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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.). University of Calgary, 1971 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of CHEMISTRY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1973 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood t h a t c o p y i n g or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a llowed without my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada - i i -ABSTRACT A study of the quenching a b i l i t i e s of a seri e s of compounds containing heavy atoms was undertaken to ascertain whether or not 0„("''E+) quenching i s subject to heavy atom enhancement. Presence of a heavy atom e f f e c t would suggest s i g n i f i c a n t quenching v i a the 0„( E + 3 -E ) t r a n s i t i o n . The fa c t that no such e f f e c t i s observed supports 8 the generally assumed quenching mode: 0„( E -> A ). z g g A systematic i n v e s t i g a t i o n of the use of overlap of absorption spectrum of quencher with emission spectrum of C^C^E*) to obtain calculated quenching rate constants was undertaken. The c o r r e l a t i o n between calculated and experimental rate constants are examined and shortcomings of the method are presented. - i i i -Production of C ^ E TABLE OF CONTENTS Page ABSTRACT 1 1 LIST OF TABLES i v LIST OF FIGURES v ACKNOWLEDGEMENTS v i INTRODUCTION 1 E l e c t r o n i c a l l y Excited Oxygen ^ Sources of Singlet Molecular Oxygen ^ Methods f or Detection of Singlet Molecular Oxygen 1 0 P h y s i c a l Quenching H EXPERIMENTAL 1 9 Flow System 1 9 Materials 22 22 Detection of 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 BIBLIOGRAPHY 68 69 - i v -LIST OF TABLES Table Page I "^O^  Transitions and Corresponding Energies ^ II Sources of Materials 23 I I I Values Obtained i n Heavy-Atom Study ^9 IV Overlap Integrals, Calculated Quenching Constants and Corresponding Experimental Quenching Constants.. 34-3 V Most Probable Mode of Quencher E x c i t a t i o n and 0 „ ( 1 E + ) Deactivation ^9-5 VI O^ C^ A ) Quenching Rate Constants (from r e f . 53) .... ^7 g VII 0 ( 1Z +) Quenching Rate Constants (from r e f . 53) 5 8 - v -20 27 LIST OF FIGURES Figure Page I Oxygen P o t e n t i a l Energy Diagram as Compiled by Gilmore (6) 3 II Schematic of Flow System III Representative Stern-Volmer P l o t (CH C l 2 ) IV Computed "induced" Emission Bands f o r the 0 2 ("*"£* -* "''A ) T r a n s i t i o n 3 ^ g V Sample of Computer Drawn Relative Overlap Integral a-d (CHF 3) 3 6 " 4 3 VI C o r r e l a t i o n Between k experimental and k calculated q q from IR absorption spectra of quenchers for 02( £ ) Quenching 4 4 VII Overlap Integral Versus 0~(^L+) Quenching Rate 46 g Constants (from r e f . 53) .... VIII Relative P o s i t i o n of the 0 o( 1Z -> 1A ) Transitions 2 g and V i b r a t i o n a l Bands of Diatomic Molecules (from r e f . 53) 5 5 - v i -ACKNOWLEDGEMENTS The author would l i k e to express her graditude to Dr. E.A. Ogryzlo for h i s guidance through t h i s study; laboratory colleague, R.D. Ashford for h e l p f u l discussion; Dr. J.A. Davidson for invaluable assistance both t e c h n i c a l l y and i n the form of discussion and s p e c i a l thanks to K.B. Storey for encouragement and moral support throughout her studies. - 1 -INTRODUCTION Excited s i n g l e t oxygen molecules were f i r s t described i n 1928 by Mulliken"'' and were subsequently detected i n : the upper atmosphere 2 3 by Herzberg ; l i q u i d oxygen by E l l i s and Kneser ; and i n emission from 4 5 gaseous discharge by Van Vleck. As early as 1939 Kautsky suggested the p o s s i b i l i t y that an e l e c t r o n i c a l l y excited, metastable s i n g l e t oxygen molecule acted as the reactive intermediate i n dye-sensitized photo-oxygenation reactions. However, u n t i l 1964 i n t e r e s t i n the properties of s i n g l e t oxygen was p r i m a r i l y the concern of as t r o p h y s i c i s t s , small molecule spectroscopists and some gas phase k i n e t i c i s t s . Recent prominance of s i n g l e t molecular oxygen as a chemical species i s due to the following: i t i s e a s i l y and abundantly generated; a v a r i e t y of detection techniques e x i s t ; and i t has s p e c i f i c chemical 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 Excited Oxygen The lowest e l e c t r o n i c configuration of oxygen KK(o-g2s) 2 (a u2s) 2 (irg2p) 2 ( T T ^ P ) 4 ( y p) 2 3 - 1 1 + 3 -can give r i s e to three states: E , A , E . Of these states E g g g g i s the ground state with "'"A and ^T/~ being excited states at 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 . Figure 1 ^ shows the pertinent p o t e n t i a l energy curves. The state i s a c t u a l l y a p a i r of A states degenerate i n energy having components of angular momentum + 2h about the 0-0 a x i s . The state i s diamagnetic, since the o r b i t a l angular momentum of the two upper electrons exactly cancel. These low l y i n g excited s i n g l e t states have e x t r a o r d i n a r i l y long r a d i a t i v e l i f e t i m e s , can s u f f e r many c o l l i s i o n s without lo s i n g t h e i r energy, and as a r e s u l t e x h i b i t the phenomenon of 'energy pooling'. Although the 1& ->- 3£ ~ 12,690 1 and 1 £ + -»• 3 E ~ 7,619 A t r a n s i t i o n s g g g g are spin forbidden both have been observed as weak absorption bands i n the near i n f r a r e d . From absorption i n t e n s i t i e s the extremely long r a d i a t i v e l i f e t i m e s of 45 min^'^ f o r "^A and 7-12 s e c ^ ' ^ for g g have been c a l c u l a t e d . 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 'magnetic-dipole' 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 smaller than those f o r allowed e l e c t r i c dipole 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 ascribed to the phenomenon of 'energy-pooling', a cooperative process whereby two excited molecules pool t h e i r energy to produce a photon with the combined energy of both molecules. The 6340 A and 7030 A bands 1 ? 12 a r i s e from combination of two 0 2 ( A ) molecules and the 4800 A band 1 1 8 arises from an 0 n ( Z ) 0 „ ( A ) p a i r . A summary of t r a n s i t i o n s and 2s g' 2 V g' corresponding energies i s given i n Table 1. These t r a n s i t i o n s were 13 14 f i r s t observed at high oxygen pressures ' and i n the atmosphere at 15 4 low sun ' and have subsequently been observed i n gas phase laboratory studies using various methods of "HD formation.H>16,17 - 3 -Figure I. Oxygen p o t e n t i a l energy diagram as compiled by Gilmore. - 5 -Table I. Tr a n s i t i o n Energy (A) oA - » 0. V 12690 2 g 2 g O.V"-* 0 0 V 7620 2 g 2 g O-^A ) + 0A\) ->0.(V) + 0 ( 3Z ) 6340 2 g 2 g 2 g 2 g 0 , (V) + 0 ( \ ) - » 0 ( V ) + 0(h) 4760 0 , (V) + 0 9 ( V)->0 (V) + 0 (V) 3800 1 6 2 g 2 g 2 g 2 g - 6 -E i n s t e i n A factors of A = 0.145 sec and A = 0.085 sec ^ ^ ® > ^ 1 + 3 -have been reported for the E -* E t r a n s i t i o n s , the f i r s t value being g g the preferred i n most studies. Badger et a l . ^ have determined a value -1 1 3 -of A = 2.58 sec for the A -»• E t r a n s i t i o n . A value of A = 2.5 x g g —3 —1 20 1 + 1 10 sec have been reported for the forbidden ''Noxon' E •+ A g g t r a n s i t i o n . Sources of Singlet Molecular Oxygen Study of any excited molecule i s r e s t r i c t e d by the methods of generation a v a i l a b l e . The v a r i e t y of a v a i l a b l e methods for production has f a c i l i t a t e d studies of s i n g l e t molecular oxygen. Thus there follows a summary of methods of formation of s i n g l e t oxygen. Singlet oxygen can be formed using chemical reactions such as: reaction of hydrogen peroxide with hypobromite or hypochlorite; decomposition of ozonides; decomposition of superperoxide ions; decomposi-t i o n of endoperoxides; and the decomposition of peroxyacetyl n i t r a t e (one of the components of smog). For a d e s c r i p t i o n of these various reactions producing s i n g l e t molecular oxygen see review a r t i c l e s references 20 and 21. Singlet oxygen molecules may also be generated with high e f f i c i e n c y by energy transfer from the excited t r i p l e t state of some s e n s i t i z e r 22-25 molecules. The key step i s the transfer of e l e c t r o n i c energy from the excited s e n s i t i z e r to the ground state oxygen molecule which r e s u l t s i n 0;, being raised to an excited s i n g l e t state. Transfer from the t r i p l e t excited state of the s e n s i t i z e r has been shown, k i n e t i c a l l y and s p e c t r o s c o p i c a l l y , to be the predominant mode of oxygen e x c i t a t i o n i n - 7 -26 27 a l l cases. ' The s i n g l e t state generated i s suggested to depend 28 upon the t r i p l e t energy of the s e n s i t i z e r thus: s e n s i t i z e r s of greater than 50 k c a l w i l l y i e l d predominantly O^ C^ E ) ; s e n s i t i z e r s of E~ ^ 8 les s than 38 k c a l can r e s u l t only i n A g ) ; and s e n s i t i z e r s of 1 1 29 intermediary energy would produce r e l a t i v e amounts of 0_( E ) and 0 „ ( A ) . 