Vibrational spectra and assignments, normal coordinate analyses, a b i n i t i o calculations, and conformational stability of the propenoyl halides

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Abstract
The infrared and Raman (3500–30 cm−1) spectra of gaseous and solid propenoyl bromide, CH2 CHCBrO, have been recorded. The Raman spectrum of the liquid has been obtained and qualitative depolarization values have been measured. The fundamental asymmetric torsion for both the s‐trans and s‐cis conformers has been observed along with several hot transitions. From these transitions the potential function governing the internal rotation has been determined and the s‐trans to s‐cis barrier is 1861 cm−1 (5.32 kcal/mol), the trans conformer being more stable by 158±20 cm−1 (452±57 cal/mol). All normal modes have been assigned for the s‐trans conformer and many of those for the s‐cis conformer, based on band contours, depolarization values, and group frequencies. A normal coordinate calculation has been carried out by utilizing a modified valence force field to calculate the frequencies and the potential energy distribution for both conformers. Temperature studies of the Raman spectra of gaseous propenoyl fluoride and chloride and of the liquid phases of propenoyl chloride and bromide have been used to obtain the enthalpy differences between the s‐trans and s‐cis conformers for these compounds. For the bromide and chloride the s‐trans conformer is clearly the more stable form but the data are inconclusive for the fluoride. Complete equilibrium geometries have been determined for both rotamers of the fluoride and chloride by ab initio Hartree–Fock gradient calculations employing both 3‐21G and 6‐31G* basis sets. The structural parameters are compared to those suggested from microwave and/or electron diffraction studies of these molecules. Both calculations are consistent with the s‐trans conformation being thermodynamically preferred for the fluoride molecule and the s‐cis conformation for the chloride, which is contrary to the experimental data for the chloride. The 3‐21G basis set was used to obtain the ab initio force constants and frequencies for the s‐trans and s‐cis conformers of both the fluoride and chloride with fixed scaling, variable scaling, and without scaling factors. These results required several reassignments of fundamentals for both conformers of both molecules. Improved vibrational data for both the fluoride and chloride were very valuable for the reassignments. These results are compared with the corresponding quantities for some similar molecules.