The molecular vibrations of sulphur hexafluoride and sulphur tetrafluoride

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Abstract
Force constants are calculated for sulphur hexafluoride employing a valence force field and a central force field (Orbital Valency Force Field [5]). Both force fields yield the same root-mean-square vibration amplitudes to two significant figures, and the calculated values agree with an experimental determination. The significance of the force constants derived from the valence force field calculation for sulphur tetrafluoride is discussed. The parallel is pointed out between the smaller value for the S-F stretching force constant in sulphur tetrafluoride as compared with sulphur hexafluoride and the increase in the S-F bond length, illustrating that the introduction of d electrons into the bonding orbitals does not affect materially the form of the relationship between force constant and bond length. The inversion vibration of sulphur tetrafluoride is discussed in relation to the calculated value of the vibration amplitudes and the configuration of the molecule as determined by electron diffraction of its vapour.