Jahn-Teller effect in vanadium, niobium and tantalum tetrafluoride molecules: ab initio study by the CASSCF method

The basic and the first exciting electronic state of the VF4, NbF4 and TaF4 molecules were studied by the self-consistent field multifunctional method in the approximation of active orbitals full space. The symmetry of these states is defined as 2E and 2T2 at tetrahedron configuration of the nuclei. The energies of electron excitation 2E → 2T2 comprise 11610 (VF4), 13450 (NbF4) and 12560 cm-1 (TaF4). In line to Jahn-Teller theorem the calculations evidenced instability of tetrahedron configuration of the molecules in the orbital-singular electronic states 2E and 2T2. The properties of the systems of the molecule potential energies were found, in keeping with deformation of tetrahedron configuration of the nuclei along the oscillation coordinates of the e symmetry, active in the Jahn-Teller effect. Equilibrium geometric configuration of the molecules with the lowest energy has a symmetry D2d as in the basic (2A1), so in the first excited electronic state. The energy of the adiabatic electronic excitation 2A1 → 2B2 is equal to 8440 (VF4), 9050 (NbF4) and 11920 cm-1 (TaF4). Deviations of molecules equilibrium geometry on the structure of the normal tetrahedron and the energy of Jahn-Teller stabilization EJT grow essentially in the series of the molecules VF4 → NbF4 → TaF4: EJT=E(Td,2E)Epy - E(D2d, 2A1)=412, 1856, 5970 cm-1; EJT=E(Td, 2T2) - E(D2d, 2B2)=3584, 6259, 6611 cm-1. The quadratic force constants, the frequencies of normal oscillations and the intensities in the IR spectra of the VF4, NbF4 and TaF4 molecules in the basic state were calculated