle v a l v e .  A  p l a c e d b e f o r e the d i s c h a r g e a r e a and heated  t o form  a m e r c u r i c o x i d e f i l m t o remove 0. the d i s c h a r g e a r e a and  An  i n l e t was  p o s i t i o n e d between  the o b s e r v a t i o n p o i n t to i n s u r e complete  d e s t r u c t i o n of oxygen atoms.  The  oxygen flow was  i n l e t v a l v e p a s t the mercury d r o p , through  c a r r i e d from the  the d i s c h a r g e a r e a where  the e x c i t e d s p e c i e s were formed, over the m e r c u r i c oxide f i l m , p a s t the NO2 122 to  i n l e t and i n t o the o b s e r v a t i o n t u b e .  cm l o n g w i t h an i n t e r n a l diameter a l l o w c o n t r o l of tube temperature.  The o b s e r v a t i o n tube  of 25 mm  and was  was  double j a c k e t e d  Quenching gas e n t e r e d  the  system v i a a t e f l o n needle v a l v e at the f r o n t end of the o b s e r v a t i o n tube.  Tube p r e s s u r e was  stopcock opening a c a p a c i t y o f 300  maintained  to the pump, a Duo 1/min.  The  between 2 and 4 mm  by a d j u s t i n g the  S e a l Vacuum Pump, Model 1376  oxygen f l o w r a t e was  the time r e q u i r e d f o r exhaust gases to empty a one f l a s k f i l l e d w i t h water when no quencher was  measured by  with  determining  l i t r e volumetric  added to the system.  - 20 -  Figure I I .  Schematic o f Flow System.  Edwards Needle  To Trap and Pump  Teflon Needle  Valve  -  22 -  Materials All  quenching m a t e r i a l s were prepared  f o r use by  degassing  u s i n g f r e e z e , pump, thaw c y c l e s , f o l l o w e d by vacuum d i s t i l l a t i o n through  o r  a d r y i n g agent such as J?2^5  the f l o w system was chemicals used and  Production of  2  a c t l v a t e  used as r e c e i v e d . their  d  alumina.  Oxygen f o r  Table I I g i v e s a l i s t  of  sources.  0„(V) g  02"^ (Ag) and 0 were formed i n an e l e c t r i c a l d i s c h a r g e produced by a Scintillonics  I n c . , Model HV15A microwave generator w i t h maximum  output power of 120 w a t t s . of  To prevent  the d i s c h a r g e a r e a the system was  the j a c k e t of the d i s c h a r g e c a v i t y . was  c o l l a p s e of the q u a r t z  c o o l e d by blowing  c o l d a i r through  Complete removal of oxygen atoms  ensured by d e p o s i t i o n of a m e r c u r i c oxide f i l m a f t e r the  a r e a f o l l o w e d by a slow f l o w of NO2 under s t u d y , 0 ("'"E ) was 2- g  O.^A 2 g  )  +  generated  O.^A 2 g  )  —>  tubing  i n t o the oxygen f l o w .  discharge  The  species  by the energy p o o l i n g p r o c e s s :  O.(V) 2 g  +  0 (3E ) 2 g  3 9  'A°  (i)  D e t e c t i o n of the E x c i t e d S p e c i e s The  r e l a t i v e c o n c e n t r a t i o n of 0„("'"E ) was determined by measuring 2 g 1 1 o e m i s s i o n i n t e n s i t i e s of E and A bands a t 7620 A and 6340 A 0  r e s p e c t i v e l y at a p o i n t about o n e - t h i r d the l e n g t h of the o b s e r v a t i o n tube from the quencher i n l e t . used to i s o l a t e the two bands.  A Zeiss variable interference f i l t e r The  chopped s i g n a l was  u s i n g a Hamamatsu TVR-213 p h o t o m u l t i p l i e r tube and  detected  f e d through  a  was  - 23 -  Table I I .  Sources of m a t e r i a l s  used.  Chemical  Source  CHC1 3  S p e c t r a l Grade, F i s h e r Chemical  CH 2 C1 2  S p e c t r a l Grade, F i s h e r Chemical  CH2Br2  Eastman Chemicals  CH 3 I  Eastman Chemicals  CH 3 Br  Matheson  CH 3 C1  A i r Products  CHF 3  Matheson  CH2F2  P e n i n s u l a r Chemicals Matheson, E x t r a Dry  °2 CH3OH  S p e c t r a l Grade, F i s h e r Chemical  i-PrOH  A n a l y t i c a l Grade, M a l i n c k r o d t  Et 2 NH  Malinckrodt  MeNH„, Me NH, Me N z z 3 E t N H 2 , E t 2 N H , NH 3  Matheson  - 24 -  T e k t r o n i x 122 p r e a m p l i f i e r i n t o a S e r i a l #240 l o c k - i n a m p l i f i e r where b o t h phase and f r e q u e n c y were compared w i t h a s i m i l a r i l y reference s i g n a l . the s i g n a l  chopped  A 10 MV Leeds Northrup r e c o r d e r was used t o r e c o r d  intensities.  Determination  of Experimental Values  R e l a t i v e r a t e c o n s t a n t s were c a l c u l a t e d from Stern-Volmer p l o t s o f 1 (I^/I) and  2  v s . [ Q ] , c o r r e c t e d f o r changes i n [O^i A ) ] .  Quencher f l o w r a t e  thus quencher c o n c e n t r a t i o n was determined by r e c o r d i n g the  pressure  drop over a known p e r i o d of time i n a s t o r a g e bulb  (various  s i z e s were used) o f known volume, on a Hewlett-Packard r e c o r d e r . 8 r a t e c o n s t a n t s were determined r e l a t i v e t o (X^* k sec  = 1.8 x 10  The —1  1 mole  ,^  f o r convenience and b e t t e r r e p r o d u c i b i l i t y .  The  gas phase i n f r a r e d a b s o r p t i o n s p e c t r a were o b t a i n e d u s i n g a  P e r k i n - E l m e r 457 spectrometer f o r the 4000 t o 250 cm ^ r e g i o n and a Cary 14 f o r t h e 10,000 t o 4000 cm ^ r e g i o n .  F o r those compounds  normally  l i q u i d a t room temperature a heated c e l l was used to o b t a i n the i n f r a r e d absorptions  i n the 10,000 t o 4000 cm ^ r e g i o n , p r e s s u r e s o f  gaseous compounds were measured u s i n g a USC p r e s s u r e of a l l compound f o r the 4000 t o 250 cm a P r e c i s i o n Pressure  gauge.  Pressures  r e g i o n were measured u s i n g  Gauge Model 145 o b t a i n e d  from Texas I n s t r u m e n t s .  - 25 -  RESULTS  Heavy Atom Study The Stern-Volmer e q u a t i o n which i s used to determine r e l a t i v e quenching r a t e c o n s t a n t s can be o b t a i n e d from the f o l l o w i n g  O.^A) 2 g  + O.^A 2 g  )  - ^ - ^  reactions:  O.