EPR Studies of Radical Pairs. The Benzoyloxy Dilemma. - ACS

12 EPR Studies of Radical Pairs. The Benzoyloxy Dilemma.

Downloaded by NORTH CAROLINA STATE UNIV on December 14, 2012 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch012

J. MICHAEL McBRIDE, MICHAEL W. VARY, and BONNIE L. WHITSEL Department of Chemistry, Yale University, New Haven, CT 06520

For the past ten years we have been using f r e e - r a d i c a l systems to study the chemistry of organic s o l i d s . T h i s work o r i g i n a t e d with s t u d i e s of the p h o t o l y s i s of s o l i d azoalkanes i n P. D. B a r t l e t t 's l a b o r a t o r y a t Harvard.(1,2) Much of our e f f o r t has been devoted to epr spectroscopy of r a d i c a l p a i r s generated i n low c o n c e n t r a t i o n by p h o t o l y z i n g s i n g l e c r y s t a l s o f such standard r a d i c a l initiators as azoalkanes and d i a c y l peroxides. In s e v e r a l cases t h i s t o o l has allowed us to determine where molecules i n a solid move during chemical r e a c t i o n and how e a s i l y they move.(3,4) Since s i n g l e - c r y s t a l epr s p e c t r a c o n t a i n information about e l e c t r o n s p i n d i s t r i b u t i o n , it is necessary to know how the e l e c t r o n s p i n i s d i s t r i b u t e d about the nuclear framework o f a r a d i c a l i n order to i n f e r how the r a d i c a l is o r i e n t e d . When the s p i n d i s t r i b u t i o n s of i s o l a t e d r a d i c a l s are known from independent sources, epr gives d e t a i l e d information about the arrangement of the r a d i c a l s i n p a i r s . N a t u r a l l y knowing t h i s arrangement can be uniquely h e l p f u l i n determining the f a c t o r s which i n f l u e n c e r e a c t i o n s of organic s o l i d s . Sometimes r a d i c a l s t r u c t u r e and s p i n d i s t r i b u t i o n are not known reliably. Epr r e s u l t s can then be u s e f u l in determining r a d i c a l s t r u c t u r e as w e l l as i n studying the arrangement o f p a i r s . T h i s paper focuses on what solid systems can tell about r a d i c a l s t r u c t u r e r a t h e r than on what r a d i c a l systems can tell about the chemistry of s o l i d s . I t s first purpose is to show what i n f o r m a t i o n about r a d i c a l s t r u c t u r e and p a i r arrangement i s contained i n s i n g l e c r y s t a l EPR s p e c t r a . I t s second is to show, using p a i r s i n c l u d i n g benzoyloxy r a d i c a l s as an example, how the s p e c t r a sometimes confuse us by p r o v i d i n g more i n f o r m a t i o n than appears to be c o n s i s t e n t with simple e x p l a n a t i o n s . EPR

of R a d i c a l P a i r s

The r a d i c a l s we study are immobilized i n p a i r s separated by 3.5 to 8 À. At these d i s t a n c e s the bonding between them i s weak enough that f o r temperatures above about 10 Κ (20 cal/mole) the

©0-8412-0421-7/78/47-069-208$05.00/0

In Organic Free Radicals; Pryor, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


12.

M C BRIDE E T A L .

EPR

Studies of Radical Pairs

209

t r i p l e t s t a t e predominates over the s i n g l e t , which i s presumably the ground s t a t e , by the s t a t i s t i c a l f a c t o r of t h r e e . The r e s i d u a l bonding remains strong enough, however, t h a t f o r most magnetic purposes (perhaps 0.01 to 0.001 cal/mole) the p a i r must be considered as a t r i p l e t s t a t e with e l e c t r o n spins exchanging r a p i d l y between the r a d i c a l s , r a t h e r than as a p a i r o f independent doublet s t a t e s each with i t s own,spin.(3) In a d d i t i o n to the normal e l e c t r o n Zeeman s p l i t t i n g there are three types of magnetic i n t e r a c t i o n s which have a strong i n f l u e n c e on the epr s p e c t r a of t r i p l e t r a d i c a l p a i r s . These are: 1) i n t e r a c t i o n between e l e c t r o n spins and nuclear s p i n s , which gives r i s e to hyperfine s p l i t t i n g (hfs) and i s d e s c r i b e d by an A tensor f o r each magnetic nucleus; 2) i n d i r e c t i n t e r a c t i o n between the e l e c t r o n s p i n s and the a p p l i e d f i e l d mediated by o r b i t a l angular momentum, which determines how much the f i e l d at the center of the p a t t e r n d i f f e r s from that f o r a f r e e e l e c t r o n , and i s d e s c r i b e d through the g tensor; and 3) i n t e r a c t i o n between the two e l e c t r o n s p i n s , which g i v e s r i s e to s p e c t r a l f i n e s t r u c t u r e (fs) and i s d e s c r i b e d by the D tensor. What information do the A, g, and D tensors contain? The s i x independent elements of each tensor (D, being t r a c e l e s s , has only f i v e ) may be transformed to y i e l d three sets of values c o n s i s t i n g of one, two, and three members. These s e t s c o n t a i n d i f f e r e n t types of information and may be determined under d i f f e r e n t c o n d i t i o n s . The set of one member (which D lacks) i s the i s o t r o p i c or average value of the i n t e r a c t i o n and may be determined from samples i n any phase. The s e t with two members d e s c r i b e s the magnitude o f the i n t e r a c t i o n ' s anisotropy and contains symmetry i n f o r m a t i o n . I t s determination u s u a l l y r e q u i r e s a r i g i d sample. The s e t o f three members d e s c r i b e s the o r i e n t a t i o n of the anisotropy, g i v i n g f o r example the s p h e r i c a l p o l a r coordinates of a molecular symmetry a x i s and the phase o f r o t a t i o n about i t . The o r i e n t a t i o n can only be determined with an o r i e n t e d sample, u s u a l l y a s i n g l e c r y s t a l . Hyperfine S p l i t t i n g . The i s o t r o p i c value o f the h f s depends on e l e c t r o n s p i n d e n s i t y at the nucleus i n q u e s t i o n . Since ρ o r b i t a l s have a node a t the nucleus, i t shows how much s p i n d e n s i t y r e s i d e s i n the atom's s o r b i t a l s . For the 2s o r b i t a l of carbon the p r o p o r t i o n a l i t y constant i s 1110 gauss/electron; f o r the Is o r b i t a l of hydrogen, 507. (5) The magnitude o f the h f s anisotropy depends on how much the s p i n d e n s i t y i n the neighborhood of the nucleus departs from s p h e r i c a l , and how much from a x i a l symmetry. Since the through-space i n t e r a c t i o n i s attenuated by the cube of the e l e c t r o n - n u c l e a r d i s t a n c e , i t samples p r i m a r i l y the immediate v i c i n i t y of the nucleus. For C-13 the a n i s o t r o p i c h f s commonly has an a x i s of symmetry and may be i n t e r p r e t e d i n terms of s p i n d e n s i t y i n the 2p o r b i t a l s o f the


