A MOLECULAR TREATMENT OF SOLVENT EFFECTS ON INTERVALENCE ELECTRON-TRANSFER

A molecular theory of the solvent effect on electron-transfer reactions in polar liquids [Matyushov, D. V. Chem. Phys. 1993, 174, 199] is extended to often encountered geometries of the donor-acceptor complex including the intersection of reactant solvation spheres and donor-acceptor distances failing below the sum of reactant radii. Expressions are derived for the reorganization energy of charge redistribution in the spherical cavity (dipole-field approximation) and for the two-cavity configuration at asymptotically large and small donor-acceptor distances. In this framework, the solvent reorganization energy E(s) is composed of the two components linked to orientational fluctuations of the solvent permanent dipoles and density fluctuations of the liquid. Calculated values for E(s) are tested on solvent-dependence data of intervalence charge-transfer energies E(op) for three biruthenium complexes and the acetylene-bridged biferrocene monocation, each of them valence localized. The plots of E(s) vs E(op) are compared with those using values of E(s) calculated from appropriate continuum theories. The plots based on the new theory are in general less scattered, and the slopes of the best-fit lines are closer to unity. As a major merit, the anomalous behavior of some solvents in the continuum description-in particular hexamethylphosphoramide and occasionally water-becomes resolved in terms of the extreme sizes, as they appear to be at opposite ends of the solvent diameter scale. The otherwise relative success of continuum theories can be traced back to two main features. First, the solvents usually dealt with are similar in molecular size. Second, there is a compensation because altering the size affects the orientational and translational parts of the solvent barrier in opposite directions. Both parts, in turn, are comparable in magnitude. Therefore, so-called Marcus-Hush effects appear to be overestimated. The new theory is also successfully tested on some temperature dependence data of solvent reorganization energies.