Solvent reorganization energy of electron transfer in weakly polar solvents

The paper reports a molecular treatment of intramolecular electron transfer (ET) reactions in weakly polar solvents. The theoretical analysis based on the perturbation expansion over solute-solvent interactions focuses on the following issues: (i) the relative contribution of induction, dispersion and dipole-dipole intermolecular forces to the ET activation parameters, (ii) the comparative participation of dipole orientational 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 intramolecular charge separation reaction. For the system considered, the dispersion and dipolar components of the energy gap vary oppositely but to a comparable magnitude with solvent polarity. Translational solvent modes were found to be increasingly important for less polar solvents. The difference in activation mechanisms in polar and nonpolar fluids is rooted in different weights of orientational and translational modes in the solvent response. The translational contribution results in a maximum in the Arrhenius coordinates experimentally reported for butyl acetate as the solvent and reproduced by the present theory. The continuum treatment of solvent effects is shown to be incapable of describing this phenomenon.