2 R a d i c a l P a i r Reactions i n M i c e l l a r Solution i n the Presence and Absence of M a g n e t i c F i e l d s
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NICHOLAS J. TURRO, JOCHEN MATTAY, and GARY F. LEHR Columbia University, Chemistry Department, New York, NY 10027 The photolyses of dibenzyl ketones in aqueous micellar solution have been shown to greatly enhance both geminate radical pair recombination and the enrichment of C in recovered ketone compared to homogeneous solution. These observations have been attributed to the combined effects of the reduced dimensionality imposed by micellization and hyperfine induced intersystem crossing in the geminate radical pairs. This latter effect is the basis of Chemically Induced Dynamic Nuclear Polarization (CIDNP), a phenomenon which is well known in homogeneous solution. The photolyses of 1,2-diphenyl-2-methyl-1-propanone and its H and C derivatives in micellar solution are now described and further demonstrate the enhanced cage and magnetic isotope effects of micellization. We report also the observation of CIDP during the photolyses of micellar solutions of several ketones, and demonstrate the validity of the radical pair model to these systems. Analyses of the CIDNP spectra in the presence and absence of aqueous free radical scavengers (e.g., Cu ) allow us to differentiate between radical pairs which react exclusively within the micelle and those that are formed after diffusion into the bulk aqueous phase. In some cases this allows us to estimate a lifetime associated with the exit of free radicals from the micelles. 13
2
13
2+
As evidenced by this symposium, the use of micelles and other organized assemblies to control the selectivity of chemical reactions has recently attracted much attention. In most of these cases, micelles or vesicles have been used as a means of separating charged intermediates formed by electron transfer reactions, thereby preventing the back reaction. The effects of the micelle or vesicle are usually dramatic. 0097-6156/82/0177-0019$05.00/O © 1982 American Chemical Society Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
20
INORGANIC REACTIONS I N ORGANIZED M E D I A
The m i c e l l e can a l s o be used to enhance the r e a c t i o n probabi l i t y o f intermediates i f they a r e seqestered i n s i d e the m i c e l l e . In t h i s case, the m i c e l l e a c t s as a r e a c t i o n v e s s e l with molecular dimensions. Below we w i l l d e s c r i b e some of our r e s u l t s on the e f f e c t of m i c e l l i z a t i o n on r a d i c a l p a i r r e a c t i o n s . We w i l l show as w e l l that the e f f e c t s o f m i c e l l i z a t i o n can be d r a m a t i c a l l y a l t e r e d by the a p p l i c a t i o n o f small e x t e r n a l magnetic f i e l d s .
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The
Cage E f f e c t
When l,2-diphenyl-2-methyl-l-propanone, J., i s i r r a d i a t e d with UV l i g h t , the primary photochemical process leads to α-cleavage from the lowest ^η,π* s t a t e to give a geminate t r i p l e t r a d i c a l pair.(A>^) In homogeneous s o l u t i o n these fragments begin to sepa r a t e ( i o - i O s ) , ® and lead e v e n t u a l l y to products which a r i s e p r i m a r i l y from scavenging o r termination r e a c t i o n s of f r e e r a d i c a l s (Scheme I ) . Table I l i s t s the chemical and quantum y i e l d s f o r these products i n benzene, a c e t o n i t r i l e and methylene c h l o r i d e . A l s o l i s t e d i n Table I a r e the corresponding y i e l d s of products when the p h o t o l y s i s i s conducted i n c a t i o n i c , a n i o n i c or non-ionic aqueous m i c e l l a r s o l u t i o n . In these cases, the higher y i e l d s of products which could a r i s e from the d i s p r o p o r t i o n a t i o n of the geminate r a d i c a l p a i r , 2 and _3, as w e l l as the lower quantum y i e l d s f o r disappearance of 1^ lead us to suspect that a substan t i a l i n c r e a s e i n cage r e a c t i o n occurs on going from homogeneous to m i c e l l a r environments. We have found that a d d i t i o n of Cu(II) to aqueous s o l u t i o n s of HDTCL leads to the s e l e c t i v e s c a v e n g i n g of f r e e r a d i c a l s which have entered the aqueous phase. P h o t o l y s i s of JL under these c o n d i t i o n s leads to the r e s u l t s l i s t e d i n Table I I . These r e s u l t s c l e a r l y i n d i c a t e the enhanced cage e f f e c t induced by p h o t o l y s i s of JL i n m i c e l l a r s o l u t i o n . From the copper scavenging r e s u l t s ( A ^ the cage e f f e c t i n HDTCL i s c a l c u l a t e d to be 30% (% cage = moles of 2 or 3/moles o f _1 consumed). 2 and _3 appear to be formed e x c l u s i v e l y w i t h i n the m i c e l l e by d i s p r o p o r t i o n a t i o n of the geminate r a d i c a l p a i r . Since cage d i s p r o p o r t i o n a t i o n i n v o l v e s r e a c t i o n of a s i n g l e t r a d i c a l p a i r , cage r e a c t i o n cannot occur p r i o r to i n t e r system c r o s s i n g (ISC) of the i n i t i a l l y geminate t r i p l e t p a i r , the r a t e of escape of the r a d i c a l s from the m i c e l l e must be slower than the r a t e of ISC. These r e s u l t s a r e i n good agreement w i t h the observed cage e f f e c t s o f other ketones s t u d i e d i n our l a b o r a tory.® (
The R a d i c a l P a i r Mechanism A simple model which accounts f o r the observed cage e f f e c t s ® i n m i c e l l a r s o l u t i o n i s diagrammed i n Figure 1. F r a g mentation of a t r i p l e t molecular species (produced by l i g h t absorption to form a s i n g l e t followed by ISC to a t r i p l e t ) pro ceeds along the r e p u l s i v e energy p o t e n t i a l u n t i l the fragments a r e
Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
TURRO
ET AL.
