Noncontinuum solvent effects upon the intrinsic free-energy barrier for electron-transfer reactions

A phenomenological electrochemical approach is outlined by which "noncontinuum" contributions to the outer-shell intrinsic barrier to electron transfer, ΔG*_os, resulting from specific reactant-solvent interactions can be estimated from the measured dependence of the formal potential upon the molecular and structural properties of the solvent. A simplified derivation, based on electrochemical half-reactions, of the conventional dielectric continuum expression is given in order to clarify the physical origins of the outer-shell intrinsic barrier and to identify likely additional noncontinuum components. Numerical calculations for ammine and other redox couples involving specific ligand-solvent interactions indicate that the noncontinuum contributions to ΔG*_os for both homogeneous and electrochemical exchange reactions can be surprisingly small (typically ≲1-2 kcal mol^-1) even when the thermodynamics of ion solvation are in severe disagreement with the dielectric continuum (Born) predictions. An additional noncontinuum component associated with vibrational distortions of outer-shell solvent may be significant for multicharged aquo complexes and other reactants engaging in strong ligand-solvent hydrogen bonding.