Electron-transfer . nonadiabatic solvent change

Electron transfer (ET) is of course accompanied by rearrangement of the solvent as shown by the horizontal displacement in Figure 26. Tradiational theories for ET fall into two cases. In the nonadiabatic case it is assumed that the rate of ET is controlled by the process of crossing from one electronic state (e.g., LE) to the other (e.g., CT) [60,61]. Alternatively in the weakly adiabatic case, it is assumed that the solvent polarization is always in equilibrium with the changing charge distribution. For this latter case transition state theory is applicable [59]. 

Nuclear reorganization consists of changes in the internal or vibrational modes of the reactants as well as changes in the nuclear polarization of the surrounding solvent molecules. The distinction between these two classes of nuclear barriers is fundamental in understanding reactivity in photoelectron transfer. With this in mind, we shall now proceed to evaluate the barriers in electron transfer (Fig. 11). The classical theory, to be discussed in the next section, emphasizes the Coulombic and nuclear, whereas in the nonclassical, nonadiabatic theories, which are discussed in Sect. 3.3, emphasis is on electronic and nuclear barriers. 

Fig. 9.1. PES of the adiabatic electron transfer reaction of Eq. (9.4) in polar solvent obtained through superposition of the terms of the initial , and the final Ef states q is the reaction coordinate that includes the solvent reorganization). The process is shown schematically as a change in the polarization of the medium when passing from the equilibrium configuration (qf) of the initial state to the equilibrium configuration (qj) of the state with the transferred electron. The electron transfer occurs in the region q. When V.j. is small, there exists a probability for the reaction trajectory to cross the transition state region without leading to product formation (nonadiabatic reaction)...

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