Jahn-Teller effect enhanced by spin-orbit coupling. From theory to Experiment:Example of niobium tetrachloride NbCl4

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 438 cm −1 , respectively. For two-mode 2 E⊗e Jahn-Teller problem the vibronic Schrodinger 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.