Ab Initio Highly Correlated Conformational Analysis of 1,2-Difluorethane and 1,2-Dichloroethane

Temperature-dependent conformational population calculations for anti and gauche forms of 1,2-dichloroethane and 1,2-difluorethane were carried out at a highly correlated level of theory (MP4(SDTQ) and CCSD(T)) employing good quality basis sets (6-311++G(3df,3pd) and aug-cc-pVQZ) for the determination of gas relative conformational energies, making use of the statistical thermodynamics formalism for the evaluation of the thermal energy correction at the MP2/6-311++G(3df,3pd) and MP2/aug-cc-pVTZ levels. In addition to the standard calculation of thermodynamic partition functions, a treatment of the lowest-frequency vibrational mode as hindered rotation and anharmonic correction to vibrational frequencies was also included. We found a good agreement between ab initio calculated conformational population values and experimental gas-phase electron diffraction data for the 1,2-dicloroethane. However, for the 1,2-difluorethane species, a reasonable agreement with the experimental anti/gauche population ratio obtained from the analysis of gas-phase far-infrared (50-370 cm(-1)) and low-frequency Raman (70-300 cm(-1)) spectra was not obtained. The results reported here indicate that, for 1,2-difluorethane, and probably other substituted alkanes where the gauche effect is of relevance, a more appropriated treatment of the low-frequency modes must be pursued in order to reproduce experimental conformational population data.