INFRARED AND RAMAN-SPECTRA, CONFORMATIONAL STABILITY, BARRIER TO INTERNAL-ROTATION, AND AB-INITIO CALCULATIONS OF 1,1-DICHLOROPROPANE

The infrared and Raman spectra of 1,1-dichloropropane are reported in the gas, liquid, amorphous solid and two distinct crystalline forms. In one of the crystalline forms (I), the trans conformer is exclusively present, whereas in the other (II), only the gauche conformer is found. Assisted by ab initio computations of the vibrational spectrum using the 3-21G basis set, complete assignments are made for both the trans and gauche conformers. Observation of the Raman spectrum of crystal form (I) and of the mid-infrared spectrum of crystal form (II) allows unique and definitive indentification of vibrations due to the trans and gauche conformers, respectively, in complete agreement with computational predictions. From the relative intensities of the Raman spectra as a function of temperature for the gas and liquid. the enthalpy differences of 216 +/- 160 cm-1 (620 +/- 450 cal mol-1) and 84 +/- 29 cm-1 (240 +/- 83 cal mol-1), respectively, were obtained. The gauche conformer is the more stable form in both the ps and the liquid. The harmonic force fields obtained with the 3-21 G basis set are reported in internal coordinates for both conformers together with scaled and unscaled frequencies calculated from these force fields. Optimized geometries obtained using the RHF/3-21G, RHF/3-21G*, RHF/6-31G* and MP2/6-31G* basis sets are reported for both conformers together with the total energies, dipole moments and rotational constants. The calculations suggest that the gauche conformer is more stable than the trans by 86 cm-1 for 3-21G, 106 cm-1 for 3-21G*, 155 cm-1 for 6-31G*, and 89 cm-1 for MP2/6-31G* bases. These results are compared with similar data for some corresponding compounds.