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

Abstract

The infrared spectra of ⁴⁰CaF₂, ⁴⁴CaF₂, ⁸⁶SrF₂, ⁸⁸SrF₂, and BaF₂ trapped in solid krypton matrices at 20°K 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 158°. For BaF₂ the apex angle has been estimated at 100°. 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.