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The potential energy constants and vibrational mean amplitudes for thionyl tetrafluoride were calculated using the Urey-Bradley Force Field (UBFF) with a view to finding out the influence of the oxygen atom on the SF4 group. It was found that the axial S-F stretching force constant is smaller than the equatorial S-F stretching force constant, which is consistent with the variation in the bond length. The behaviour of the oxygen atom is found to be similar to the lone pair of electrons in sulphur tetrafluoride. The vibrational mean amplitudes calculated from spectroscopic data are in very good agreement with the electron diffraction data. The large root mean square amplitude of vibration for equatorial-equatorial non-bonded fluorine atoms (0.099 Ǻ) in sulphur tetrafluoride indicates the possibility of incomplete inversion as pointed out by Chantry and Ewing.

An attempt has been made to modify the UBFF for chlorine trifluoride by taking into account the presence of lone pairs of electrons, on the lines suggested by Pariseau, Wu, and Overend. It was found that the lone-pair-bond-pair interaction is less than the lone-pair-lone-pair interaction which is considerably lower than the stretching force constant for the lone pair of electrons. An approximate relation between the above interactions was obtained.

The potential energy constants of thionyl fluoride and sulphuryl fluoride were calculated using a modified UBRF. The behaviour of the oxygen atom is found to be similar to that of the lone pair of electrons in thionyl fluoride. The vibrational mean amplitudes of thionyl fluoride and sulphuryl fluoride were also calculated from the vibrational spectral data.