SOLVENT REORGANIZATION ENERGY OF ELECTRON-TRANSFER IN WEAKLY POLAR-SOLVENTS

The paper reports a molecular treatment of intramolecular electron tra nsfer (ET) reactions in weakly polar solvents. The theoretical analysi s based on the perturbation expansion over solute-solvent interactions focuses on the following issues: (i) the relative contribution of ind uction, dispersion and dipole-dipole intermolecular forces to the ET a ctivation parameters, (ii) the comparative participation of dipole ori entational and molecular translational solvent modes in activating ET, and (iii) the effect of solvent molecularity on temperature variation of ET rates. The theory is tested on the experimentally studied intra molecular charge separation reaction. For the system considered, the d ispersion and dipolar components of the energy gap vary oppositely but to a comparable magnitude with solvent polarity. Translational solven t modes were found to be increasingly important for less polar solvent s. The difference in activation mechanisms in polar and nonpolar fluid s is rooted in different weights of orientational and translational mo des in the solvent response. The translational contribution results in a maximum in the Arrhenius coordinates experimentally reported for bu tyl acetate as the solvent and reproduced by the present theory. The c ontinuum treatment of solvent effects is shown to be incapable of desc ribing this phenomenon.