Radical pair kinetics in the hydrogen abstraction of benzophenone derivatives in micellar solutions, studied by pulsed microwave irradiation

The hydrogen abstraction reaction of benzophenones in micellar interiors wa s studied by the effect of a short, resonant microwave pulse on the recombi nation kinetics under magnetic fields of approximately 0.3 T, This techniqu e allows all the significant rate constants in the reaction to be determine d, in particular the singlet state recombination rate constant, which is ve ry difficult to accurately determine by other techniques. This is determine d to be 1.90 x 10(7) s(-1) for benzophenone (BP) and 1.83 x 10(7) s(-1) for decafluorobenzophenone in sodium dodecyl sulfate (SDS) micelles, indicatin g that this value is not limited by the rate of spin mixing, and thus that the kinetic approach used is justified. It also indicates that recombinatio n is limited by reencounter frequency and is not dependent upon substitutio n in the ketyl radical. All kinetic parameters are extracted for each syste m using a combination of single pulse and pulse shift measurements. Data ar e also presented for the reactions of BP in SDS solutions of high salt conc entration, in polyoxyethylene dodecyl ether (Brij 35), and in hexadecyltrim ethylammonium chloride micelles. The rate constants reveal useful informati on on the effects of the both the micelle interior and the nature of ketyl radical on the recombination kinetics.