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

The ground and first excited electronic states of VF 4 , NbF 4 , and TaF 4 molecules were studied by the multiconfiguration complete-active-space self-consistent-field (CASSCF) method. For the tetrahedral nuclear configuration, the symmetry of these states is 2 E and 2 T 2 , respectively. The 2 E → 2 T 2 electron excitation energies were determined: 11 610, 13 450, and 12 560 cm - 1 for VF 4 , NbF 4 , and TaF 4 respectively. In agreement with the Jahn-Teller theorem, calculations demonstrated the instability of the tetrahedral configurations of the molecules in orbitally degenerate electronic states 2 E and 2 T 2 . Characteristics were determined for sets of molecular potential energy surfaces corresponding to the deformation of the tetrahedral nuclear configuration along the Jahn-Teller-active vibrational coordinates of symmetry e. The lowest energy equilibrium geometric configuration of molecules has the D 2 d symmetry both in the ground ( 2 A 1 ) and first excited ( 2 B 2 ) electronic states. The 2 A 1 → 2 B 2 adiabatic electron excitation energy is 8440, 9050, and 11920 cm - 1 for VF 4 , NbF 4 , and TaF 4 , respectively. Jahn-Teller distortions of the tetrahedral structure and Jahn-Teller stabilization energies E J T substantially increase in the series of molecules VF 4 → NbF 4 → TaF 4 : E J T = E(T d , 2 E) -E(D 2 d , 2 A 1 ) = 412, 1856, and 5970 cm - 1 , respectively; E J T = E(T d , 2 T 2 ) - E(D 2 d , 2 B 2 ) = 3584, 6259, and 6611 cm - 1 , respectively. Quadratic force constants, the harmonic frequencies and normal modes of all the vibrations, and IR intensities were calculated for VF 4 , NbF 4 , and TaF 4 molecules in the ground electronic state.