RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, ABINITIO CALCULATIONS AND VIBRATIONAL ASSIGNMENT FOR BROMODIFLUOROACETYL CHLORIDE

The Raman (1800-10 cm-1) and infrared (2000-30 cm-1) spectra of bromodifluoroacetyl chloride, CBrF2CClO, are reported for the gas and polycrystalline solid. Additionally, the Raman spectrum of the liquid along with qualitative depolarization ratios have also been obtained. These data have been interpreted on the basis of an equilibrium between the gauche and the trans conformers (bromine atom trans to the chlorine atom) in the gas and liquid phases. From a study of the Raman spectrum of the liquid at various temperatures, a value of 851 +/- 56 cm-1 (2.43 +/- 0.07 kcal mol-1) was obtained for DELTAH with the gauche conformer the more stable form. A similar study of the gas gave a value of 725 +/- 150 cm-1 (2.07 +/- 0.43 kcal mol-1), again with the gauche rotamer the more stable conformer. Hence the gauche conformer is the predominate rotamer in the gas and liquid and the only conformer present in the annealed solid. Aided by ab initio and scaled computations of the vibrational spectrum with the STO-3G* basis set and, for the trans conformer at the 4-31G*/MIDI-4* level of the theory, complete assignments of observed bands to fundamental vibrations of the gauche conformer are made, whereas partial assignment is reported for the trans conformer. The fundamental torsional mode for the gauche conformer was observed at 46 cm-1 but excited-state transitions were not observed so the potential function governing the conformational interchange could not be determined. Optimized geometries, dipole moments, unscaled and scaled vibrational frequencies and harmonic force fields are reported for both conformers. With the STO-3G* basis set, the gauche conformer is calculated to be more stable than the trans conformer by 17 cm-1 (48 cal mole-1). These results are compared with the corresponding quantities for some similar molecules.