A relativistic effective core potential ab initio study of molecular geometries and vibrational frequencies of lanthanide trihalides LnX3 (Ln = Gd, Lu; X = F, Cl)

The equilibrium geometries and harmonic vibrational frequencies of some lanthanide trihalides LnX3 (Ln = Gd, Lu; X = F, Cl) have been investigated using ab initio relativistic effective core potential Hartree-Fock, complete active space multiconfigurational self-consistent field and second-order Møller-Plesset calculations. In accordance with experimental data, the ab initio results indicate a pyramidal (C3v) arrangement for LnF3. Conversely, the planar (D3h) geometries are more stable for LnCl3. The role of electron correlation appears crucial to predict the trigonal shape of fluorides since the simpler Hartree-Fock approach indicates planar structures. The calculated LnX bond lengths are close to the experimental values. The calculated frequencies are in good agreement with the IR data for all the molecules. Differences are found only for the ν2 vibration due to the fluxional behaviour of the molecules. The ν1 and ν3 stretching modes for both fluoride and chloride molecules increase linearly along the lanthanide series.