Equilibrium geometries and harmonic vibrational frequencies of lanthanum trihalides LaX 3 (X  F, Cl). A relativistic effective core potential ab initio MO study

Abstract

The equilibrium geometries and the harmonic vibrational frequencies of lanthanum trihalides, LaF 3 and LaCl 3, 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 (C 3v) arrangement for LaF 3. Conversely, the planar (D 3h) geometry is found to be more stable for LaCl 3. The electronic factors affecting the equilibrium structures are discussed and compared to results of identical calculations made for the simpler, purely σ-bonded, LaH 3.