RAMAN AND FAR-INFRARED SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT AND ABINITIO CALCULATIONS OF 2,3-DICHLOROPROPENE

The far-infrared spectrum (375 to 30 cm-1) of gaseous 2,3-dichloropropene, CH2 = C(CH2Cl)Cl, was recorded at a resolution of 0.06 cm-1. The fundamental asymmetric torsional mode is observed at 91.9 cm-1 with eight excited states failing to low frequency for the s-cis (chlorine atom eclipsing the double bond) conformer. For the higher energy gauche conformer, the asymmetric torsion is estimated to be near 85 cm-1. Utilizing the s-cis torsional frequency, the gauche dihedral angle and the enthalpy difference between conformers, the potential function governing the interconversion of the rotamers was calculated. The determined potential function coefficients are V1 = 1047 +/- 96, V2 = -619 +/- 68, V3 = 1216 +/- 50 and V4 = 26 +/- 22 cm-1, with an enthalpy difference between the more stable s-cis and gauche conformers of 245 +/- 30 cm-1 (0.70 +/- 0.09 kcal mol-1). This function gives values of 1035 +/- 10 cm-1 (2.96 +/- 0.03 kcal mol-1), 2015 +/- 150 cm-1 (5.76 +/- 0.43 kcal mol-1) and 787 +/- 10 cm-1 (2.25 +/- 0.03 kcal mol-1) for the s-cis to gauche, gauche to gauche and gauche to s-cis barriers, respectively. From the relative intensities of the Raman lines of the liquid at 1633 cm-1 (gauche) and 1649 cm-1 (s-cis) as a function of temperature, the enthalpy difference is found to be 334 +/- 39 cm-1 (0.95 +/- 0.11 kcal mol-1), but now with the gauche conformer more stable. Also, the more polar gauche conformer remains in the crystalline solid. The Raman spectrum of the gas was recorded from 3500 to 70 cm-1 and, utilizing these data and the previously reported infrared data, a complete vibrational analysis is proposed for both conformers. The conformational stability, barriers to internal rotation, fundamental vibrational frequencies and structural parameters which were 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.