2 g 2 g Certain reactions of ozone r e s u l t i n formation of e l e c t r o n i c a l l y 30 o excited molecular oxygen. Use of X = 2537 A r e s u l t s i n production of e i t h e r O-C^ A ) or O-C^ E ) from primary processes. Secondary ^ 8 ^ 8 processes inv o l v i n g energy transfer from 0(^"D) r e s u l t i n formation of 0„("'"E ) . Ozone photolysis i s not generally considered useful for ^* § generating large amounts of s i n g l e t oxygen. 31 Vacuum UV f l a s h photolysis has been used to photochemically produce 0„ (''"E ) . This method i s highly dependent upon a high degree of s e n s i t i v i t y of the detection method, however, i t o f f e r s the advantage that the secondary reactions with ^ ( ^ E g ) ° ^ atomic oxygen or ozone often encountered i n microwave discharge may be neglected as a r e s u l t of the miniscule concentration of 0 produced. The method of generation used i n t h i s study was e l e c t r i c ( s p e c i f i c a l l y microwave) discharge through a stream of oxygen. E l e c t r i c discharge through a stream of molecular oxygen produces appreciable quantities of e l e c t r o n i c a l l y excited 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 discharge 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 flow system i s excited to the "*"A 8 1 35 state i n radiofrequency discharge tubes, some E i s also produced, 8 however, th i s i s r a p i d l y relaxed to . Most "*"E i s formed downstream 8 § - 8 -from the discharge by the energy pooling process: 0 2 ( \ ) + 0 2 (1A g ) - > + O ^ ) ( i ) (to be dealt with i n greater d e t a i l i n the following s e c t i o n ) . Although there seems to be no great d i f f e r e n c e between the e x c i t a t i o n properties of radiofrequency and microwave discharges, i t i s easier to couple higher power into discharge with a microwave source and i s easier to maintain a stable discharge over a wider pressure range. To study ^0^ i t i s necessary to free the system of oxygen atoms produced i n the discharge, t h i s can be accomplished by interposing a f i l m of mercuric 36 oxide between the discharge and observation point and/or by adding an 37 excess of N02 to the oxygen flow. In the absence of foreign gases added to the flow both "*"A and ^E w i l l decay p r i m a r i l y by wall „ . . 38 c o l l i s i o n s . The buildup to a steady state concentration of ° 2 ^ ^ g ^ ^n a s t r e a m a f t e r a discharge can be accounted for by the energy pooling process: °2 ( 1 v + ° 2 ( 1 y ~ ^ v 1 ^ + ° 2 ( 3 y ( i ) 3 9 ' 4 0 Three energy-pooling processes have been proposed: (1) cooperative emission or r a d i a t i v e pooling; (2) cooperative transfer or ,termolecular tra n s f e r ; and (3) energy disproportionation. Examples of cooperative * 12,16 transfer processes are: - 9 -0 9 ( X A ) + 0 „ ( 1 A ) > OA3l+) + 0 (3E ) + hv (ii) 2 g 2 g 2 g 2 g 0 9 ( V ) + 0.( 1A C T) — > 0,(3E") + O . ( V ) + h v ( i i i ) 2 g 2 g 2 g 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: 209(1A ) + V > 20, ( V ) + V* (iv) 2 g 2 g V = violanthrone 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 is the method whereby a steady state concentration of 0 9(^E +) is built up after a discharge flow system. 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. The invariance of [0.(^£+)] with 2 g 37 43 time ' can be explained on the basis of steady state equilibrium by (i) and deactivation reactions. - 10 -Methods for Detection of Singlet Molecular Oxygen The presence of s i n g l e t molecular oxygen may be detected by d i r e c t observation i f possible 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. A review of gas phase detection techniques follows. 44 EPR has been used by F a l i c k et a l . to observe the AM = 1 t r a n s i t i o n for the J = 2 state of O-C^ A ) . The r e s u l t s obtained were used to: (1) 2 g show that approximately 10% of discharged gases i s 0^~k and (2) study r e l a t i v e r eaction rates of ethylene with O-C^ A ) and with atomic oxygen. The paramagnetic property of the "'"A state of oxygen molecule i s a product 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 detection methods to study 0„(^A ) concentrations i n an oxygen flow system. ^ 8 Isothermal calorimetry can also be used to detect 02("'~A ) and to 45 determine concentrations of excited oxygen i n flow systems. Arnold by use of two isothermal calorimeters i n conjunction with observation of the 1.27 u band has shown that a cobalt coated detector destroys greater than 95% of °2^^g' present. Luminescence spectroscopy i s the detection method used i n t h i s study. C h a r a c t e r i s t i c s i n g l e and double molecule simultaneous t r a n s i t i o n s may be employed for i d e n t i f i c a t i o n and detection of the excited species. The si n g l e molecule t r a n s i t i o n s [(0-0) bands] at 7620 1 and 1.27 u for 02(^2^) and ®2^^gJ r e s P e c t l v e l y a r e expected to be the strongest. At 1 O o high 0 „ ( A ) concentration the dimol emission bands at 6340 A and 7030 A ^ 8 41 43 may be detected. ' Uncertainty over absolute rate constant for the reaction O.^A ) + 0 o(1A ) > 20 0(V) + hv 6340 A 2 g 2 g 2 g - 11 -l i m i t s the use of dimol emission spectroscopy to the study of r e l a t i v e 1 46 47 0 „ ( A ) concentration. ' Use of emission spectroscopy permits 2- 8 1 1 detection of both £ and A over wide pressure ranges. . 8 8 Deactivation of Singlet Molecular Oxygen E l e c t r o n i c a l l y excited molecules can be deactivated e i t h e r by r a d i a t i v e emission or v i a various non-radiative pathways. The low A factors and long r a d i a t i v e l i f e t i m e s of excited s i n g l e t molecular oxygen make r a d i a t i v e emission a n e g l i g i b l e process compared to non-r a d i a t i v e processes of d e a c t i v a t i o n . Non-radiative deactivation can occur v i a : (1) e l e c t r o n i c energy t r a n s f e r ; (2) chemical reactions; (3) p h y s i c a l quenching. E l e c t r o n i c energy transfer from excited s i n g l e t oxygen states of E^"*" and "^A i s seldom observed because few molecules have e l e c t r o n i c a l l y 8 g excited states below the e x c i t a t i o n energies of 22.5 k c a l ( A ) and 38.5 k c a l (^E+) of these two low-lying s t a t e s . However, e l e c t r o n i c e x c i t a t i o n v i a dimol processes has been observed i n a number of 12,41,48 systems. The only chemical reactions of s i n g l e t molecular oxygen which have 11 31 37 49 50 been w e l l studied are those with ozone » » > » a n ( j those with •» 44,45,50,52 o l e f i n s . Physical Quenching Phy s i c a l quenching i s a process whereby e l e c t r o n i c e x c i t a t i o n may be degraded i n t o nuclear motion within the system. Quenching studies of 0 o(^A ) have shown i t to be somewhat immune to de a c t i v a t i o n , most 2V g' "^A quenching constants are approximately 10^ times smaller than - 12 -corresponding "*"E+ c o n s t a n t s . " * " E+, more e a s i l y deactivated and thus somewhat easier to study was the species examined i n th i s work. In the absence of a quenching gas the steady state concentration of O^C^E*) maintained i n a flow system i s given by the following reactions: 0,(V) + O.