ORGANIC F R E E RADICALS


210

atom. The s p l i t t i n g by a C-13 nucleus o f an e l e c t r o n l o c a l i z e d i n i t s ρ o r b i t a l would vary over a range o f 97 gauss. (5) For hydrogen the i n t e r p r e t a t i o n i s i n terms o f s p i n d e n s i t y on i t s neighbor. The o r i e n t a t i o n a l i n f o r m a t i o n can then t e l l which 2p o r b i t a l of carbon bears the e l e c t r o n s p i n . For r a d i c a l s i n a t r i p l e t p a i r the observed h f s i s the average o f what i t would be f o r the i n d i v i d u a l r a d i c a l s . Since the s p i n of a nucleus on one r a d i c a l has n e g l i g i b l e d i r e c t i n t e r a c t i o n with the e l e c t r o n s p i n on the other, the h f s i s h a l f of the value f o r the i s o l a t e d r a d i c a l . The C-13 h f s o f p a i r s o f bridgehead t r i p t y c y l r a d i c a l s p r o v i d e s an example. P a i r s which p e r s i s t a t -100°C i n a photolyzed s i n g l e c r y s t a l o f the r e l a t e d d i a c y l peroxide show s i g n a l s with s a t e l l i t e doublets from s p l i t t i n g by bridgehead C-13 i n n a t u r a l abundance.{6) The i s o t r o p i c value o f t h i s s p l i t t i n g i s 49.9 G and the a n i s o t r o p y shows approximate a x i a l symmetry with a range o f about 38 G. The i s o l a t e d r a d i c a l thus has (2 χ 49.9 / 1110) = 9% s p i n d e n s i t y i n the 2s o r b i t a l o f the bridgehead carbon and (2 χ 38 / 97) = 77% d e n s i t y i n i t s 2p o r b i t a l . Thus some 86% o f the s p i n d e n s i t y i s l o c a l i z e d on the bridgehead carbon i n an sp^ h y b r i d . The d i r e c t i o n of the symmetry a x i s shows how the o r b i t a l i s o r i e n t e d i n the s o l i d , and the e x i s t e n c e o f a s i n g l e p a i r o f s a t e l l i t e s shows t h a t the two r a d i c a l s i n a p a i r are c l o s e enough t o a n t i p a r a l l e l t h a t t h e i r bridgehead carbons g i v e e q u i v a l e n t splitting. S p i n - O r b i t - F i e l d Coupling. Even a nondegenerate r a d i c a l can acquire o r b i t a l angular momentum when an a p p l i e d magnetic f i e l d mixes e l e c t r o n i c a l l y e x c i t e d s t a t e s with the ground s t a t e by the H»L i n t e r a c t i o n . ( 7 ) In an LCAO-MO p i c t u r e t h i s mixing i s the r e s u l t a n t o f c o n t r i b u t i o n s from each atomic ρ o r b i t a l i n the s i n g l y occupied MO, which a f t e r r o t a t i o n by 90° about the a p p l i e d f i e l d d i r e c t i o n would o v e r l a p a ρ o r b i t a l i n another MO o f s i m i l a r energy. For example, i f the s i n g l y occupied MO had a l a r g e c o e f f i c i e n t from ρχ of a p a r t i c u l a r atom, and a vacant MO had a l a r g e c o e f f i c i e n t f o r py o f the same atom, a f i e l d i n the ζ d i r e c t i o n would tend to mix them through the 1^ operator, which " r o t a t e s " Ρχ i n t o i n py (at the same time r o t a t i n g py i n t o - i n p^, and d e s t r o y i n g p ) . The magnitude o f the mixing depends d i r e c t l y on the extent of r o t a t i o n and the amount o f o v e r l a p i t generates (that i s on the f i e l d s t r e n g t h and the product o f the c o e f f i c i e n t s of coupled AOs i n the two MOs) and i n v e r s e l y on the energy gap between the MOs. The s i g n o f the mixing and the sense o f the o r b i t a l angular momentum depend on the s i g n o f the energy gap (that i s on whether mixing i s with a lower-energy doubly occupied o r b i t a l or with a higher vacant one). The i n f l u e n c e o f the r e s u l t i n g o r b i t a l angular momentum on the e l e c t r o n s p i n through L'S^ depends on the s t r e n g t h o f the s p i n - o r b i t c o u p l i n g constant o f those atoms whose ρ o r b i t a l s make a s i g n i f i c a n t c o n t r i b u t i o n t o the mixing. The f o l l o w i n g expression f o r the e f f e c t i v e g-value z


12.

M C BRIDE E T A L .

summarizes these

EPR

211


effects: g = 2.0023 -

Σ I

2

n

where 2.0023 i s the g value f o r the f r e e e l e c t r o n ; Ψ i s the s i n g l y occupied MO; ψ i s another MO; ζ)ς i s the s p i n - o r b i t c o u p l i n g constant o f atom k; L i s the o r b i t a l angular momentum operator which r o t a t e s the ρ o r b i t a l s o f atom k about the a p p l i e d f i e l d d i r e c t i o n ; and C i s the amount o f ψ mixed i n t o φ . 0

