The Outer-Sphere Barrier

In PET, the rate can be markedly affected by the solvent polarity. With the formation of each new charge-transfer intermediate, solvent dipoles undergo reorientation in response to the new charge distribution on the reactants [49]. The solvent response influences the free-energy barrier of the reaction by altering the potential energy surface of the electron transfer. We consider this facet of solvent motion in this section. In a later section, we examine dynamical solvent effects. 

measures the energy barrier involved as the solvent dipoles change. For spheroidal molecules, the following expression was first used by Marcus 

Equation (33) gives approximate estimates of ks for spherical reactants (Table 5). These values are undoubtedly overestimates [16,50]. However, various attempts have been made to provide more realistic values. For example, in some models, the reactants are visualized as oblate ellipsoids, separated by a distance d in the transition state [39]. Other models take into account the charge 

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