CH-stretching overtone spectra of a fast rotating methyl group. I. TolueneC6D5CH3

The CH-stretching overtone spectra of the methyl group in gaseous toluene C 6D5CH3 are recorded with conventional Fourier transform near infrared spect roscopy in the Deltav(CH)=1-4 regions and by intracavity laser photoacousti c spectroscopy in the Deltav(CH)=5 and 6 regions. All spectra exhibit a com plex structure. They are analyzed with a theoretical model which takes into account, within the adiabatic approximation, the coupling of the anharmoni c CH stretch vibrations, described by Morse potentials, with the quasifree internal rotation of the methyl group and with isoenergetic combination sta tes involving the six angle deformation modes of the methyl group. Using un iquely determined canonical potential-energy and kinetic-energy matrices al lows us to solve the problem of indeterminacy caused by the description of molecular vibrations in such redundant coordinates. A set of Fermi resonanc e parameters and their variation with the internal rotation coordinate are determined from the fitting of the Deltav(CH)=1-6 overtone spectra. At Delt av=1 and 2, the vibrational energy is expressed in a normal mode basis. Fer mi resonance phenomena involving mainly the HCH bending modes lead to stron g intramolecular vibrational energy redistribution. At this energy, a Corio lis coupling through internal rotation, which gives rise to a characteristi c asymmetric top vibration-rotation profile, further perturbs the vibration al modes perpendicular to the rotation axis. From the second overtone (Delt av=3), the vibrational energy is described in a local mode basis and pertur bations due to Fermi resonance vanish progressively up to Deltav=5. At Delt av=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 s uccessfully describes the relative intensity and frequency of each peak wit hin a given overtone. (C) 2001 American Institute of Physics.