The prediction of electrochemical reactivities from contemporary theory: some comparisons with experiment

The application of current theoretical treatments of electron transfer to outer-sphere electrochemical reactions are considered with regard to the numerical prediction of rate parameters from thermodynamic and structural data. Formalisms based on a "semiclassical" treatment for the Franck-Condon barrier together with an "encounter preequilibrium" model for the preexponential factor are summarized and related to the more widely considered treatments for homogeneous redox reactions. Comparisons are made between the theoretical predictions and experimental rate parameters for representative inorganic outer-sphere reactions at electrode surfaces, and with relative reactions in homogeneous solution. The effects of altering the electrode material and the outer-shell solvent are also considered. Although the measured rate parameters for several reactions at mercury electrodes are in reasonable agreement with the theoretical predictions, significant and even large discrepancies are seen for a number of cases. Likely reasons for these findings are discussed, including nonadiabaticity and specific solvation effects.