CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT, AND RHF/STO-3G-STAR CALCULATIONS OF 2-BROMOPROPENOYL FLUORIDE

The far-infrared spectrum of gaseous 2-bromopropenoyl fluoride, CH2CBrCFO, has been recorded at a resolution of 0.10 cm-1 in the region 350-35 cm-1. The fundamental asymmetric torsional frequencies of the more stable s-trans (two double bonds oriented trans to one another) and the high-energy s-cis conformations have been observed at 61.90 and 44.27 cm-1, respectively, each with several excited states appearing at low frequencies. From these data, the asymmetric torsional potential function governing internal rotation about the C-C bond has been determined. The potential coefficients are V1 = -17 +/- 12, V2 = 1506 +/- 48, V3 = 321 +/- 19, V4 = -43 +/- 14, and V5 = -48 +/- 7 cm-1. The s-trans-s-cis and s-cis-s-trans barriers have been determined to be 1684 +/- 35 and 1428 +/- 21 cm-1, respectively, with an energy difference between the conformations of 256 +/- 79 cm-1 (732 +/- 226 cal/mol). From studies of the Raman spectrum at variable temperatures, the conformational enthalpy difference has been determined to be 247 +/- 60 cm-1 (706 +/- 172 cal/mol) and 340 +/- 22 cm-1 (972 +/- 63 cal/mol) for the gas and liquid, respectively. A complete assignment of the vibrational fundamentals observed from the infrared spectra (3500-50 cm-1) of the gas and solid and the Raman spectra (3200-10 cm-1) of all three physical states is proposed. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing the STO-3G* basis set. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.