Today, existing methods of recycling spent nuclear fuel do not solve the problem of separation of transuranium elements and lanthanides. Electrolysis of molten salts is promising method for separation of lanthanides and actinides. In our previous works we studied the electrochemical behavior of the Sm(III)/Sm(II) redox couple in alkali chlorides melts. In these works. Was suggested an outer-sphere mechanism of electron transfer in samarium-containing melts.

Quantum-chemical studies of the electron transfer mechanism in molten salts based on a direct calculation of the transition state are faced with practically insurmountable computational difficulties. As our experience shows, model systems assigned to investigate the mechanism of charge transfer should include, in addition to the electroactive complex, two more of its coordination spheres. The search for a transition state by standard methods will require enormous computer time and is almost unrealistic.

In this paper, we proposed approach, which is based on the analysis of frontier molecular orbitals (FMO) under various deformations of the initial structure. The aim of this work was a quantum-chemical analysis of the electron transfer process in a model system 18NaCl+Na₃SmF₆ by the FMO method.

The geometry optimization of structures was performed with the Firefly program package, partially based on the source code of the GAMESS(US) program, by the density functional theory DFT/RHF/UHF method with the use of the B3LYP hybrid functional. For the F and Cl atom quasi-relativistic basis set Stuttgart RLC ECP (ECP – effective core potential) was used; for Na – CRENBL ECP; for Sm, the Stuttgart/Cologne group basis set with 4f electrons included in the core was used. In all cases, the search for the optimized geometry was accompanied by control calculation of vibrational frequencies. Because we were interested the state of the complex SmF₆^3- near the cathode surface, on one side of the model system, a flat boundary layer consisting of 12-16 chlorine and sodium ions was formed. The samarium complex was in contact with one of the sodium cations belonging to the boundary layer. All such systems were applied an electric field of 10⁹ V/m, simulating the field of the cathode.

Based on the electrochemical experimental data and the quantum-chemical analysis by FMO in a 18NaCl+Na₃SmF₆ system, a mechanism for the charge transfer was confirmed. As a result, it was found the structure of transition state. For the transition state, the value of the energy barrier was calculated, which was in a good agreement with the experimental data. The high efficiency of FMO approach allows recommending this method as the instrument for testing hypotheses on the electron transfer mechanism in molten salts.