ИСТИНА

One of the major research goals in the field of heterogeneous electron transfer (ET) kinetics is the experimental verification of the quantum mechanical theory of ET in polar solvents [1]. The pioneering works of M. Weaver have allowed to gain insight into the complexity of even the simplest one electron transfer reactions, which kinetics was demonstrated to be dictated by both static and dynamic solvent effects as well as by the electrode/reactant orbital overlap and reaction layer structure [2]. A vast number of attempts to reach molecular level understanding of solvent effect on electrode processes was performed for heterogeneous as well as homogeneous ET for different reactants (transition and noble metal complexes with various ligands, metallocene derivatives, etc.). Unfortunately, not only model estimates of the rate constants are uncertain, but also the experimental data often exhibits a pronounced scatter due to the difficulties in estimating very high rate constants. In this work ferrocene/ferrocenium redox couple was selected as a good model system for theoretical consideration due to the sphericity of the reactant, insignificant molecular rearrangements in the reactant’s molecule after the ET and reversibility of the process. The kinetic data for Fc+/Fc in a series of nonaqueous solvents were frequently used in the analysis of solvent dynamic effects on the ET rates. The most popular strategy in this analysis employs the construction of the plots of the apparent rate constants vs. the solvent longitudinal relaxation times, with the linearity of this plot being generally considered to point to the adiabaticity of the Fc+/Fc reaction and predominant kinetic control by the dynamics of solvent relaxation. In this work, we were able to construct a semi-quantitative description for the Fc+/Fc redox process in a series of molecular and ionic solvents (aprotic solvents, room-temperature ionic liquids and alcohols), based on the combination of molecular modeling and continuum-level estimates in the framework of quantum mechanical theory of charge transfer [3, 4]. This allowed us to revisit the classical test for the reaction adiabaticity. The common approach was shown to be particularly misleading for solvents with a complex dielectric behavior. The molecular-level description of the reaction layer structure in both molecular and ionic solvents was also shown to be essential for the diagnostics of the ET regime and rate constants estimation. The results obtained illustrate the current level of agreement between the predicted and experimental rate constant values. References [1] A.M. Kuznetsov, J. Ulstrup, Electron transfer in chemistry and biology: An introduction to the theory, John Wiley & Sons Ltd, Chichester, UK, 1999. [2] M.J. Weaver. Chem. Rev., 92 (1992) 463-480. [3] V.A. Nikitina, S.A. Kislenko, R.R. Nazmutdinov, M.D. Bronshtein, G.A. Tsirlina. J. Phys. Chem. C, 118 (2014) 6151-6164. [4] V.A. Nikitina, A.V. Rudnev, R.R. Nazmutdinov, G.A. Tsirlina, T. Wandlowski. J. Electroanal. Chem., (2017) doi: 10.1016/j.jelechem.2017.08.006