^A ) 09<V) + 0 (V) (i) 2 g 2 g 2 g 2 g 1 + k 0 2( E g) + wal l — — V products (v) [ ° 2 ( V ) ] = ^ to 2 ( 1V ] 2 ( v i ) and the emission i n t e n s i t y from "^ E i s given by: I = k[0 (V)] o 2 g - K ^ I O , ^ ) ]2 ( v i i ) w ^ S Upon addition of a quencher gas the following reaction must be added: 1 + k"0 0 ( E ) + Q s > products ( v i i i ) ^ 8 and the steady state concentration i s now given by: k [0 (XA ) ] 2 [ ° 2 < X ) ] = k + kn[g] ( 1 X ) W Ij and the ^ Z emission i n t e n s i t y by: - 13 -k k . [ 0 9 ( 1 A ) ] 2 d 2 R - (x) Q k w + k Q[Q] The r a t i o of emission with and without quencher i s : = 1 + [ Q ] ( x i ) w From the Stern-Volmer p l o t s of I0 /Iq v s« [Q] a s t r a i g h t l i n e i s obtained having a slope of k^/k^, which, used i n conjunction with k w determinations w i l l give k_. k can be calculated from absolute measurements of Q w [CLC^A )] i n the absence of quencher. ^ 8 R e s t r i c t i n g study to p h y s i c a l quenchers with ground state s i n g l e t s 1 + there are then two possible mechanisms whereby 0^(. £ ) may be quenched: (1) O . c V ) + 2 M > ° O ( 1 A J + ^ 2 g I g (2) 0 ( V ) + hi. > °2(3h) + 1 + 3 -Process (2) involves the t r a n s i t i o n of 0_( E ) —> 0~( E ) which i s ^ g z g spin forbidden unless the perturber i s paramagnetic and i s thus expected to be i n e f f e c t i v e f or diamagnetic quenchers. Although i t has generally been assumed that quenching by diamagnetic species w i l l r e s u l t i n "^E —> "^A t r a n s i t i o n , ^ ' ^ ^ there has been no d i r e c t evidence g g reported to support t h i s . The best i n d i r e c t evidence comes from the d i r e c t photo-excitation experiments of Evans. It has previously been mentioned that "^A i s p a r t i c u l a r l y immune to d e a c t i v a t i o n . This i s probably a r e s u l t of two f a c t o r s : (1) the - 14 -1 3 -energy gap f o r the A £ t r a n s i t i o n i s considerably greater than that of 1Z+ '-»- "'"A t r a n s i t i o n ; (2) the process O-^A ) 0 „ (3E ~ ) g g 2 g 2 g i s forbidden by various s e l e c t i o n r u l e s , i . e . S = 0, i t i s spin forbidden, ft = 0 + 1, £ «-> A t r a n s i t i o n s are forbidden, u <-> g or g g odd terms combine only with even and v i c e versa, i . e . a dipole must be created or destroyed i n the t r a n s i t i o n . However, only the spin s e l e c t i o n rule w i l l n e c essarily hold true f o r molecules undergoing c o l l i s i o n s . Thus ^A^ can be expected to be much less susceptible to quenching than . the ^ "E s t a t e . However, unexpectedly high quenching 53 1 rates have been observed f o r quenching of 0_( A ) by the paramagnetic 2 g species N02 , NO. These anomalously high values are a t t r i b u t e d to a paramagnetic e f f e c t , whereby the presence of a paramagnetic species enhances spin o r b i t a l coupling thus making the formerly spin forbidden 1 3 A —>- E t r a n s i t i o n formally allowed. There are now believed to be three major modes of 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 occur v i a formation of a charge-transfer complex ; (2) quenching by atomic species has been suggested to involve transformation of 53 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 ; (3) quenching by molecules has been suggested to involve transformation of e l e c t r o n i c -u «.. -> 53,59 e x c i t a t i o n to v i b r a t i o n a l energy. 57 58 Ackerman et a l . and Ogryzlo and Tang i n t h e i r respective studies on s u l f i d e s and amines have both postulated formation of a charge-transfer complex between 0„("'"A ) and the quencher, amine/sulfide. 2- g It was suggested that formation of a contact p a i r occurs between the amine/sulfide and O-C^ A ) and further that the r e s u l t i n g zero order state 2 g - 15 -then mixes with the s i n g l e t charge-transfer state ('''CT). A s i m i l a r 3 _ t r i p l e t zero order state of C^ C £ ) and quencher would then mix with 3 the t r i p l e t charge-transfer state ( CT). Because these charge-transfer states are very close i n energy, a small spin- o r b i t i n t e r a c t i o n due to the nitrogen/sulfur would then allow the necessary m u l t i p l i c i t y change. The process can be written as: 1[ 0 / VI I + X < 1 C I > ] S ^ c t L ' W * + "<3CI>) where: X represents the s u l f i d e or amine A,u represent the amount of charge-transfer character mixed int o the zero order s t a t e s ; are dependent upon the energy of the charge transfer states and therefore upon i o n i z a t i o n p o t e n t i a l of the quencher. Thus since log a IP(q), a c o r r e l a t i o n between log k^ and IP i s expected i f the above mechanism holds; 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 of s i n g l e t molecular oxygen Davidson 53 and Ogryzlo suggested that the observed quenching rates of the atomic species He and Ar represent the basic rate of a process 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 process -*• i s transformed S 8 into t r a n s l a t i o n a l motion of both quencher and oxygen molecule. They have further suggested that higher quenching rates observed for molecules can be associated with 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 quencher. K e a r n s ^ recently completed a study of the r a d i a t i o n l e s s decay of O-C^ A ) i n s o l u t i o n , and presented a theory f o r the quenching of 0„ (^"A ) by solvent i n terms of intermolecular e l e c t r o n i c to v i b r a t i o n a l energy - 16 -t r a n s f e r . Quenching e f f i c i e n c i e s could, according to h i s theory, 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 of IR overtone and combination absorption bands of solvents at 7880 cm ^ and 6280 cm ^ which correspond 1 3 -to the (0-0) and (0-1) A -»• E t r a n s i t i o n . It i s assumed that the g g 3 ^ 1 quenching process involves second order i n d i r e c t mixing of E and A 8 8 1 + 3 - 1 + states through the E state by solvent i n t e r a c t i o n where E and E g g g are mixed v i a intramolecular spin o r b i t coupling. Because E and A 8 8 mixing i s also important i n ®2^^gJ quenching the matrix elements used to account for observed 0_(^A ) quenching are also applicable to 0~(^~Z ) ^ g ^ g quenching. The quenching rate constant can be expressed as: 2ir x B c n 2 2 h mn where: i and f r e f e r to i n i t i a l and f i n a l s t a t e s . T 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 of the solvent. v i b - e cosf 1 i e coso. . "I B • = < * ( A)! £ 5—1- | > 6 ± 1 R±2 R. i s the distance from the centre of the solvent to the l oxygen nucleus 6 . i s the angle between the two -1 1 1 3 BgQ = 140 cm (matrix element coupling E and E) F i s the Franck-Condon factor f o r t r a n s i t i o n from the zeroth m v i b r a t i o n a l l e v e l of A to the mth v i b r a t i o n a l l e v e l of ground ^"L e l e c t r o n i c s t a t e . M = that f a c t o r which determines the i n t e n s i t y of 0-n solvent n t r a n s i t i o n s i n the IR region. Analysis of gas and s o l u t i o n phase quenching constants indicates that for both and "^A , rate constants obtained i n one phase may be used 8 8 to c a l c u l a t e quenching constants i n the other phase. 53 Davidson and Ogryzlo have recently presented a s i m i l a r theory for gas phase quenching of C- (^l"1") which involves the conversion of e l e c t r o n i c e x c i t a t i o n energy of the excited species i n t o v i b r a t i o n a l e x c i t a t i o n of the quencher. If t h i s i s so, would be expected, to a f i r s t approximation, to obey the r e l a t i o n s h i p : k = c 1 J e „ ( v ) f _ (v)dv ( x i i i ) o q ° 2 where f (v) i s the r e l a t i v e emission i n t e n s i t y of t r a n s i t i o n 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 constant; and i n t e g r a t i o n i s over the l i m i t s of O.