A theoretical and experimental approach to the adiabaticity of diffusional electron transfer processes. Electroreduction of 2-nitropropane on mercury microelectroelectrodes

• Modelling of degree of adiabaticity for diffusional electrode processes is considered.
• A potential energy curve formalism has been employed in the kinetic model.
• 2-Nitropropane reduction on mercury is studied via temperature-variable voltammetry.
• Cyclic voltammograms are recorded in DMSO solutions at temperatures 22.0–44.5 °C.
• The adiabatic character depends on the size of the supporting electrolyte cation.

A theoretical and experimental approach to the degree of adiabaticity of electrode processes is considered for the case where the electroactive species of the redox couple move freely in solution. Within a transition state-like framework, the adiabatic effect on the activation energy of electron transfer is included through the theory developed by Schmickler (W. Schmickler, J. Electroanal. Chem. 204 (1986) 31). The effect on the probability of electron tunnelling is also incorporated according to the Landau–Zener formalism (L. Landau, Phys. Z. Sowjetunion, 1932 [29]; C. Zener, Proc. R. Soc. London A 140 (1933) 660). In applying both aspects, it is recognised that the electron transfer takes place over a range of distances from the electrode surface.The theory is applied to the study of the electroreduction kinetics of 2-nitropropane in fully-supported DMSO solutions on mercury hemispherical microelectrodes of 23 μm radius. By fitting of experimental cyclic voltammetry, the standard rate constant (k0) is determined at different temperatures and for different supporting electrolytes. The reorganization energy and degree of adiabaticity from the variation of k0 with temperature are evaluated.