η


k

n

c

n

-

η

L

·

The i s o t r o p i c value of the s h i f t i n g from the f r e e e l e c t r o n value shows the magnitude o f s p i n - o r b i t - f i e I d c o u p l i n g averaged over a l l f i e l d d i r e c t i o n s . I t g i v e s q u a l i t a t i v e information about whether there i s s i g n i f i c a n t s p i n d e n s i t y on atoms of high s p i n - o r b i t c o u p l i n g and whether there e x i s t low-energy vacant o r high-energy doubly occupied o r b i t a l s . The anisotropy of g shows what the extremes i n s p i n - o r b i t - f i e l d c o u p l i n g are and g i v e s information about molecular symmetry. The o r i e n t a t i o n of the g tensor i s e s p e c i a l l y v a l u a b l e , s i n c e by showing the f i e l d d i r e c t i o n s which g i v e strong and weak c o u p l i n g i t helps a s s i g n the ground s t a t e and l o w - l y i n g e x c i t e d s t a t e s o f the r a d i c a l . Such an a p p l i c a t i o n i s e x e m p l i f i e d by benzoyloxy r a d i c a l below. For r a d i c a l s i n a t r i p l e t p a i r the observed g tensor i s the average of the a p p r o p r i a t e l y o r i e n t e d tensors o f the c o n s i t i t u e n t f r e e r a d i c a l s . Thus i f the g tensor o f one member o f the p a i r i s known, t h a t of the other may be found by s u b t r a c t i n g the known tensor from twice the observed tensor. Electron-Electron S p l i t t i n g . The f i n e s t r u c t u r e s p l i t t i n g i n a r a d i c a l p a i r , u n l i k e h f s and g, i s not an average o f values f o r the separate r a d i c a l s . Since i t a r i s e s from i n t e r a c t i o n o f the r a d i c a l s , i t depends more on t h e i r s e p a r a t i o n than on t h e i r i n d i v i d u a l o r i e n t a t i o n s . To f i r s t order the f s l i n e s e p a r a t i o n i s 3D: 3D = < ( l - 3 c o s 6 ) / r > · 27,853 gauss A , 2

3

3

where r i s the d i s t a n c e between e l e c t r o n spins and θ i s the angle between the s p i n - s p i n v e c t o r and the a p p l i e d f i e l d . ( 8 ) The average i s taken over a l l p a i r s of s i t e s between the two r a d i c a l s and weighted according to the product of the s p i n d e n s i t i e s o f the sites. I f only one s i t e i s important i n each r a d i c a l the " p o i n t d i p o l e " approximation h o l d s , and the l a r g e s t s p l i t t i n g i s observed when the f i e l d l i e s along the r a d i c a l - r a d i c a l v e c t o r so t h a t the average i n the 3D expression i s - 2 / r . Spin d e r e a l i z a t i o n i n the r a d i c a l s w i l l a f f e c t 3D i n obvious ways, and c o n t r i b u t i o n s from p a i r s of s i t e s with opposite s p i n d e n s i t y may p a r t i a l l y c a n c e l c o n t r i b u t i o n s from p a i r s with the same s i g n . The 3



212

p o i n t - d i p o l e approximation overestimates the "average" s p i n - s p i n d i s t a n c e when the spins are d e l o c a l i z e d i n d i r e c t i o n s p e r p e n d i c u l a r t o t h e i r average s e p a r a t i o n . I f the s p i n d i s t r i b u t i o n and o r i e n t a t i o n o f the r a d i c a l s i s known, 3D i s r e a d i l y c a l c u l a b l e f o r any f i e l d d i r e c t i o n and I n t e r r a d i c a l vector.


9 Since (l-3cos θ) averages t o zero over a s p h e r i c a l l y symmetric s e t o f s p i n - s p i n v e c t o r s w i t h i n a s p e c i e s , D has no i s o t r o p i c value and i s unobservable by epr f o r a r a p i d l y tumbling sample. The anisotropy o f D t e l l s by how much the r e l a t i v e s p i n d i s t r i b u t i o n departs from s p h e r i c a l symmetry. I f a s p h e r i c a l d i s t r i b u t i o n i s d i s t o r t e d by s t r e t c h i n g along one d i r e c t i o n and compressing along the others t o preserve the average s p i n - s p i n s e p a r a t i o n , the s p l i t t i n g measured with the f i e l d i n the s t r e t c h i n g d i r e c t i o n becomes negative (the average cos 6 increases). S p l i t t i n g s measured with f i e l d s i n the plane p e r p e n d i c u l a r t o the s t r e t c h i n g become p o s i t i v e but, o f course, remain equal t o one another. I f the c y l i n d r i c a l symmetry i s broken by a d d i t i o n a l s t r e t c h i n g along some d i r e c t i o n i n the plane, 3D i n that d i r e c t i o n becomes l e s s p o s i t i v e . The magnitude o f the anisotropy o f D (the f a m i l i a r "D" and "Ε" values) depends on the symmetry and i n t e r r a d i c a l d i s t a n c e o f the p a i r , while the o r i e n t a t i o n o f the tensor assigns d i r e c t i o n s t o the d i s t o r t i o n s . The f s o f r a d i c a l s separated by 6 t o 10 Â i s l e s s s e n s i t i v e than h f s t o d e t a i l s o f t h e i r i n d i v i d u a l i n t e r n a l s p i n d i s t r i b u t i o n s and, u n l i k e g, i s i n s e n s i t i v e t o the energy gaps between ground s t a t e s and v a r i o u s e x c i t e d s t a t e s . Given a reasonably accurate model f o r the s p i n d i s t r i b u t i o n s and o r i e n t a t i o n s o f two r a d i c a l s , the f s g i v e s p r e c i s e i n f o r m a t i o n about the i n t e r r a d i c a l v e c t o r . F o r example the p a i r o f t r i p t y c y l r a d i c a l s mentioned above has an a x i a l l y symmetric f s with a maximum (negative) s p l i t t i n g o f 319 G. For p o i n t d i p o l e s t h i s would correspond t o a s e p a r a t i o n o f 5.58 The s p l i t t i n g s measured with an accuracy o f 2.5 G f i x the length o f t h i s v e c t o r w i t h i n 0.02 Â and i t s d i r e c t i o n w i t h i n I . The d i r e c t i o n o f maximum s p l i t t i n g l i e s w i t h i n 5° o f the symmetry a x i s o f the C-13 h f s , so the i n t e r a c t i o n should be c a l c u l a t e d f o r e l e c t r o n s i n a n t i p a r a l l e l sp^ h y b r i d o r b i t a l s r a t h e r than f o r p o i n t d i p o l e s centered on the n u c l e i . Using S l a t e r o r b i t a l s we c a l c u l a t e a l a r g e r s e p a r a t i o n o f 6.22 λ between the bridgehead n u c l e i . So the f s provides h i g h l y p r e c i s e geometrical i n f o r m a t i o n , but t o i n t e r p r e t i t p r o p e r l y we must know the s p i n d i s t r i b u t i o n w i t h i n the r a d i c a l s . 2

e

£ or

? The Benzoyloxy Dilemma

Benzoyloxy i s one questionable. Despite e l e c t r o n i c ground s t a t e Various semi-empirical

r a d i c a l f o r which the s p i n d i s t r i b u t i o n i s i t s chemical importance n e i t h e r the nor the e q u i l i b r i u m geometry i s c e r t a i n . and ab i n i t i o SCF-MO methods, with and


12.

