Geometry and Vibrational Spectra of Alkaline-Earth Dihalides, Ii. CaF2, SrF2 and BaF2

摘要

The infrared spectra of 40CaF2, 44CaF2, 86SrF2, 88SrF2 and BaF2 trapped in solid krypton matrices at 20K are reported. From precise measurements of the shifts of the vibrational modes on isotopic substitution upper and lower limits for the apex angles of these molecules can be established. The mean values of the apex angle based on these limits are found to be 140 for CaF2, 108 for SrF2, and a reanalysis of the earlier data for MgF2 gives 150. For BaF2 the apex angle has been estimated at 100C. Using a valence force field a set of force constants have been obtained from the measured fundamental frequencies, as well as approximate values for the anharmonic corrections to these vibrational modes. The statistical entropies calculated from the vibrational frequencies and molecular geometries are in excellent agreement with the thermodynamic measurements. (Author)