Perturbation mediated forbidden transitions in the in-plane rocking mode of O-18 substituted methanol: Very high-resolution Fourier transform spectroscopy using globar and synchrotron radiation sources in the 10-μm region

In this work, very high-resolution Fourier transform spectrum of methyl alcohol and its oxygen-18 substituted variant species have been recorded in the wide range of 1.6-200 mu m using a precisely controlled corner cube spectrometer developed at the University of Oulu at a resolution approaching the Doppler width of the lines with excellent signal to noise (S/N) ratio. Later some of the regions have been re-recorded using the synchrotron radiation-based Bruker spectrometer at the Canadian Light Sources with extreme accuracy. The region covers the fundamental torsional band, C-O stretch band, CH3-rocking band, COH-bending band, the CH3-deformation bands, the CH3-stretch bands, and numerous other fundamental, combination, and overtone bands. The density of quantum levels in these regions makes the states highly perturbed by nearby levels. Despite the congestion due to overlapping bands the high resolution and enhanced intensity allowed assignments of some difficult bands in the most congested and complicated part of the spectra as presented in this report. In this paper, the results for new transition bands connected with the in-plane CH3 rocking mode and COH-bending modes are presented. In these transitions, one quantum of torsion is involved in the upper and state and/or lower states. For the O-18 substituted methanol strong transitions to a torsionally excited rocking vibrational state originating from the ground vibrational state have been found which should be normally forbidden. These transitions gather intensity due to a strong "Fermi" type interaction with the COH-bending mode lying immediately above. To gain some insight into the problem similar regions of the spectrum for the parent species (C-12 variant) have also been revisited. Since the strong C-O stretching band is downshifted due to the presence of the heavier O-18 species, the CH3-Rocking and COH-bending are rather peaceful making this work feasible. It was possible to identify many transitions which are normally forbidden without the interaction. Detail assignments have been presented in an atlas of about 3800 highly precise spectral lines for the first time. This work should be valuable for the fundamental understanding of vibrational relaxation pathways of the molecules under consideration and the study of the extreme abundances of this molecule in interstellar space. The details can be found in the text. To save journal pages majority of the supporting data had been gathered in an Appendix.