^Z) + 0 ( 3 Z ) 2  g  2  g  (i)  k  02(  1 + E ) + wall  ——»  02(  1 + E ) + Q  —3—>  products  (ii)  products  (iii)  k  In the absence of quencher, 0 2 ( ^ E + ) e m i s s i o n i n t e n s i t y i s g i v e n by:  x  o •  k  r'tQ2 Vi ( 1  2  ( v i i )  w upon a d d i t i o n of quencher, the e x p r e s s i o n f o r 0.("''E ) becomes: 2- g  k k [o th )] Q  k  w  + k [Q]  2  w  q  The r a t i o o f e m i s s i o n w i t h and w i t h o u t quencher r e s u l t s i n the S t e r n Volmer e q u a t i o n :  k I  Plotting  I  c/ Q  "  1  +  t  t  Q  (xi)  ]  w  ( I / I Q ) a g a i n s t quencher c o n c e n t r a t i o n w i l l r e s u l t i n a Q  s t r a i g h t l i n e having  a s l o p e e q u a l to the r e l a t i v e quenching  constant.  At l e a s t f o u r s e t s o f data were c o l l e c t e d f o r each quencher s t u d i e d (a d a t a s e t c o n s i s t e d o f the measurement o f the 0 _ ( ^ Z + ) i n t e n s i t y a t f o u r d i f f e r e n t quencher c o n c e n t r a t i o n s ) . computer program was  emission A. l e a s t  squares  used to o b t a i n the s l o p e f o r each d a t a s e t and  subsequent l e a s t squares a n a l y s i s of the s l o p e s r e s u l t e d i n a b e s t f o r the r e l a t i v e quenching r a t e c o n s t a n t . Stern-Volmer p l o t s o b t a i n e d quenching c o n s t a n t s constants  An example t y p i c a l o f  value  the  i n t h i s study i s shown i n F i g u r e I I I .  obtained  a  The  i n t h i s study are g i v e n i n T a b l e I I I .  Rate  f o r the f o l l o w i n g compounds c o u l d not be determined because  of system l i m i t a t i o n s :  CHI^,  CH,^*  CHBr^.  That the v a r i a t i o n i n k^  v a l u e s i s so s m a l l i n d i c a t e s t h a t the d e a c t i v a t i o n o f 0„("'"E + ) i s not 2 g s u b j e c t t o heavy-atom e f f e c t . T h i s r e s u l t supports the assumption t h a t d e a c t i v a t i o n goes w i t h 0„( 2  1 + E ) g  the 0 „ ( ^ E + ) -»• O-C^A 2 g 2  3 0„( E ) transition occurring. 2 g  ) t r a n s i t i o n r a t h e r than  the  g  Quenching Mechanism Study I t has been s u g g e s t e d ^ t h a t the quenching r a t e c o n s t a n t  can  be  p r e d i c t e d from the o v e r l a p o f the a b s o r p t i o n s p e c t r a o f the quencher spectrum o f 0 „ ( ^ " E + ) i n i t s t r a n s i t i o n z g u s i n g the f o l l o w i n g e x p r e s s i o n :  with  the e m i s s i o n  k  q  to 0_(^A ) , £ g  (xiii)  - 27 -  Figure I I I .  Stern-Volmer p l o t f o r CH^C^ O  ,^  , •  ,O  (each symbol:  r e p r e s e n t s a data s e t ) .  7.0-  5  7(5  S  20~  25  30  35  CQ3 moles f 1 Ho" ) 7  40  45~  50  55  - 29 -  Table I I I .  Quenching r a t e c o n s t a n t s o f CH X (X = h a l o g e n ) . n m  Quencher  k  (l.mole  sec  )  F  Cl  Br  CHX3  3.77 x 1 0 7  3.49 x 1 0 7  —  CH 2 X 2  7.87 x 1 0 7  1.42 x  io  8  2. 22 x  io  8  1.04  io  8  1.20 x  io  8  CH3X CH. 4  4.5 x 1 0 7  see r e f erence i61l.  3  x  I  1.32 x 1 0 8  - 30 -  An expansion o f t h i s e x p r e s s i o n i n terms o f quencher e x t i n c t i o n 1 + 1 c o e f f i c i e n t s a t 0 „ ( E -*• A ) t r a n s i t i o n f r e q u e n c i e s was used by Merkel 2 g g 60 1 and Kearns i n t h e i r study o f 0.( A ) d e a c t i v a t i o n i n s o l u t i o n and ^ 8 subsequently by Davidson and O g r y z l o  i n t h e i r study o f gas phase  quenching o f 0 2 ^ g ^ ^  Davidson and O g r y z l o proposed  hydrocarbons.  an expansion o f ( x i i i ) i n terms o f o v e r l a p i n t e g r a l s i n an attempt t o o b t a i n a b e t t e r f i t between c a l c u l a t e d and e x p e r i m e n t a l quenching c o n s t a n t v a l u e s than was observed u s i n g an expansion i n terms o f extinction  coefficients:  k  q  = c [ F C . x o v e r l a p @ 5237 cm l  -1  + F C 0 x o v e r l a p @ 3755 cm  -1  I  + F C 3 x o v e r l a p @ 2297 cm"1 + FC^ x o v e r l a p @ 865 c m - 1 (xv) Both these s t u d i e s used the t h e o r e t i c a l Franck-Condon f a c t o r s by  determined  Nicholls^2: 0-0 t r a n s i t i o n @ 5237 cm"1  0.97  0-1 t r a n s i t i o n @ 3755 cm" 1  2.32 x 1 0 ~ 2  0-2 t r a n s i t i o n @ 2297 cm"1  3.70 x 1 0 ~ 4  865 cm" 1  5.09 x 1 0 ~ 6  0-3 t r a n s i t i o n @ f o r the  O-C^E* 2  g  ^A ) t r a n s i t i o n . g  However, t h e r e i s some q u e s t i o n  as to t h e v a l i d i t y o f u s i n g these v a l u e s s i n c e they were c a l c u l a t e d 63 f o r the f r e e oxygen m o l e c u l e .  Infrared absorption studies  have shown  t h a t c o l l i s i o n induced t r a n s i t i o n s have d i f f e r e n t Franck-Condon f a c t o r s from f r e e molecule t r a n s i t i o n s .  However, s i n c e c o l l i s i o n induced v a l u e s  a r e almost i m p o s s i b l e t o o b t a i n , Davidson and Ogryzlo^"'" o b t a i n e d a ' b e s t - f i t ' w i t h t h e i r hydrocarbon d a t a by s i m p l y v a r y i n g the FranckCondon f a c t o r s .  - 31 -  In t h i s s t u d y , o v e r l a p s i n the 0-0,  0-1,  were c a l c u l a t e d f o r a v a r i e t y of quenchers. O g ' r y z l o ^ expansion  0-2,  •*• O^C^A 2. g  0-3  regions  Using the Davidson  and  i n terms of o v e r l a p i n t e g r a l s quenching r a t e  c o n s t a n t s f o r these quenchers were c a l c u l a t e d . + 0_("4; ) 2 g  and  S i n c e the shapes of  the  ) e m i s s i o n bands have not y e t been r e p o r t e d , t h e i r  s t r u c t u r e s were c a l c u l a t e d u s i n g f r e e m o l e c u l e s p e c t r o s c o p i c c o n s t a n t s and assuming t h a t the s e l e c t i o n r u l e s f o r changes i n r o t a t i o n a l quantum numbers are those c h a r a c t e r i s t i c s of induced 0,+  1,+  2 and  transitions, i.e.  that a l l t r a n s i t i o n s occur w i t h e q u a l  probability.  F i g u r e IV shows a p l o t o f the computer c a l c u l a t e d oxygen ("4]+ t r a n s i t i o n band s t r u c t u r e .  