A model for the effect of ion pairing on an outer sphere electron transfer

Abstract

<p>Our theory for the effect of ion pairing on electron transfer explains why the chloride ion catalyses copper deposition.</p>

Copper Deposition from Chloride-Containing Aqueous Solutions: Catalysis and the Role of the Water Structure

“…Thus, for the case of Cu + and [CuCl 2 ] − , for which the minimum PMF is found at about 3.0 Å, the most stable structure is localized within the valley near the upper edge of the peak at ≈2.5 Å, where both Cu + and [CuCl 2 ] − form a coordinated structure involving the molecules of the highly ordered water layer, which resembles the well-known classical Stern water monolayerthe Helmholtz ion layer picture of aqueous double layer theory. In the same way, Cu 2+ and [CuCl 2 ] are stably positioned at ≈4.0 Å, where characteristic solvated configurations are found in accord with our previous work . Therefore, the similarity in the profiles of the PMFs is highly conditioned by the interaction between the water and the surface.…”

“…Recently, we have also found that ion pairing (i.e., the formation of mixed complexes) in the solution bulk affects strongly the ET activation barrier . It was shown that the formal expression for the activation energy (including the effect of bond breaking) is not affected by ion pairing.…”

“…The standard basis set 6-31g­(d,p) was used to describe the electrons in the Cl, O, and H atoms. In our model calculations, for the electronic transmission coefficient, the geometry of each complex species (previously optimized in the solution bulk, as described in ref ) was fixed. The open-shell systems were treated in terms of unrestricted formalism.…”

“…We also employ quantum chemical modeling to calculate the electronic transmission coefficients for Cu 2+ /Cu + and [CuCl 2 ]/[CuCl 2 ] − redox couples in the framework of the modern theory of charge transfer in condensed media . Finally, we estimate the rate constants for the first ET step combining the data of MD simulations, quantum chemical calculations, as well as the results obtained by us previously, see ref . A similar approach was employed recently for the analysis of heterogeneous kinetics of the Fc/Fc + redox couple in various solvents .…”

Asymmetric and Anharmonic Electrode Kinetics: Evaluation of a Model for Electron Transfer with Concerted Rupture of Weak, Inner Shell Interactions

“…Both Savéant [36,37] and Schmickler et al [38] have described this process using Morse curves, even though this might not be the most appropriate model for ion pairing. There is limited justification for our approach since ion pairing interactions have been described as the summation of a 12-6 potential with an R À 1 Coulombic energy, [39] although this moves to an R À 3 term, [40] if the ion pairing interaction is considered as occurring over larger distances, so that it is to be dominated by dipole-dipole type interactions (which vary as R À 6 , and which integrate over a surface to afford an R À 3 term). [30,31] For metallic electrodes, where the driving force for an heterogeneous electron transfer is…”