Conformational Stability, Barriers to Internal Rotation, Vibrational Assignment, and RHF/STO-3G* Calculations of 2-Bromopropenoyl Fluoride

Article abstract of DOI:10.1021/j100162a026

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 lower 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.