Infrared Spectra of CF2CHD and CF2CD2: Scaled Quantum-Chemical Force Fields and an Equilibrium Structure for 1,1-Difluoroethylene
journal contributionposted on 02.09.2010, 00:00 by Donald C. McKean, Mark M. Law, Peter Groner, Andrew R. Conrad, Michael J. Tubergen, David Feller, Michael C. Moore, Norman C. Craig
Infrared (IR) spectra in the gas phase are reported for CF2CHD and CF2CD2 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 CF2CH2 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 F2CCH2 is determined and compared favorably with the structure obtained from new high-level ab initio calculations. Centrifugal distortion constants are predicted for the five isotopic species, and those for F2CCH2 are compared with values fit to microwave spectra.