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 $\chi$ tensor.