Short-distance electron transfer reactions

Long-Distance Electron Transport by Sequential Short-Distance Electron Transfer Reactions Is a Diffusive Process... 

In the following sections the effect of pressure on different types of electron-transfer processes is discussed systematically. Some of our work in this area was reviewed as part of a special symposium devoted to the complementarity of various experimental techniques in the study of electron-transfer reactions (124). Swaddle and Tregloan recently reviewed electrode reactions of metal complexes in solution at high pressure (125). The main emphasis in this section is on some of the most recent work that we have been involved in, dealing with long-distance electron-transfer processes involving cytochrome c. However, by way of introduction, a short discussion on the effect of pressure on self-exchange (symmetrical) and nonsymmetrical electron-transfer reactions between transition metal complexes that have been reported in the literature, is presented. 

The situation is fundamentally different from that for outer sphere electron transfer reactions where, according to Marcus theory, the solvent reorganization determines the reaction. In contrast, the model calculations discussed in this section indicate that the energy of activation for the ion transfer step is not related to the electron exchange with the electrode, since the crossing between the two diabatic energy states of 1 and 1° occurs only at such short distances where the ion has already surpassed the solvent barrier. Contrary to the approach discussed here, Xia and Berkowitz [235] assumed the validity of the outer sphere mechanism from the outset. The analysis of the dependence of the solvent barrier on external electric field and temperature indicates that a in Eq. (17) is indeed not constant but depends on temperature. 

According to the classical theory of simple electron-transfer reactions, the reactants get very close to the electrode surface, and then electrons can tuimel over the short distance (tenths of a nanometer) between the metal and the activated species in the solution phase. 

---