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. I n one of the crystalline forms (I), the trans conformer is exclusively present, whereas in the other (II), only the gauche conformer is foun d. Assisted by ab initio computations of the vibrational spectrum usin g the 3-21G basis set, complete assignments are made for both the tran s 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 a nd gauche conformers, respectively, in complete agreement with computa tional predictions. From the relative intensities of the Raman spectra as a function of temperature for the gas and liquid. the enthalpy dif ferences 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 conf ormer is the more stable form in both the ps and the liquid. The harmo nic force fields obtained with the 3-21 G basis set are reported in in ternal coordinates for both conformers together with scaled and unscal ed frequencies calculated from these force fields. Optimized geometrie s 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 en ergies, dipole moments and rotational constants. The calculations sugg est 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 som e corresponding compounds.