Volumes of activation (ΔVel) 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, ΔVel 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 ΔVel 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 ΔVel 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 ΔVel for the oxidation of the uncharged molecule ferrocene at various electrodes in acetonitrile were unsuccessful, although ΔVel was again seen to be clearly positive.Key words: electrode kinetics, volumes of activation, nonaqueous electron transfer, solvent dynamics.
Synthesis of an elusive, stable 2-azaallyl radical guided by electrochemical and reactivity studies of 2-azaallyl anions