CH stretching anharmonicity in CH2CF2 and CH2CCl2

Abstract The CH stretching anharmonicity in CH 2 CF 2 and CH 2 CCl 2 has been investigated through application of the local mode model to vibration levels up to 14500 cm −1 . Good reproduction of observed data is achieved in each case, but whereas a “normal” anharmonicity constant of ≈−58 cm −1 is obtained for CH 2 CCl 2 , an anomalous value of ≈−52 cm −1 is determined for CH 2 CF 2 . By comparison of the spectra, the cause of the problem is traced to two critical misassignments due to the effects of Fermi resonances at different levels of excitation in CH 2 CF 2 . A “normal” anharmonicity constant is then obtained. The Fermi resonances cannot be quantified satisfactorily, due to lack of sufficient observations and their complexity. The local mode parameters determined for each molecule are supported by available structural information.