ABSTRACT
Established and new elements of the theory of electron transfer at electrochemical interfaces are overviewed. This extends particularly to current-over voltage relations at metal, semimetal, semiconductor, and high-temperature superconductor electrodes in the diabatic limit of weak interaction between the electrode and reacting molecule. A simple view of the conditions for the prevalence of the adiabatic or diabatic limits is also introduced, and attention to the electronic tunneling factor is given. The latter exhibits a new feature different from electron transfer in homogeneous solution, i.e., the electron density of the metal electrode expands on negative and contracts on positive surface charging. This facilitates and hampers the tunneling process, respectively.
Views of electrochemical electron transfer can be extended to the two-electrode configuration of in situ scanning tunneling microscopy (STM) of redox molecules. The potential Gibbs free energy surfaces in electron transfer modes of a large redox molecule such as a transition metal complex or a redox metalloprotein in the in situ STM configuration are constructed, and it is shown that this configuration offers approaches to experimental distinction between different mechanisms for tunneling through the adsorbed molecule. The mechanisms are particularly sequential two-step electron transfer and coherent or resonance tunneling. It is finally shown that recent in situ STM data for adsorbed and covalently bound metalloporphyrins and for adsorbed azurin (a blue single-copper protein) exhibit features according to these views.
