Vibrational spectra and assignments, normal coordinate analyses,abinitiocalculations, and conformational stability of the propenoyl halides

摘要

The infrared and Raman (3500ndash;30 cmminus;1) spectra of gaseous and solid propenoyl bromide, CH2CHCBrO, 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 theshyphen;transandshyphen;cisconformers has been observed along with several hot transitions. From these transitions the potential function governing the internal rotation has been determined and theshyphen;transtoshyphen;cisbarrier is 1861 cmminus;1(5.32 kcal/mol), thetransconformer being more stable by 158plusmn;20 cmminus;1(452plusmn;57 cal/mol). All normal modes have been assigned for theshyphen;transconformer and many of those for theshyphen;cisconformer, 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 theshyphen;transandshyphen;cisconformers for these compounds. For the bromide and chloride theshyphen;transconformer 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 fluorideand chloride byabinitioHartreendash;Fock gradient calculations employing both 3hyphen;21G and 6hyphen;31Gast; 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 theshyphen;transconformation being thermodynamically preferred for the fluoride molecule and theshyphen;cisconformation for the chloride, which is contrary to the experimental data for the chloride. The 3hyphen;21G basis set was used to obtain theabinitioforce constants and frequencies for theshyphen;transandshyphen;cisconformers 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.