Non-local and overscreening effects in the kinetics of heterogeneous charge transfer reactions

Abstract

The effects of solvent spatial dispersion including the overscreening effect and field penetration into a metal on the kinetic parameters of heterogeneous charge-transfer reactions are studied. The calculations are based on the exactly solvable ‘sharp-boundary model’ of the interface. The analytical expressions derived using the mean-field theory for the phenomenological Hamiltonian of the liquid are adopted for the dielectric function of the solvent. The parameters entering these expressions are chosen to reproduce the results of molecular dynamics computer simulations presented in the literature. We consider the (Cp) 2Co +/(Cp) 2Co electrochemical reaction and acetonitrile (AN) and dimethyl sulfoxide (DMSO) as the solvents. The smeared Born sphere (SBS) model of an ion is used for calculations of the reorganization Gibbs energies at infinite electrode–reactant separation. The extent of the smearing of the ion excess charge is fitted to obtain the electrostatic part of the solvation Gibbs energy of an ion in the bulk of the solvent. The effective Born sphere (EBS) and renormalized smeared Born sphere (RSBS) models of an ion are suggested for calculations of the reorganization and activation Gibbs energies near an electrode. It is shown that the reorganization Gibbs energy and the activation Gibbs energy decrease with decrease of the electrode–reactant separation in accordance with Marcus theory (in which the non-local effects are neglected). However, the reorganization and activation Gibbs energies are found to be larger than those given by the Marcus formula and agree rather well with the estimates obtained from the experimental data.