M C BRIDE E T A L .

EPR

213


without s p i n and s p a c i a l r e s t r i c t i o n s , have been used t o study acyloxy r a d i c a l s . ( 9 ^ 1 0 ) They agree t h a t most of the s p i n d e n s i t y (in a l l cases g r e a t e r than 75%) r e s i d e s on oxygen, but how i s i t d i s t r i b u t e d between the oxygens? I f the r a d i c a l i s symmetrical (c2v)· oxygens should of course share the s p i n d e n s i t y e q u a l l y . I f , however, the r a d i c a l possesses o n l y a plane of symmetry ( C ) , as would a c a r b o x y l i c a c i d with the hydroxyl hydrogen removed, the s p i n would be unevenly d i s t r i b u t e d . F i g u r e 1 i l l u s t r a t e s the question by showing the form o f the molecular o r b i t a l s which are s i n g l y occupied i n v a r i o u s s t a t e s o f the formyloxy r a d i c a l . ( 2 0 For the C geometry the candidates are a π o r b i t a l ( a , the out-of-phase combination of the oxygen ρ o r b i t a l s ) , and two σ o r b i t a l s (a^, the in-phase, and b , the outof-phase combinations o f i n - p l a n e oxygen ρ o r b i t a l s ) . For the C geometry the π p o s s i b i l i t y i s a", a d i s t o r t e d v e r s i o n o f a , and the σ p o s s i b i l i t y i s a , a d i s t o r t e d sum o f a-^ and b . In e i t h e r geometry the o r b i t a l s which are not s i n g l y occupied are f i l l e d . Since d i s t o r t i o n from C t o C mixes a^ with b , i t i s not s u r p r i s i n g that a l l molecular o r b i t a l c a l c u l a t i o n s have p r e d i c t e d t h a t the lowest-energy Σ r a d i c a l i s the A' s t a t e . (9.'i£) energy o f the Π r a d i c a l i s l e s s s e n s i t i v e t o d i s t o r t i o n , and SCF-MO methods d i f f e r i n a s s i g n i n g i t s e q u i l i b r i u m geometry. Newton has found that even with an STO 4-31G b a s i s s e t , the C formyloxy r a d i c a l shows doublet i n s t a b i l i t y , meaning t h a t the c a l c u l a t e d energy i s minimized by an unsymmetrical e l e c t r o n d i s t r i b u t i o n . ( 1 0 ) T h i s i s p h y s i c a l l y unreasonable and i s but one of many symptoms o f the inadequacy o f single-determinant methods as a p p l i e d to t h i s system. The 5-15 kcal/mole d i f f e r e n c e s they p r e d i c t among the lowest s t a t e s and geometries vary i n s i g n as w e l l as magnitude and are c e r t a i n l y untrustworthy. At l e a s t u n t i l CI c a l c u l a t i o n s become a v a i l a b l e , ( 1 0 ) we can depend on theory o n l y to give us the form o f the o r b i t a l s i n the v a r i o u s s t a t e s . Only experimental data would allow a c o n f i d e n t choice between a Σ s t a t e with most o f the s p i n on one oxygen and a Π s t a t e with equal s p i n on both. To understand such chemical p r o p e r t i e s o f the benzoyloxy r a d i c a l as the r a t e s o f d e c a r b o x y l a t i o n and of oxygen scrambling one should know i t s e l e c t r o n i c s t r u c t u r e . We had hoped by studying the epr s p e c t r a o f p a i r s i n c l u d i n g t h i s r a d i c a l t o determine i t s ground s t a t e e x p e r i m e n t a l l y . t

n

e


g

2 v

2

2

s

2

1

2

2 v

s

2

2

T

h

e

2 v

A c e t y l Benzoyl Peroxide Methyl Motion and the D Tensor. P h o t o l y s i s o f c r y s t a l l i n e a c e t y l benzoyl peroxide with 0-18 i n the peroxy p o s i t i o n s g i v e s methyl benzoate with most of the 0-18 bound to methyl r a t h e r than scrambled between the carbonyl and ether p o s i t i o n s . ( 1 1 ) In attempting to understand t h i s d i s c r i m i n a t i o n we s t u d i e d the epr spectrum o f the methyl-benzoyloxy r a d i c a l p a i r (M-B) trapped i n a photolyzed c r y s t a l of the peroxide a t low temperature.(3)




214

The f i r s t row o f Table I expresses i n s e v e r a l ways the two items o f n o n - o r i e n t a t i o n a l information from the e l e c t r o n - e l e c t r o n f s of M-B. The f i r s t two e n t r i e s , Ζ and X, are the extreme values o f s p l i t t i n g ( i n gauss) and correspond t o the maximum and minimum components, r e s p e c t i v e l y , o f the average s p i n - s p i n d i s t a n c e . The t h i r d entry, Y, i s the s p l i t t i n g i n the t h i r d orthogonal d i r e c t i o n , and i t s d i f f e r e n c e from X shows by how much the d i s t r i b u t i o n departs from a x i a l symmetry. The next two e n t r i e s express t h i s same information i n wave numbers as the D and Ε parameters. The l a s t entry i s the s e p a r a t i o n ( i n &) between p o i n t d i p o l e s which would give the same Z. As d i s c u s s e d above t h i s provides an upper l i m i t t o the d i s t a n c e between d e l o c a l i z e d s p i n s .

Table I . E l e c t r o n - E l e c t r o n S p l i t t i n g s o f R a d i c a l P a i r s . Pair M-B M-P P-B P-P B-B B-B'

Ζ -375 -259 -329 -197 -1256 -1320

X

Y

D

Ε

Dist.

