Conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of propionyl fluoride

Abstract

AbstractThe Raman (3100–10 cm−1) and infrared spectra (3100–30 cm−1) of propionyl fluoride in the gas and solid phases were recorded. Additionally, the Raman spectrum of the liquid and the qualitative depolarization ratios were also obtained. From these data a complete vibrational assignment is proposed for an s‐trans/gauche equilibrium in the gas and liquid phases, with the s‐trans conformer (the CH3 moiety is s‐trans to the fluorine atom) being the most stable form in both of these physical states and the only form present in the solid phase. A new potential function has been calculated from the data previously obtained from the far‐infrared spectrum of the gas. The determined potential coefficients are V1 = 328 ± 6, V2 = 239 ± 8, V3 = 417 ± 3, V4 = 30 ± 3 and V6 = −4 ± 2 cm−1, and values of 720 ± 8 (2.06 ± 0.02 kcal mol−1) and 297 ± 9 cm−1 (0.85 ± 0.02 kcal mol−1), respectively, were obtained for the s‐trans to gauche and gauche to gauche barriers to internal rotation of the CFO moiety. Additionally, the s‐trans conformer was determined to be 448 ± 17 cm−1 (1.28 ± 0.05 kcal mol−1) more stable than the gauche conformer. From the study of the Raman spectrum obtaind for the liquid at different temperatures, a value of 267 ± 12 cm−1 (0.76 ± 0.03 kcal mol−1) was determined for ΔH between the two conformers. The structural parameters, conformational stabilities, barriers to internal rotation and fundamental vibrational frequencies which were determined experimentally are compared with those obtained from ab initio Hartree‐Fock gradient calculations employing both the RHF/3‐21G and RHF/6–31G* basis sets, and with the corresponding quantities obtained for some similar molecules.