Effect of the Electrode Continuum of States in Adiabatic and Nonadiabatic Outer-Sphere and Dissociative Electron Transfers. Use of Cyclic Voltammetry for Investigating Nonlinear Activation-Driving Force Laws

Abstract

The impact of taking into account the multiplicity of electrode electronic states on the kinetics of outer-sphere and dissociative electron transfers is discussed under the approximation that the electronic coupling energy and the density of states are not strongly dependent on the energy of the electronic states with emphasis on practical consequences. It is shown that, in most cases of practical interest, the activation-driving force laws may be derived from the classical Marcus−Hush quadratic relationship by application of a simple and small correction. Under the same approximation, the passage from adiabatic to fully nonadiabatic behaviors can be estimated as a function of the electronic coupling energy, showing that the reaching of complete adiabaticity requires rather modest values of this factor. In the same pragmatic vein, we discuss how cyclic voltammetry can be used to derive nonlinear kinetic laws from experimental data. It is often believed that the extraction of kinetic information from the cyclic voltammetric raw data requires that the form of the kinetic law be known a priori, consequently causing a preference for potential-step or impedance techniques where this question does not arise. It will be shown that simple treatments of the raw data, both in the case where the reactants are attached to the electrode surface or free to move in the solution, can be used to circumvent this apparent difficulty making cyclic voltammetry a tool as efficient as the above-mentioned techniques for this purpose.