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

The infrared (3200-400 cm-1) and Raman (3200-10 cm-1) spectra have been recorded for gaseous and solid 1-chloro-2-fluoroethane. Additionally, the Raman spectrum of the liquid has been recorded, and qualitative depolarization values have been obtained. These data have been assigned and interpreted on the basis that the molecule exists in the trans (two halogen atoms oriented trans to one another) and gauche conformations in the vapor and liquid and exclusively in the gauche conformation in the solid. From studies of the Raman spectrum of the gas at variable temperatures the trans conformation has been determined to be more stable than the gauche by 287 +/- 58 cm-1 (821 +/- 170 cal/mol). However, similar studies for the liquid are consistent with the gauche conformation being more stable than the trans by 342 +/- 22 cm-1 (978 +/- 63 cal/mol). The asymmetric torsional fundamentals in the far-infrared spectrum of the gas at a resolution of 0.10 cm-1 have been observed at 138.3 and 127.9 cm-1 for the more stable trans and high-energy gauche conformers, respectively. From these data the asymmetric torsional potential function governing the internal rotation about the C-C bond has been determined, and the potential coefficients are V1 = 392 +/- 8, V2 = 150 +/- 8, V3 = 1632 +/- 13, V4 = 212 +/- 7, and V6 = -132 +/- 4 cm-1. This potential is consistent with trans-to-gauche, gauche-to-gauche, and gauche-to-trans barriers of 1780, 1737, and 1493 cm-1, respectively, and a dihedral angle, angle FCCCl, for the gauche conformer of 68-degrees. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing both the 3-21G* and 6-31G* basis sets. Additionally, complete equilibrium geometries have been determined for both rotamers. The results are discussed and compared with the corresponding quantities obtained for some related molecules.