Microwave Spectrum, Barrier to Internal Rotation, Structural Information, and Quadrupole Coupling of 1-Chloro-1,1-difluoroethane

The microwave spectra of two isotopic species, CH3–CF2 35Cl and CH3–CF2 37Cl, have been investigated in the region 8–40 GHz. A partial determination of structure strongly suggests a shortening in C–C, C–F, and C–Cl distances compared to singly or doubly substituted fluoro- or chloroethanes. Vibrationally excited states were observed showing doublets due to the hindered internal rotation of the methyl group. An analysis of these doublets gives a barrier to internal rotation of 4400 ± 100 cal/mole. The chlorine quadrupole coupling tensor is nearly symmetric about the C–Cl bond. This implies a pitfall in the application of simple resonance theory to molecules of this type; with simple resonance theory the shortened C–Cl bond is explained in terms of considerable double-bond character for the C–Cl bond, which in turn would call for an asymmetric χ tensor.