STRUCTURAL AND THERMOCHEMICAL AB-INITIO STUDIES OF LANTHANIDE TRIHALIDE MOLECULES WITH PSEUDOPOTENTIALS

Structural and vibrational frequency studies have been carried out for 30 lanthanide trihalide molecules LnX(3) (Ln = La-Lu; X = Cl, F) usin g ab initio quasi-relativistic effective core potentials at Hartree-Fo ck and Moller-Plesset post-Hartree-Fock levels of theory. A planar D-3 h geometry was obtained for all trichloride molecules. In accordance w ith previous theoretical works, a C-3v, structure was clearly establis hed for the majority of the trifluorides. Calculated bond lengths and vibrational frequencies are in good agreement with the experimental da ta. Therefore, thermochemical functions have been estimated from therm ostatistics calculations combined with our quantum mechanics results. The electronic partition function is crucial in order to be in accorda nce with experimental data tables, especially for temperatures higher than 500 K. A simple example of thermochemical equilibrium clearly pro ves the validity of the estimated functions in the case of the lanthan um halides.