Electro-donicity of the solvents and kinetics of simple electrode reactions: The Eu(III)/Eu(II) couple in common solvents

Abstract

The kinetics of the Eu(III)/Eu(II) system at mercury electrodes has been investigated in water, acetonitrile (AN), N,N-dimethylformamide (DMF) dimethylsulphoxide (DMSO) and hexamethylphosphoramide (HMPA). Formal potentials E f o move towards more negative values in the order AN<H 2O<DMF<DMSO <HMPA, indicating increasing solvation ability of the solvents in agreement with their donor numbers (DN). Logarithms of standard rate constants corrected for the double-layer effect do not show any straightforward relation with the basicity of the solvents used. However, λ i, the reorganization energy of the inner sphere, correlates with DN similarly to E f o. It has been suggested that λ i determines mostly the reaction kinetics in the case of basic solvents, while λ 0, the reorganization energy of the solvent around the ion, with its primary solvation sphere, is essential for solvents of low basicity.