^E*) O.^A ) 2 g 2 V g' progression. which can be separated as: kq = F Cl /eq( v ) fO (v)dv + FC2Jeq(v>f0 (v)dv + (xiv) where FC i s the Franck-Condon factor for the t r a n s i t i o n and the i n t e g r a t i o n i s over a p a r t i c u l a r band i n the progression and f (v) i s the r e l a t i v e CL emission i n t e n s i t y within the band. ° 2 In t h i s preliminary report they studied only the overlap i n the 5240 cm the (0-0) band region. In contrast to the homonuclear diatomics, the i n f r a r e d absorptions of other molecules may be more important. The rather poor c o r r e l a t i o n obtained was p a r t i a l l y a t t r i b u t e d to a d d i t i o n a l modes of v i b r a t i o n and consequent absorptions due to combination bands. - 18 -As did Merkel and K e a r n s , ^ Davidson and Ogryzlo"^ u t i l i z e d free molecule Franck-Condon factors i n t h e i r i n i t i a l s t u d i e s . However, Franck-Condon values f o r a molecule involved i n the quenching process very l i k e l y d i f f e r from those f o r a free molecule, t h i s has been observed i n a subsequent study by Davidson and Ogryzlo.^"'' Before a d e t a i l e d quantitative theory could be developed, consider-ably more data i s required p a r t i c u l a r l y a determination of r e l i a b l e Franck-Condon f a c t o r s . Thus the object of the present study was twofold: (1) the systematic study of quenching a b i l i t i e s of a serie s of molecules containing heavy atoms to unambiguously show the presence or absence of a heavy atom e f f e c t ; (2) the determintion of the complete IR spectra over the energy regions of 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 varied ser i e s of quenchers, the majority 53 of whose k^ have been previously reported, i n an e f f o r t to e m p i r i c a l l y determine a set of r e l i a b l e Franck-Condon f a c t o r s . The major aim of the study was to further c l a r i f y the above proposed mechanism for physical quenching of 0~(^~T,+) and to determine, i f p o s s i b l e , r e l i a b l e Franck-8 Condon factors f o r the (0-0), (0-1), (0-2) and (0-3) 1E+ 1A t r a n s i t i o n s . 8 8 - 19 -EXPERIMENTAL Flow System Figure II shows a schematic of the flow system used i n t h i s study. Oxygen was admitted to the system v i a an Edwards needle valve. A drop of mercury was placed before the discharge area and heated to form a mercuric oxide f i l m to remove 0. An i n l e t was positioned between the discharge area and the observation point to insure complete destruction of oxygen atoms. The oxygen flow was c a r r i e d from the i n l e t valve past the mercury drop, through the discharge area where the excited species were formed, over the mercuric oxide f i l m , past the NO2 i n l e t and i n t o the observation tube. The observation tube was 122 cm long with an i n t e r n a l diameter of 25 mm and was double jacketed to allow control of tube temperature. Quenching gas entered the system v i a a t e f l o n needle valve at the front end of the observation tube. Tube pressure was maintained between 2 and 4 mm by adjusting the stopcock opening to the pump, a Duo Seal Vacuum Pump, Model 1376 with a capacity of 300 1/min. The oxygen flowrate was measured by determining the time required f o r exhaust gases to empty a one l i t r e volumetric f l a s k f i l l e d with water when no quencher was added to the system. - 20 -Figure I I . Schematic of Flow System. Edwards Needle To Trap and Pump Teflon Needle Valve - 22 -Materials A l l quenching materials were prepared f o r use by degassing using f r e e z e , pump, thaw c y c l e s , followed by vacuum d i s t i l l a t i o n through a drying agent such as J?2^5 o r a c t l v a t ed alumina. Oxygen for the flow system was used as received. Table II gives a l i s t of chemicals used and t h e i r sources. Production of 0„(V) 2 g 02"^  (Ag) and 0 were formed i n an e l e c t r i c a l discharge produced by a S c i n t i l l o n i c s Inc., Model HV15A microwave generator with maximum output power of 120 watts. To prevent collapse of the quartz tubing of the discharge area the system was cooled by blowing cold a i r through the jacket of the discharge c a v i t y . Complete removal of oxygen atoms was ensured by deposition of a mercuric oxide f i l m a f t e r the discharge area followed by a slow flow of NO2 into the oxygen flow. The species under study, 0 ("'"E ) was generated by the energy pooling process: 2- g O.^A ) + O.^A ) —> O.(V) + 0 ( 3E ) 3 9 ' A ° (i) 2 g 2 g 2 g 2 g Detection of the Excited Species The r e l a t i v e concentration of 0„("'"E ) was determined by measuring 2 g 1 1 o 0 emission i n t e n s i t i e s of E and A bands at 7620 A and 6340 A respe c t i v e l y at a point about one-third the length of the observation tube from the quencher i n l e t . A Zeiss v a r i a b l e interference f i l t e r was used to i s o l a t e the two bands. The chopped s i g n a l was detected using a Hamamatsu TVR-213 photomultiplier tube and fed through a - 23 -Table I I . Sources of materials used. Chemical Source CHC13 Spectral Grade, Fisher Chemical CH2C12 Spectral Grade, Fisher Chemical CH 2Br 2 Eastman Chemicals CH3I Eastman Chemicals CH3Br Matheson CH3C1 A i r Products CHF3 Matheson CH 2F 2 Peninsular Chemicals ° 2 Matheson, Extra Dry CH3OH Spectral Grade, Fisher Chemical i-PrOH A n a l y t i c a l Grade, Malinckrodt Et 2NH Malinckrodt MeNH„, Me NH, Me N z z 3 Matheson EtNH 2, Et 2NH, NH3 - 24 -Tektronix 122 preamplifier into a S e r i a l #240 l o c k - i n a m p l i f i e r where both phase and frequency were compared with a s i m i l a r i l y chopped reference s i g n a l . A 10 MV Leeds Northrup recorder was used to record the s i g n a l i n t e n s i t i e s . Determination of Experimental Values Relative rate constants were calculated from Stern-Volmer plots of 1 2 (I^/I) vs. [Q], corrected f or changes i n [O^i A )] . Quencher flowrate and thus quencher concentration was determined by recording the pressure drop over a known period of time i n a storage bulb (various s i z e s were used) of known volume, on a Hewlett-Packard recorder. The 8 —1 rate constants were determined r e l a t i v e to (X^* k = 1.8 x 10 1 mole sec , ^  f o r convenience and better 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 absorption spectra were obtained using a Perkin-Elmer 457 spectrometer f or the 4000 to 250 cm ^ region and a Cary 14 f o r the 10,000 to 4000 cm ^ region. For those compounds normally l i q u i d at room temperature a heated c e l l was used to obtain the i n f r a r e d absorptions i n the 10,000 to 4000 cm ^ region, pressures of gaseous compounds were measured using a USC pressure gauge. Pressures of a l l compound f o r the 4000 to 250 cm region were measured using a P r e c i s i o n Pressure Gauge Model 145 obtained from Texas Instruments. - 25 -RESULTS Heavy Atom Study The Stern-Volmer equation which i s used to determine r e l a t i v e quenching rate constants can be obtained from the following reactions: O.^A) + O.^A ) - ^ - ^ O.^Z) + 0 ( 3 Z ) ( i ) 2 g 2 g 2 g 2 g 1 + k 0 2 ( E ) + wall — — » products ( i i ) 1 + k 0 2 ( E ) + Q —3—> products ( i i i ) In the absence of quencher, 0 2(^E+) emission i n t e n s i t y i s given by: xo • k r ' t Q 2 ( 1 V i 2 ( v i i ) w upon addition of quencher, the expression for 0.