ELECTRONIC COUPLING ENERGY AND SOLVENT REORGANIZATION ENERGY IN THE NONADIABATIC INTERMOLECULAR ELECTRON-TRANSFER REACTIONS

The quenching of the first excited singlet states of a series of subst ituted carbazole molecules and substituted phenol and tocopherol deriv atives by methylene bromide has been examined at 298 K in acetonitrile , ethanol and 3-methylpentane. The first-order rate constant of electr on transfer (ET) is separated from the diffusion rate constant by appl ying the Fuoss-Eigen formalism. The first-order rates of the ET are co rrelated with the free energy changes for ET through the use of the se miquantum mechanical ET theory proposed by Onuchic for outer-sphere el ectron transfer dynamics. The fluorescence quenching of the phenols an d tocopherols is diffusion-influenced, so that the rates are only tent atively analyzed by the nonadiabatic theory. The solvent reorganizatio n energy (As) obtained in acetonitrile and ethanol are generally highe r than that calculated from the Marcus dielectric continuum model. The electronic coupling matrix elements are relatively small ( < 50 cm-1) and decrease exponentially with the center-to-center intermolecular D -A distances. These results are consistent with the idea that the rate -determining step in the reactions between vitamins and free radicals might involve an electron transfer oxidation.