Conformational stability, barriers to internal rotation, normal coordinate analyses, and vibrational assignments of bromoacetyl halides

Abstract

The Raman (3200-10 cm −1) and IR (3200-30 cm −1) spectra of bromoacetyl fluoride, bromoacetyl chloride and bromoacetyl bromide in the gaseous and annealed solid phases have been recorded. Additionally, the Raman spectra and the qualitative depolarization measurements for each molecule in the liquid phase have been obtained. All of these data indicate that these bromoacetyl halides exist in mixtures of two conformers, with the trans rotamer (halogen atoms oriented trans to one another) being the thermodynamically preferred, and the gauche conformation being the high energy form. From the studies of Raman spectra at different temperatures, the enthalpy differences have been determined for the gaseous and liquid phases, respectively, to be 185 ± 23 cm −1 (0.53 ± 0.06 kcal mol −1) and 249 ± 27 cm −1 (0.71 ± 0.08 kcal mol −1) for bromoacetyl fluoride, 391 ± 130 cm −1 (1.12 ± 0.37 kcal mol −1) and 317 ± 27 cm −1 (0.91 ± 0.08 kcal mol −1) for bromoacetyl chloride, and 341 ± 48 cm −1 (0.98 ± 0.14 kcal mol −1) and 286 ± 26 cm −1 (0.82 ± 0.07 kcal mol −1) for bromoacetyl bromide. Potential functions have been calculated for the fluoride and chloride from the observed asymmetric torsional frequencies and values of 580 ± 11 cm −1 (1.66 ± 0.03 kcal mol −1), 359 ± 8 cm −1 (1.03 ± 0.02 kcal mol −1), and 387 ± 16 cm −1 (1.11 ± 0.04 kcal mol −1) for the trans to gauche, gauche to gauche to gauche and gauche to trans barrier, respectively, were determined for bromoacetyl fluoride. The corresponding quantities were 967 ± 30 cm −1 (2.76 ± 0.08 kcal mol −1), 696 ± 67 cm −1 (1.99 ± 0.19 kcal mol −1), and 663 ± 30 cm −1 (1.61 ± 0.08 kcal mol −1) for bromoacetyl chloride. These potential functions are consistent with the dihedral angle of the gauche conformation being 120° and 117° for the fluoride and chloride, respectively. Complete vibrational assignments have been carried out based on the observed IR band contours, the depolarization ratios, and group frequencies. The results are compared to those for other haloacetyl halides.