199 135 173 98 943 1024

176 125 156 92 313 296

0.0175 0.0121 0.0154 0.0092 0.0587 0.0617

0.0003 0.0001 0.0003 0.0001 0.0098 0.0113

5.30 5.99 5.53 6.56 3.54 3.48

More o f the f s information i s presented i n F i g u r e 2D, where a heavy arrow i n d i c a t i n g the d i r e c t i o n o f maximum s p i n separation (Z) i s shown r e l a t i v e t o the framework o f the peroxide p r e c u r s o r i n two orthogonal p r o j e c t i o n s . The length o f t h i s arrow i s 5.30 Â, corresponding t o the p o i n t d i p o l e l i m i t f o r the observed s p l i t t i n g . The forked l i n e i n d i c a t e s the d i r e c t i o n o f minimum s p i n - s p i n separation (X). Since the s i t e o f s p i n i n methyl i s obvious, we could use the arrow t o l o c a t e the r a d i c a l r e l a t i v e t o benzoyloxy, i f we knew the s p i n d i s t r i b u t i o n and o r i e n t a t i o n o f the l a t t e r . On the assumption that benzoyloxy i s C and t h a t i t i s immobile i n the c r y s t a l , we suggested e a r l i e r t h a t the methyl r a d i c a l moves from i t s p o s i t i o n i n s t a r t i n g peroxide by 2.4 Â. The motion can be v i s u a l i z e d approximately i n F i g u r e 2 by t r a n s l a t i n g the heavy arrow so t h a t i t s o r i g i n i s centered between the oxygens o f benzoyloxy. Benzoyloxy d e r e a l i z a t i o n would shorten the p o i n t d i p o l e arrow by about 0.1 λ . While t h i s motion seems t o e x p l a i n the preference f o r methyl attack a t the o r i g i n a l p e r o x i d i c oxygen, i t i s s u r p r i s i n g i n l i g h t of the f s o f the methyl-phenyl r a d i c a l p a i r (M-P), which can be generated from M-B by p h o t o l y s i s a t 800 nm. T h i s f s i s summarized i n the second entry o f Table I , and the p o i n t d i p o l e v e c t o r i t p r e d i c t s i s i l l u s t r a t e d by the l i g h t arrow i n F i g u r e 2D. The o r i g i n o f t h i s arrow i s more c e r t a i n than t h a t o f M-B, s i n c e the 2 v



12.

M C BRIDE E T A L .

EPR


or

IT

Figure 1. Orbitals that might be singly occu pied, low energy states of an acyloxy radical in the C and C geometries. The plane of the drawing is the nodal plane of the it orbitals. The most likely candidates for single occu pancy in the ground state are a (v) and α! (σ). 2v

8

2

c h ^ D Figure

οχ 9

2.

M-B and M-P tensors relative to acetyl ben zoyl peroxide. The two orthogonal projections (above and below) show the orientation of the D tensors of M-B and M-P (left) and of the g tensor of benzoyloxy (right). D. Open circles denote oxygen atoms; a filled circle, the methyl group. The heavy line with a forked tail shows the direction of minimum spin-spin separation of M-B. The heavy arrow represents the point dipole approximation to the spin-spin vector of M-B in length and direction. The light arrow presents the same information for M-P. g. The principal directions for the g shift of benzoyloxy are denoted by heavy vectors 2-A long. The filled and open triangles denote the most and least positive g shifts, respectively (see Table II).


215


216

phenyl r a d i c a l i s mostly l o c a l i z e d . (13) The arrow's head f a l l s very near the o r i g i n a l methyl p o s i t i o n . I f methyl moves by 2.4 Â i n forming M-B, i t must move back again when benzoyloxy decarboxyl a t e s . T h i s coincidence i s conceivable, but s u s p i c i o u s . The geometry of M-P would have been l e s s s u r p r i s i n g i f we had assigned benzoyloxy a C Σ e l e c t r o n i c s t a t e w i t h most o f i t s s p i n d e n s i t y on the oxygen which was o r i g i n a l l y c a r b o n y l . The o r i g i n of the heavy arrow would then have been as shown i n F i g u r e 2D, and methyl would undergo only modest displacements from peroxide to M-B to M-P. I f the C Σ benzoyloxy r a d i c a l had i t s s p i n on the oxygen which was o r i g i n a l l y peroxy, the arrow would o r i g i n a t e on t h a t oxygen, and the excursion o f the methyl r a d i c a l would be even l e n g t h i e r than f o r the Π. I f benzoyloxy r a d i c a l had a Σ ground s t a t e , one might expect the two geometries t o c o e x i s t and give a doubling o f the peaks i n the epr spectrum, which we have not observed. But the l a t t i c e f o r c e s which impose a 5-6 kcal/mole b a r r i e r on r a d i c a l recombination could e a s i l y b i a s an e q u i l i b r i u m toward one of the Σ geometries. I f we wished t o be parsimonious about the amount o f methyl motion i n the c r y s t a l , we would choose the Σ benzoyloxy r a d i c a l with s p i n on the o r i g i n a l carbonyl oxygen, although the d i r e c t i o n of minimum s p i n - s p i n d i s t a n c e i s more c o n s i s t e n t with the Π than with t h i s Σ r a d i c a l .


g

s

The cf. Tensor o f Benzoyloxy. By s u b t r a c t i n g the i s o t r o p i c g value of methyl r a d i c a l from twice the g tensor of M-B, we determined an approximate g tensor f o r benzoyloxy. S h i f t s o f i t s p r i n c i p a l values from 2.0023 are presented i n the f i r s t row o f Table I I . Since the s h i f t s are p o s i t i v e , f i l l e d o r b i t a l s must predominate over vacant ones i n mixing with the s i n g l y occupied orbital. Table I I . Partner CH » Ph-

94 99

3

PhC0 2

a)

iso

86(89)

Benzoyloxy g S h i f t s .

a

Ζ

Y

X 205 221 207(239) 212(209)

18 15 -64 (-58) 9(20)

59 61 113(83) 37(36)

4

g s h i f t = (g - 2.0023) χ 10 , where g i s c a l c u l a t e d from twice the g tensor observed f o r the r a d i c a l p a i r of benzoyloxy w i t h a p a r t n e r minus the g tensor of the p a r t n e r . The p a r e n t h e t i c a l values i n the PhC0 * e n t r i e s are f o r Β-Β', the others f o r B-B. The l a s t two rows correspond to d i f f e r e n t tensor assignments, one of which i s i n c o r r e c t , as explained under The B-B P a i r s i n the t e x t . 2


12.

M C BRIDE E T

EPR

AL.

