Infrared spectra obtained in gas and liquid argon phases are reported for 12CF2-12CH2 and 12CF2-13CH2. These spectra firmly establish the positions of nu3(A1) and nu6(A2) for both isotopomers. Using anharmonicity constants from MP2 calculations, Fermi resonances affecting nu1(A1), nu2(A1), nu3, and nu8(B1) are analyzed. Deperturbed fundamental frequencies from these analyses are used in conjunction with unaffected fundamentals and ab initio anharmonicity data to predict all 12 “observed” harmonic frequencies. A Darling-Dennison type resonance between 2nu6 and nu11 + nu12 is diagnosed, the calculation of which from ab initio data requires modification of the existing second-order treatment of such constants, where Fermi resonance type terms are also present. Predictions are made of many overtone and combination band frequencies, aiding assignment of observed spectra. From the isolated CH stretching frequency obtained here of 3125.4 cm-1, the C-H equilibrium bond length is predicted to be 1.0762(11) Å.
McKean, D. C., van der Veken, B., Herrebout, W., Law, M. M., Brenner, M. J., Nemchick, D. J., & Craig, N. C. (2010). Infrared spectra of 12CF2=12CH2 and 12CF2=13CH2, quantum-chemical calculations of anharmonicity, and analyses of resonances. Journal of Physical Chemistry A, 114(18), 5728-5742. https://doi.org/10.1021/jp100438z