Influence of specific reactant-solvent interactions on intrinsic activation entropies for outer-sphere electron-transfer reactions

The physical basis of the solvent contribution to the intrinsic activation entropy, ΔS*_int, for outer-sphere electron-transfer reactions in homogeneous solution is examined in terms of the entropic parameters for the constituent electrochemical half-reactions. A relationship for calculating ΔS*_int is derived which takes into account specific reactant-solvent interactions for the isolated redox centers by employing electrochemical reaction entropy data. This relation yields rather larger and more structure-sensitive values of ΔS*_int, than those deduced on the basis of the usual dielectric continuum treatment. These considerations indicate that the more negative values of ΔS*_int typically extracted from experimental kinetic data arise largely from the modification to the specific reactant-solvent interactions within the precursor complex caused by the proximity of the other redox center.