("''E ) becomes: 2- g k k [o th )] 2 Q k + k [Q] w w q The r a t i o of emission with and without quencher r e s u l t s i n the Stern-Volmer equation: k Ic/IQ " 1 + t t Q ] w (xi) P l o t t i n g ( I Q / I Q ) against quencher concentration w i l l r e s u l t i n a s t r a i g h t l i n e having a slope equal to the r e l a t i v e quenching constant. At le a s t four sets of data were c o l l e c t e d f o r each quencher studied (a data set consisted of the measurement of the 0_(^Z+) emission i n t e n s i t y at four d i f f e r e n t quencher concentrations). A. l e a s t squares computer program was used to obtain the slope f o r each data set and a subsequent l e a s t squares analysis of the slopes resulted i n a best value for the r e l a t i v e quenching rate constant. An example t y p i c a l of the Stern-Volmer p l o t s obtained i n t h i s study i s shown i n Figure I I I . The quenching constants obtained i n t h i s study are given i n Table I I I . Rate constants for the following compounds could 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^ values i s so small indicates that the deactivation of 0„("'"E+) i s not 2 g subject to heavy-atom e f f e c t . This r e s u l t supports the assumption that deactivation goes with the 0 „ ( ^ E+) -»• O-C^ A ) t r a n s i t i o n rather than the 2 g 2 g 1 + 3 -0 „ ( E ) 0 „ ( E ) t r a n s i t i o n occurring. 2 g 2 g Quenching Mechanism Study I t has been suggested^ that the quenching rate constant can be predicted from the overlap of the absorption spectra of the quencher with the emission spectrum of 0„(^"E+) i n i t s t r a n s i t i o n to 0_(^A ) , z g £ g using the following expression: k q ( x i i i ) - 27 -Figure I I I . Stern-Volmer p l o t f o r CH^C^ (each symbol: O , ^  , • , O represents a data s e t ) . 7.0-5 7(5 S 20~ 25 30 35 40 45~ 50 55 CQ3 moles f 1 Ho"7) - 29 -Table I I I . Quenching rate constants of CH X (X = halogen). n m Quencher k (l.mole sec ) F Cl Br I CHX3 3.77 x 107 3.49 x 107 — CH2X2 7.87 x 107 1.42 x io 8 2. 22 x io 8 CH3X 1.04 x io 8 1. 20 x io 8 1.32 x 10 8 CH. 4 4.5 x 107 3 see ref erence i61l. - 30 -An expansion of t h i s expression i n terms of 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 at 0 „ ( E -*• A ) t r a n s i t i o n frequencies was used by Merkel 2 g g 60 1 and Kearns i n t h e i r study of 0.( A ) deactivation i n so l u t i o n and ^ 8 subsequently by Davidson and Ogryzlo i n t h e i r study of gas phase quenching of 0 2 ^ g ^ ^ hydrocarbons. Davidson and Ogryzlo proposed an expansion of ( x i i i ) i n terms of overlap i n t e g r a l s i n an attempt to obtain a better f i t between calculated and experimental quenching constant values than was observed using an expansion i n terms of ex t i n c t i o n c o e f f i c i e n t s : -1 -1 k = c[FC. x overlap @ 5237 cm + FC 0 x overlap @ 3755 cm q l I + FC 3 x overlap @ 2297 cm"1 + FC^ x overlap @ 865 cm- 1 (xv) Both these studies used the t h e o r e t i c a l Franck-Condon factors determined by N i c h o l l s ^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 10~2 0-2 t r a n s i t i o n @ 2297 cm"1 3.70 x 10~4 0-3 t r a n s i t i o n @ 865 cm"1 5.09 x 10~6 f o r the O-C^ E* A^ ) t r a n s i t i o n . However, there i s some question 2 g g as to the v a l i d i t y of using these values since they were calculated 6 3 for the free oxygen molecule. Infrared absorption studies have shown that 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 factors from free molecule t r a n s i t i o n s . However, since c o l l i s i o n induced values are almost impossible to obtain, Davidson and Ogryzlo^"'" obtained a ' b e s t - f i t ' with t h e i r hydrocarbon data by simply varying the Franck-Condon f a c t o r s . - 31 -In t h i s study, overlaps i n the 0-0, 0-1, 0-2, and 0-3 regions were calculated f o r a v a r i e t y of quenchers. Using the Davidson and Og'ryzlo^ expansion i n terms of overlap i n t e g r a l s quenching rate constants for these quenchers were c a l c u l a t e d . Since the shapes of the 0_("4;+) •*• O^ C^ A ) emission bands have not yet been reported, t h e i r 2 g 2. g structures were calculated using free molecule spectroscopic constants and assuming that the s e l e c t i o n rules 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 t r a n s i t i o n s , i . e . J = 0,+ 1,+ 2 and that a l l t r a n s i t i o n s occur with equal p r o b a b i l i t y . Figure IV shows a plot of the computer calculated oxygen ("4]+ A^ ) t r a n s i t i o n band structure. The overlap i n t e g r a l s were calculated with the use of a computer program and are l i s t e d i n Table IV. Figure Va-d shows an example of the overlap i n t e g r a l s obtained i n t h i s study. The following expression was used to c a l c u l a t e values of quenching constants which were subsequently entered into Table IV: k q = c(0.97 x overlap i n the 0-0 region + 2.32 x 10 -4 x overlap i n the 0-1 region + 3.70 x 10 —6 x overlap i n the 0-2 region + 5.09 x 10 x overlap i n the 0-3 region] (xvi) c f i x e d to make k^(calculated) for CyH^ equal to k^(experimental). Values of experimental rate constants were plot t e d against the calculated values i n Figure VI. From a comparison of t h i s p l o t and the p l o t 53 obtained by Davidson and Ogryzlo (see Figure VII) i t i s evident that the c o r r e l a t i o n obtained i s not improved by including overlap - 32 -Figure IV. Computed "induced" emission bands f o r the 0 „ - > "*"A ) t r a n s i t i o n . 2 g g 1.0-. 9 -. 8 -CO -7 54.00 5200 5 0 0 0 3900 3700 3500 2 5 0 0 2300 2100 1100 9 0 0 700 V (cm l) (note compressed scale) Table IV. Overlap i n t e g r a l s and cal c u l a t e d quenching constants. Quencher k q (expt'l) (l.mole "'"sec ^) 0,0 overlap x 10" 2 0,1 overlap x 10~ 3 0,2 overlap x 10~ 3 0,3 overlap x 10" 4 k (calc) q -1 -1 (l.mole sec ) CH30H 2.06 x 10 9 3.56 14.1 0.0348 2.43 4.8 x EtOH 1.59 x 10 9 0.868 7.64 4.50 3.68 1.9 x i s o - C ^ O H 1.58 x 10 9 Q a 6.31 5.94 3.47 3.14 5.4 x CH3NH 1.1 x 10 9 3 2.20 0.0428 0.395 12.7 1.5 x (CH 3) 2NH 2 5.6 x 10 8 3.6 x 10 8 3 8.9 x 10 8 a 7.79 x 10 8 5.8 x 10 8 3 2.76 1.57 2.42 3.88 2.2 x (CH 3) 3N 3.43 1.60 3.51 8.47 2.6 x C 2H 5NH 2 4.21 0.327 0.351 4.64 2.9 x (C 2H 5) 2NH 6.13 1.23 2.81 1.51 4.4 x (C 2H 5) 3N 3.36 0.432 0.0428 0.649 2.1 x NH 3 1.2 x 10 9.85 0.154 0.141 17.3 7.0 x -O CH 3I 1.32 X io 8 0.513 0.405 1.27 1.82 4.3 X 10 7 CH 2Br 2 2.22 X 10 8 0.430 1.35 2.66 0.509 5.3 X io 7 CH 3Br 1.20 X io 8 0.00 0.119 1.74 0.639 2.4 X io 6 CHC13 3.49 X io 7 0.017 0.185 0.999 13.1 5.0 X io 6 CH 2C1 2 1.42 X io 8 0.490 0.628 0.780 5.97 4.5 X 10 7 CH3C1 1.04 X 10 8 0.561 0.408 2.07 0.249 4.6 X io 7 CHF 3 3.77 X 10 7 0.435 0.368 20.2 0.139 4.2 X 10 7 CH 9F 0 7.87 X io 7 1.17 .0.0 1.62 0.000274 8.1 X io 7 Table IV (Continued) Quencher k (expt'l) n qi "I (l.mole sec ) 0,0 overlap x 10~2 0,1 overlap x 10~3 0,2 overlap x 10"3 0,3 overlap x 10~4 k (calc) q _ i _ i (l.mole sec ) C H4 C2H6 C3H8 C4H10 ^C3H6 C4H10 S H 1 2 C r 4.4 x 10 1.9 x 10 2.7 x 10 2.9 x 10 3.7 x 10 3.8 x 10 4.5 x 10 5.2 x 10 8 0.022 1.72 2.74 3.71 6.09 3.71 5.31 6.72 0.226 1.32 1.44 1.64 0.951 1.64 2.63 4.05 0.0068 1.16 1.28 1.53 1.13 1.53 2.8 3.96 0.0000 5.3 X 0.79 1.4 X 0.455 2.1 X 0.272 2.8 X 6.91 4.4 X 0.222 2.8 X 1.19 4.1 X U) 1.27 5.3 x 10 8 C6H14 O a 5.5 x 10 8 5.8 x 10 8 5.8 x 10 8 6.56 7.62 8.43 3.41 4.36 4.54 3.63 3.35 4.20 1.15 1.60 2.04 5.1 x 10 8 6.0 x 10 8 6.6 x 10 8 C7H16 6.0 x 10 8 7.77 4.03 4.28 1.09 6.0 x 10 8 Reference 53 and references th e r e i n . Hydrocarbon k^ values from reference 61. - 36 -Figure Va. Computer drawn r e l a t i v e overlap i n t e g r a l f o r CHF 0-0 region. - LZ -- 38 -Figure Vb. Computer drawn r e l a t i v e overlap i n t e g r a l f o r CHF^, 0-1 region. - 6C -- 40 -Figure Vc. Computer drawn r e l a t i v e overlap i n t e g r a l f o r CHF3> 0-2 region. - 41.-- 42 -Figure Vd. Computer drawn r e l a t i v e overlap i n t e g r a l for CHF GT3 region. - tv -- 44 -Figure VI. Cor r e l a t i o n between (experimental) and k (calculated by use of overlap method). 8 b7j CO to A o 1 A • SD T i 20" 6 7 § 9 1 0 T T -8 A Hydrocarbons (ref.59) k q ( e x p f | ) l O I mole" sec • Heavy-atom containing compounds o Alcohols and Amines 12 13 14 15 16 17 18 - 46 -Figure VII. Overlap i n t e g r a l versus 0 2 ( E ) quenching rate constants (from r e f . 