Spectra and structure of organophosphorus compounds. LVI. Vibrational spectra, conformational stability, ab initio calculations, and vibrational assignment of fluoromethyl phosphonic difluoride

Abstract

Abstract The Raman (3200 to 10 cm −1 ) and infrared (3200 to 30 cm −1 ) spectra of fluoromethyl phosphonic difluoride, FCH 2 P(O)F 2 , in the gas and solid phases have been recorded. Additionally, the Raman spectrum of the liquid along with qualitative depolarization ratios have been obtained. These data show that there is an equilibrium between the trans (fluorine atom of the fluoromethyl group trans to the oxygen atom) and gauche conformers in the gas and liquid phases, with the trans conformer being the more stable form in both of these physical states and the only form present in the crystalline solid. From the study of the Raman spectrum of the liquid as a function of temperature, a value of 157 ± 36 cm −1 (448 ± 103 cal/mol) has been determined for ΔH . A complete vibrational assignment is proposed for both conformers based on infrared band contours, Raman depolarization data, group frequencies, and normal coordinate calculations. The conformational stabilities, barriers to internal rotation, force constants, infrared and Raman intensities and fundamental vibrational frequencies, along with the structural parameters, have been obtained from RHF/3-21G ∗ , RHF/6-31G ∗ , and/or MP2/6-31G ∗ ab initio calculations. The calculated Raman intensities reproduce the observed Raman spectrum remarkably well. Many of these results are compared to the corresponding quantities for some similar molecules.