Observation of the Turnover between the Solvent Friction (Overdamped) and Tunneling (Nonadiabatic) Charge-Transfer Mechanisms for a Au/Fe(CN)63-/4-Electrode Process and Evidence for a Freezing Out of the Marcus Barrier

Abstract

By variation of the electronic coupling strength, the transition between the solvent-controlled regime (in which the electron-transfer rate constant depends on the solvent friction) and the nonadiabatic electron-transfer limit was observed for the Au/Fe(CN)63-/4- redox system. The solvent friction regime was demonstrated for a bare Au electrode by showing that the apparent standard rate constant was inversely proportional to the viscosity in water/glucose solutions containing 1 M KCl. The magnitude of the electronic coupling between the Au and the redox species was reduced by preparing n-alkanethiol-coated gold electrodes (Au−S−(CH2)n-1−CH3 with n = 2, 4, 6, 8) of different thicknesses. For the case of a Au electrode coated by an ethanethiol monolayer (n = 2) the rate constant exhibited a fractional viscosity dependence, whereas the electrodes with n = 4, 6, and 8 methylenes in the film showed no viscosity dependence. This trend is indicative of an overall gradual turnover between the two regimes. In the nonadiabatic regime the distance dependence of the electronic coupling decay is 1.04 Å-1, and its extrapolated value at the closest electrode−reactant distance is 3.5 kcal mol-1. Analysis of the kinetic data, together with some results available in the literature, determines the intrinsic parameters of the charge-transfer step in both regimes. Corrections for the significant variation in the reactive site potential near the electrode (at the outer Helmholtz plane, OHP) and the reorganization free energy with the charge-transfer distance are taken into account. Evidence for a freezing out of the Marcus barrier (lowering by a factor of 2) was found for the process at the bare Au electrode, in accordance with theoretical prediction (Zusman, L. D. Chem. Phys. 1983, 80, 29).