Marcus kinetic theory reorganization energy

Figure 4.2 illustrates the parabolic free-energy surfaces as a function of reaction coordinate. In particular, three different kinetic regimes are shown in accordance with the classical Marcus theory. The reorganization energy, X, represents the change in free energy upon transformation of the equilibrium conformation of the reactants to the equilibrium conformation of the products when no electron is... 

Some difficulties in comparing the experimental kinetic data with the outer-sphere reorganization energy calculated from the Marcus formula (28) result from several assumptions made in this theory. The reactant was assumed to have a spherical shape with a symmetric charge distribution. No field penetration into the metal was considered. Also, the spatial dispersion of the dielectric permittivity of the medium was not taken into account. In fact, the positions and orientations of dipoles around a given ion are correlated with each other therefore the reorientation of one dipole, under the influence of the external field, changes to some extent the reorientation of other dipoles within the distance defined by the correlation length. 

We would like to present somewhat more extensively the results of the work of Dzhavakhidze et al. [Ill], who studied the role of the spatial dispersion of the solvent dielectric permittivity and field penetration into a metal in determining the kinetics of electrode reactions. Considering the particular case of the field penetration effect on the reorganization energy, they found [111] that the AG value obtained is greater than predicted by the Marcus theory. Moreover, under some eonditions the dependence of AG on the reactant-electrode distance d) exhibits an anti-Mar-cusian behavior. 

It is commonly assumed that solvent reorganization will dominate electron transfer kinetically. Depending on the thermodynamics of the electron exchange, it is possible to quantitatively predict a relationship between the free energy of activation for electron transfer and the free energy associated with solvent reorganization based on Marcus theory. 

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