EQUILIBRIUM GEOMETRIES AND HARMONIC VIBRATIONAL FREQUENCIES OF LANTHANUM TRIHALIDES LAX3 (X=F, CL) - A RELATIVISTIC EFFECTIVE CORE POTENTIAL AB-INITIO MO STUDY

The equilibrium geometries and the harmonic vibrational frequencies of lanthanum trihalides, LaF3 and LaCl3, have been studied using ab initio relativistic effective core potential Hartree-Fock, complete active space multiconfigurational self-consistent field and configuration interaction calculations. In accordance with experimental data, present ab initio results indicate a pyramidal (C3v) arrangement for LaF3. Conversely, the planar (D3h) geometry is found to be more stable for LaCl3. The electronic factors affecting the equilibrium structures are discussed and compared to results of identical calculations made for the simpler, purely sigma-bonded, LaH3.