217


How the g s h i f t s help choose the ground s t a t e can be seen by examining Figure 1. The C Π s t a t e ( A ) would be s t r o n g l y mixed with B by a f i e l d i n the oxygen-oxygen d i r e c t i o n , s i n c e the oxygen o r b i t a l s of a r o t a t e d about t h a t a x i s give high o v e r l a p with those of b2 (they have the wrong phase to o v e r l a p with a i ) . A f i e l d along the molecular symmetry a x i s would i n p r i n c i p l e mix a^ with a , but i n f a c t the shape of the SL^ o r b i t a l i s such t h a t very l i t t l e overlap would r e s u l t , and only a small g s h i f t would be expected. A f i e l d p e r p e n d i c u l a r to the molecular plane would not mix the Π s t a t e with any other s t a t e , and would g i v e no g shift. Thus a Π r a d i c a l should d i s p l a y a l a r g e g s h i f t along the 0-0 d i r e c t i o n , a small g s h i f t along the symmetry a x i s , and no g s h i f t p e r p e n d i c u l a r to the nuclear plane. The C Σ s t a t e ( Α·) would be s t r o n g l y mixed with A" by a f i e l d along the bond connecting the r a d i c a l oxygen to the carboxyl carbon. A f i e l d p e r p e n d i c u l a r to the bond and i n the n u c l e a r plane would not mix the A* s t a t e , since i t would l i e along the a x i s of the spin-bearing ρ o r b i t a l of oxygen. A f i e l d perpendi c u l a r to the plane would mix A' with higher-energy Σ s t a t e s . Thus the Σ r a d i c a l would show a strong g s h i f t along the C-0 s i n g l e bond, a weak s h i f t p e r p e n d i c u l a r to the plane, and no s h i f t i n the t h i r d orthogonal d i r e c t i o n . The o r i e n t a t i o n of the g tensor shown i n F i g u r e 2g i s j u s t what would be expected f o r the A* benzoyloxy r a d i c a l with s p i n on the o r i g i n a l peroxy oxygen. Kim, K i k u c h i , and Wood have used t h i s agreement to argue f o r a Σ benzoyloxy r a d i c a l with s p i n on t h i s oxygen. (14) T h e i r i n t e r p r e t a t i o n may w e l l be c o r r e c t , but i t i s d i s t u r b i n g that t h i s assignment i s the one f o r which the D tensor r e q u i r e s maximum motion of the methyl r a d i c a l . As we suggested e a r l i e r , the Σ r a d i c a l with s p i n on the o r i g i n a l carbonyl oxygen, which gives the least-motion i n t e r p r e t a t i o n of the D tensor, gives the worst agreement with the g tensor.(3) Thus the D and g tensors give c o n f l i c t i n g i n d i c a t i o n s o f the geometry of M-B and of the ground s t a t e f o r benzoyloxy. To decide on a r a d i c a l p a i r geometry which w i l l help us understand the s o l i d s t a t e r e a c t i o n we must r e s o l v e t h i s c o n f l i c t . Unfortunately the a d d i t i o n a l data we have been able to c o l l e c t have served more t o sharpen the c o n f l i c t than to r e s o l v e i t . 2 v

2

2

2

2


2

2

2

s

2

2

2

Dibenzoyl

Peroxide.

The P-B P a i r . P a r t l y to assess the r e l i a b i l i t y o f the experimental tensors f o r M-B, we determined D and g tensors f o r the analogous phenyl-benzoyloxy p a i r (P-B) i n photolyzed d i b e n z o y l peroxide. This s p e c i e s , l i k e M-P, was f i r s t reported by Lebedev. (15) We used perdeuterated c r y s t a l s to remove phenyl h f s from the h i g h l y overlapped s p e c t r a of four symmetry-related v e r s i o n s of each r a d i c a l p a i r (there would have been e i g h t , each corresponding to l o s s of one of the C 0 groups i n the Ρ2^2^2^ u n i t c e l l , i f i t were not f o r an approximate, n o n - c r y s t a l l o g r a p h i c 2


ORGANIC F R E E


218

RADICALS

two-fold symmetry a x i s , which almost converts a screw a x i s t o a t r a n s l a t i o n ) . Box has determined the D tensor f o r the phenylphenyl p a i r (P-P) i n d i b e n z o y l peroxide. (16) F i g u r e 3 presents our D and g tensors f o r P-B, and Box's D tensor f o r P-P i n the same format as F i g u r e 2. Comparison o f F i g u r e s 2 and 3 and o f the corresponding data i n Tables I and I I shows t h a t the tensors o f the p a i r s with methyl and phenyl as p a r t n e r have almost the same magnitude and the same o r i e n t a t i o n i n the molecular frame, although the c r y s t a l l i n e environments are completely d i f f e r e n t . T h i s i n c r e a s e s our confidence i n the experimental t e n s o r s . More importantly i t r e q u i r e s t h a t whatever motion methyl undergoes i n a c e t y l benzoyl peroxide, phenyl must undergo i n d i b e n z o y l peroxide. Far from r e s o l v i n g our dilemma about the ground s t a t e o f benzoyloxy r a d i c a l , t h i s a d d i t i o n a l i n f o r m a t i o n has made i t more acute. I f we favor the Σ r a d i c a l with s p i n on the o r i g i n a l peroxy oxygen, which i n i t s i n i t i a l o r i e n t a t i o n i s c o n s i s t e n t w i t h the g tensor, the D tensor f o r c e s us to accept t h a t i n t h e i r d i f f e r e n t c r y s t a l s M-B and P-B p a i r s undergo i d e n t i c a l l a r g e (about 3 A) displacements r e l a t i v e to the peroxide geometry, although the M-P and P-P p a i r s have geometries very s i m i l a r to those i n the peroxide. I f we favor the Π r a d i c a l , the g tensor i s anomalous and no longer a t t r i b u t a b l e t o experimental e r r o r , and the methyl or phenyl e x c u r s i o n i s s t i l l s u b s t a n t i a l . Again the Σ r a d i c a l with s p i n on the o r i g i n a l carbonyl carbon allows minimal r a d i c a l motion but seems u n l i k e l y t o have the observed g t e n s o r . The B-B P a i r s . While measuring the s p e c t r a o f P-B we n o t i c e d weak doublets with g r e a t e r e l e c t r o n - e l e c t r o n s p l i t t i n g than P-B and no h f s , even i n undeuterated samples. T h e i r strong g anisotropy, r e s i s t a n c e to s a t u r a t i o n a t low temperature, and r a p i d decay a t 35 Κ (P-B can be observed a t 63 K) confirmed our i n f e r e n c e t h a t they were due t o benzoyloxy-benzoyloxy p a i r s (B-B). Box has mentioned observing B-B a f t e r s u b j e c t i n g a s o l i d s o l u t i o n of d i b e n z o y l peroxide i n d i b e n z o y l d i s u l f i d e t o i o n i z i n g r a d i a t i o n . His B-B recombined at a few degrees above 4.2 K."(16) A f t e r b r i e f p h o t o l y s i s at 20 Κ and c o o l i n g t o 6 Κ we observed e i g h t doublets r a t h e r t h a t the four r e q u i r e d by symmetry f o r a s i n g l e s p e c i e s . Thus there are two d i f f e r e n t metastable arrangements of the B-B p a i r . One p a i r , which we denote B-B', converts r a p i d l y to the other on warming to 25 K. We determined D and g tensors f o r both B-B* and B-B i n hopes o f answering two questions. What can these p a i r s t e l l us about the ground s t a t e of benzoyloxy? How can r a d i c a l p a i r s be s t a b l e when they are not separated by an i n t e r v e n i n g molecule, such as CO^ o r 1^? Since the c r y s t a l l i n e u n i t c e l l contains four symmetryr e l a t e d molecules, s p e c t r a o f four B-B o r i e n t a t i o n s are measured simultaneously i n one c r y s t a l mounting. However t h i s experimental e f f i c i e n c y comes a t a high p r i c e , because there i s no d i r e c t method f o r determining which o f four symmetry-related tensors goes M



