Internal dynamics contributions to the CH stretching overtone spectra of gaseous nitromethane NO2CH3

Abstract

The methyl CH stretching overtone spectra of gaseous nitromethane NO2CH3 have been recorded with Fourier transform infrared conventional near infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. They all exhibit a complex structure with, at ΔvCH=1 and 2, a characteristic asymmetric top vibration-rotation profile which vanishes as vibrational energy increases. In addition, in these two lower energy spectra, the perpendicular stretching vibrations exhibit a widely spaced fine structure profile resulting from a Coriolis coupling induced by the methyl internal rotation. These excited spectra have been analyzed with a theoretical model which takes into account, in the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by a Morse potential, with the quasi-free internal rotation of the methyl group and with isoenergetic combination states involving methyl bending modes. Till Δv=3, the theoretical treatment yields normal modes. From the third overtone, the vibrational energy is seen to be localized and thus the calculations proceed in a local mode basis from Δv=4–6. Most of the parameters of this model and their variation with the internal rotation coordinate θ are the same as those used to account for the overtone spectra of the monohydrogenated species. Fermi resonance phenomena, also modeled with θ dependent parameters, lead to only weak IVR localized at the second overtone where only the two first tiers are effective and at the third overtone where three tiers must be considered to give a good reproduction of the experimental spectral features. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone.