Electron transfer, activation control reorganization energy

It has been shown so far that internal and external factors can be combined in the control of the electron-transfer rate. Although in most cases a simple theoretical treatment, e.g. by the Marcus approach, is prevented by the coincidence of these factors, it is clear that the observed features for the isoenergetic self-exchange differ by the electronic coupling and the free energy of activation. Then it is also difficult to separate the inner- and outer-sphere reorganization energies. 

If a catalyst (C) which can interact with one of the products of electron transfer (e.g., A in Scheme 2) is introduced into the D-A system, the factors to control the rate of electron transfer, AG°et and 2, may be altered. The AG°et value is shifted in the negative direction when the activation barrier of electron transfer is reduced to accelerate the rates of electron transfer as shown in Scheme 2, where C forms a complex with A. Since most organic compounds such as n acceptors in particular have small reorganization energies, the change of redox potentials due to the interaction of the corresponding radical anions with C may be the main factor to accelerate the rates of electron transfer. Thus, any catalyst C that can stabilize the products of electron transfer thermodynamically by complexation may act as an... 

The homogeneous outer sphere electron transfer reactions in solution occur at a rate that is noticeably Icj er than the diffusion rate. This peculiar behaviour has been explained through a three-step mechanism formation of a precursor complex from the separated reactants, actual electron transfer within this complex to form a successor complex and dissociation of the latter complex into separated products. The reaction rate is usually controlled by the electron transfer step, this step being governed by the Franck-Condon principle. This principle is embodied in classical electron transfer theories using an activated-complex formalism in which the electron transfer occurs at the intersection of two potential energy surfaces, one for the reactants and the other for the products. This implies that the second step necessarily involves the reorganization of the solvent before and after the electron transfer itself is produced. So, it is obvious that solvent must play an essential role in the rate of electron transfer reactions in solution. 

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