Solvent Polarity and Polarization

Depending on the solvent polarity and redox potentials of a donor and an acceptor, the ions resulting from electron transfer may remain associated either as a contact IRP or as a solvent-separated IRP. In the contact pair, back electron transfer can take place. For such electron back-transfers, the solvent reorganization energy is less than 5% of the total reorganization energy (Serpa and Arnaut 2000). 

Solvent reorganization energy should be taken into account when, during the reaction, a charge moves from one to another portion of the molecule. For instance, in the acetophenone series, such charge transfer takes place according to the following sequence  

The buzzword polarity, derived from the dielectric approach, is certainly the most popular word concerning solvent effects. (It is the basis for the famous rule of thumb similia similibus solventur, i.e., like dissolves like in English.) We should add like helps like. Let us compare toluene and n-hexane as solvents. At temperatures when both solvents have comparable viscosity, the reaction between 1,2,4,5-tetrafluorobenzene and its anion-radical proceeds in different ways (Werst 1993), depending on the solvent. In toluene, the reaction consists of electron exchange  

in toluene, no dimer anion-radicals are observed. Encounters between anion-radicals and neutral molecules result in electron transfer instead. Charge transfer from the solute anion-radicals to toluene is unlikely, since toluene is a poor electron acceptor. Consistent with this fact, no differences were observed in the anion-radical ESR-coupling constants in toluene when compared to those in -hexane (Werst 1993). Charge (not electron) transfer from the toluene molecules to the 

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