Radical Pair Reactions in Micellar Solution
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2.
Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
21
Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
H
6 6 3.3 6
0.10 0.18 0.16
10 18 22
0.95 0.97 0.73 0.73
a)
(b)
(C)
17
0.19
22
0.88
0.03 0.02
4.0 2.8 2.6
0.11 0.11 0.15
0.02
0.03
3.5
0.16
—
1
>
I
(c) Dodecyltrimethylainmonium chloride (d) Sodium dodecyl sulfate
>
% g
Ω
3
17
0.19
30
0.67
— — —
0.02
2.2
0.16
§
15
0.17
24
0.75
0.07 7
0.18
18
0.06
0.09
9
0.17
18
0.03
0.07
8
0.18
21
0.03
Φ
5 %
Φ
4 %
Φ
(b) Hexadecyltrimethylammonium bromide
23
0.19
27
3
to to
(a) Hexadecyltrimethylammonium chloride
B r i j 35
SDS
W )
DDTC1
HDTBr
23
3.4
0.13
15
%
0.87
HDTC1*
3
CH CN
C
Φ
%
+-ket
2
Relative Yields (based on ketone consumed) and Quantum Yields for the Photo Products of JL i n Homogeneous and Micellar Solutions.
Solvent or Detergent
Table I»
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Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
0.20
30
0.21
0.0
0.00
Yields of scavenging products: benzoic acid, >33% (φ >_0.23); α,α dimethylbenzyl chloride, 26% (φ = 0.18); a,a dimethylraethyl benzyl alcohol, 23% (φ = 0.16).
29
(b)
M).7
Not measured.
(b>
(a)
5
trace (a)
0.20
(a)
0.20
(a)
4
M).7
0.01 (a)
0.19
(a)
0.20
(a)
2
M).7
0.015
(a)
0.21
(a)
0.19
(a)
M).7
0.4
0.02
2.2
0.16
23
0.16
22
M).73
0
Cu(II)/l
Table i l . Relative Yields (based on ketone consumed) and Quantum Yields for the Photo Products of 1 i n HDTC1 Micellar Solution i n the Presence of CuCl
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INORGANIC REACTIONS IN ORGANIZED MEDIA
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24
Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
2.
TURRO E T
AL.
