A MOLECULAR TREATMENT OF SOLVENT EFFECTS ON INTERVALENCE ELECTRON-TRANSFER

A molecular theory of the solvent effect on electron-transfer reaction s in polar liquids [Matyushov, D. V. Chem. Phys. 1993, 174, 199] is ex tended to often encountered geometries of the donor-acceptor complex i ncluding the intersection of reactant solvation spheres and donor-acce ptor distances failing below the sum of reactant radii. Expressions ar e derived for the reorganization energy of charge redistribution in th e spherical cavity (dipole-field approximation) and for the two-cavity configuration at asymptotically large and small donor-acceptor distan ces. In this framework, the solvent reorganization energy E(s) is comp osed of the two components linked to orientational fluctuations of the solvent permanent dipoles and density fluctuations of the liquid. Cal culated values for E(s) are tested on solvent-dependence data of inter valence charge-transfer energies E(op) for three biruthenium complexes and the acetylene-bridged biferrocene monocation, each of them valenc e localized. The plots of E(s) vs E(op) are compared with those using values of E(s) calculated from appropriate continuum theories. The plo ts based on the new theory are in general less scattered, and the slop es of the best-fit lines are closer to unity. As a major merit, the an omalous behavior of some solvents in the continuum description-in part icular 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 conti nuum theories can be traced back to two main features. First, the solv ents usually dealt with are similar in molecular size. Second, there i s a compensation because altering the size affects the orientational a nd 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 als o successfully tested on some temperature dependence data of solvent r eorganization energies.