VIBRATIONAL OVERTONE SPECTROSCOPY AND INTERNAL DYNAMICS IN GASEOUS NITROMETHANE NO2CH2D

The CH-stretching overtone spectra of the methyl group in gaseous nitr omethane NO2CH2D have been recorded with conventional Fourier transfor m near-infrared spectroscopy in the Delta upsilon(CH) = 1 - 4 regions and by intracavity laser photoacoustic spectroscopy in the Delta upsil on(CH) = 5 and 6 regions. All spectra exhibit a complex structure; the y have been analyzed with a theoretical model which takes into account , within the adiabatic approximation, the coupling of the anharmonic C H stretch vibrations, described by Morse potentials, with the quasifre e internal rotation of the methyl group and with isoenergetic combinat ion states involving methyl bending modes. Most of the parameters of t his model, and their variation with the internal rotation coordinate ( theta), are identical to ones used to account for the overtone spectra of the monohydrogenated and perhydrogenated species. Fermi resonance phenomena, also modeled with theta dependent parameters, lead to only weak intramolecular vibrational energy redistribution. This simple cal culation successfully describes the relative intensity and frequency o f each peak within a given overtone. The vibrational energy is seen to be localized at lower energy in the dihydrogenated compound [at the s econd overtone (upsilon = 3)] than in the perhydrogenated one [at the third overtone (v = 4)]. The overtone vibrations obtained from the cal culations can be considered as normal modes up to Delta upsilon = 2 an d as local modes from Delta upsilon = 3 to 6. However, at Delta upsilo n = 3, the existence of a transitional regime, where normal and local modes coexist, can be demonstrated. The CH/CD interbond coupling shift s the overtone spectra toward high for some additional weaker features in the high overtone spectra (Delta upsilon = 5 and 6). (C) 1998 Amer ican Institute of Physics. [S0021-9606(98)01222-7].