Electrochemical concerted proton and electron tranfers. Potential-dependent rate constant, reorganization factors, proton tunneling and isotope effects
Cyrille Costentin, Marc Robert, Jean-Michel Savéant*
Contribution from the Laboratoire d'Electrochimie Moléculaire, Université de Paris 7-Denis Diderot, 2 place Jussieu,75251 Paris Cedex 05, France
Electrochemistry, through techniques like cyclic voltammetry, can provide a quite effective access to CPET in terms of diagnosis and quantitative kinetic characterization. The relationships expressing the electrochemical rate constant as a function of the electrode potential are derived. Besides the CPET standard potential, it depends on two main factors. One is the reorganization energy, which is the sum of an intramolecular contribution and a solvent reorganization energy. This last term appears to be the sum of proton and electron transfer contributions. Procedures are proposed to model and estimate these factors. The pre-exponential factor is a distinctive feature of CPET reactions. It indeed mainly depends upon proton tunneling through the activation barrier. Procedures for estimating this factor,
and accordingly, the H/D kinetic isotope effect are described.