The accelerating effect of anions for electrode reactions has been known for a long time, but it is much less appreciated that these effects can sometimes be caused by traces of anions. We have demonstrated that the Cu § + reaction is strongly catalyzed by trace amounts of chloride ions in the solution. The Cu+/Cu reaction was found to be unaffected by chloride ions. These experimental results were also substantiated by theoretical calculations. We have investigated the electronic coupling for homogeneous electron-transfer reactions that are approximate models for electron transfer in the copper deposition: (i) outer-sphere reaction (water-water bridge), and (fi) innersphere reaction (chloride bridge). For Cu++/Cu+ electron transfer we found increased coupling for the chloride bridge, which we attribute to the closer approach found for this complex compared to the water bridge, while for CuUCu electron transfer, coupling was not increased for the chloride bridge reaction.

We describe experiments on the temperature dependence of the rate of the ferrous-ferric electron transfer reaction at a gold electrode and compare them with a detailed molecular dynamics simulation which is used to predict the rate. We find from the experiments that the temperature dependence of the rate has the Arrhenius form over the temperature range from 25 to 275~ and that the transfer coefficient is independent of temperature in this range. The molecular dynamics simulations are used in two ways to extract activation energies and transfer coefficients for comparison with experiment. In one of these methods, we assume parabolic dependence of the energies for the product and reactant in a reaction coordinate which is not specified a priori. In the other method, we use a quantum mechanical calculation extrapolated from the very short molecular dynamics time scale to times characteristic of the electron transfer rate. The assumption of parabolic dependence of the energies gives an estimate for the activation energy which is consistent with experiment. The transfer coefficient calculated using this assumption is also consistent with experiment. The activation energy and the transfer coefficient from the quantum mechanical calculation are both lower than the experimental values. The quantum mechanical method, together with a molecular orbital calculation of the electron transfer matrix element, permits a theoretical estimate of the absolute value of the rate, which is also compared with the experimental result. These results show that the ferrous-ferric reaction, which is a single-step outer-sphere charge-transfer reaction, follows the classical Butler-Volmer equation at temperatures up to 275~ and that earlier results on other reactions giving a temperature dependent transfer coefficient are likely to arise from elementary steps other than outer-sphere charge transfer.

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We describe results of experiment and theory of the cuprous–cupric electron transfer rate in an aqueous solution at a copper electrode. The methods are similar to those we reported earlier for the ferrous–ferric rate. The comparison strongly suggests that, in marked distinction to the ferrous–ferric case, the electron transfer reaction is adiabatic. The model shows that the activation barrier is dominated by the energy required for the ion to approach the electrode, rather than by the energy required for rearrangement of the solvation shell, also in sharp distinction to the case of the ferric–ferrous electron transfer at a gold electrode. Calculated activation barriers based on this image agree with the experimental results reported here.

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