J =  ^A  )  The o v e r l a p i n t e g r a l s were c a l c u l a t e d w i t h  the use of a computer program and  Figure  Va-d  shows an example of the o v e r l a p i n t e g r a l s o b t a i n e d i n t h i s s t u d y .  The  f o l l o w i n g e x p r e s s i o n was  used to c a l c u l a t e v a l u e s of quenching  c o n s t a n t s which were subsequently  kq  are l i s t e d i n T a b l e IV.  e n t e r e d i n t o Table  = c(0.97 x o v e r l a p i n the 0-0 x o v e r l a p i n the 0-1  IV:  r e g i o n + 2.32  r e g i o n + 3.70  x  10  x  10  -4 —6  x o v e r l a p i n the 0-2  r e g i o n + 5.09  x o v e r l a p i n the 0-3  region]  c f i x e d to make k ^ ( c a l c u l a t e d ) f o r  x  10 (xvi)  C  y  H  ^  e q u a l to  k^(experimental). Values  of e x p e r i m e n t a l  values i n Figure VI.  r a t e c o n s t a n t s were p l o t t e d a g a i n s t the From a comparison of t h i s p l o t and  calculated  the p l o t  53 o b t a i n e d by Davidson and O g r y z l o  (see F i g u r e V I I ) i t i s e v i d e n t  t h a t the c o r r e l a t i o n o b t a i n e d i s not improved by i n c l u d i n g o v e r l a p  - 32 -  F i g u r e IV.  Computed "induced" e m i s s i o n bands f o r the 0 „ - > 2 g  "*"A ) g  transition.  1.0.9.8CO  -7  54.00  5200  5000  3900 V  (cm  3700 l  )  3500  2500  (note compressed  2300  2100  scale)  1100  900  700  T a b l e IV.  Quencher  O v e r l a p i n t e g r a l s and c a l c u l a t e d quenching  k  q  (expt'l)  0,0 o v e r l a p  constants.  0,1 o v e r l a p  0,2 o v e r l a p  0,3 o v e r l a p  k  (calc) -1 -1 (l.mole sec ) q  (l.mole "'"sec ^)  x 10"  2  CH 0H  2.06 x 1 0  9  3.56  EtOH  1.59 x 1 0  9  0.868  iso-C^OH  1.58 x 1 0 Q a 1.1 x 1 0 5.6 x 1 0  3  CH NH  9  3  (CH ) NH 3  9  2  2  1.9 x  6.31  5.94  3.47  3.14  5.4 x  2.20  0.0428  0.395  0.327  0.351  4.64  2.9 x  6.13  1.23  2.81  1.51  4.4 x  3.36  0.432  0.0428  0.649  2.1 x  9.85  0.154  0.141  8  0.513  0.405  1.27  1.82  4.3  X  8  0.430  1.35  2.66  0.509  5.3  X  8  0.00  0.119  1.74  0.639  2.4  X  7  0.017  0.185  0.999  5.0  X  io io io  8  0.490  0.628  0.780  5.97  4.5  X  10  2.07  0.249  4.6  X  io  0.139  4.2  X  10  0.000274  8.1  X  io  8  3  X  io  2.22  X  10  CH Br  1.20  X  CHC1  3.49  X  1.42  X  io io io  CH C1  1.04  X  10  8  0.561  0.408  CHF  3.77  X  10  7  0.435  0.368  7.87  X  io  3  CH Br 2  2  3  3  CH C1 2  3  3  CH F 9  0  2  1.5 x  4.21  8  1.32  CH I  12.7  a  1.2 x 10  3  3.68  2.6 x  NH  3  4.50  8.47  5.8 x 1 0  5  7.64  3.51  (C H ) N 2  4.8 x  1.60  7.79 x 1 0  2  2.43  3.43  (C H ) NH 5  0.0348  2.2 x  8  2  4  3.88  8.9 x 1 0  2  x 10"  2.42  C H NH 5  3  1.57  3.6 x 1 0  2  14.1  x 10~  2.76  (CH ) N 3  3  3  8  8  3  3  x 10~  7  1.17  .0.0  20.2 1.62  17.3  13.1  -O  7.0 x  10  7  7  6  6  7  7  7  7  T a b l e IV (Continued)  Quencher  C H  4  C  2H6  C  3H8  C  4H10  ^C3H6 C  4H10  SH12 C  C  r  6H14  O a C  k (expt'l) q n i " Isec ) (l.mole  x 10~  2  0,1 o v e r l a p x 10~  3  0,2 o v e r l a p x 10"  3  0,3 o v e r l a p x 10~  4  k  (calc) q _ i _ i (l.mole sec )  4.4 x 10  0.022  0.226  0.0068  0.0000  5.3  X  1.9 x 10  1.72  1.32  1.16  0.79  1.4  X  2.7 x 10  2.74  1.44  1.28  0.455  2.1  X  2.9 x 10  3.71  1.64  1.53  0.272  2.8  X  3.7 x 10  6.09  0.951  1.13  6.91  4.4  X  3.8 x 10  3.71  1.64  1.53  0.222  2.8  X  4.5 x 10  5.31  2.63  2.8  1.19  4.1  X  U)  5.2 x 10  5.5 x 10  5.8 x 10  5.8 x 10  7H16  0,0 o v e r l a p  6.0 x 10  8  8  8  8  8  Reference 53 and r e f e r e n c e s  6.72  4.05  3.96  1.27  5.3 x 10  6.56  3.41  3.63  1.15  5.1 x 10  7.62  4.36  3.35  1.60  6.0 x 10  8.43  4.54  4.20  2.04  6.6 x 10  7.77  4.03  4.28  1.09  6.0 x 10  therein.  Hydrocarbon k^ v a l u e s from r e f e r e n c e 61.  8  8  8  8  8  - 36 -  Figure Va.  Computer drawn r e l a t i v e o v e r l a p i n t e g r a l 0-0 r e g i o n .  f o r CHF  - LZ -  - 38 -  F i g u r e Vb.  Computer drawn r e l a t i v e o v e r l a p i n t e g r a l 0-1 r e g i o n .  f o r CHF^,  - 6C -  - 40 -  Figure Vc.  Computer drawn r e l a t i v e o v e r l a p i n t e g r a l 0-2 r e g i o n .  f o r CHF 3 >  - 41.-  - 42 -  F i g u r e Vd.  Computer drawn r e l a t i v e o v e r l a p i n t e g r a l GT3 r e g i o n .  f o r CHF  - tv -  - 44 -  Figure VI.  C o r r e l a t i o n between  (experimental)  and k  ( c a l c u l a t e d by use o f o v e r l a p method).  8 b7j CO to  o  A  1  A  • SD 6  A Hydrocarbons (ref.59) • Heavy-atom containing compounds o A l c o h o l s and Amines  7  §  k (expf|)lO q  9  -8  10TT  I mole" sec  12  13  14  15  16  17  18  Ti  20"  - 46 -  Figure V I I .  Overlap i n t e g r a l v e r s u s 0 2 ( E ) quenching constants  (from r e f . 5 3 ) .  rate  - 47 -  8  10  M  " relative overlap integral A Designates those compounds containing rm or OK Number .Compound 1  2 3 k  5 6  7 8 9  10 11 12 13  14 15  16 17 18 •19 20 21 22  C2H6 CH . cyclo-C-jH^ n-C^Hjo iso-Ci^Hjo 3  8  cyclo-C6H  |2  CH^-cyclo-C^Hcj n-C7H1o CH^-cyclc-CfcH*1 NHn CH3NHX  (CH^NH C^cOrU  CHnOH C 0  2  - 48 -  i n t e g r a l s i n the 0-1, 0-2 and 0-3 r e g i o n s i n the c a l c u l a t i o n s . r e s u l t suggests t h a t better  This  the l i m i t a t i o n s of the model are so g r e a t that a  c o r r e l a t i o n cannot be o b t a i n e d w i t h i t .  U s i n g a s i m p l e r seemingly l e s s q u a n t i t a t i v e to p r e d i c t  method i t i s p o s s i b l e  how the e l e c t r o n i c e x c i t a t i o n o f O ^ ^ E * ) i s d i s t r i b u t e d  i n v i b r a t i o n a l e x c i t a t i o n i n O^C^A ) and v a r i o u s quencher modes. make these p r e d i c i t o n s and and  o v e r t o n e s were c o n s i d e r e d .  