12
J. Phys. Chem. 1981, 85,12-14
The Importance of Geminate Pairs in the Mechanism of Photochemically Induced Dynamic Electron Polarization S. King Wong,” Tak-Ming Chiu, and James R. Bolton’ Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 587 (Received October 14, 1980)
In the chemically induced dynamic electron polarization (CIDEP) radical-pair mechanism (RPM), a geminate triplet radical pair is predicted to generate the same qualitative polarization pattern but a larger electron polarization than a free random pair does; nevertheless, almost all the radical-pair CIDEP previously observed has been attributed to free pairs. This letter provides the first clear experimental evidence to support the theory that in the event of producing painvise radicals, as is commonly the case in photolysis, the geminate pairs dominate the polarization process. Pairs of the acetone ketyl (dimethylhydroxymethyl)radicals were chosen for study. Geminate triplet pairs are obtained from hydrogen abstraction by excited triplet acetone from 2-propanol,while free random pairs are presumably generated from meetings of two independently produced ketyl radicals from hydrogen abstraction by tert-butoxyl radicals from 2-propanol. Both systems were studied by flash photolysis with EPR detection with time resolution of -1.5 PS. Introduction That chemically induced magnetic polarization (CIMP) may provide unique mechanistic details of chemical reactions is well re~ognized.l-~Chemically induced dynamic nuclear polarization (CIDNP) appears to be understood better than chemically induced dynamic electron polarization (CIDEP), probably because there have been many more experimental investigations of CIDNP. Two major mechanisms have been proposed to interpret the observed CIDEP phenomena, namely, the radical-pair mechanism (RPM)4s5and the triplet mechanism (TM).6 To obtain mechanistic details of photochemical reactions one must be able to distinguish one from the other. The triplet mechanism, in which the polarization originates from the triplet sublevels of the photoexcited precursor and thence is transferred to the product free radicals by rapid reactions, has been the subject of some critical experimental It is characterized by the generation of the same initial polarization (often emission) for both doublet radicals and by the dependence of observed polarization on concentration of the reacting partner. The radical-pair mechanism, the qualitative feature of which is well recognized from the usual emission observed for low-field lines and enhanced absorption for the high-field lines, involves subtle and complicated interactions and diffusion kinetics of the pair of radicals.’-3 There are two distinct processes which can contribute to the radical-pair mechanism. The geminate process (G process) is one in which a radical pair is created in a correlated way from a single reaction and the free random process (F process) is one in which the two components of the radical pair are uncorrelated with their mechanistic origin. In the early development of RPM in CIDNP, Closs and Trifunacg in 1969 demonstrated the existence of these two possible contributions in CIDNP and further showed that the triplet geminate pairs (G pairs) generate the same but larger polarization than independently produced radicals with uncorrelated spins (F pairs). AdrianlO successfully accounted for this observation theoretically. Very recently, Closs and Miller1’ have demonstrated elegantly the separation of G and F processes in CIDNP within a photochemical system by laser flash photolysis with NMR de-
‘Publication No. 254 from the Photochemistry Unit.
tection. However, the experimental distinction of these two RPM contributions in CIDEP has never clearly been demonstrated partly because early experimental efforts in photo-CIDEP were devoted to the distinction and separation of TM and RPM.12-15 While the current RPM theories of CIDEP permit geminate radical-pair processes to generate electron polarization, most of RPM CIDEP observations made so far have been attributed either to F pairs or just to RPM without further comment as if there were only one process. Furthermore, the recent reviews on CIDEP2b93J6 have concluded that most of the observed RPM CIDEP signals originate from F pairs. This letter reports for the first time clear experimental evidence showing that the geminate pairs contribute most of the polarization in photo-CIDEP as compared with free random phase pairs. Experimental Section A Varian E-12 EPR spectrometer was employed in (1)A. R. Lepley and G. L. Closs, Ed., “Chemically Induced Magnetic Polarization”, Wiley, New York, 1973. (2)(a) J. K. S. Wan, S. K. Wong, and D. A. Hutchinson, Acc. Chem. Res., 7,58(1974);(b) J. K. S. Wan and A. J. Elliott, ibid, 10,161 (1977). (3)L. T.Muus, P. W. Atkins, K. A. McLauchlan, and J. B. Pedersen, Ed., “Chemically Induced Magnetic Polarization”, Reidel, Dordrecht, Holland, 1977. (4)(a) R. Kaptein and L. J. Oosterhoff, Chem. Phys. Lett., 4, 195 (1969); (b) R. Kaptein and L. J. Oosterhoff, ibid., 4, 214 (1969). (5)(a) G.L. Closs, J. Am. Chem. SOC.,91,4552(1969); (b) G.L.Gloss and A. D. Trifunac, ibid., 92,2183 (1970). (6)(a) S. K. Wong and J. K. S. Wan, J. Am. Chem. SOC.,94,7197 (1972); (b) S. K. Wong, D. A. Hutchinson, and J. K. S. Wan, ibid., 95, 622 (1973); (c) J. Chem. Phys., 58, 985 (1973). (7)(a) A. J. Dobbs, Mol. Phys., 30,1073 (1975); (b) P. W. Atkins, A. J. Dobbs, G. T. Evans, K. A. McLauchlan, and P. W. Percival, ibid., 27, 769 (1974);(c) P.W. Atkins, A. J. Dobbs, and K. A. McLauchlan, Chem. Phys. Lett., 22, 209 (1973). (8)(a) F.J. Adrian, J.Chem. Phys., 61,4875(1974);(b) B.B.Adeleke, K. Y. Choo, and J. K. S. Wan, J. Chem. Phys., 62,3822 (1975); (c) A. J. Dobbs and K. A. McLauchlan, Chem. Phys. Lett., 30, 257 (1975). (9)(a) G.L. Closs and A. D. Trifunac, J. Am. Chem. SOC.,91,4554 (1969); (b) ibid., 92,2186 (1970). (10)F.J. Adrian, J. Chern. Phys., 54, 3912 (1971). (11)G.L. Closs and R. J. Miller, J. Am. Chem. SOC.,101,1639(1979). (12)B.B.Adeleke and J. K. S. Wan, J. Chem. SOC.,Faraday Trans. 1, 72, 1799 (1976). (13)J. B. Pedersen, C. E. M. Hansen, H. Parbo, and L. T. Muus, J. Chem. Phys., 63,2398 (1975). (14)P. W. Atkins, A. J. Dobbs, and K. A. McLauchlan, J.Chem. SOC., Faraday Trans. 2,71,1269 (1975). (15)P. B. Ayscough, G . Lambert, and A. J. Elliott, J. Chem. SOC., Faraday Trans. 1, 72, 1770 (1976). (16)P.J. Hore, C. G . Joslin, and K. A. McLauchlan, Chem. SOC.Rev., 8,29 (1979).