53). - 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 1 3 14 1 5 16 17 18 • 1 9 2 0 2 1 2 2 C2H6 C3H8 . cyclo-C-jH^ n-C^Hjo iso-Ci^Hjo cyclo -C6H | 2 CH^ -cyclo-C^ Hcj n-C 7H 1 o CH^ -cyclc-CfcH* 1 NHn C H 3 N H X (CH^NH C ^ c O r U CHnOH C 0 2 - 48 -i n t e g r a l s i n the 0-1, 0-2 and 0-3 regions i n the c a l c u l a t i o n s . This r e s u l t suggests that the l i m i t a t i o n s of the model are so great that a better c o r r e l a t i o n cannot be obtained with i t . Using a simpler seemingly le s s quantitative method i t i s possible to p r e d i c t how the e l e c t r o n i c e x c i t a t i o n of 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 various quencher modes. To make these predicitons the fundamental frequencies and binary combinations and overtones were considered. I t was assumed that: (1) the O^ C^ E"**) 2 g and quencher were only weakly coupled, thus the use of separated states i s allowed; and (2) the strength of coupling i s dependent upon 1 1 the energy difference between the t r a n s i t i o n s 0 o ( A ) 0„( A. ) and J 2 g 2 g v=z Q -> Q , where z, x, y are the degrees of v i b r a t i o n a l e x c i t a t i o n H va=x,vb=y J & and a, b are the normal v i b r a t i o n a l modes of the quencher. Table V. Most probable mode of quencher e x c i t a t i o n and u2 ^ ^ g ^ de e x c i t a t i o n . Quencher k Fundamental Important (l.mole-^ frequency s e c ~ l ) (degeneracy) combinations AE (cm ) overtones and (C^ tra n s i t i o n ) Most probable e x c i t a t i o n and deactivation mode vj 2917 CH C ± i 4 5 x 107 3 V2 1 5 3 4 ( 2 ) 4 ^ X i U v 3 3019(3) v 4 1306(3) v : 838(3755) v 3 736(3755) 0 o(V") _ + CH. + 0 o( 1A ) - + CH. . 2 g v=0 4 2 g v=l 4 v 3 = l + 0 „ (1A ) , + CH. . 2 g v=l 4 vj = l C2H6 c n v x 2954 v 2 1388 v 3 995 289 v 5 2896 8 a V6 " 7 6 2.0 x 10 v 7 2969(2) v 8 1468(2) v 9 1190(2) v 1 02985(2) v n1469(2) via 822(2) vj 801(3755) v 5 859(3755) v 7 786(3755) v 1 0770(3755) + C2H6 + v V v - 1 + C2 H6 v 1 0 = 1 °2 < \ > v - l + C2 H6 v 7=l -> ° 2 < V - 1 + C2H6 v 1 = l -> 02 < V v - l + C2 H6 V 5 - 1 c 3 V " ( A ) 3.7 x 10 8 a v x 3038 v 2 1479 v 3 1188 \) k 1126 v 5 1070 3103 854 v 8 3025(2) v 9 1438(2) v 1 01029(2) v n 866(2) v 1 23082(2) v 1 31188(2) vlk 739(2) » 1 717(3755) v 6 652(3755) v 8 730(3755) v 1 2673(3755) , L + , + C3H6 v 6=l °2 < \ > v - l + C3 H6 v 8=l 02 < V v = l + C3 H6 v 1 = l ° 2(\ > v . l + C3 H6 v 1 2 = l Table V. (Continued) Quencher q -1 (l.mole.. ) sec Fundamental frequency (degeneracy) Important overtones and combinations AE (cm ) Most probable e x c i t a t i o n and deactivation mode (0^ t r a n s i t i o n ) CH30H c n 2.2 x 10' CH 3NH 2 c n 1.1 x 10 V i 3681 v 2 3000 v 3 2844 1477 v 5 1455 v 6 1345 v 7 1060 v 8 1030 vg 2960 v 1 01477 v n 1 1 6 5 vi2^250 9 b 3361 2961 2820 1623 1473 1430 1130 1040 780 v 1 03427 v n 2 9 8 5 v 1 21465 v 1 31419 v l l t1195 v 1 5 286 v l v 2 v3 vk v 5 v 7 v8 v 9 v-L+\>k 5158 V!+v5 5136 VI++VKJ5050 v i 74(3755) \>±+\>u 81(5239) vi+v 5 103(5239) V! 394(3755) v 1 0 328(3755) v 4+v 1 0190(5239) 0 0 ( 1 i : + ) + CH.OH 2 g 3 0„(1A ) + CH.OH . 2 g v=l 3 vj=l V ^ g W ^ 2 01 o °2< Ag>v=l + C H 3 N H 2 vio-1 * 02 ( 1 A g>v-l + ^ 2 v i - 1 NH c i i 1.2 x 10 vj 3337 .9 b v 2 950 v 3 3444(2) 1627(2) v 3+Vi* 5071 418(3755) 311(3755) V3+V!, 168(5239) ^3 0 ( h*) + NH 2 • g 3 0„(1A ) + NH . 2 g v=l 3 v 3 = l O ^ 1 ^ ) , , ^ + NH„ •> 0 o( XA„) "g v=0 g v=l + NH 3 Vj-1 Table V. (Continued) Quencher k Fundamental Important ? -1 frequency (l.mole - ^ 3 . s e c ~ l ) (degeneracy) combinations AE (cm ) overtones and (0^ t r a n s i t i o n ) Most probable e x c i t a t i o n and deactivation mode CH 3C1C 1 1 1 x 108 o v3 C H 2 B r 2C 1 1 2.22 x 10 vk 2933 v 2 1252 ^3 533 822(3755) v 4 3060(2) v 4 695(3755) ^5 1436(2) ^6 882(2) v i 2972 v 2 1305 V3 611 vi 783(3755) v<+ 3056(2) v 4 699(3755) ^5 1443(2) ^6 953(2) 2937 v 2 1355 V3 732 vi 818(3755) vi+' 3039(2) vi+ 716(3755) V5 1452(2) ^6 1017(2) vi 3009 v 2 1382 ^3 588 i+ 169 vi 746(3755) V5 1095 V6 682(3755) V6 3073 v 7 812 V8 1195 v 9 653 0 0(V") _ + CH.I •* 0_( 1A ) + CH.I , 2 ,g v=0 3 2 g v=l 3 v i =1 + 0_(1A ) + CH.I . 2 g v=l 3 vi t=l 0,(1Z+) _ n + CH Br -»• 0 „ (1A ) + CH„Br 2 g v=0 3 2 g v=l 3 v i = l 0 „ (1A ) + CH Br 2 g v=l 3 vi+=l ° o (l z +) n + C HQC 1 ° O (1 a > i + C H^C 1 i 2 g v=0 3 2 e v=l 3 vti=l 0 ( A ) + CH„C1 . 2 g v=l 3 v i = l °2 ( 1 Z g > v-0 + C H 2 B r 2 * ° 2 ( l A g ) v - l + C H 2 B r 2 v 1 = l - 0 2 ( \ ) v = 1 + CH 2Br 2 v 6 - l Table V. (Continued) Quencher k Fundamental Important AE (cm 1) Most probable e x c i t a t i o n and deactivation mode n ? -1 frequency overtones and (CL t r a n s i t i o n ) sec"1) (degeneracy) combinations 1 V ! 2999 v 2 1467 0 o ( X E ) + CH0C10 + 0 o( XA ) -• + CH„C1, v 3 717 2V g v=0 2 2 2X g'v=l 2 2 v i = l TH n c i i i A? v i n 8 V l + 2 8 2 V l 756(3755) + O.^A ) + CH 9CI_ _ CH2C12 1.42 x 10 V s 1 1 5 3 V g 7 1 5 ( 3 7 5 5 ) 2 g v-1 2 2 v 6 - l v 6 3040 v 7 898 v 8 1268 v 9 758 V i 2993 7 ll i n m + ^ ~ ° < V w + ^ n-i CH 2F 2 7.87 x 107 > J „ 4 v 6 671(3755) - °2< Vv-1 + C H2 F2 v6-l v 6 3084 v 7 1217 v 8 1435 Vg 1090 v : 3034 v 2 680 C H C l 3c l i 3.49 x 107 ll 12fQ(2) V ! 721(3755) o / ^ ) ^ + CHC13 - 0 2 ( 1 A g ) v = 1 + CHC13 v i = 1 v 5 774(2) v 6 261(2) v x 3036 v 2 1117 c i i 7 v 3 700 i J . i CHF3 3.8 x 10 V i f 1 3 7 2 ( 2 ) V l 719(3755) 0 2 ( E ) = ( ) + CHF3 - O ^ A ) v = 1 + CHF3 y = ]_ v 5 1152(2) 1  v6 507(2)  SL b e Ref. 61 and references therein; Ref. 53; fundamental v i b r a t i o n a l frequences, ( i ) Ref. 64, ( i i ) Ref. 65. - 53 -DISCUSSION Heavy-Atom Study Of the two possible modes of deactivation of 0_(''"E+): (1) 0 ( V ) + *M , „ , . , z g z s (2) 0 _ ( V ) + > + ^ 2 g 2 g i t has been generally assumed that p h y s i c a l quenching goes v i a process (1) as was previously stated. Although there i s no d i r e c t evidence to support t h i s assumption, i n d i r e c t evidence"*"* and analysis of quenching a b i l i t i e s of various species do give an i n d i c a t i o n of the most probable path. When a heavy atom or paramagnetic species on one molecule i n t e r a c t s with the e l e c t r o n i c structure of a second molecule, t h i s enhances the p r o b a b i l i t y of intersystem crossing by inducing s p i n - o r b i t coupling which relaxes the forbidden nature of the t r a n s i t i o n . Applying t h i s to the quenching process, i f quenching involved intersystem crossing the observed quenching rate constant for a compound containing a heavy atom or a paramagnetic species would be unexpectedly high. For quenching 1 1 3 -of 0 „ ( A ) which involves the A £ t r a n s i t i o n , a paramagnetic 2 g g g 53 e f f e c t has been observed. From Table VI i t i s evident that the quenching constants f o r the heavy atom Xe and for the paramagnetic species 0^ are greater than would normally be expected. However, examination of quenching rate constants of paramagnetic species f o r CL ("4;+) deactivation seems somewhat contradictory. 0 , NO, and N02 are a l l paramagnetic species, 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. Table VII shows the quenching constant values of seri e s of quenchers containing these paramagnetic species. That NO i s observed to be such a good quencher, may be a t t r i b u t e d to e i t h e r a paramagnetic/heavy atom e f f e c t or to the fact that the NO induced t r a n s i t i o n s e x h i b i t greater overlap with the 0„(^"Z+ "*"A ) t r a n s i t i o n bands than do the 0„ t r a n s i t i o n s (see Figure 2 g g 2 V I I I ) . The NO(0,2) t r a n s i t i o n i s very close to the 1Z -* "^(0,1) and the N0(0,3) t r a n s i t i o n i s close to the *E "^(0,0) t r a n s i t i o n , the (0,1) and e s p e c i a l l y the (0,0) O^^Z -> "''A) t r a n s i t i o n s are expected to be important i n the overlap model of the quenching process. A systematic i n v e s t i g a t i o n of a seri e s of halogenated hydrocarbons was undertaken to obtain d e f i n i t i v e evidence concerning a heavy atom e f f e c t on ° 2 ^ ^ g ) quenching. The quenching rate constants obtained i n t h i s study, Table I I I , show no evidence of a heavy atom e f f e c t . Absence of a heavy atom e f f e c t combined with the fact that 0 2 i s a poor quencher, i s a good i n d i c a t i o n that O ^ ^ g ^ <lu e riching does not occur v i a intersystem crossing but rather that i t i s v i a the i n t e r n a l conversion process: (i) o 9 (V) + XM —y v V ) + 1 m g ^ g - 55 -Figure VIII. Relative p o s i t i o n s of the G^ C E -> A ) t r a n s i t i o n s and v i b r a t i o n a l bands of diatomic molecules (from ref. 53). - 56 -500 1000 2CCO 3000 4000 50 0 0 6000 frequency(cm" ) - 57 -Table VI. Quenching rate constants f o r quenching of O^C^A ). Quencher k (l.mole '''sec "S q He 4.8 Ar 5.3 Kr 5 Ze 20 H 2 2.7 x 103 N 2 50 0 2 1.2 x 103 (k values from reference 53). q - 58 -Table VII. Quenching rate constants for quenching of 0 0 ( E ) . Quencher k (l.mole q -1 -1, sec ) H2 4 X io8 HD 1.5 X io 8 D2 1.2 X io 7 N2 1.3 X io 6 ° 2 1 X io 5 CO 2 X io 6 NO 2.6 X io 7 co2 2.0 X 10 8 N02 1.7 X io 7 so2 1 X io 6 (k values from reference 53). q - 59 -Quenching Mechanism Study Physical quenching i s a process whereby an e l e c t r o n i c a l l y excited species c o l l i d e s with an atom or molecule and i s deactivated with transfer of 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. As shown i n the previous section p h y s i c a l quenching of 0„("4;+) can be assumed to involve the t r a n s i t i o n 0„("4]+) -> 0„(^A ). 