Ferrocene/Ferrocenium Redox Couple at Au(111)/Ionic Liquid and Au(111)/Acetonitrile Interfaces: A Molecular-Level View at the Elementary Act

Abstract

A quantum mechanical theory is employed to describe heterogeneous ferrocene (Fc)/ferrocenium (Fc+) electron transfer across Au(111)/[bmim][BF4] and Au(111)/acetonitrile interfaces. Classical molecular dynamics simulations were performed to calculate the potential of mean force for Fc and Fc+ and to estimate the solvent reorganization energy. The structure of the reaction layer and the solvent shell of Fc and Fc+ as derived from molecular dynamics are thoroughly investigated. The molecular structure of ferrocene and ferrocenium species, as well as the reactant–electrode orbital overlap are addressed on the basis of a quantum chemical approach. The experimental dielectric spectra for both types of solvents are used for quantum corrections of the outer-sphere reorganization energy as well as for estimations of the effective frequency factor in the limit of strong and weak electronic coupling. The dependence of the electronic transmission coefficient on the electrode–reactant distance is calculated for several orientations of the ferrocenium cation relative to the electrode surface which was represented by a cluster. Emphasis is put on the molecular nature of the elementary act and its qualitatively interesting features for both interfaces. The electron transfer rate constants are calculated and discussed in the viewpoint of available experimental data.