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 substituted carbazole molecules and substituted phenol and tocopherol derivatives by methylene bromide has been examined at 298 K in acetonitrile, ethanol and 3-methylpentane. The first-order rate constant of electron transfer (ET) is separated from the diffusion rate constant by applying the Fuoss-Eigen formalism. The first-order rates of the ET are correlated with the free energy changes for ET through the use of the semiquantum mechanical ET theory proposed by Onuchic for outer-sphere electron transfer dynamics. The fluorescence quenching of the phenols and tocopherols is diffusion-influenced, so that the rates are only tentatively analyzed by the nonadiabatic theory. The solvent reorganization energy (As) obtained in acetonitrile and ethanol are generally higher 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.