FAR-INFRARED SPECTRUM, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, ABINITIO CALCULATIONS, AND VIBRATIONAL ASSIGNMENT OF PROPIONYL BROMIDE

The Raman (3200 to 10 cm-1) and IR spectra (3200 to 30 cm-1) of propionyl bromide, CH3CH2CBrO, of the gas and solid have been recorded. Additionally, the Raman spectrum of the liquid has been recorded with qualitative depolarization measurements. All of these data indicate that propionyl bromide exists as a mixture of two conformers in the fluid phases with one being a high energy gauche form and the other the thermodynamically preferred s-trans rotamer (Br trans to CH3) which is the only form remaining in the annealed solid. From the studies of the Raman spectra at different temperatures, the enthalpy differences have been determined to be 403 +/- 28 cm-1 (1.15 +/- 0.08 kcal mol-1) and 221 +/- 25 cm-1 (0.63 +/- 0.07 kcal mol-1) for the gas and liquid, respectively, with the s-trans conformer being the more stable form in both phases. The fundamental asymmetric torsions for both the s-trans and gauche conformers have been observed in the far-IR spectrum of the gas at 75.68 and 71.38 cm-1, respectively, with each having several excited state transitions falling to lower frequencies. A potential function has been calculated from the observed asymmetric torsional frequencies with the following potential coefficients: V1 = 627 +/- 46, V2 = -239 +/- 39, V3 = 744 +/- 14, V4 = -17 +/- 6, and V6 = 14 +/- 4 cm-1. From this potential function, values of 711 +/- 8, 1119 +/- 58, and 459 +/- 9 cm-1 were obtained for the s-trans to gauche, gauche to gauche and gauche to s-trans barrier, respectively, with an enthalpy difference of 252 +/- 99 cm-1. A complete vibrational assignment supported by normal coordinate calculations has been proposed for both conformers based on the observed IR band contours, the depolarization data, and group frequencies. Additionally, ab initio calculations have been carried out with the STO-3G* basis set from which the fundamental vibrational frequencies, conformational stabilities, barriers to internal rotation, and structural parameters have been obtained. The results of this study are compared to similar ones for the corresponding propionyl fluoride and propionyl chloride molecules.