Infrared and Raman spectra, conformational stability, vibration assignment, and ab initio calculations for 3-bromo-3,3-difluoropropene

Abstract

The infrared spectra (3200–50cm−1) of gaseous and solid and Raman spectra (3200–10cm−1) of the liquid with qualitative depolarization ratios and solid 3-bromo-3,3-difluoropropene CH2CHCBrF2 have been recorded. Both the gauche and cis conformers have been identified in the fluid phase, but the gauche conformer is thermodynamically more stable than the cis rotamer and it is the only rotamer present in the spectrum of the annealed solid. Variable temperature (−105 to −150°C) studies of the infrared spectra of the sample dissolved in liquid krypton has been carried out. From these data, the enthalpy difference has been determined to be 281±28cm−1 (3.36±0.33kJmol−1), with the gauche conformer as the more stable rotamer, which is in agreement with the ab initio predictions at all levels of calculations. It is estimated that there is only 11% of the cis conformer present at an ambient temperature. A complete vibration assignment is proposed for the gauche conformer which is based on infrared band contours, depolarization values, and group frequencies which is supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G (d) calculations. The conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from the ab initio calculations. The results are discussed and compared with the corresponding properties of some similar molecules.