Knowledge Bank

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 $Kmax\prime =8$ 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 $6.2cm-1$ for the OH-stretch state is about two-thirds of the ground state value of $9.1cm-1$, implying a substantially increased torsional potential barrier height.