AB initio Calculations of the structure and spectra of chromium, molybdenum, and tungsten tetrafluorides. nontetrahedral molecular structure of WF4

The properties of MF4 molecules (M = Cr, Mo, W) are investigated by the restricted Hartree-Fock method using Moller-Plesset second-order perturbation theory and by the second-order configuration interaction method using the multiconfigumtion wave function derived in a complete active space approximation, in wide bases complemented with polarization d and f functions. Relativistic effective potentials are used to describe the core electrons. For CrF4 and MoF4, the tetrahedral configuration of nuclei in the electronic state of3k2 symmetry is energetically most favorable. In the WF4 molecule, the least-energy structure is a D2hstructure in the singlet state 1A1. The D4h,(1 A1g) and Td ( 3A2) configurations in the WF4 molecule are higher on the energy scale than the ground state by 4 and 6305 cm’1 and are saddle points. For all of the analyzed configurations of MF4 molecules, the geometrical parameters, the vibrational spectra, and the energies of vertical electronic transitions are found. The chemical bonding is analyzed and a simple model is proposed to explain the variation of the relative energies of states in the series CrF4→MoF4→WF4.