Force constants and fundamental frequencies of the fare earth trihalide molecules

Abstract

The effective nuclear charge model has been applied to the rare earth trihalides. A set of the effective nuclear charge values of the rare earth atoms has been evaluated from the bond stretch force constants obtained by normal coordinate analysis of the infrared spectra of the lanthanide trifluorides. The force constants and the fundamental frequencies of all the lanthanide trihalides have been calculated by means of the effective nuclear charge model. The calculated results are in agreement with the experimental data. The results show that the most probable configuration of the lanthanide trihalides is pyramidal with C 3ν symmetry, although the geometry of the lanthanide trifluoride molecules is very close to planar. It is also found that the frequency ν 2 (symmetric deformation) and the frequency ν 4 (antisymmetric deformation) will vary with the bond angle (XLnX).