CONFORMATIONAL-ANALYSIS, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT, AND ABINITIO CALCULATIONS OF 1,2-DIFLUOROETHANE

The far infrared (50-370-cm-1) and low frequency Raman (70-300-cm-1) spectra of gaseous 1,2-difluoroethane have been recorded. The fundamental asymmetric torsional frequencies of the more stable gauche (two fluorine atoms oriented gauche to one another) and the high energy trans conformations have been observed in the far infrared spectrum of the gas at 147.0 and 116.7 cm-1, respectively. For the trans conformer, four Q-branches arising from transitions between excited torsional vibrational states have also been observed. With use of these data, along with the enthalpy difference and the gauche torsional dihedral angle, the potential function governing internal rotation about the C-C bond has been determined with the following potential coefficients: V1 = 557 +/- 14 cm-1, V2 = -846 +/- 12 cm-1, V3 = 1163 +/- 4 cm-1, V4 = 51 +/- 4 cm-1, and V6 = -23 +/- 2 cm-1. This potential is consistent with an energy difference of 280 +/- 30 cm-1 (0.80 +/- 0.09 kcal/mol), a gauche dihedral angle of 71.0-degrees, and tram to gauche, gauche to gauche, and gauche to trans barriers of 741,2000, and 1021 cm-1, respectively. The conformational stability, barriers to internal rotation, and vibrational frequencies are compared to those obtained from ab initio calculations utilizing the RHF/6-311++G** basis set and to the corresponding results previously obtained for some similar molecules.