RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, VIBRATIONAL ASSIGNMENT, AND AB-INITIO CALCULATION OF CIS-1,3-DICHLOROPROPENE

The Raman (3200-10 cm-1) and infrared (3200-20 cm-1) spectra of gaseou s and solid cis-1,3-dichloropropene, cis-ClHC=CHCH2Cl, have been recor ded. Additionally, the Raman spectrum of the liquid with qualitative d epolarization data has also been obtained. These spectral data have be en interpreted on the basis that the molecule exists predominantly in the gauche (allylic chlorine atom oriented gauche to the double bond) conformation in all physical phases. Some bands in the infrared spectr um of the gas could be due to a second higher energy conformer but the lack of corresponding Raman bands casts doubt on such an assignment. Ab initio Hartree-Fock gradient calculations employing the RHF/6-31G basis set are consistent with two minima that correspond to the two eq uivalent gauche forms for the potential surface for the internal rotat ion about the C-C bond. These minima occur at dihedral angles, less th an with right parenthesis through(dih) ClCCC, of 122-degrees and 238-d egrees (0-degrees corresponds to the syn conformation) and the gauche to gauche barrier at the 180-degrees transition state, which correspon ds to the anti structure, is 342 cm-1. The barrier from the gauche min ima to the syn (0-degrees) structure, which is also a transition state , is 2425 cm-1. Complete equilibrium geometries for the gauche structu re have been determined with the RHF/3-21G, RHF/6-31G*, and MP2/6-31G basis sets. A normal coordinate analysis utilizing a harmonic force field calculation with the RHF/3-21G basis set has been carried out f or the gauche conformer. The results are discussed and compared with t he corresponding quantities obtained for some similar molecules.