Self-exchange processes

The cross relation has proven valuable to estimate ET rates of interest from data tliat might be more readily available for individual reaction partners. Simple application of tire cross-relation is, of course, limited if tire electronic coupling interactions associated with tire self exchange processes are drastically different from tliose for tire cross reaction. This is a particular concern in protein/protein ET reactions where tire coupling may vary drastically as a function of docking geometry. 

The reorganization energy of a self-exchange reaction is denoted A(0) (from the fact that AG° = 0) and is an important quantity in the Marcus theory, where it can be shown that the activation free energy of a self-exchange reaction, AG(0), is equal to X.(0)/4. It is also possible to measure rate constants of self-exchange processes experimentally and thus get access to (0) via this relationship. 

The qualitative elements of Marcus theory are readily demonstrated. For example, the process of transferring an electron between two metal ions, Fe2+ and Fe3 +, may be described schematically by Fig. 33 (Eberson, 1982 Albery and Kreevoy, 1978). The reaction may be separated into three discrete stages. In the first stage the solvation shell of both ions distorts so that the energy of the reacting species before electron transfer will be identical to that after electron transfer. For the self-exchange process this of course means that the solvation shell about Fe2+ and Fe3+ in the transition state must be the same if electron transfer is not to affect the energy of the system. In the second phase, at the transition state, the electron is transferred without... 

Figure 1 shows the double-well potential which is often used to represent the electron transfer act in oxidation-reduction. The special case of a self-exchange process was chosen for simplicity. (For a full discussion of the issues being discussed in relation to the potential energy diagram, see Ref. 11. Ref. 13 gives a more compact treatment.) The implications of the diagram may not be immediately obvious, and the... 

Coni(en)3]3+/[Con(en)3]2+ couple, E° = -0.02 V versus the normal hydrogen electrode (NHE) [18]. Such an electron transfer is especially slow. In particular, for the self-exchange process ... 

It is quite amazing that the ET rates are so rapid (>10 s ) in the radical anions of 3-5, considering that the free energy change, AG, for ET in these systems is zero consequently, the free energy of activation for the reaction is at its maximum value (this follows from Eq. 2 the nominator in the exponent is equal to for a self-exchange process and it is not reduced by the presence of an exergonic AG<, term). 

Another example involves the Ru3+aq/Ru2+aq system, where the self-exchange process may be distinguished through having different oxygen isotopes present in the two aquated... 

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