Self-exchange, electron transfer process

The coordination of copper ion by macrocyclic quadri- and quinque-dentate thi-oethers makes the Cu VCu reduction process easier, and also increases the rate of the redox self-exchange electron transfer process [55]. For example, [Cu(13-aneS4)] undergoes a one-electron reduction at 0.52 V versus NHE [58] (cf. E°(Cu /Cu ) = 0.15V versus NHE for the aquated ion in water) and the rate constant for the corresponding self-exchange electron transfer is 3 x 10 s [59] (to be compared... 

The self-exchange electron-transfer (SEET) process, in which a radical is trapped by the parent molecule, has been studied using the intersecting-state model (ISM).91 Absolute rate constants of SEET for a number organic molecules from ISM show a significant improvement over classical Marcus theory92-94 in the ability to predict experimental SEET values. A combination of Marcus theory and the Rips and Jortner approach was applied to the estimation of the amount of charge transferred in the intramolecular ET reactions of isodisubstituted aromatic compounds.95... 

Figure 1.13 Potential energy diagrams describing electron transfer processes according to Marcus theory. (A) Self-exchange (B) Cross Reaction.

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 theoretical results obtained for outer-sphere electron transfer based on self-exchange reactions provide the essential background for discussing the interplay between theory and experiment in a variety of electron transfer processes. The next topic considered is outer-sphere electron transfer for net reactions where AG O and application of the Marcus cross reaction equation for correlating experimental data. A consideration of reactions for which AG is highly favorable leads to some peculiar features and the concept of electron transfer in the inverted region and, also, excited state decay. 

Thus the Marcus theory gives rise to a free energy relationship of a type similar to those commonly used in physical organic chemistry. It can be transformed into other relationships (see below) which can easily be subjected to experimental tests. Foremost among these are the remarkably simple relationships that were developed (Marcus, 1963) for what have been denoted cross reactions. All non-bonded electron-transfer processes between two different species can actually be formulated as cross reactions of two self-exchange reactions. Thus the cross reaction of (59) and (60) is (61), and, neglecting a small electrostatic effect, the relationship between kn, k22 and kl2... 

The examples presented in this chapter clearly demonstrate that electron transfer reactions exhibit a characteristic pressure dependence that can be employed to gain further insight into the mechanism of the electron transfer process. The pressure dependence of self-exchange reactions can be used to develop the theoretical interpretation of the observed volumes of activation because the overall reactions involve no net volume change. In the case of nonsymmetrical reactions, the volume profile treatment can reveal information regarding the reorganization involved in going from the reactant to the transition and product... 

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