# FOURIER-TRANSFORM SPECTRA OF THE OH-STRETCH FUNDAMENTAL OF COOLED METHANOL

Please use this identifier to cite or link to this item: http://hdl.handle.net/1811/14141

Files Size Format View
1997-WF-10.jpg 197.1Kb JPEG image

 Title: FOURIER-TRANSFORM SPECTRA OF THE OH-STRETCH FUNDAMENTAL OF COOLED METHANOL Creators: Xu, Li-Hong; Lu, Z. Issue Date: 1997 Publisher: Ohio State University Abstract: Absorption spectra of the O-H stretching band of methanol have been recorded from 3600-3800 $cm^{-1}$ on the modified Bomem DA8.002 Fourier Transform spectrometer in the laboratory of Dr. J. W. C. Johns at NRC, Ottawa, at a resolution of 0.0025 $cm^{-1}$. Spectra were taken both at room temperature of 297 K and at 190 K in a cooled cell. The two spectra are very similar, with the cooled spectrum having a narrower linewidth and less underlying grass” in the background. The low-resolution spectrum of this region shows a single OH stretch fundamental displaying recognizable parallel-band structure with fairly clear R and P branches and a sharp Q branch centered at 3681 $cm^{-1}$. At high resolution, however, each R(J) and P(J) parallel multiplet is seen to be diluted by the perpendicular transitions also expected for the OH-stretch mode, and by transitions to the numerous lower-lying vibrational fundamental and combination states unavoidable in the region concerned. The cooled 190 K FTIR spectrum was used for detailed spectroscopic investigation. More than 50 subbands of both a-and b-types have been assigned and confirmed up to $K^{\prime}_{\max} = 8$ for A and E torsional symmetries, with only a few missing series still to be found. Although most of the subbands were difficult to follow to high J due to level crossing resonances, our assignments are firmly supported by ground-state combination differences known to high accuracy. Some what unexpectedly, the observed parallel and perpendicular transitions are equally strong, yet the latter are the main ones previously reported in the $literature^{[a-c]}$. Our experimental intensity ratios between the parallel and perpendicular transitions yield an average $\Delta \mu_{a}/\Delta \mu_{b}$ transition moment ratio of about 1:1 for the OH-stretch vibration. Torsional energies were evaluated for the OH-stretch mode by adding the infrared subband origin wavenumbers to reduced ground state energies with the K-rotational dependence removed. The plot of torsional energy against K displays a regular since-curve three-fold oscillation pattern, apart from a few levels which are shifted due to interactions with dark background state. The $K=0$ torsional A-E splitting of $6.2 cm^{-1}$ for the OH-stretch state is about two-thirds of the ground state value of $9.1 cm^{-1}$, implying a substantially increased torsional potential barrier height. Description: $^{a}$ R. C. Lee, R. H. Hunt, E. K. Plyler, and D. M. Dennison, {J. Mol. Spectrosc}. \textbf{57}, 138-154 (1975). $^{b}$ P. Carrick, R. F. Curl, M. Dawes, E. Koester, K. K. Murray, M. Petri, and M. L. Richnow, {J. Mol. Struct}. \textbf{223}, 171-184 (1990). $^{c}$ I. Kleiner, G. T. Franser, J. T. Hougen, and A. S. Pine, J. Mol. Spectrosc. \textbf{147}, 155-172 (1991). Author Institution: Department of Physical Sciences, University of New Brunswick; Steacie Institute for Molecular Sciences, National Research council Canada URI: http://hdl.handle.net/1811/14141 Other Identifiers: 1997-WF-10