I t was assumed t h a t :  i s a l l o w e d ; and (2) the s t r e n g t h o f c o u p l i n g i s dependent upon  energy d i f f e r e n c e J  1 between the t r a n s i t i o n s 0 ( A ) 2 g o  1 0„( A. ) and 2 g v=z  Q -> Q , where z, x, y a r e the degrees of v i b r a t i o n a l va=x,vb=y H  and  combinations  (1) the O^C^E"**) 2 g quencher were o n l y weakly c o u p l e d , thus the use o f s e p a r a t e d  states the  the fundamental f r e q u e n c i e s and b i n a r y  To  J  &  a, b a r e the normal v i b r a t i o n a l modes o f the quencher.  excitation  T a b l e V.  Most p r o b a b l e mode o f quencher e x c i t a t i o n and u 2 ^ ^ g ^  Quencher  k  Fundamental  (l.mole-^ s e c ~ l )  frequency (degeneracy)  Important overtones and combinations  AE (cm ) (C^ t r a n s i t i o n )  vj 2917  CH  C ± i  4  4 5x 1 0 73 2 X i U V  ^  1 5 3 4  ( ) 2  v 3 3019(3) v 4 1306(3)  v x 2954 v 2 1388 v3 995 289 v 5 2896 8 a 6 "76 2.0 x 10 v 7 2969(2) v 8 1468(2) v 9 1190(2) v102985(2) vn1469(2) v i a 822(2) V  C  H  cn  2 6  v: v3  838(3755) 736(3755)  (A)  lk  Most p r o b a b l e e x c i t a t i o n and d e a c t i v a t i o n mode  1 02(V") _ + CH. + 0 ( Ag v)= l- + 4CH.v = l . o g v=0 4 2 o  3  + 0 „ ( 1 A ) , + CH. . 2 g v=l 4 vj=l  +  C  2H6 + v V v - 1  °2<\>v-l  vj 801(3755) v 5 859(3755) v 7 786(3755) v10770(3755)  vx v2 v3 \) k v5  c3V"  3038 1479 1188 1126 1070 3103 8 a 854 3.7 x 10 v 8 3025(2) v 9 1438(2) v101029(2) v n 866(2) v123082(2) v131188(2) v 739(2)  deexcitation.  ->  ° 2 < V - 1 ->  0  2<Vv-l  ,L+,  °2<\>v-l » 1 717(3755) v 6 652(3755) v 8 730(3755) v 1 2 673(3755)  0  2<Vv=l  ° 2  (  \ > v . l  +  C  2H6  +  C  2H6 v7=l  +  C  2H6 v  v  10=1  1 =  l  +  C  2H6 V5-1  +  C  3H6 v6=l  +  C  3H6 v8=l  +  C  3H6 v  +  C  1 =  l  3H6 v12=l  Table V. (Continued) Quencher  Fundamental frequency (degeneracy)  -1 (l.mole.. sec ) q  3681 3000 2844 1477 v 1455 v 1345 v 1060 v 1030 v g 2960 v 1477 v 1165 vi2^250  Important overtones and combinations  AE (cm ) (0^ t r a n s i t i o n )  Most p r o b a b l e e x c i t a t i o n and d e a c t i v a t i o n mode  Vi  v v  CH 0H c n 3  2  3  5  2.2 x 10'  6  v-L+\> 5158 V!+v 5136 k  5  7  vi  74(3755) 81(5239) 0 ( i : ) + CH.OH 2 g 3 103(5239)  \>±+\>u  vi+v  5  1  0„( A ) + CH.OH . 2 g v=l 3 vj=l 1  +  0  8  10  n  01 o  3361 2961 3 2820 vk 1623 5 1473 1430 9 b v 1130 1.1 x 10 8 1040 v 780 v 3427 v 2985 v 1465 v 1419 v 1195 v 286 l v  v  2  v  v  CH NH c n 3  2  7  VI++VKJ5050  v  V! 394(3755) v 328(3755) v +v 190(5239) 1 0  4  V ^ g W  ^  2  °2< Ag>v=l  * 2(1Ag>v-l  10  0  9  +  +  C H  3  N H  ^  2 vio-1  2  vi-1  10  n  12  13  llt  1 5  NH  cii  v j 3337 .9 b v 950 1.2 x 10 v 3444(2) 1627(2) 2  3  v +Vi* 5071 3  418(3755) 311(3755) ^3 V3+V!, 168(5239)  0 ( h*) 2 •g  +  NH 3  0„( A ) + NH . 2 g v=l 3 v =l 1  3  + NH O ^ ^ ) , , ^ + NH„ •> 0 ( A „ ) "g v=0 g v=l 3 Vj-1 1  X  o  Table V. (Continued) Quencher  k ? -1 (l.mole s e c ~ l )  Fundamental frequency 3 ^ . (degeneracy) 2933 v 2 1252 ^3 533 v 4 3060(2) ^5 1436(2) 882(2) ^6  CH3C1  C11  1 x 10  8  o CH2Br2  C 1 1  2.22 x 10  vi v2 V3 v<+ ^5 ^6  2972 1305 611 3056(2) 1443(2) 953(2)  v2 V3 vi+' V5 ^6  2937 1355 732 3039(2) 1452(2) 1017(2)  v i 3009 v 2 1382 v 588 ^3 3 169 v vi+ 1095 V5 V6 3073 v7 812 1195 V8 v9 653 k  Important overtones and  AE (cm ) (0^ t r a n s i t i o n )  Most p r o b a b l e e x c i t a t i o n and d e a c t i v a t i o n mode  combinations  020 (V") ,g v4  1 _ + CH.I • * 0 _ ( Ag)v = l + CH.I v=0 3 2 3  ,  v i =1  1  + 0_( A ) + CH.I . 2 g v=l 3 vi t =l  822(3755) 695(3755)  1  +  1  0 , ( Z ) _ n + CH Br -»• 0 „ ( A ) + CH„Br 2 g v=0 3 2 g v=l 3 vi=l v i 783(3755) v 4 699(3755)  0„(1A ) + CH Br 2 g v=l 3 vi+=l  °2o (  l z +  n+ g) v=0  C  H  v i 818(3755) vi+ 716(3755)  3 C  1  ° O2 (  1  a  i l+ e> v =  C H  ^ C3 1  v tii = l  0 ( A ) + CH„C1 . 2 g v=l 3 vi=l  °2 v i 746(3755) V6 682(3755)  Q  ( 1 Z  g>v-0  +  C H  2Br2 * ° 2 - 0  ( l A  2  g v-l )  ( \ )  v = 1  +  C H  2Br2 v  1 =  + CH2Br2  l  v6-l  T a b l e V.  (Continued)  Quencher  k n  Fundamental frequency (degeneracy)  ? -1 sec"1)  V! v  v i i  i1.42 A? vx i10n  8  v  2  7.87 x 10  9  Vi  2993  7 8  >J„  7  v  v v  7  8  3.49 x 1 0 7  v  :  v  2  ll v v  5  6  vx cii CHF 3  3.8 x 10  7  v v  3  Vif  v v  SL  2  v  + O.^A 2 g v-1  7 1 5 ( 3 7 5 5 )  +  n 4  )  756(3755)  m  6  ^  9  +  ~ °< V w - °2< Vv-1  671(3755)  + CH 2CI_2 v_- l  +  C H  6  n-i  ^  2 2 v6-l F  3084 1217 1435 1090  6  Vg  CHCl3cli  V g  3040 898 1268 758  6  0 o (V X E ) + CH 0 C1 0 + 0 o (X X A ) -• + CH„C1, 2 g v=0 2 2 2 g'v=l 2 2 vi=l V l  1 1 5 3  ll i  7 2  2 8 2  V s  v v v  CH F  3  Vl+  1  2999 1467 717  2  TH n c CH 2 C1 2  AE (cm 1 ) Most p r o b a b l e e x c i t a t i o n and d e a c t i v a t i o n mode (CL t r a n s i t i o n )  Important overtones and combinations  3034 680  f  12  Q(2)  V!  721(3755)  o / ^ ) ^  + CHC1 3 - 0 2 ( 1 A g ) v = 1 + CHC1 3  v  i  =  774(2) 261(2) 3036 1117 700 1 3 7 2 ( 2 )  5  1152(2)  6  507(2)  R e f . 61 and r e f e r e n c e s t h e r e i n ;  V l  719(3755)  i  J .  02( E )  = ( )  + CHF 3 - O ^ A  i  )  v = 1  + CHF 3  y  =]  _ 1  b e R e f . 53; fundamental v i b r a t i o n a l f r e q u e n c e s , ( i ) R e f . 64, ( i i ) R e f . 65.  1  - 53  -  DISCUSSION  Heavy-Atom Study p o s s i b l e modes of d e a c t i v a t i o n o f 0_(''"E+):  Of the two  (1)  (2)  0 ( V ) z g  +  0_(V)  +  2  i t has  g  *M  ,  „  >  , . ,  z  s  2  g  +  ^  been g e n e r a l l y assumed t h a t p h y s i c a l quenching goes v i a p r o c e s s  (1) as was  previously stated.  Although t h e r e i s no d i r e c t  to support t h i s assumption, i n d i r e c t evidence"*"* and  evidence  a n a l y s i s of  quenching a b i l i t i e s of v a r i o u s s p e c i e s do g i v e an i n d i c a t i o n  of the most  probable path. When a heavy atom or paramagnetic s p e c i e s on one molecule i n t e r a c t s with  the e l e c t r o n i c s t r u c t u r e of a second m o l e c u l e , t h i s enhances  probability  the  of i n t e r s y s t e m c r o s s i n g by i n d u c i n g s p i n - o r b i t c o u p l i n g which  r e l a x e s the f o r b i d d e n n a t u r e of the t r a n s i t i o n .  