CH-stretching overtone spectra of a fast rotating methyl group. I. Toluene C6D5CH3

Abstract

The CH-stretching overtone spectra of the methyl group in gaseous toluene C6D5CH3 are recorded with conventional Fourier transform near infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. All spectra exhibit a complex structure. They are analyzed with a theoretical model which takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by Morse potentials, with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving the six angle deformation modes of the methyl group. Using uniquely determined canonical potential-energy and kinetic-energy matrices allows us to solve the problem of indeterminacy caused by the description of molecular vibrations in such redundant coordinates. A set of Fermi resonance parameters and their variation with the internal rotation coordinate are determined from the fitting of the ΔvCH=1–6 overtone spectra. At Δv=1 and 2, the vibrational energy is expressed in a normal mode basis. Fermi resonance phenomena involving mainly the HCH bending modes lead to strong intramolecular vibrational energy redistribution. At this energy, a Coriolis coupling through internal rotation, which gives rise to a characteristic asymmetric top vibration-rotation profile, further perturbs the vibrational modes perpendicular to the rotation axis. From the second overtone (Δv=3), the vibrational energy is described in a local mode basis and perturbations due to Fermi resonance vanish progressively up to Δv=5. At Δv=6, a strong spectral perturbation is again observed. But, the efficient Fermi resonance phenomena are now essentially related to combination states involving HCH and HCC bending combination modes. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone.