Simultaneous two-electron transfer in electrode kinetics

The analysis of the mechanism of electrode reactions is usually conducted with the implicit assumption that electrons must be transferred one at a time. This often necessitates the assumption of the existence of intermediates that are highly unstable, such as the monovalent ions of different metals formed in the course of electrodeposition. In the present paper this practice is questioned. Calculations, based on the theory of electron transfer developed by Marcus and by Dogonadze and Levich, show that the conditions under which simultaneous 2-e(-) transfer will be the predominant pathway are mild. Even moderate instability of the supposed intermediate (formed by the transfer of the first electron) can tilt the balance in favor of the simultaneous 2-electron route. With a somewhat higher degree of instability, even a simultaneous 3-e(-) transfer could be the dominant mechanism. The concept of simultaneity is discussed by extending the Frank -Condon principle for multiple electron transfer. Even if electrons tunnel across the interface one at a time, the transfer of two electrons could be considered simultaneous, if it happens within a short time interval, estimated to be 0.5-5 fs. It is concluded that simultaneous 2-e(-) transfer can be a common pathway in electrode reactions, and should always be considered as a viable alternative in the analysis of the mechanism of these processes. (C) 2002 Elsevier Science B.V. All rights reserved.