Molecular solvent-dynamical effects on activated electron-transfer kinetics: How important is solvent friction?

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

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.

Pressure effects and solvent dynamics in the electrochemical kinetics of the tris(hexafluoroacetylacetonato)ruthenium(III)/(II) couple in nonaqueous solvents

Rate constants and reactant diffusion coefficients for the Ru(hfac)30/– electrode reaction have been measured at 25°C as functions of pressure (0-200 MPa) in acetone, acetonitrile, methanol, and propylene carbonate. In sharp contrast to the negative volumes of activation ΔVex‡ found for the corresponding bimolecular self-exchange reaction in organic solvents, the volumes of activation ΔVel‡ for the electrode reaction are markedly positive, ranging from 8 to 12 cm3 mol–1. The volumes of activation ΔVdiff‡ for reactant diffusion (which can be equated to the volume of activation ΔVvisc‡ for viscous flow) range from 12 to 19 cm3 mol–1. For the Debye solvents acetonitrile and acetone at least, ΔVel‡ is given within the experimental uncertainty by ΔVdiff‡ + (ΔVex‡/2). In this relation, the numerical value of ΔVdiff‡ represents indirectly the dominant contribution of solvent dynamics (solvent friction) to ΔVel‡, and ΔVex‡/2 represents the pressure dependence of the free-energy barrier height for the electrode reaction. It is proposed that solvent friction is important in nonaqueous electrode processes but not in the corresponding bimolecular self-exchange reactions because the free-energy activation barrier is twice as high in the latter.Key words: electrode reaction kinetics, solvent dynamics, electron transfer mechanisms, pressure effects, volume of activation.