Infrared and raman spectra, vibrational assignment, and conformational stability of methacryloyl bromide

Abstract

The IR (3500 to 20 cm −1) and Raman (3200 to 10 cm −1) spectra have been recorded for gaseous and solid methacryloyl bromide, CH 2C(CH 3)C(O)Br. Additionally, the Raman spectra of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the s- trans conformer (the two double bonds are oriented trans to one another) is the thermodynamically preferred rotamer in the gaseous and liquid phases and the only rotamer present in the spectra of the annealed solid. A second high energy conformer is present in the fluid phases at ambient temperature and spectral features are consistent with it being the gauche conformer but the possibility that it is the s- cis conformer cannot be completely eliminated. From a temperature study of the Raman spectrum of the liquid an enthalpy difference of 326±12 cm −1 (932±34 cal mol −1) has been determined. The methyl and asymmetric torsional modes for the s- trans conformer have been assigned at 166.6 and 48 cm −1, respectively, from the IR spectrum of the vapor. The threefold barrier to internal rotation of the methyl group has been calculated to be 662±2 cm −1 (1.89 kcal mol −1). A complete vibrational assignment is proposed based on IR band contours, depolarization values and group frequencies. These results are compared to similar quantities in some related molecules.