Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignment of fluoromethyl phosphonothioic dichloride

The Raman spectra from 3100 to 10 cm(-1) of the gaseous, liquid and solid phases and the infrared spectra from 3100 to 30 cm(-1) of the vapour and solid phases of fluoromethyl phosphonothioic dichloride, FCH2P(S)Cl-2, were recorded. The spectral data were interpreted on the basis of a conformational equilibrium between the symmetric anti conformer (fluorine atom trans with respect to the sulfur atom) and the gauche conformer in the vapour and liquid phases, with the anti conformer being the more stable rotamer and the only conformer remaining in the annealed solid. From a variable-temperature study of the Raman spectrum of the liquid, a value of 250 +/- 20 cm(-1) (2.99 +/- 0.24 kJ mol(-1)) was determined for the enthalpy difference between the two conformers in the liquid phase. A vibrational assignment for both conformers is proposed. Ab initio calculations were carried out at RHF/3-21G*, RHF/6-31G* and MP2/6-31G* levels and the calculated equilibrium structures, vibrational wavenumbers, infrared and Raman intensities and the potential function governing the conformational equilibrium are reported. The calculated Raman intensities were graphically compared with the experimental spectra. The results were compared with the corresponding results for some similar molecules. (C) 1998 John Wiley & Sons, Ltd.