Effects of solvent polarization relaxation on nonadiabatic outersphere electron transfer reactions in ultrafast dipolar solvents

Since the important work of Efrima and Bixon [J. Chem. Phys. 70, 3531 (1979)], it is believed that solvent polarization relaxation is usually too slow ~compared to the rate of electron transfer! or the amplitude of energy fluctuation too large to have any noticeable effect on the dynamics of the nonadiabatic (NA) electron transfer reactions. On the other hand, recent studies have demonstrated that solvent polarization relaxation in several common dipolar liquids can proceed at a rate much faster than that anticipated in the earlier studies. This calls for a re-examination of the role of solvent dynamics on NA electron transfer reactions in these ultrafast solvents. In this paper, the results of such studies are presented for NA reactions in water and acetonitrile. It is found that because of ultrafast solvation, many NA reactions may lie in the dynamic region where the solvent effects are just beginning to be important. The present study further reveals the following new results. (i) In the case of high barrier reactions in solvents such as acetonitrile, the polarization relaxation in the reactant well can contribute significantly to the total rate of the nonadiabatic electron transfer reaction. (ii) In water, on the other hand, the reactive friction is still sufficiently high to make energy diffusion in the reactant well efficient and so, the solvent effects are predicted to be negligible. This is in accord with the earlier theoretical suggestions and is demonstrated here clearly for real systems. (iii)We find an interesting limiting situation where the long time rate can be significantly larger than the rate given by the Marcus expression