Influence of molecular and medium effects on two-electron processes

Electron transfer (E.T.) reactions in which there is an overall stoichiometry of two or more electrons are common and may be found in a number of important chemical and biological processes. Many of these reactions occur through mechanisms in which chemical reactions (C) are coupled to the E.T. process (E). For example, the classic quinone/hydroquinone couple follows an electrochemical reaction–chemical reaction–electrochemical reaction–chemical reaction mechanism, combining a pair of electron and proton transfers, displaying two-electron Nernstian = behavior. This chapter restricts itself to the simpler case of E.T. reactions of two-electron stoichiometry that are uncomplicated by coupled chemical reactions. This will facilitate discussion of the three principal factors affecting the relative ordering and degree of potential separation of the successive one-electron transfers of the Electron transfer/Electron transfer (EE) mechanism (Equation 11.1): changes in electronic and molecular structure, differences in solvation energies, and differences in ion-pairing energies, between members of the E.T. series. Furthermore, only cases in which structure change and medium effects are thought to be concomitant (i.e., concerted) with E.T. will be considered. Thus, “square schemes” [1], in which reversible structure changes “follow” E.T., are not covered. Note also that, although the principles discussed in this chapter relate also to systems having more than two sequential E.T. reactions, “super” multielectron transfer processes such as those involving C60 will not be systematically covered