0022-3654/81/2085-0012$01.90/00 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 1, 1981
Letters
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conjunction with a home-built 2-MHz field modulation detection unit which had an instrument rise time of 1.5 ps for kinetic studies.17 The flash system was a PRA Model 61OC pulsed light source with a Xenon Corp. Model N722C flash lamp with a flash duration (fwhm) of -1.5
13
I
PS.
For the acetone system measurements a solution consisting of acetone (Fisher spectro grade), 2-propanol (Fisher spectro grade), doubly distilled water in a volume ratio of 1:1:2,and hydrochloric acid (4 mL of 12 M HC1 per 100 mL of aqueous mixture) was flowed slowly at 0.3 mL/min through a quartz aqueous solution flat cell in the cavity. For the (t-BuO), system, the flowing solution was a 1:l mixture of di-tert-butyl peroxide ((t-BuO),, Canlab reagent grade) and 2-propanol (Fisher). Nitrogen gas was bubbled through all solutions before and during the experiment. All EPR signals were digitized by a Nicolet 2090111 transient recorder interfaced with a Nicolet 1180 computer for signal averaging and processing. Results and Discussion We chose two photochemical systems for study, (1)the photolysis of acetone in 2-propanol (acetone system) and (2) the photolysis of di-tert-butyl peroxide in 2-propanol ((t-BuO)2system), anticipating that the first system would yield us geminate radical pairs of acetone ketyl radicals while the second system would initially produce free random-phase ketyl radicals.18 CH3-C-CH3
II 0
CH3-C-CH3
I OH
-I- CH3-CH-CH3
I OH
+
hu
geminate pairs
CH3-C-CH3
I
OH
11 CH3-6
-CH3
I OH
+
CH3-6
-CH3
I OH
-2’-BuoH eventually producinq free random-phase pairs
*CH3CH-cH3 bH -t
2t-Bu0.
Only the acetone ketyl radicals are detected in steadystate electron paramagnetic resonance (EPR) studies for both systems. We have studied the kinetics of these two photochemical systems by a newly constructed 1.5-pstime resolution 2-MHz modulation unit.lg The results shown in Figure 1 for the acetone system and the (t-BuO), system are very different, reflecting distinct origins. I t is evident that the acetone system (Figure la) produces a strong emission/absorption (E/A) pattern characteristic of a hyperfine interaction mechanismZ0as would be expected for a triplet pair of ketyl radicals. The comparison of the above system with the (t-BuO), system (Figure lb) is most illustrative. In the (t-BuO), system E/A CIDEP is visible but is much weaker even though the total concentration of ketyl radicals produced is similar to that of the acetone system.21 One (17) A. R. McIntosh and J. R. Bolton, unpublished results. (18) A. Henne and H. Fischer, J.Am. Chern. Soc., 99,300 (1977). The photochemical background for our system can be obtained quickly from thiR work. (19) The general technique of flash photolysis electron paramagnetic resonance has been reviewed. (See, for example, J. R. Bolton and J. T. Warden in “Creaton and Detection of the Excited State”, Vol. 2, W. R. Ware, Ed., Marcel Dekker, New York, 1974, Chapter 2.) Further improvements of the time resolution (-1 ps) have been made to include direct detection without magnetic field modulation and 2-MHz modulation detection. (20) F. J. Adrian, Chapter V in ref 3.
Figure 1. Time dependence of EPR signals at magnetic-field positions corresponding to the maximum of each hyperfine component (the labels -1, -6, -15, 20, 4-15, +6, +1 are used to indicate EPR lines of ketyl radicals from low field (minus sign) to high field (plus sign) of corresponding degeneracy for slx-equivalent protons: only five components shown) of the ketyl radical from (a) flash photolysis of acetone in 2-propanol (10000 flashes for each component except the 20-fold which is 50 000 flashes, but they are normalized) and from (b) flash photolysis of di-fed-butyl peroxide In 2-propanol (1:l by volume). The number of flashes is 50 000 for each component.
would expect the CIDEP results to be similar for both systems if RPM CIDEP arose only from the F process. Thus, the strong RPM CIDEP observed in the acetone system must originate from the unique geminate-pair process. Let us now examine the progression in time of molecular events for the G and F processes to see why the G process should be much more important than the F process. The EPR and CIDEP observations are on an ensemble of ketyl radicals which have a certain spin polarization. The characteristics of the spin polarization contain some information on the prior events. In the acetone system hydrogen abstraction by the excited acetone from 2propanol takes place in
