CH-stretching Overtone Spectra of a Fast Rotating Methyl Group: 2-CH₃ and 2-CHD₂ Pyridines

Abstract

The CH-stretching overtone spectra of the methyl group in gaseous 2-CH(3) and 2-CHD(2) methylpyridines are recorded with conventional Fourier transform near-infrared spectroscopy in the Deltav(CH) = 1-4 regions and by intracavity laser photoacoustic spectroscopy in the Deltav(CH) = 5 and 6 regions. All spectra exhibit a complex structure. They are analyzed with a theoretical model that incorporates, 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. The molecular vibrations are calculated in terms of redundant internal coordinates in an unambiguous canonical form. A simultaneous analysis of different isotopic derivatives is thus achieved. The Fermi resonance coupling parameters are those previously determined for toluene. The technique of diabatic rotations is used to disentangle the multiple avoided crossings occurring along the internal rotation coordinate theta in the calculated spectra, which become rapidly very dense owing to the low symmetry of the system. This simulation is successful in reproducing the experimental spectra. In addition, the transferrability of the Fermi resonance coupling parameters between two parent molecules is demonstrated.

Infrared spectra of methyl isocyanate isolated in Ar, Xe and N2 matrices

“…It is well-known that the vibrational spectra of methyl-containing compounds can be complicated by occurrence of two types of anharmonic interactions: (i) resonances between m CH and the first overtone (2 b CH) and/or combination tones of methyl bending (b CH) modes [60,61] and (ii) coupling of m CH with sCH 3 modes [62,63], giving rise to polyads of levels in the 3000-2800 cm À1 range [64]. The assignment of the particular modes in such spectra usually requires studies of the partially deuterated CHD 2 or CH 2 D analogues [65,66] and sophisticated theoretical treatment [67,68]. It is not new that MIC is amongst the molecules exhibiting a strong resonance between the m CH and 2b CH modes [69].…”

Vibrational Autodetachment−Intramolecular Vibrational Relaxation Translated into Electronic Motion

“…This assumption could in principle be justified by the argument that the ν s (CH 3 ) vibrational mode and indeed all CH stretching modes are so much higher in energy than the torsion in the anion that coupling between these modes will be rather ineffective, since it would be of high order. However, several examples from the IVR literature indicate that methyl torsion plays an important role in IVR for other, related systems. ,,− ,,− Figure shows a fit of this model without internal rotor excitation of neutral CH 3 NO 2 to the VAD spectrum.…”

Combined IR/NIR and Density Functional Theory Calculations Analysis of the Solvent Effects on Frequencies and Intensities of the Fundamental and Overtones of the C═O Stretching Vibrations of Acetone and 2-Hexanone

“…In recent years, the rapid development of NIR spectrometers and progress in quantum chemical calculation studies of overtones, combinations, and anharmonicities have been attracted a great attention of numerous research group and permitted to achieve a lot of promising results. − In the literature, the overtone studies primarily focus on the CH and OH stretching modes, less on NH and very few on SH stretching vibrations. This results from increasing difficulty in recording of the overtone spectra.…”