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

Abstract

The properties of MF₄ molecules (M = Cr, Mo, W) are investigated by the restricted Hartree-Fock method using Möller-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 D_2h structure in the singlet state ¹A₁. The D_4h,(¹ A_1g) and T_d ( 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.