The far-infrared spectrum (375 to 30 cm-') of gaseous 2,bdichloropropene, CH,=C(CH,Cl)CI, was recorded at a resolution of 0.06 cm-'. The fundamental asymmetric torsional mode is observed at 91.9 cm-' with eight excited states falling to low frequency for the s-cis (chlorine atom eclipsing the double bond) conformer. For the higher energy gauche conformer, tbe asymmetric torsion is estimated to be near 85 cm-'. 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 V , = 1047 f %, V, = -619 f 68, V, = 1216 f 50 and V, = 26 f 22 cm-', with an enthalpy difference between the more stable s-cis and gauche conformers of 245 f 30 cm-' (0.70 f 0.09 kcal mol-'). This function gives values of 1035 f 10 cm-' (2.% f 0.03 kcal mol-'), 2015 & 150 cm-' (5.76 f 0.43 kcal mol-') and 787 f 10 cm-'(2.25 f 0.03 kcal mol-') for the s-cis to guuche, gauche to gauche and gauche to s-cis barriers, respectively. From the relative intensities of the Raman lines of the liquid at 1633 cm-' (~Q u c~) and 1649 cm-' (s-cis) as a function of temperature, the enthalpy difference is found to be 334 f 39 cm-' (0.95 f 0.11 kcal mol-'), but now with tbe gauche conformer more stable. A h , the more polar gauche conformer remains in the crystalline solid. The Raman spectrum of the gas was recorded from 3500 to 70 cm-' 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, f d a m e n t d 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.