Vibrational analysis and ab initio molecular orbital calculations of the molecular force constants of gaseous CF 4 and SiF 4

Abstract

The molecular potential constants of gaseous CF 4 and SiF 4 have been evaluated using approximate force fields and ab initio quantum mechanical methods. A recently developed three-parameter approximate force field, called the adjusted valence force field (AVFF), is found to give force constants for CF 4 and SiF 4 that are in reasonably good agreement with published values based on detailed analysis of vibrational data (including isotopic frequencies and Coriolis constants) and other three-parameter approximate force fields, such as the Urey—Bradley force field (UBFF) and the orbital valence force field (OVFF). In the ab initio studies, force constants were calculated at several basis set levels; correlation effects were included by applying second-order Möller Plesset theory. At the highest level of the theory (6–31 G* basis set), the ab initio force constants for CF 4 are in very good agreement with the most accurately derived values reported in the literature. The ab initio force constants computed for SiF 4 are in reasonable accord with accurately determined values, but the agreement is not as good as that obtained for CF 4.