Applying  t h i s to  quenching p r o c e s s , i f quenching i n v o l v e d i n t e r s y s t e m c r o s s i n g  the  the  observed quenching r a t e c o n s t a n t f o r a compound c o n t a i n i n g a heavy atom or a paramagnetic s p e c i e s would be u n e x p e c t e d l y h i g h . 1 of 0 „ ( A ) which i n v o l v e s the 2 g  A  1 g  3  £  g  For quenching  t r a n s i t i o n , a paramagnetic  e f f e c t has been o b s e r v e d .  53  From Table VI i t i s e v i d e n t t h a t the  quenching c o n s t a n t s f o r the heavy atom Xe and  f o r the paramagnetic  s p e c i e s 0^ are g r e a t e r than would n o r m a l l y be  expected.  However, examination s p e c i e s f o r CL and N0 2  ("4;+)  o f quenching r a t e c o n s t a n t s o f paramagnetic  d e a c t i v a t i o n seems somewhat c o n t r a d i c t o r y .  0 ,  NO,  a r e a l l paramagnetic s p e c i e s , however, 0^ i s a poor quencher of  O^C^Zg) whereas NO  i s a r e l a t i v e l y good quencher.  T a b l e V I I shows the  quenching c o n s t a n t v a l u e s o f s e r i e s of quenchers c o n t a i n i n g these paramagnetic s p e c i e s . may  That NO  i s observed  to be such a good quencher,  be a t t r i b u t e d t o e i t h e r a paramagnetic/heavy atom e f f e c t o r to the  f a c t t h a t the NO 0 „2 ( ^ " Z + g  induced  t r a n s i t i o n s e x h i b i t greater overlap with  "*"A ) t r a n s i t i o n bands than do the 0 „2 t r a n s i t i o n s g  the  (see F i g u r e  V I I I ) . The NO(0,2) t r a n s i t i o n i s v e r y c l o s e to the 1 Z -* "^(0,1) and N0(0,3) t r a n s i t i o n i s c l o s e t o the *E and e s p e c i a l l y the (0,0) O^^Z important  "^(0,0) t r a n s i t i o n , the  -> "''A) t r a n s i t i o n s are expected  i n the o v e r l a p model o f the quenching p r o c e s s .  i n v e s t i g a t i o n o f a s e r i e s of halogenated  A systematic  hydrocarbons was  undertaken °2^^g)  The quenching r a t e c o n s t a n t s o b t a i n e d i n t h i s s t u d y , T a b l e  I I I , show no evidence of a heavy atom e f f e c t . e f f e c t combined w i t h the f a c t t h a t 0 2  Absence of a heavy atom  i s a poor quencher, i s a good  <l u e r iching does not occur v i a i n t e r s y s t e m c r o s s i n g  i n d i c a t i o n that O ^ ^ g ^  but r a t h e r t h a t i t i s v i a the i n t e r n a l c o n v e r s i o n p r o c e s s :  (i)  (0,1)  to be  to o b t a i n d e f i n i t i v e evidence c o n c e r n i n g a heavy atom e f f e c t on quenching.  the  o 9 ( V ) + M —y X  g  vV) + ^  g  1m  - 55 -  Figure VIII.  Relative  positions  of the G^C  E ->  and v i b r a t i o n a l bands of d i a t o m i c (from r e f .  53).  A ) transitions molecules  - 56 -  500  1000  2CCO  3000 frequency(cm" )  4000  50 0 0  6000  - 57 -  Table V I .  Quenching r a t e c o n s t a n t s f o r quenching of O^C^A  Quencher  (l.mole '''sec "S  k q  He  4.8  Ar  5.3  Kr  5  Ze  20  H2 N2 02  (k v a l u e s from r e f e r e n c e 5 3 ) . q  2.7  x  103  x  103  50 1.2  ).  - 58 -  T a b l e V I I . Quenching r a t e c o n s t a n t s f o r quenching o f 0 0 ( E ) .  Quencher  k  q  (l.mole  X  io  8  HD  1.5  X  io  8  2  1.2  X  io  7  1.3  X  io  6  1  X  io  5  CO  2  X  io  6  NO  2.6  X  io  7  2.0  X  108  1.7  X  io  7  1  X  io  6  D  N  2  2  °2  co  2  N0 2  so  q  sec  4  H  (k  -1  2  v a l u e s from r e f e r e n c e 5 3 ) .  -1, )  - 59 -  Quenching  Mechanism Study  P h y s i c a l quenching i s a p r o c e s s whereby an e l e c t r o n i c a l l y  excited  s p e c i e s c o l l i d e s w i t h an atom o r molecule and i s d e a c t i v a t e d w i t h t r a n s f e r o f e l e c t r o n i c e x c i t a t i o n to t r a n s l a t i o n a l , r o t a t i o n a l , or v i b r a t i o n a l energy. of  + 0„("4; ) can 2 g  As shown i n the p r e v i o u s s e c t i o n p h y s i c a l  be assumed to i n v o l v e the t r a n s i t i o n  The q u e s t i o n which now and how  a r i s e s i s how  the "*"E  and  quenching  + 0„("4] ) -> 0„(^A ). 2 g 2 g  A  s t a t e s are coupled  the degree of c o u p l i n g can be measured or p r e d i c t e d .  Before  these q u e s t i o n s can be t a c k l e d i t i s n e c e s s a r y to review what i s known c o n c e r n i n g 0,("*"£ ) quenching. 2g +  V a r i o u s s t u d i e s o f 0 C^E"*") quenching have l e d to the f o l l o w i n g 2 g conclusions: o  (1)  In g e n e r a l , quenching i s dependent  the quencher  upon the e x t e n t to which  induces mixing o f the "'"E-^'A w a v e f u n c t i o n s .  Variations  i n k^ have been r e l a t e d t o the magnitude qf the i n t e r m o l e c u l a r i n t e r a c t i o n s between the oxygen m o l e c u l e and the as deduced  from Lennard-Jones  '  quencher,  p o t e n t i a l parameters  f o r the  i n t e r a c t i o n between two n o n p o l a r molecules or from Stockmayer p o t e n t i a l parameters  (2)  f o r i n t e r a c t i o n between two p o l a r molecules  The h i g h quenching e f f i c i e n c y of p o l a r molecules has been  a t t r i b u t e d to 'long-range' p e r t u r b i n g e f f e c t s v i a the i n t e r a c t i o n o f the d i p o l e moment o f the p e r t u r b e r (quencher) w i t h the oxygen e l e c t r o n s ( 2 0 ) .  (3)  That hydrogen  c o n t a i n i n g molecules are most e f f e c t i v e  quenchers has been a t t r i b u t e d to i n t e r m o l e c u l a r between the H and oxygen  (66).  interaction  (45)  - 60 -  (4)  Quenching e f f i c i e n c y i n c r e a s e s m o n o t o n i c a l l y  with  i n c r e a s e i n the magnitude o f the quencher fundamental v i b r a t i o n a l frequency  (67).  In a p r e l i m i n a r y study o f t h e quenching mechanism, Davidson and 53 Ogryzlo transfer  have proposed t h a t quenching by atomic s p e c i e s i n v o l v e s of electronic  e x c i t a t i o n energy to t r a n s l a t i o n a l motion o f 0^  and quencher whereas quenching by molecules v i b r a t i o n a l e x c i t a t i o n o f t h e quencher.  involves transfer to  A c o r r e l a t i o n between  funda-  mental v i b r a t i o n a l f r e q u e n c i e s and quenching e f f i c i e n c i e s was 53 attempted f o r d i a t o m i c s p e c i e s factors e.g.  a r e important  to determine whether t h e Franck-Condon  i n the quenching p r o c e s s .  Symmetric d i a t o m i c s  0^, ^2 have no d i p o l e moment i n both s t a t i c and v i b r a t i o n a l  s t a t e s and a r e t h e r e f o r e n o t expected to v i b r a t i o n and r o t a t i o n .  t o have i n f r a r e d a b s o r p t i o n s due  However, b i n a r y c o l l i s i o n s induce  d i s t o r t i o n s which r e s u l t i n a c t i v a t i o n o f v i b r a t i o n a l bands. absorptions attributed have been o b s e r v e d .  molecular Weak IR  to d i p o l e moments induced by i n t e r m o l e c u l a r f o r c e s  F o r induced  IR a b s o r p t i o n s the primary  of i n d u c t i o n seems t o be q u a d r i p o l a r i n t e r a c t i o n .  These  mechanism  induced  t r a n s i t i o n s o f homonuclear d i a t o m i c s were used i n t h e c o r r e l a t i o n .  The  o b t a i n e d c o r r e l a t i o n showed an e x p o n e n t i a l i n c r e a s e i n quenching r a t e c o n s t a n t w i t h i n c r e a s e i n fundamental v i b r a t i o n a l frequency  o f quencher  f o r t h e homonuclear d i a t o m i c s , w i t h h e t e r o n u c l e a r d i a t o m i c s h a v i n g r a t e c o n s t a n t s g e n e r a l l y g r e a t e r than those o b t a i n e d from t h e homonuclears. Relatively  good quenching by the n o n p o l a r  d i a t o m i c s suggests  (IR i n a c t i v e )  homonuclear  the presence o f a p o l a r i z a t i o n e f f e c t , r e s u l t i n g  from i n d u c t i v e i n t e r a c t i o n s  o p e r a t i v e d u r i n g c o l l i s i o n s , i n the quenching  - 61 -  process.  That d i p o l a r molecules are more e f f e c t i v e i n i n d u c i n g a d i p o l e  than are n o n p o l a r m o l e c u l e s would suggest t h a t d i p o l a r m o l e c u l e s are 53 more e f f e c t i v e quenchers  than n o n p o l a r m o l e c u l e s  T h i s was  observed.  G e n e r a l t r e n d s observed i n quenching by m o l e c u l e s can be to:  attributed  (1) the a b i l i t y o f the molecule to absorb the energy r e l e a s e d i n  the  -> ''"A  "^E -»- ^A  transition;  (2) the Franck-Condon f a c t o r s c o n t r o l l i n g the  t r a n s i t i o n ; w h i l e the s u b t l e r t r e n d s appear to be a r e s u l t of the  c l o s e n e s s o f the energy match between energy r e l e a s e d i n the oxygen t r a n s i t i o n and quencher  vibrational excitation.  (These f a c t o r s were  p r e v i o u s l y used to p r e d i c t the most p r o b a b l e mode of e x c i t a t i o n i n quencher.)  ^2^^g^ Qu^ching  c a n  D e  c o n s i d e r e d to proceed v i a the  f o l l o w i n g r e a c t i o n scheme:  1  E+ g  where Q  +  Q  k — ^ I" -1  [ 1 E + 'Q] g  k —?-X  I  1  k —3-»  1  A  + Q*  (xvii)  g  -2  i s v i b r a t i o n a l l y excited k_^  l A -Q] g  - k^ >> k^ >> ^-_2  :  quencher. m o s t  c o l l i s i o n s w i l l not  result  i n f o r m a t i o n o f the complex w i t h v i b r a t i o n a l l y e x c i t e d modes i n 1 * quencher; d i s s o c i a t i o n o f the complex A 8 i s more p r o b a b l e 1 * 1 + than i s the r e a c t i o n ( A *Q -*• E «Q) because the d e n s i t y of 2 ^g g ± + s t a t e s o f complex A «Q i s g r e a t e r than t h a t of complex [ E *Q]. g 1 + 1 * g - The r a t e d e t e r m i n i n g s t e p [ E > Q ] - * - [ A * Q ] i n v o l v e s a radiationless - kq Examining  transition.  = k1k2/k_1. t h i s scheme f o r ° 2 ^ £ g ) d e a c t i v a t i o n , the f i r s t step i s noted  to be the f o r m a t i o n , upon c o l l i s i o n of 0 _ ( ^ E + ) and a quencher 2g  molecule,  of a m o l e c u l a r e n t i t y o f a t r a n s i e n t n a t u r e known as a c o l l i s i o n  complex.  - 62 -  T h i s c o l l i s i o n complex can be l i k e n e d to a l a r g e molecule or a p o l y n u c l e a r complex and  thus the r a t e determining  an i n t r a m o l e c u l a r r a d i a t i o n l e s s t r a n s i t i o n .  step can be t r e a t e d as And  the formalism  developed  68 by Robinson and F r o s c h f o r r a d i a t i o n l e s s r e l a x a t i o n i n a polynuclear complex can be used to t r e a t the quenching of O-C^E" "). 2 g 1  Current t h e o r i e s of r a d i a t i o n l e s s t r a n s i t i o n s are based on  the  i m p l i c i t assumption that a c o u p l i n g between the e x c i t e d s p e c i e s and medium i s e s s e n t i a l f o r o c c u r r e n c e of a r a d i a t i o n l e s s t r a n s i t i o n , medium b e i n g r e q u i r e d to p r o v i d e a s i n k f o r the d i s s i p a t i o n of energy and  to secure energy c o n s e r v a t i o n r e s t r i c t i o n s .  n e c e s s a r y to examine the c o u p l i n g i n t e r a c t i o n and  It i s  to p r e s e n t  f o r o b t a i n i n g a measure of the degree of c o u p l i n g .  the  the  excitation now  a method  In t h e i r p r e l i m i n a r y  s t u d i e s b o t h Davidson and O g r y z l o " * 3 a n d M e r k e l and K e a r n s ^ assumed a d i p o l e - d i p o l e i n t e r a c t i o n and used o v e r l a p of a b s o r p t i o n s p e c t r a of quenchers w i t h e m i s s i o n  spectrum of the e x c i t e d s p e c i e s to o b t a i n a  measure of the degree of  coupling.  