Conformational stability from temperature-dependent FT-IR spectra of liquid rare gas solutions, barriers to internal rotation, vibrational assignment, and ab initio calculations for 3-chloropropene

The infrared spectra of 3-chloropropene, (allyl chloride) CH2=CHCH2Cl, dissolved in liquid argon (94-124 K), liquid krypton (117-167 K), and liquid xenon (161-221 K) at concentrations of about 1 x 10(-2) M are reported. In argon the solubility is too low to determine the enthalpy difference between the conformers. For liquid krypton and xenon the temperature dependence of the infrared spectra in the CH2 deformation region has been used to obtain an enthalpy difference of 96 +/- 5 cm(-1) (1.15 +/- 0.06 kJ/mol) for the krypton solution and 147 +/- 20 cm(-1) (1.76 +/- 0.24 kJ/mol) for the xenon solution with the gauche conformer the more stable rotamer. The asymmetric torsional transitions have been remeasured in the far-infrared spectrum and the fundamental for the cis conformer is observed at 147.3 cm(-1) with four successive hot bands falling to lower frequency and the fundamental for the gauche conformer is observed at 102 cm(-1) with four accompanying hot bands. Utilizing the enthalpy and torsional data along with the gauche dihedral angle (ClCCC) of 120.0 degrees, the potential function governing the conformational interchange has been determined. The cis to gauche, gauche to gauche, and gauche to cis barriers have been determined to be 881, 699, and 1005 cm(-1), respectively. The structural parameters, conformational stability, infrared intensities, Raman activities, and depolarization ratios, along with the vibrational frequencies, have been obtained from ab initio calculations at the MP2/6-31G(d) level. These data are compared to the corresponding experimental quantities, and the overall results are compared and contrasted to those for some similar molecules.