Molecular force fields of octahedral XF₆ molecules

The orbital valency force field (OVFF) is applied to the fifteen hexafluorides whose vibrational frequencies are known, and the best-fit force constants are calculated. The OVFF showed definite advantages over the more widely used UBFF model in that (a) the angular coordinates contain no redundancy; (b) the agreement with the observed frequencies in the angle deformation modes is far superior; and (c) the least squares calculation procedure yields rapidly-converging sets of force constants for all fifteen hexafluorides. The f_1u force constants of some of the hexafluorides are determined from their infrared band envelopes and are compared with the calculated values. The angle distortion constant D and the repulsion constant F are found to have strong dependence upon the number of nonbonding electrons in the 4d and 5d transition metal series. This effect is discussed in terms of the repulsion between the nonbonding and bonding electrons and the Coulombic forces between partially ionic fluorine atoms.