Volumes of activation for electrode processes of various charge-types in nonaqueous solvents

Abstract

Volumes of activation (ΔV‡el) are reported for electron transfer at a Pt electrode of Mn(CN-cyclo-C6H11)62+/+ in acetonitrile, acetone, methanol, and propylene carbonate, and of Fe(phen)33+/2+ in acetonitrile. In all cases, ΔV‡el is markedly positive, whereas for the homogeneous self-exchange reactions of these couples in the same solvents the corresponding parameter is known to be strongly negative. The rate constants for the electrode reactions correlate loosely with the mean reactant diffusion coefficients (i.e., with solvent fluidity) and the ΔV‡el values with the volumes of activation for diffusion (i.e., for viscous flow), consistent with solvent dynamical control of the electrode reaction rate in organic solvents. A detailed analysis of ΔV‡el values of the kind presented for a couple with an uncharged member (Zhou and Swaddle, Can. J. Chem. 79, 841 (2001)) fails, however, either because of ion-pairing effects with these more highly charged couples or because of breakdown of transition-state theory in predicting the contribution of the activational barrier. Attempts to measure ΔV‡el for the oxidation of the uncharged molecule ferrocene at various electrodes in acetonitrile were unsuccessful, although ΔV‡el was again seen to be clearly positive.Key words: electrode kinetics, volumes of activation, nonaqueous electron transfer, solvent dynamics.