Energy surfaces, reorganization energies, and coupling elements in electron transfer

The effects of changes in the shapes and intersections of the reactant and product free-energy surfaces on the vertical reorganization parameter and the free energy of activation for an electron self-exchange reaction are considered. Parabolic free energy surfaces provide a very good description of the inner-shell reorganization process even when the stretching force constants for the oxidized and reduced forms of the redox couple differ by a factor of two. The activation energy depends on the reorganization criterion and the contributions of the individual reactants to the inner-shell barrier are quite sensitive to the model used. Optical charge transfers and the consequences of reactant, product, and transition state stabilization in weakly interacting and very strongly interacting systems are also considered. Relationships between the equilibrium constant for the comproportionation reaction forming the mixed-valence complex and the optical charge transfer parameters are presented. (C) 1999 Elsevier Science S.A. All rights reserved.