Highly excited vibrational states of molecules by thermal lensing spectroscopy and the local mode model. I. CHCl3, CHBr3, CH2Cl2, CH2Br2

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Abstract
Visible absorption spectra (14 700–19 100 cm−1) measured with the thermal lensing spectrometer and near‐infrared absorption (5 600–14 000 cm−1) measured with a conventional spectrophotometer are reported for CHCl3, CHBr3, CH2Cl2, and CH2Br2. Relatively strong absorption peaks are identified as overtones of the C–H stretching vibrations in these compounds. The overtone spectra are analyzed in terms of the local mode (LM) model, which treats the molecule as a set of loosely coupled anharmonic oscillators localized on individual C–H bonds. Relatively less intense peaks are observed and are assigned as combinations of a local mode C–H vibration and some lower frequency (normal) mode of the molecule. Fermi resonance is seen to occur in the spectra of CHCl3 and CHBr3, resulting in anomalously high intensities for the combination bands involved. Significant anharmonic local–normal coupling constants are seen for these same molecules. These coupling constants appear to indicate the presence of strong physical coupling of the two motions involved. The importance of these coupled motions in the intramolecular relaxation of vibrational energy is suggested.