Making use of the continuum approximation Fermi g o l d e n r u l e ) and  ( a l s o r e f e r r e d to as  the assumption t h a t the c o l l i s i o n complex i s  s i m i l a r to a p o l y n u c l e a r complex, the r a t e constants  f o r the r a d i a t i o n l e s s  t r a n s i t i o n i n the quenching scheme can be expressed as  k2  *  (64):  2  | j Ie|  a = i n t e r a c t i o n energy between i n i t i a l and  6 = ^Jtt'j'^  the  (xviii)  f i n a l states  - 63 -  The e x p r e s s i o n M e r k e l and R e a m s used i n t h e i r study o f O^C^A deactivation hot  1  =  is solution^  )  i s o b t a i n e d from ( x v i i i ) and the r e l a t i o n s h i p  T ., where x ... i s the v i b r a t i o n a l r e l a x a t i o n vib vib  time of the  solvent:  2.17 T  k„  .,  5  *  ^  0  |3|  (xix)  2  68 Relating  a to the d e n s i t y o f s t a t e s  p  the e x p r e s s i o n used by Davidson  and O g r y z l o i n t h e i r study o f gas phase d e a c t i v a t i o n  O^i^Z) by  of  h y d r o c a r b o n s ^ 1 can be o b t a i n e d :  k2  *  |  B  Assuming d i p o l e - d i p o l e  k  92  =  |  2  (xx)  interaction  (xx) becomes:  3 -n r l K• Je 2l (• S < ^ m1^ >< Q |n '^o |r 'Qo> ) 2 ( x x i ) 'o mn  = e l e c t r o n i c 0^ w a v e f u n c t i o n ( i s expected to remain c o n s t a n t for a l l quenchers). 1  1  2  (< A I Z >) °  = Franck-Condon  1  for  o v e r l a p between v i b r a t i o n a l w a v e f u n c t i o n s  1  Z -* A >  ( < Q j | QQ )  transition.  i s related  t o the p r o b a b i l i t y o f o p t i c a l  of quencher t o the n t h v i b r a t i o n a l  excitation  level.  which p a r a l l e l s e q . ( x i i ) used by M e r k e l and K e a r n s . ^ Since k  q  = k.k 0 /k , 12—1  ,  k  q  =  1  k , -1  I„  2T70  —r\v  1  i 2 . „1 ,  6 , 1 ( Z < A1 el m mn  i l  ^IGUI-.  Z ><Q — 1 o n'6r  N  2  Q >) o  ,  ...  (xxn)  - 64 -  and can be approximated by:  00  o  if  c ( v ) f _ (v)dv q C>2  (xiii)  the f o l l o w i n g assumptions a r e made: (1)  8  does n o t change w i t h quencher,  (2)  p i s n o t o f major importance i n the quenching p r o c e s s ,  (3)  and k_^ do n o t change a p p r e c i a b l y w i t h quencher.  Of these assumptions the v a l i d i t y o f (2) would be most l i k e l y questioned.  to be  A p r e l i m i n a r y survey undertaken by Davidson i n d i c a t e s  t h a t the c o n t r i b u t i o n o f the d e n s i t y o f s t a t e s i s outweighed by o t h e r factors specifically  the Franck-Condon  factors.  U s i n g an e x p a n s i o n o f  ( x i i i ) i n terms o f o v e r l a p i n t e g r a l s , i . e . e q u a t i o n ( x v i ) , quenching r a t e c o n s t a n t s were c a l c u l a t e d .  F i g u r e VI shows the c o r r e l a t i o n o b t a i n e d  between c a l c u l a t e d and e x p e r i m e n t a l quenching c o n s t a n t s .  The poor  c o r r e l a t i o n suggests s e r i o u s shortcomings i n the o v e r l a p model u s e d . Examination of the assumptions made and o f the i n h e r e n t shortcomings of the model i n d i c a t e s t h a t the c o r r e l a t i o n , a l t h o u g h p o o r , i s the b e s t t h a t c o u l d be expected w i t h t h i s model. The assumptions made a r e summarized: (1)  IR a b s o r p t i o n i n t e n s i t y was assumed t o be p r o p o r t i o n a l to  the a b i l i t y  of the quencher to accommodate the e l e c t r o n i c  energy l o s t i n the 02(^2 -»• "''A) t r a n s i t i o n .  excitation  Use o f IR a b s o r p t i o n s  does n o t c o n s i d e r p o s s i b l e c o n t r i b u t i o n from IR i n a c t i v e  bands.  Use o f the d a t a a v a i l a b l e made i t d i f f i c u l t t o c o n s i d e r induced absorptions.  - 65 -  (2)  In c a l c u l a t i n g the 0 o ( E ) e m i s s i o n spectrum 2  that:  selection rules  i t was  assumed  g  f o r changes i n r o t a t i o n a l quantum  numbers are those c h a r a c t e r i s t i c o f induced t r a n s i t i o n s , i . e . AJ = 0,+l>+2; a l l t r a n s i t i o n s o c c u r w i t h e q u a l (highly (3)  unlikely).  I t was  assumed t h a t f r e e molecule s t a t e s  c o l l i s i o n p r o c e s s e s , t h i s i s not n e c e s s a r i l y indeed reason to b e l i e v e the  There i s controlling  transitions  differ  (63).  i n the model a r e :  The c o l l i s i o n which o c c u r s i n the quenching p r o c e s s w i l l  undoubtedly  perturb the f r e e m o l e c u l e s t a t e s  free molecule state p r o p e r t i e s  Physical  quenching  matter of convenience assumed v a l i d . rules  true.  E ->• "'"A t r a n s i t i o n f o r c o l l i s i o n induced t r a n s i t i o n s  Inherent shortcomings  (2)  are v a l i d i n  t h a t the Franck-Condon f a c t o r s  from those f o r f r e e molecule  (1)  probability  making the use of  invalid.  i s a nonphotonic p r o c e s s , however as a  i n a n a l y s i s , p h o t o n i c s e l e c t i o n r u l e s were  This introduces considerable error  since  selection  f o r p h o t o n i c p r o c e s s e s a r e o f t e n o p p o s i t e to those f o r non-  photonic processes (i)  (69), f o r example:  The Born-Openheimer (B-0) a p p r o x i m a t i o n h o l d s f o r  p h o t o n i c p r o c e s s e s but occurence of a nonphotonic r e s u l t s i n the breakdown of the B-0  process  a p p r o x i m a t i o n because  of d i s t o r t i o n o f the m o l e c u l a r geometry r e s u l t i n g  from  collision. (ii)  (+) «-* (-)  i s a l l o w e d , (+)  (+) and  (-) 4" (")  a  not a l l o w e d f o r p h o t o n i c p r o c e s s e s but f o r nonphotonic p r o c e s s e s (+)  (+) and  (-) «-»- (-)  are a l l o w e d .  