Infrared and raman spectra, conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of difluoroacetyl chloride

Abstract

The Raman (3100–10 cm −1) and infrared (3100–30 cm −1) spectra of difluoroacetyl chloride, CHF 2CClO, in the gas and solid phases have been recorded. Additionally, the Raman spectrum of the liquid with qualitative depolarization ratios has been obtained. From these data, a trans/gauche equilibrium is proposed in the gas and liquid phases, with the trans conformer (hydrogen atom eclipsing the oxygen atom and trans to the chlorine atom) the more stable form in the gas, but the gauche rotamer is more stable in the liquid and is the only form present in the annealed solid. From the study of the Raman spectrum of the gas at different temperatures, a value of 272 ± 115 cm −1 (778 ± 329 cal mol −1) was determined for Δ H, with the trans conformer the more stable form. Similar studies were carried out on the liquid and a value of 109 ± 9 cm −1 (312 ± 26 cal mol −1) was obtained for Δ H, but now the gauche conformer is the more stable form. A potential function for the conformational interchange has been determined with the following potential constants: V 1 = 397 ± 23, V 2 = −101 ± 5, V 3 = 474 ± 3, V 4 = −50 ± 3, and V 6 = 10 ± 2 cm −1. This potential has the trans rotamer more stable by 179 ± 31 cm −1 (512 ± 89 cal mol −1) than the gauche conformer. A complete vibrational assignment is proposed for both conformers based on infrared band contours, Raman depolarization data, group frequencies and normal coordinate calculations. The experimental conformational stability, barriers to internal rotation, and fundamental vibrational frequencies 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 to the corresponding quantities obtained for some similar molecules.