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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

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Title: FOURIER-TRANSFORM SPECTRA OF THE OH-STRETCH FUNDAMENTAL OF COOLED METHANOL
Creators: Xu, Li-Hong; Lu, Z.
Issue Date: 1997
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.
URI: http://hdl.handle.net/1811/14141
Other Identifiers: 1997-WF-10
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