Reactant solvation and kinetics of simple charge-transfer electrode reactions in mixed solvents

Abstract

The rate of the interfacial charge transfer in the V(III)/V(II) system at the mercury electrode in H 2O+DMPU 1 1 Abbreviations: DMPU, 1,3-dimethyl-3,4,5,6-tetrahydro-2-(1 H)-pyrimidinone; DMF-dimethylformamide; AN = acetonitrile; HMPA = hexamethylphosphorotriamide. +0.4 mol dm −3 HClO 4 mixtures was found to increase when x DMPU was raised from 0.1 to 0.8. The solvatochromism of the electronic d-d transitions within the vanadium ions was used to leam about their resolvation with DMPU molecules. It was found that, under the experimental conditions used, the charge transfer occurs at the electrode surface which is already largely covered by DMPU (θ DMPU = 0.9 to 1.0) and the interfacial kinetics are determined mainly by the solvation state of reactant in the bulk solution. Our observation of a close correspondence between log k s, and the reactant inner-shell composition can be considered as a result of the prevailing non-continuum contributions to the activation energy arising from the elimination of specific short-range, quo-ligand-solvent (hydrogen bond) interactions by coordinated DMPU molecules which are not hydrogen-bond donors. It seems that generally such effects could be kinetically significant also in other aqueous solutions of aprotic solvents (e.g. DMF, AN, HMPA).