Infrared and raman spectra, conformational stability, normal coordinate analysis, ab initio calculations, and vibrational assignment of difluoroacetyl fluoride

Abstract

The Raman (3100–10 cm −1) and i.r. (3200–400 cm −1) spectra of difluoroacetyl fluoride, CHF 2CFO, have been recorded for the gaseous and solid phases. Additionally, the Raman spectrum of the liquid phase has been recorded and qualitative depolarization values have been obtained. Based on these data, it is shown that both the gauche and trans (hydrogen atom trans to the fluorine atom) conformers exist in the fluid phases. For the sample dissolved in liquified xenon, a variable temperature study gave an enthalpy difference of 88 ± 18 cm −1 (251 ± 50 cal mol −1) with the gauche conformer the more stable rotamer. This result is at variance with the ab initio calculations where the trans conformer is predicted to be more stable when electron correlation to second order is utilized. At ambient temperature the gauche conformer is the predominant rotamer in the liquid phase and is the only conformer present in the solid. Complete assignments of observed bands to fundamental vibrations of the gauche conformer are reported and a nearly complete assignment is obtained for the trans conformer. The C-H distance ( r 0 has been obtained from the isolated carbon-hydrogen stretching frequencies for the trans and gauche forms with values of 1.090 and 1.093 Å, respectively. Additionally, other r 0 parameters have been obtained from the previously reported rotational constants for five isotopomers of difluoroacetyl fluoride. All the experimental data are compared with the corresponding quantities obtained from ab initio RHF/3-21G, RHF/6-31G∗, MP2/6-31G∗ and MP2/6-31++G∗ calculations. From the RHF/3-21G calculation the gauche conformer is predicted to be the more stable conformer by 222 cm −1, but the trans conformer is predicted to be more stable by 105, 19 and 56 cm − from the RHF/6-31G∗, MP2/6-31G∗ and MP2/6-31++G∗ calculations, respectively. The structural parameters, vibrational frequencies and harmonic force field have been calculated for both conformers and the values are compared to the experimental data where appropriate.