Theory of Electron-Transfer Reactions and of Related Phenomena

Abstract

Data on electron exchange reactions have provided insight into factors influencing rates of electron-transfer reactions in solution. The present paper has the twofold purpose of discussing some of these factors and of describing applications of these exchange data and theory to other phenomena. The reaction rate depends upon the extent of reorganization of bond lengths (angles) in the reactants and of solvent reorientation outside them. The reorganization is facilitated or hindered in a comparatively simple way by a favorable or unfavorable standard free energy of reaction. The rate depends, too, on coulombic and other interactions, as evidenced perhaps by certain salt effects, though probably only by a few orders of magnitude typically. The observed variation of rates of some 15 orders of magnitude is best attributed primarily 10 differences in the vibrational reorganization term, a factor calculable from bond lengths and force constants when known. A remaining factor, non-adiabaticity, is at present of uncertain importance. Arrhenius frequency factors in chemical and electrochemical exchange rate constants would provide the most direct information, but can be complicated or even dwarfed by solvent reordering effects in the coulombic interaction. Available data are few. They provide examples where a non-adiabatic effect is minor. There appear to be no known examples where it is major (Fe2+ - Fe3+ could be a candidate but its mechanism is apparently uncertain). Reorganization in reactants and in solvent occurs in a variety of related phenomena, and related concepts will be applied to treat them. In turn, chemical exchange data have useful applications to the latter. These areas include electrochemical exchange reactions, chemiluminescent electron-transfer reactions (between positive and negative aromatic ions, for example), and redox reactions of the solvated electron. An explanation for the chemiluminescent reactions will be based on the possible "inverse ΔF" effect, discussed several years ago by the author. A related phenomenon involving solvent "orientation strain" occurs in light absorption or emission by polar solutes in polar solvents, and the theoretical approach used by the author for treating it is closely related to that used for the exchange reactions.

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