Infrared Spectra of CF2═CHD and CF2═CD2: Scaled Quantum-Chemical Force Fields and an Equilibrium Structure for 1,1-Difluoroethylene

Infrared (IR) spectra in the gas phase are reported for CF2═CHD and CF2═CD2 in the region 350−4000 cm−1. Ab initio calculations of an harmonic force-field and anharmonicity constants have been made with an MP2/aug-cc-pVTZ model. These enable a number of Fermi resonances in each species to be analyzed and a complete set of “observed” harmonic frequencies to be derived. The latter are combined with similar data for CF2═CH2 in a scaling of the model harmonic force field to both anharmonic and harmonic frequencies. Inspection of the scale factors reveals minor defects of the model, evident in the out-of-plane wagging modes and in the CF stretch/CF stretch interaction force constant. Fermi resonance treatments involved in all isotopomers studied are compatible with the overall force-field refinement results. The treatment leaves a small anomaly in the 13C shift on ν1. Improved microwave spectra are reported for five isotopic species, and a semiexperimental equilibrium structure for F2C═CH2 is determined and co...