Jahn-Teller effect enhanced by spin-orbit coupling. From theory to Experiment:Example of niobium tetrachloride NbCl 4 Valery V. Sliznev * , Sergey V. Smorodin, Georgiy V. Girichev Ivanovo State University of Chemistry and Technology, Research Institute for Thermodynamics and Kinetics of Chemical Processes,153460, Ivanovo, Russia article info Article history: Received 17 April 2019 Received in revised form 30 May 2019 Accepted 3 June 2019 Available online 4 June 2019 Keywords: Niobium chloride Jahn-teller effect Spin-orbit coupling Vibronic levels Thermal average parameters abstract The ground and lower-lying excited electronic states of NbCl 4 molecule were systematically studied by the complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (MCQDPT2) methods. Scalar-relativistic effects and spin-orbit coupling (SOC) have been taken into account employing the third-order Douglas-Kroll-Hess (DKH) Hamiltonian and full Breit-Pauli operator, respectively. The high-symmetrical T d structure with spin-orbit ground 2 G 3/ 2 state undergoes Jahn-Teller distortion along all non-totally symmetric vibrational coordinates. As a result the compressed ( 2 E 1/2 state) and elongated ( 2 E 3/2 state) D 2d structures correspond to minimum and first order saddle point on the lower sheet of adiabatic potential energy surface (APES). The height of the warping barrier in the trough of APES and the Jahn-Teller stabilization energy are 179 and 438cm 1 , respectively. For two-mode 2 E5e Jahn-Teller problem the vibronic Schr€ odinger equation has been solved in the diabatic basis. It was shown, the topology of the lower sheet determined mainly the energies and wave functions ground and few low-lying vibronic levels. The original methodology of the simulation of gas-phase electron diffraction (GED) data was developed. This methodology is based on the analytic description of some APES numerically obtained. The computed radial distribution function and thermal average parameters were compared with the experimental GED data. A good agreement between experimental and theoretical results supports the reliability of the theoretical data. © 2019 Elsevier B.V. All rights reserved. 1. Introduction The niobium chlorides attract the considerable interest because of their important role as a metal transporter in the chloride method of refinement in metallic niobium production. The niobium is widely used as the construction material for different technical equipment, which operate at high temperature in aggressive me- dium. To simulate chemical equilibrium by statistical thermody- namic methods while improving technological processes involving the gas phase of niobium tetrachloride, it is necessary to know its molecular structure and nuclear dynamics. On the other hand, the geometric and electronic structure of niobium tetrachloride molecule deserves a specific attention from the point of view of theoretical chemistry. For describing of NbCl 4 structure it is necessary to develop an approach that takes into account the presence of low-lying excited electron states, as well as the mani- festation of the spin-orbit coupling and the Jahn-Teller effect. According to mass spectrometric study [1], the vapor over solid niobium tetrachloride in 500e600  C temperature range contains only NbCl 4 molecular form. The molecular structure of free NbCl 4 molecule has been studied by gas-phase electron diffraction (GED) [1]. The analysis of diffraction intensities of NbCl 4 was carried out in terms of the intramolecular potential function including vibronic interaction. The authors [1] suppose T d configuration can be slightly distorted to D 2d structure due to the dynamic JahneTeller effect. The JahneTeller stabilization energy estimated in Ref. [1] for NbCl 4 is less than 306 cm 1 . The only work [2] employing CASSCF method reports the geometrical and electronic structure of V, Nb, and Ta tetrafluorides. According to Ref. [2] the T d configuration of V, Nb and Ta tetraflu- orides possesses two-fold degenerate orbital 2 E ground state and undergoes Jahn-Teller distortion to D 2d structures. The compressed D 2d structure with the 2 A 1 electronic state corresponds to mini- mum on the lower sheet of Jahn-Teller potential energy surface. The available up to date information about geometrical and electronic structure of the niobium tetrachloride is poor. The effects * Corresponding author. E-mail address: sliznev@isuct.ru (V.V. Sliznev). Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc https://doi.org/10.1016/j.molstruc.2019.06.010 0022-2860/© 2019 Elsevier B.V. All rights reserved. Journal of Molecular Structure 1195 (2019) 598e605