2 g 2 g 2 g The question which now a r i s e s i s how the "*"E and A states are coupled and how the degree of coupling can be measured or predicted. Before these questions can be tackled i t i s necessary to review what i s known concerning 0,("*"£+) quenching. 2- g Various studies of 0oC^E"*") quenching have led to the following 2 g conclusions: (1) In general, quenching i s dependent upon the extent to which the quencher induces mixing of the "'"E-^ 'A wavefunctions. Variations i n k^ have been rel a t e d to the magnitude qf the intermolecular ' i n t e r a c t i o n s between the oxygen molecule and the quencher, as deduced from Lennard-Jones p o t e n t i a l parameters for the i n t e r a c t i o n between two nonpolar molecules or from Stockmayer p o t e n t i a l parameters f o r i n t e r a c t i o n between two polar molecules (45) (2) The high quenching e f f i c i e n c y of polar molecules has been a t t r i b u t e d to 'long-range' perturbing e f f e c t s v i a the i n t e r a c t i o n of the dipole moment of the perturber (quencher) with the oxygen electrons (20). (3) That hydrogen containing 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 intermolecular i n t e r a c t i o n between the H and oxygen (66). - 60 -(4) Quenching e f f i c i e n c y increases monotonically with increase i n the magnitude of the quencher fundamental v i b r a t i o n a l frequency (67). In a preliminary study of the quenching mechanism, Davidson and 53 Ogryzlo have proposed that quenching by atomic species involves transfer of 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 motion of 0^ and quencher whereas quenching by molecules involves transfer to v i b r a t i o n a l e x c i t a t i o n of the quencher. 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 frequencies and quenching e f f i c i e n c i e s was 53 attempted for diatomic species to determine whether the Franck-Condon factors are important i n the quenching process. Symmetric diatomics e.g. 0^, ^ 2 have no dipole moment i n both s t a t i c and v i b r a t i o n a l states and are therefore not expected to have i n f r a r e d absorptions due to v i b r a t i o n and r o t a t i o n . However, binary c o l l i s i o n s induce molecular 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 of v i b r a t i o n a l bands. Weak IR absorptions a t t r i b u t e d to dipole moments induced by intermolecular forces have been observed. For induced IR absorptions the primary mechanism of induction seems to be quadripolar i n t e r a c t i o n . These induced t r a n s i t i o n s of homonuclear diatomics were used i n the c o r r e l a t i o n . The obtained c o r r e l a t i o n showed an exponential increase i n quenching rate constant with increase i n fundamental v i b r a t i o n a l frequency of quencher fo r the homonuclear diatomics, with heteronuclear diatomics having rate constants generally greater than those obtained from the homonuclears. R e l a t i v e l y good quenching by the nonpolar (IR inacti v e ) homonuclear diatomics suggests the presence of 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 inductive i n t e r a c t i o n s operative during c o l l i s i o n s , i n the quenching - 61 -process. That dipolar molecules are more e f f e c t i v e i n inducing a dipole than are nonpolar molecules would suggest that dipolar molecules are 53 more e f f e c t i v e quenchers than nonpolar molecules This was observed. General trends observed i n quenching by molecules can be a t t r i b u t e d to: (1) the a b i l i t y of the molecule to absorb the energy released i n the -> ''"A t r a n s i t i o n ; (2) the Franck-Condon factors c o n t r o l l i n g the "^ E -»- ^ A t r a n s i t i o n ; while the subtler trends appear to be a r e s u l t of the closeness of the energy match between energy released i n the oxygen t r a n s i t i o n and quencher v i b r a t i o n a l e x c i t a t i o n . (These factors were previously used to p r e d i c t the most probable mode of e x c i t a t i o n i n quencher.) ^2^^g^ Qu^ching c a n D e considered to proceed v i a the following r e a c t i o n scheme: k k k 1E+ + Q — ^ [1E+'Q] — ? - X l1 A -Q] —3-» 1A + Q* ( x v i i ) g I" g I g g -1 -2 where Q i s v i b r a t i o n a l l y excited quencher. k_^ - k^ >> k^ >> ^-_2: m o s t c o l l i s i o n s w i l l not r e s u l t i n formation of the complex with v i b r a t i o n a l l y excited modes i n 1 * quencher; d i s s o c i a t i o n of the complex A i s more probable 1 * 1 + 8 than i s the reaction ( A *Q -*• E «Q) because the density of 2 ^g g ± + states of complex A «Q i s greater than that of complex [ E *Q]. g 1 + 1 * g - The rate determining step [ E > Q ] - * - [ A * Q ] involves a r a d i a t i o n l e s s t r a n s i t i o n . - k q = k 1k 2/k_ 1. Examining t h i s scheme for ° 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 formation, upon c o l l i s i o n of 0_(^E+) and a quencher molecule, 2- g of a molecular e n t i t y of a transient nature known as a c o l l i s i o n complex. - 62 -This c o l l i s i o n complex can be likened to a large molecule or a poly-nuclear complex and thus the rate determining step can be treated as an intramolecular r a d i a t i o n l e s s t r a n s i t i o n . And the formalism developed 68 by Robinson and Frosch 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 treat the quenching of O-C^E"1"). 2 g Current theories 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 coupling between the excited species and the medium i s e s s e n t i a l f o r occurrence 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 , the medium being required to provide a sink for the d i s s i p a t i o n of e x c i t a t i o n energy and to secure energy conservation r e s t r i c t i o n s . It i s now necessary to examine the coupling i n t e r a c t i o n and to present a method fo r obtaining a measure of the degree of coupling. In t h e i r preliminary studies both Davidson and O g r y z l o " *3a n d Merkel and K e a r n s ^ assumed a dipole-dipole i n t e r a c t i o n and used overlap of absorption spectra of quenchers with emission spectrum of the excited species to obtain a measure of the degree of coupling. Making use of the continuum approximation (also referred to as the Fermi golden rule) and the assumption that the c o l l i s i o n complex i s s i m i l a r to a polynuclear complex, the rate constants for 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 (64): k2 * | j Ie|2 ( x v i i i ) a = i n t e r a c t i o n energy between i n i t i a l and f i n a l states 6 = ^Jtt'j'^ - 63 -The expression Merkel and Reams used i n t h e i r study of O^C^A ) deactivation i s s o l u t i o n ^ i s obtained from ( x v i i i ) and the r e l a t i o n s h i p hot 1 = 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 time of the vib vib solvent: 2.17 T ., 0 k„ * 5 ^ | 3 | 2 (xix) 68 Relating a to the density of states p the expression used by Davidson and Ogryzlo i n t h e i r study of gas phase deactivation of O^i^Z) by hydrocarbons^1 can be obtained: k 2 * | B |2 (xx) Assuming dipole-dipole i n t e r a c t i o n (xx) becomes: k9 = 3 r l K J2( S < ^  1^ >< Q | ^ | Q > ) 2 (xxi) 2 -n • e l • m ' o n ' o r ' o mn = e l e c t r o n i c 0^ wavefunction ( i s expected to remain constant for a l l quenchers). 