NONLOCAL ELECTROSTATIC EFFECTS ON ELECTRON-TRANSFER ACTIVATION-ENERGIES - SOME CONSEQUENCES FOR AND COMPARISONS WITH ELECTROCHEMICAL AND HOMOGENEOUS-PHASE KINETICS

The combined role of field penetration into the metal electrode and solvent spatial correlations (nonlocal electrostatics) upon the outer-shell free energy of activation, ΔG*e, for electrochemical exchange reactions as described by a recent treatment (ref 4) are explored in relation to the activation free energy, ΔG*h, of homogeneous self-exchange processes by means of illustrative numerical calculations and by comparisons with experimentally derived barriers. The latter are obtained from solvent-dependent kinetic data for cobaltocenium-cobaltocene (Cp2Co+/0) electrochemical exchange and homogeneous self-exchange, along with optical electron-transfer energies for biferrocenylacetylene cation (BFA+). The predicted values of ΔG*e in relation to ΔG*h are examined for systematic variations in spatial and solvent parameters. In contrast to the expectations of the conventional dielectric continuum treatment, consideration of metal field penetration yields a predicted destabilization of the electrochemical transition state (i.e., increase in ΔG*e) so that typically ΔG* ≳ ΔG*h. The inclusion of solvent spatial correlations increases the degree of divergence from the continuum predictions. Comparison with ΔG*h values derived from the BFA+ optical electron-transfer data, ΔG*h(op), indicate the significance of solvent spatial correlations, especially in hydrogen-bonded media. Experimental estimates of ΔG*e in seven solvents are derived from the Cp2Co+/0 electrochemical rate constants, the required preexponential factors being estimated from the knowledge, based on a solvent dynamical analysis (ref 8c), that essentially adiabatic pathways are followed. These ΔG*e estimates lie in the range 5 ± 0.5 kcal mol-1, which when compared with ΔG*h(op) as well as the calculated ΔG*e values supports the predicted absence of stabilizing metal-reactant image interactions. Values of (ΔG*e - ΔG*h) for Cp2Co+/0 exchange are also extracted in nine solvents by combining the electrochemical and homogeneous rate data; this procedure exploits the additional detailed information that has been obtained recently for the self-exchange preexponential factors (ref 9d). These experimentally derived (ΔG*e - ΔG*h) estimates are uniformly small and positive (≲2 kcal mol-1), in harmony with the theoretical predictions based on the combined effects of metal field penetration and nonlocal solvent electrostatics. © 1990 American Chemical Society.