The frequency factor for outer-sphere electrochemical reactions

The conventional formulation of the frequency factor A_n^e for outer-sphere electrochemical reactions based on a "collisional" model is compared with an alternative "encounter preequilibrium" treatment whereby A_n^e is determined by the effective frequency of surmounting the free energy barrier within a previously assembled precursor state. The latter model is shown to provide a more appropriate description for electrochemical reactions under most conditions, in harmony with analogous considerations for homogeneous redox processes. Noticeably (ca. 20-fold) larger values of A_n^e are derived from the encounter preequilibrium as compared to the collisional model. Numerical relationships between the kinetics of corresponding electrochemical and homogeneous reactions and of competing inner- and outer-sphere electrochemical pathways that are significantly different from the conventional relations are also predicted on the basis of the encounter preequilibrium model. The relationships between A_n^e and experimental frequency factors obtained from the temperature dependence of electrochemical rate constants are also considered in the light of these considerations.