On alternative activation mechanisms in electron-transfer reactions in solution

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Abstract
The model for electron-transfer kinetics in solution is considered. In one model the appropriate energetic condition for charge transfer is met by a small number of vibration-rotation states in thermal equilibrium with the solution. Collisional activation (CA) between ion in the solution and the solvent is the origin of such states. In another model CA is neglected and the appropriate energy states are regarded as being reached by the fluctuations in the energy of the ion, as a result of its interaction with many surrounding solvent molecules; this is the energy fluctuation (EF) model. The dependence of the charge-transfer rate upon the interfacial potential difference for the two models is outlined, and the differences between the models is discussed. Comparison with spectroscopic data for H3O+ in solution suggests that the energy distribution in the vibration-rotation levels in this ion is continuous and the classical modes of vibration exist in water. A supposed discontinuity, which would have annulled the deduction of Tafel's law, was an origin of the EF model. In the EF model, the applicability of the Born-Landau equation, ΔF 0 = e 2/2r(1/εopt - 1/εstat) is assumed. However, we show that this depends on a sufficiently large difference of the energy of an electron, trapped in the medium and bound to atoms in it. This difference is great if the medium is a solid, but not if it is a liquid. States suitable for acceptance or donation of electrons from ions to metals arise (at the equilibrium potential) much more frequently as a result of the equilibrium of the H3O+ ion with the solvent heat sink than those by electro-static fluctuation.

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