1 1 2 (< A I Z >) = Franck-Condon overlap between v i b r a t i o n a l wavefunctions ° 1 1 f o r Z -* A t r a n s i t i o n . ( < Q j | Q Q>) i s related to the p r o b a b i l i t y of o p t i c a l e x c i t a t i o n of quencher to the nth v i b r a t i o n a l l e v e l . which p a r a l l e l s eq. ( x i i ) used by Merkel and K e a r n s . ^ Since k = k.k0/k , q 1 2 — 1 , 1 2T70 I „ i 2 . „ 1 , i l ^ I G U I - . N 2 , . . . k = —r- 6 , ( Z < A Z ><Q — Q >) (xxn) q k , \v 1 e l 1 m1 o n'6r 1 o -1 mn - 64 -and can be approximated by: 00 c (v)f_ (v)dv ( x i i i ) o q C>2 i f the following assumptions are made: (1) 8 does not change with quencher, (2) p i s not of major importance i n the quenching process, (3) and k_^ do not change appreciably with quencher. Of these assumptions the v a l i d i t y of (2) would be most l i k e l y to be questioned. A preliminary survey undertaken by Davidson indicates that the contribution of the density of states i s outweighed by other factors s p e c i f i c a l l y the Franck-Condon f a c t o r s . Using an expansion of ( x i i i ) i n terms of overlap i n t e g r a l s , i . e . equation ( x v i ) , quenching rate constants were c a l c u l a t e d . Figure VI shows the c o r r e l a t i o n obtained between calculated and experimental quenching constants. The poor c o r r e l a t i o n suggests serious shortcomings i n the overlap model used. Examination of the assumptions made and of the inherent shortcomings of the model indicates that the c o r r e l a t i o n , although poor, i s the best that could be expected with t h i s model. The assumptions made are summarized: (1) IR absorption i n t e n s i t y was assumed to be proportional 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 e x c i t a t i o n energy l o s t i n the 02(^2 -»• "''A) t r a n s i t i o n . Use of IR absorptions does not consider possible contribution from IR i n a c t i v e bands. Use of the data a v a i l a b l e made i t d i f f i c u l t to consider induced absorptions. - 65 -(2) In c a l c u l a t i n g the 0 o ( E ) emission spectrum i t was assumed 2 g that: s e l e c t i o n rules 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 of 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 occur with equal p r o b a b i l i t y (highly u n l i k e l y ) . (3) It was assumed that free molecule states are v a l i d i n c o l l i s i o n processes, t h i s i s not necessarily true. There i s indeed reason to believe that the Franck-Condon factors c o n t r o l l i n g the E ->• "'"A t r a n s i t i o n for c o l l i s i o n induced t r a n s i t i o n s d i f f e r from those f o r free molecule t r a n s i t i o n s (63). Inherent shortcomings i n the model are: (1) The c o l l i s i o n which occurs i n the quenching process w i l l undoubtedly perturb the free molecule states making the use of free molecule state properties i n v a l i d . (2) Physical quenching i s a nonphotonic process, however as a matter of convenience i n a n a l y s i s , photonic s e l e c t i o n rules were assumed v a l i d . This introduces considerable error since s e l e c t i o n rules f or photonic processes are often opposite to those f o r non-photonic processes (69), f o r example: ( i ) The Born-Openheimer (B-0) approximation holds f o r photonic processes but occurence of a nonphotonic process r e s u l t s i n the breakdown of the B-0 approximation because of d i s t o r t i o n of the molecular geometry r e s u l t i n g from c o l l i s i o n . ( i i ) (+) «-* (-) i s allowed, (+) (+) and (-) 4" (") a r e not allowed f o r photonic processes but f o r nonphotonic processes (+) (+) and (-) «-»- (-) are allowed. - 66 -( i i i ) u g and g u (a dipole must be created or destroyed i n t r a n s i t i o n ) i n photonic processes whereas u g but u ->• u and g -> g are allowed i n nonphotonic processes. The use of photonic s e l e c t i o n rules i s necessitated by the f a c t that there has not as yet 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 experimentally measurable quantities using non-photonic s e l e c t i o n r u l e s . From th i s study i t appears that the p o s s i b i l i t i e s of the overlap model have quickly been exhausted. Thus i t i s necessary to e i t h e r modify the model or to consider a new method of attack. In developing the model, the rate determining step was considered a 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 a polynuclear species, i . e . i n the c o l l i s i o n complex, however, for convenience, the c a l c u l a t i o n s were c a r r i e d out using spectroscopic data for the separated molecules. A more correct method would be 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. Rigorous c a l c u l a t i o n s of t h i s type would necessitate density of state determina-tions which quickly becomes a major project for complicated systems. Of the various approaches to density of state c a l c u l a t i o n s , Haarhoff's method seems most useful.7^* More rigorous c a l c u l a t i o n s would also require determination of p o t e n t i a l energy curves for the system and determination 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 . Studies of the d i s t r i b u t i o n of v i b r a t i o n a l energy following energy transfer would be of assistance i n drawing the p o t e n t i a l energy curves. Preliminary experiments have been undertaken i n t h i s laboratory to observe IR emission of the excited quencher a f t e r energy transfer has occurred and have met with some success. 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 transfer needed to determine - 67 -p o t e n t i a l energy curves. A second approach which may be considered i s the p o s s i b i l i t y of constructing p o t e n t i a l energy surfaces f o r the c o l l i d i n g 0„(^"Z+) and i n d i v i d u a l quencher molecules. Thus the quenching mechanism could be considered i n terms of p o t e n t i a l energy surfaces 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. This method has proved quite successful 3 for such simple systems as the quenching of Na ( P) by atoms and simple d i a t o m i c s .7 1 To apply t h i s to the more complicated system of O^i^Z) quenched by polyatomics would r e s u l t i n unwieldy equations which,if solvable^would produce n dimensional surfaces where n > 3 depending upon the number of atoms i n the system. A l t e r n a t i v e l y a number of si m p l i f i c a t i o n s could 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 loss i n accuracy that t h i s treatment would be no more useful than the previous overlap treatment. - 68 -SUMMARY AND CONCLUSIONS The heavy atom study was undertaken to present d e f i n i t i v e evidence that 0„("'"Z+) deactivation occurs v i a an i n t e r n a l conversion process 2 g rather than v i a intersystem crossing. From the r e s u l t s i t i s evident that there i s no heavy atom e f f e c t on the quenching of 0 ("4;+) and ^ g thus the assumption that quenching proceeds with i n t e r n a l conversion i s v a l i d . The quenching mechanism study was undertaken to thoroughly test 53 61 the overlap model proposed by Davidson and Ogryzlo. ' As was shown, i t i s impossible to r e f i n e the model to a point where accurate predictions can be made f o r a l l types of quenchers as a r e s u l t of the assumptions used and the model shortcomings. Other treatments which may be more e f f e c t i v e include: (1) rather than using•. = separated species to obtain t r a n s i t i o n p r o b a b i l i t i e s , the c o l l i s i o n complex could be used and the Robinson-Frosch formalism more rigorously followed. 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