12.

M C BRIDE E T A L .

219

EPR Studies of Radical Pairs

with a p a r t i c u l a r molecular o r i e n t a t i o n . The p r e v i o u s r a d i c a l p a i r s were s u f f i c i e n t l y separated that the p o i n t d i p o l e approximation permitted a unique choice o f the c o r r e c t t e n s o r . For B-B (and B-B') two of the four choices may be excluded as g i v i n g s p i n - s p i n extension i n d i r e c t i o n s n e a r l y orthogonal t o reasonable ones. The remaining two p o s s i b i l i t i e s are p l o t t e d i n F i g u r e s 4 and 5 f o r B-B. P l o t s f o r B-B' analogous t o F i g u r e s 4 and 5 would be p r a c t i c a l l y i n d i s t i n g u i s h a b l e from 4 and 5, except i n the end-on view o f 5, where the B-B' v e c t o r s are i n c l u d e d and i n d i c a t e d by squares. In both f i g u r e s we have s t a r t e d the s p i n - s p i n arrow from the carbonyl oxygen, which seemed t o be the best o r i g i n f o r the M-B and the P-B p a i r s i n terms of p r e d i c t i n g minimal motion. As before the arrow should terminate near the center of s p i n d e n s i t y o f the o t h e r r a d i c a l i n the p a i r , which i n t h i s case i s a l s o benzoyloxy. The g tensors o f the second benzoyloxy i n B-B and B-B' were c a l c u l a t e d as b e f o r e by s u b t r a c t i n g the g tensor o f the f i r s t (assumed e q u i v a l e n t t o t h a t found from the P-B p a i r ) from twice the observed tensor. The next to l a s t row o f Table I I corresponds t o the tensor assignment o f Figure 4, and the l a s t row to t h a t of F i g u r e 5. The arrow o f Figure 4D suggests t h a t the s p i n d e n s i t y i n the second benzoyloxy r a d i c a l , as i n the f i r s t , r e s i d e s mostly near the o r i g i n a l carbonyl oxygen. I t does r e q u i r e some motion o f t h i s atom, which could be achieved by t w i s t i n g the carboxy group about the C-phenyl bond. The same sense o f r o t a t i o n i s suggested by the o r i e n t a t i o n of the g tensor f o r t h i s r a d i c a l . The magnitudes o f the p r i n c i p a l g s h i f t s (Table II) are s i m i l a r t o those o f the previous benzoyloxy r a d i c a l s , but d i f f e r from them by much more than they d i f f e r from one another. I f t h i s i s the c o r r e c t tensor assignment, the s t a b i l i t y o f the B-B p a i r could perhaps be a t t r i b u t e d t o the t w i s t , which separates the peroxy oxygens enough t h a t t h e i r bonded a t t r a c t i o n cannot overcome a l a t t i c e b a r r i e r t o the t w i s t back. The a t t r a c t i o n may be p a r t i c u l a r l y weak i f most of the s p i n r e s i d e s on the o r i g i n a l carbonyl oxygens. B-B and B-B' are remarkably s i m i l a r , d i f f e r i n g by l e s s than 0.4 λ i n p o i n t d i p o l e vectors and 6° i n g o r i e n t a t i o n . The source o f a b a r r i e r between two such s i m i l a r geometries i s not obvious. As f o r the M-B and P-B p a i r s , the D and g tensors suggest s p i n l o c a l i z a t i o n on d i f f e r e n t oxygens. The tensor assignment o f F i g u r e 5 suggests t h a t the average s p i n p o s i t i o n o f the second benzoyloxy r a d i c a l i s very near the carbonyl carbon, as would be expected f o r a C radical. Furthermore the g s h i f t s f o r t h i s r a d i c a l i n B-B are p r e c i s e l y those t h a t would be p r e d i c t e d f o r an almost unmoved Π s t a t e , both i n magnitude and i n o r i e n t a t i o n . The g o r i e n t a t i o n o f the corresponding r a d i c a l i n B-B' d i f f e r s from t h i s by r o t a t i o n o f I I about the C-phenyl bond, as shown i n the end-on view o f F i g u r e 5. I t i s remarkable t h a t the p r i n c i p a l v a l u e s o f these g tensors are so s i m i l a r t o those of the other r a d i c a l , i f t h a t r a d i c a l i s i n a Σ s t a t e . I f t h i s assignment i s c o r r e c t , the b a r r i e r t o recombi2 v

e


ORGANIC FREE RADICALS


220

D

g

Figure 3. P-B and P-P tensors relative to dibenzoyl peroxide. Presentation analogous to that of Figure 2, except that the double-headed arrow representing the spin-spin vector of P-P is centered between the substituted ring carbons of the peroxide.

Figure 4. B-B tensors relative to dibenzoyl peroxide. Tensor assignment of the next-to-last row in Table II. Presentation analogous to Figure 2, except that the middle diagram in the g column is an end-on view obtained by rotating the lower diagram by 90° to the right about a vertical axis in the page.



12.

M C BRIDE E T A L .


Figure 5. B-B tensors relative to dibenzoyl peroxide. Tensor assignment of the last row in Table II. Presentation analogous to Figure 4, except that the end-on view includes the g tensor of B-B' denoted by squares instead of triangles.


