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 gaseous and solid cis-1,3-dichloropropene, cis-ClHC=CHCH2Cl, have been recorded. Additionally, the Raman spectrum of the liquid with qualitative depolarization data has also been obtained. These spectral data have been 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 spectrum 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 equivalent gauche forms for the potential surface for the internal rotation about the C-C bond. These minima occur at dihedral angles, less than with right parenthesis through(dih) ClCCC, of 122-degrees and 238-degrees (0-degrees corresponds to the syn conformation) and the gauche to gauche barrier at the 180-degrees transition state, which corresponds to the anti structure, is 342 cm-1. The barrier from the gauche minima to the syn (0-degrees) structure, which is also a transition state, is 2425 cm-1. Complete equilibrium geometries for the gauche structure 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 for the gauche conformer. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.