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

The far infrared spectrum (375 to 30 cm-1) of gaseous propionyl chloride has been recorded at a resolution of 0.10 cm-1. A substantial number of bands have been assigned to both the symmetric and asymmetric torsional modes for both the s-trans (oxygen atom eclipsing the methyl group) and the higher energy gauche conformers. From these data both the symmetric and asymmetric torsional potential functions have been calculated. The potential function coefficients for the asymmetric torsion are calculated to be (in cm-1) V1 = 872 +/- 40, V2 = 164 +/- 31, V3 = 644 +/- 10, and V4 = 18 +/- 6, with an enthalphy difference between the more stable s-trans and the gauche conformers of 491 +/- 81 cm-1 (1.40 +/- 0.23 kcal/mol). This function gives values of 760 +/- 6 cm-1 (2.17 +/- 0.02 kcal/mol), 1026 +/- 39 cm-1 (2.93 +/- 0.11 kcal/mol), and 269 +/- 11 cm-1 (0.77 +/- 0.03 kcal/mol) for the s-trans to gauche, gauche to gauche, and gauche to s-trans barriers, respectively. From this potential function, the dihedral angle of the gauche conformer is calculated to be 108.5 +/- 0.5-degrees. From the studies of the Raman spectra at different temperatures, the enthalpy difference was determined to be 596 +/- 45 cm-1 (1.70 +/- 0.13 kcal/mol) and 607 +/- 50 cm-1 (1.74 +/- 0.14 kcal/mol) for the gas and liquid, respectively. The barrier to methyl rotation for the s-trans conformer is found to be 950 +/- 2 cm-1 (2.72 kcal/mol). The r(o) structural parameters for the heavy atom skeleton of the s-trans conformer have been calculated from the previously reported rotational constants of the Cl-35 and Cl-37 isotopic species. The conformational stability, barriers to internal rotation, fundamental vibrational frequencies, and structural parameters that have been determined experimentally are compared to those obtained from ab initio Hartree-Fock gradient calculations employing both the 3-21G* and 6-31G* basis sets and to the corresponding quantities for some similar molecules.