POSSIBLE EXPERIMENTAL-EVIDENCE FOR MOLECULAR SOLVATION EFFECTS IN SIMPLE HETEROGENEOUS ELECTRON-TRANSFER REACTIONS

The activation enthalpy, DELTA-H*ex, for electroreduction of cobaltocenate anion was determined at the standard potential in five solvents. These results are higher than the corresponding activation enthalpies of the reduction of cobaltocenium cation to the molecule in the same solvent. According to the traditional treatment of electron-transfer theory, the activation parameters for the two reactions should be the same unless their internal reorganization energies are different. The present observation can also be attributed to molecular differences in solvation of the activated complexes. It is shown that the observed difference in the experimental activation enthalpies can be estimated using the mean spherical approximation when the polarization parameter includes consideration of the "stickiness" in ion-solvent interactions.