CH-stretching overtone spectra of a fast rotating methyl group. II. Toluenes C6D5CH2D and C6D5CHD2

Abstract

The CH-stretching overtone spectra of the methyl group in gaseous toluene C6D5CH2D and C6D5CHD2 are recorded with conventional Fourier transform near infrared spectroscopy in the Δv=1–4 regions and by intracavity laser photoacoustic spectroscopy in the Δv=5 and 6 regions. The spectra are analyzed with a theoretical model that takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving methyl bending and rocking modes. A simultaneous successful reconstruction of the CH stretching overtone spectra of three isotopic derivatives of the methyl group of toluene is obtained with a single set of parameters from Δv=1–6 (18 spectra). The modifications caused by the partial deuteration of the rotating methyl group on its IVR mechanisms are analyzed. In both methyl deuterated compounds, the CH/CD interbond coupling induces a shift of the overtone spectra towards high frequencies and the appearance of additional features in the high energy overtone spectra corresponding to [(n−1)νCH+1νCD] resonant combination states. In toluene C6D5CHD2, this shift is progressively amplified until Δv=4 by Fermi resonance phenomena involving mainly HCD bending modes. From Δv=3, the effect of these anharmonic interactions appears as an extra structure in the low energy side of the spectra. The “tuning” of these interacting states into resonance is reached at Δv=5 and causes a strong intramolecular vibrational energy redistribution. In toluene C6D5CH2D, at Δv=1 and 2, the CH stretching spectra are perturbed by Fermi resonance with HCH bending mode “doorway” states. A transitional regime between normal and local mode is detected at Δv=2, which requires a more elaborate model. At higher energy, the HCH bending mode combinations move farther out of resonance. From Δv=4–6, the perturbation of the spectra is then increasingly due to Fermi resonance phenomena involving HCD bending modes.