Radical Pair Reactions in Micellar Solution
25
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separated to a d i s t a n c e where the exchange i n t e r a c t i o n of the un p a i r e d e l e c t r o n s i s n e g l i g i b l e . At t h i s p o i n t , the s i n g l e t (S) and t r i p l e t (T) energies are e s s e n t i a l l y degenerate and ISC w i t h i n the r a d i c a l p a i r can occur. In homogeneous s o l u t i o n the random movement of the r a d i c a l fragments most o f t e n leads to i r r e v e r s i b l e s e p a r a t i o n of the p a i r , ® and we t h e r e f o r e expect most of the products to a r i s e from u n c o r r e l a t e d f r e e r a d i c a l s . The enhanced cage e f f e c t i n m i c e l l a r s o l u t i o n s a r i s e s because the chemical p o t e n t i a l of the micelle-water i n t e r f a c e provides a r e f l e c t i n g w a l l i n the r e g i o n of the S and Τ degeneracy. The r a d i c a l fragments are prevented from d i f f u s i n g beyond s e v e r a l angstroms from t h e i r o r i g i n a l l o c a t i o n , and the chances of a reencounter are t h e r e f o r e increased. The r a d i c a l p a i r thus experiences a longer period of time to e f f e c t ISC from Τ to S. The mechanism by which S and Τ can i n t e r c o n v e r t i s based on the r a d i c a l p a i r model of Chemically Induced Dynamic Nuclear Polarization (CIDNP).® According to t h i s theory, n u c l e a r mag n e t i c moments which are coupled to the unpaired e l e c t r o n s by hyperf i n e i n t e r a c t i o n s ( A f ) can a l t e r the r a t e at which the e l e c t r o n spins l o s e t h e i r phase coherence, i . e . , undergo ISC. In the p r e s ence of an a p p l i e d magnetic f i e l d greater than the h y p e r f i n e s p l i t t i n g s , the components of the t r i p l e t s u r f a c e s p l i t i n t o T+, T and T_ l e v e l s , i n which the degeneracy of T+ and S i s removed (Figure 2). The e f f e c t of a strong a p p l i e d f i e l d then i s to i n h i b i t ISC of ^2/3 of the t r i p l e t r a d i c a l p a i r s from the ensemble which could (at zero f i e l d ) ISC to S. Several p r e d i c t i o n s based on the r a d i c a l p a i r m o d e l ® can be made with regard to the p r o b a b i l i t y of cage r e a c t i o n of a r a d i c a l p a i r : (a) f o r a t r i p l e t r a d i c a l p a i r , the cage e f f e c t at h i g h f i e l d (H >Ahf ) w i l l be l e s s than that observed at low or zero f i e l d (H Ζ A ). (b) intersystem c r o s s i n g w i l l be f a s t e s t f o r r a d i c a l p a i r s with the l a r g e s t magnitude of h y p e r f i n e coupled n u c l e a r s p i n s . (c) a l a r g e r hyperfine c o u p l i n g w i l l r e s u l t i n f a s t e r intersystem crossing. (d) the maximum intersystem c r o s s i n g r a t e occurs at an a p p l i e d f i e l d approximately equal to the h y p e r f i n e s p l i t t i n g s . I t was recognized ΟΛ) very e a r l y i n the development of the r a d i c a l p a i r model that a n a t u r a l consequence of (b) was that the s e p a r a t i o n of nuclear s p i n isotopes from non-magnetic isotopes was possible. Since the cage r e a c t i o n i n homogeneous s o l u t i o n c o n t r i butes very l i t t l e towards the o v e r a l l product y i e l d s , the "magne t i c i s o t o o e e f f e c t " i s not very s i g n i f i c a n t i n homogeneous s o l u tion. \ A ® In m i c e l l a r s o l u t i o n , the extent of cage r e a c t i o n can be s u b s t a n t i a l l y increased. Below we w i l l examine the consequen ces of an a p p l i e d magnetic f i e l d on the photoreactions of ketones in micellar solution. n
s
Q
0
s
0
h f s
Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
INORGANIC REACTIONS IN ORGANIZED MEDIA
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26
Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
2.
TURRO E T A L .
Magnetic
Radical Pair Reactions in Micellar Solution
27
F i e l d E f f e c t s on Cage E f f i c i e n c i e s
F i g u r e 3 shows the e f f e c t of a p p l i e d magnetic f i e l d on the cage e f f e c t f o r 1 and some of i t s i s o t o p i c isomers. The higher cage e f f e c t f o r "^-^C enriched ketone (compound _8 i n F i g u r e 3) c o r r e l a t e d w e l l w i t h p r e d i c t i o n ( b ) . In t h i s case, we have added an a d d i t i o n a l s p i n 1/2 nucleus with a l a r g e h f s ( A =128 G a u s s ) ® to one of the r a d i c a l fragments. Likewise, the lower cage e f f e c t observed f o r the deuterated ketones (6 and J_ i n F i g u r e 3) c o r r e l a t e s w e l l with p r e d i c t i o n (c) and the r e l a t i v e h f s of *H and (1:4). I t s noteworthy that 2^ shows no mass i s o t o p e e f f e c t f o r the d i s p r o p o r t i o n a t i o n r e a c t i o n . ® With the a p p l i c a t i o n of a magnetic f i e l d , the cage e f f e c t f o r _1 drops s u b s t a n t i a l l y . The f u l l e f f e c t (30% decrease) i s achieved with the a p p l i c a t i o n of a f i e l d of only a few hundred Gauss, the magnitude of the f i e l d commonly a v a i l a b l e from a magnetic s t i r r i n g bar. A s i m i l a r decrease i s observed f o r
