KINETICS AND CIDEP OF RADICALS DURING PHOTO-REDUCTION OF ACETONE WITH 2-PROPANOL BY EFFECT MODULATED ESR SPECTROSCOPY

ESR spectroscopy with modulated radical initiation is used to analyze quantitatively chemical lifetimes and CIDEP enhancements of 2-propyl-2-ol radicals, formed by photoreduction of acetone with 2-propanol in aqueous solution at T = 16°C. The bimolecular termination rate constant of the radicals is found to be diffusion controlled and to depend on the hyperfine state as a consequence of T0S mixing in F-pairs. CIDEP enhancements built up in geminate and in F-pairs are separated. Their relative dependence on the hyperfine state agrees with microscopic theory, which however, fails to reproduce the absolute enhancements by a factor of 4. The polarizations indicate equal reactivities towards photoreduction for the three sublevels of the 3nπ* state of acetone, and conservation of the electron spin polarization upon radical formation. The initial separation of the species in the geminate pair is found to lie within the strong exchange region, since geminate and F-pairs show equal RPM polarizations. The CIDEP enhancements limit the rate constant k22 for abstraction of the hydroxylic hydrogen of 2-propanol by 3nπ* acetone to k22 < 105 dm3 mol-1 s -1. © 1979.