A coupled-cluster study of lanthanum trihalide structures

Coupled-cluster calculations were carried at the CCSD(T)-level in order to determine the structures and vibrational spectra of the lanthanum trihalides LaF₃ and LaCl₃. Two different approaches were employed to describe scalar relativistic effects; the quasi-relativistic Effective Core Potential approach and the all-electron Douglas-Kroll approach. We find LaF₃ to be of pyramidal structure and LaCl₃ to be planar. The obtained vibrational frequencies accord well with experimental measurements except for the v₂ mode. This disagreement is attributed to the intrinsic uncertainty of harmonic frequencies of this fluxional mode as extracted from spectra which were obtained at high temperature and for matrix isolated species.