221


222

n a t i o n o f the r a d i c a l s may be due t o a f a i l u r e of a Σ-Π r a d i c a l p a i r s t a t e t o c o r r e l a t e with the ground s t a t e o f peroxide.


Summary and

Conclusions

Although we have now measured D and g tensors f o r f o u r d i f f e r e n t r a d i c a l p a i r s c o n t a i n i n g the benzoyloxy r a d i c a l , we cannot yet be c e r t a i n of the r a d i c a l ' s e l e c t r o n i c s t a t e . I f we consider only the g t e n s o r s , we would f e e l c o n f i d e n t t h a t the r a d i c a l i n the M-B and P-B p a i r s i s i n a Σ s t a t e with i t s s p i n on the oxygen which was o r i g i n a l l y p e r o x i d i c . However, the D tensors would then i n d i c a t e t h a t the other r a d i c a l , methyl or phenyl, must undergo s u r p r i s i n g l y l a r g e (and s i m i l a r ) motion a f t e r the f i r s t d e c a r b o x y l a t i o n but r e t u r n to i t s i n i t i a l l o c a t i o n a f t e r the second. I f we considered only the D tensors o f these p a i r s , we would conclude that the benzoyloxy i s i n a Σ s t a t e w i t h s p i n on the o r i g i n a l carbonyl oxygen, and t h a t very l i t t l e r a d i c a l motion occurs during the d e c a r b o x y l a t i o n s . The g tensors do not seem c o n s i s t e n t with t h i s i n t e r p r e t a t i o n . An unmoved Π benzoyloxy r a d i c a l would not g i v e such f l a g r a n t disagreement with one o f the tensors as one o r the other o f the Σ r a d i c a l s does. Neither would i t g i v e such good agreement w i t h the other tensor. We could cease t r y i n g to understand the nature o f the benzoyloxy r a d i c a l i n M-B and P-B and assume only t h a t i t s s p i n i s near the o r i g i n a l s i t e of the carbonyl oxygen and that i t has the observed g tensor. T h i s might r e s u l t from some u n a n t i c i p a t e d motion or d i s t o r t i o n o f the r a d i c a l . I f we then r i s k supposing t h a t one o f the benzoyloxy r a d i c a l s o f B-B has these same p r o p e r t i e s , we may determine the g tensor and s p i n l o c a t i o n of the other. The same i s true o f B-B'. In e i t h e r instance one o f two p l a u s i b l e i n t e r p r e t a t i o n s i s i n c o r r e c t . E i t h e r 1) the second r a d i c a l has the same anomalous p r o p e r t i e s as the f i r s t and has i t s carboxyl group t w i s t e d about the C-phenyl bond, or 2) the second r a d i c a l i s i n a Π e l e c t r o n i c s t a t e and i s e s s e n t i a l l y unmoved i n B-B but t w i s t e d by 11° about the C-phenyl bond i n B-B'. Obviously we must do much more work before c l a i m i n g t h a t we understand acyloxy r a d i c a l s and t h e i r behavior i n s o l i d s . Still the work d e s c r i b e d above shows t h a t s i n g l e - c r y s t a l epr s p e c t r o scopy of r a d i c a l p a i r s can provide unique, i f not always unambigu ous, information about the s t r u c t u r e and chemistry o f f r e e radicals. Acknowledgments T h i s work has been supported by the t i o n (DMR 76-01996) and by a C a m i l l e and Scholar Grant. We thank Dr. M a r s h a l l D. E. Wood f o r h e l p f u l d i s c u s s i o n s o f t h e i r

N a t i o n a l Science Founda Henry Dreyfus TeacherNewton and P r o f e s s o r D. unpublished work.


12.

M C BRIDE E T A L .


223


Abstract Low-temperature epr spectra yield A, g, and D tensors for radical pairs generated by uv photolysis of single crystals of diacyl peroxides. The precise, but sometimes ambiguous, information these tensors contain about the structure of the radicals and about their arrangement in pairs is discussed with examples. Pairs including a benzoyloxy radical together with methyl, phenyl, or another benzoyloxy radical are perplexing in that D tensors seem to indicate that electron spin resides mostly on the original carbonyl oxygen, while g tensors seem to indicate that most of the spin resides on the original peroxy oxygen. The radicals in the benzoyloxy-benzoyloxy pair may be i n different electronic states. Literature Cited

1.

Nelsen, S. F., and Bartlett, P. D., J . Am. Chem. Soc., (1966), 88, 137. 2. Bartlett, P. D., and McBride, J . Μ., Pure Appl. Chem., (1967), 15, 89. 3. Karch, N. J., Koh, E. T., Whitsel, B. L . , and McBride, J . Μ., J. Am. Chem. Soc., (1975), 97, 6729. 4. Jaffe, A. B., Skinner, K. J., and McBride, J . M., J . Am. Chem. Soc., (1972), 94, 8510. 5. Wertz, J . Ε., and Bolton, J . R., "Electron Spin Resonance," McGraw Hill, New York (1972), Table C. 6. Reichel, C. L . , and McBride, J . M., J . Am. Chem. Soc., (1977), 99, 6758. 7. Carrington, Α., and McLachlan, A. D., "Introduction to Magnetic Resonance," Harper and Row, New York, 1967 Ch. 9. 8. Ibid., Ch. 8. 9. Kikuchi, Ο., Tetrahedron Lett., (1977), 2421, and references therein. 10. Newton, M. D., personal communication and work in progress. 11. Karch, N. J., and McBride, J . M., J . Am. Chem. Soc., (1972), 94, 5092. 12. Whitsel, B. L . , Ph. D. Dissertation, Yale Univ., (1977). 13. Kasai, P. H., Clark, P. Α., and Whipple, Ε. B., J . Am. Chem. Soc., (1970), 92, 2640. 14. Yim, M. B., Kikuchi, O., and Wood, D. Ε., J . Am. Chem. Soc., in press. 15. Barchuk, V. I., Dubinsky, Α. Α., Grinberg, O. Ya., and Lebedev, Ya. S., Chem. Phys. Lett., (1975), 34, 476 16. Box, H. C., Budzinski, Ε. Ε., and Freund, H. G., J . Am. Chem. Soc., (1970), 92, 5305. RECEIVED December 23, 1977.


EPR Studies of Radical Pairs. The Benzoyloxy Dilemma. - ACS

Recommend Documents