Adiabatic and nonadiabatic outersphere electron transfer reactions in methanol: effects of the ultrafast solvent polarization modes

Abstract

Recent studies have demonstrated that the solvation dynamics in common dipolar liquids like water and acetonitrile is dominated by an initial ultrafast Gaussian component which seems to account for about 60%-70% of the total energy relaxation. Methanol, on the other hand, exhibits a rather different behavior with a much smaller amplitude of the initial Gaussian component and the relaxation is primarily caused by a much slower exponential decay. In the present study, we have investigated the role of these solvent modes on both adiabatic and nonadiabatic outersphere electron transfer reactions in methanol. It is found that the rate of the adiabatic barrier crossing is greatly enhanced due to the ultrafast solvation. For nonadiabatic reactions, the relative importance of the solvent dynamic modes increases enormously compared to the situation when only the slow, overdamped modes are included. Another important conclusion is that because of the dominance of the inertial modes, the rate of electron transfer reaction is almost independent of the longitudinal relaxation time, τ_L, of the solvent. The results of the present study are compared with those obtained earlier by us for water and acetonitrile to elucidate the underlying difference in the high frequency polar response of these liquids.