r  e  - 66 -  (iii) u  g and g  u  (a d i p o l e must be c r e a t e d o r d e s t r o y e d  i n t r a n s i t i o n ) i n p h o t o n i c p r o c e s s e s whereas u  g but  u ->• u and g -> g are a l l o w e d i n nonphotonic p r o c e s s e s .  The use o f p h o t o n i c s e l e c t i o n r u l e s i s n e c e s s i t a t e d by the f a c t t h a t t h e r e has not as y e t been developed  a r e l a t i o n s h i p between t h e o r e t i c a l  t r a n s i t i o n moment and e x p e r i m e n t a l l y measurable q u a n t i t i e s u s i n g n o n photonic s e l e c t i o n  rules.  From t h i s study i t appears t h a t the p o s s i b i l i t i e s o f the o v e r l a p model have q u i c k l y been e x h a u s t e d .  Thus i t i s n e c e s s a r y  modify the model or to c o n s i d e r a new the model, the r a t e d e t e r m i n i n g  method o f a t t a c k .  step was  considered a  to e i t h e r In d e v e l o p i n g  radiationless  t r a n s i t i o n i n a p o l y n u c l e a r s p e c i e s , i . e . i n the c o l l i s i o n however, f o r convenience,  the c a l c u l a t i o n s were c a r r i e d out u s i n g  s p e c t r o s c o p i c data f o r the s e p a r a t e d m o l e c u l e s . would be  complex,  A more c o r r e c t method  to undertake c a l c u l a t i o n s f o r the c o l l i s i o n complex.  c a l c u l a t i o n s o f t h i s type would n e c e s s i t a t e d e n s i t y o f s t a t e t i o n s which q u i c k l y becomes a major p r o j e c t f o r c o m p l i c a t e d  Rigorous determina-  systems.  Of the v a r i o u s approaches to d e n s i t y o f s t a t e c a l c u l a t i o n s , H a a r h o f f ' s method seems most u s e f u l . 7 ^ *  More r i g o r o u s c a l c u l a t i o n s would a l s o  r e q u i r e d e t e r m i n a t i o n of p o t e n t i a l energy curves f o r the system d e t e r m i n a t i o n of the t r a n s i t i o n p r o b a b i l i t i e s .  S t u d i e s of the  and distribution  o f v i b r a t i o n a l energy f o l l o w i n g energy t r a n s f e r would be o f a s s i s t a n c e i n drawing the p o t e n t i a l energy c u r v e s .  P r e l i m i n a r y experiments have  been undertaken i n t h i s l a b o r a t o r y to observe  IR e m i s s i o n of  the  e x c i t e d quencher a f t e r energy t r a n s f e r has o c c u r r e d and have met some s u c c e s s .  with  Further studies should provide information concerning  v i b r a t i o n a l energy d i s t r i b u t i o n a f t e r energy t r a n s f e r needed to determine  - 67 -  p o t e n t i a l energy c u r v e s . A second approach which may of c o n s t r u c t i n g p o t e n t i a l  be c o n s i d e r e d i s the p o s s i b i l i t y  energy s u r f a c e s f o r the c o l l i d i n g 0„(^"Z ) +  i n d i v i d u a l quencher m o l e c u l e s .  and  Thus the quenching mechanism c o u l d be  c o n s i d e r e d i n terms o f p o t e n t i a l energy s u r f a c e s and the p r o b a b i l i t y of  t r a n s i t i o n between them.  T h i s method has proved q u i t e  successful  3 f o r such s i m p l e systems as the quenching o f Na diatomics.71  ( P) by atoms and simple  To a p p l y t h i s t o the more c o m p l i c a t e d system o f  O^i^Z)  quenched by p o l y a t o m i c s would r e s u l t i n unwieldy e q u a t i o n s w h i c h , i f s o l v a b l e ^ w o u l d produce n d i m e n s i o n a l s u r f a c e s where n > 3 depending upon the number o f atoms i n the system. A l t e r n a t i v e l y  a number of  s i m p l i f i c a t i o n s c o u l d be employed to ensure r e l a t i v e ease i n c a l c u l a t i o n s , however, t h i s would r e s u l t i n such a l o s s i n a c c u r a c y t h a t t h i s treatment would be no more u s e f u l than the p r e v i o u s o v e r l a p t r e a t m e n t .  - 68 -  SUMMARY AND CONCLUSIONS  The heavy atom study was undertaken to p r e s e n t d e f i n i t i v e e v i d e n c e +  t h a t 0„("'"Z ) d e a c t i v a t i o n o c c u r s v i a an i n t e r n a l c o n v e r s i o n p r o c e s s 2 g r a t h e r than v i a i n t e r s y s t e m c r o s s i n g .  From the r e s u l t s i t i s e v i d e n t  t h a t there i s no heavy atom e f f e c t on the quenching o f 0 ^  ("4;+)  and  g  thus the assumption t h a t quenching proceeds w i t h i n t e r n a l c o n v e r s i o n i s valid. The quenching mechanism study was undertaken to t h o r o u g h l y t e s t 53 61 the o v e r l a p model proposed by Davidson and O g r y z l o .  '  As was  shown,  i t i s i m p o s s i b l e t o r e f i n e the model to a p o i n t where a c c u r a t e p r e d i c t i o n s can be made f o r a l l types o f quenchers as a r e s u l t o f the assumptions used and the model s h o r t c o m i n g s . Other treatments which may be more e f f e c t i v e (1)  include:  r a t h e r than using•. = s e p a r a t e d s p e c i e s to o b t a i n  transition  p r o b a b i l i t i e s , the c o l l i s i o n complex c o u l d be used and the RobinsonF r o s c h f o r m a l i s m more r i g o r o u s l y f o l l o w e d . of s t a t e and energy l e v e l (2)  ( T h i s would i n c l u d e d e n s i t y  calculations);  each quenching r e a c t i o n c o u l d be i n d i v i d u a l l y t r e a t e d u s i n g  p o t e n t i a l energy s u r f a c e s and the p r o b a b i l i t y o f s u r f a c e  transitions.  Of these two methods the f i r s t h o l d s most promise o f s u c c e s s .  - 69 -  BIBLIOGRAPHY  1.  M u l l i k e n , R.S., Phys. Rev., 32, 88 (1928).  2.  H e r z b e r g , G., N a t u r e , 133, 759 (1934).  3.  E l l i s , J.W., K n e s e r , H.O., Z. P h y s i k , 86, 583 (1933).  4.  Van V l e c k , J.H., A s t r o p h y s . J . , 80, 161 (1934).  5.  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