ON THE DIFFERENCES IN THE MAGNITUDE OF THE OBSERVED SOLVENT EFFECT INTHE KINETICS OF SIMPLE HETEROGENEOUS ELECTRON-TRANSFER REACTIONS

Solvent effects have been examined for 18 simple heterogeneous redox r eactions involving one-electron oxidation of a molecule to form a mono cation or one-electron reduction to form a monoanion. It is shown that the degree of dependence of the rate constant on the longitudinal rel axation time of the solvent decreases with the experimental value of t he standard rate constant. This observation is discussed within the co ntext of contemporary theory for the effect of the solvent on the pre- exponential factor for the rate constant. It is concluded that, as the importance of solvent fluctuations on the frequency of crossing the G ibbs activation energy barrier for electron transfer decreases, there is a corresponding decrease in the electron transmission coefficient.