Structure, force fields, and vibrational spectra of cerium tetrahalides

The geometrical structure, force fields, and vibrational spectra of CeX4 (X = F, Cl, Br, I) were investigated by second, third, and fourth order Moeller-Plesset perturbation theory, CISD+Q configuration interaction method, and the CCSD(T) coupled cluster method. Calculations on CeF4 were also performed by multiconfiguration second order perturbation theory MCQDPT2/CASSCF. The wave function of the ground state of CeX4 molecules was found to be appreciably non-one-configurational; this property increases from cerium fluorides to iodides and leads to the divergence of the series of Moeller-Plesset perturbation theory. The calculated data point to a tetrahedral equilibrium nuclear configuration in CeX4 molecules. The energy barriers to the inversion of the tetrahedral CeX4 molecules via the square configurations are high enough, 74-89 kJ/mol. The calculated vibration frequencies, effective internuclear distances, and mean amplitudes of nuclear vibrations in CeF4 agree with IR and Raman spectroscopic and high-temperature gas-phase electron diffraction data