Analysis of the torsion-rotation spectra of the isotopic methanol molecules

Abstract

The high resolution infrared spectra of CH3OH, CH3OD, CD3OH and CD3OD observed by D. R. Woods and C. W. Peters in the region from 400 to 900 cm-1 have been analyzed to obtain the molecular moments of inertia, the barrier height and the Kirtman perturbation constants. The first step consisted in identifying as many Q branch origins as possible. Between 25 and 40 origins were determined for each isotopic molecule with an accuracy of about 0.03 cm-1. These data were combined with Q branch origins found by R. M. Lees and J. G. Baker from their very accurate measurements of the microwave spectra of these molecules. A nonlinear least squares solution yielded values for the constants. With these constants the spectrum was recomputed and found to agree with the observed spectrum to the order of the experimental errors. In all, 176 data points were described by 40 constants-10 for each molecule.In the second phase of the work a tentative theory is proposed which relates the perturbation constants of the four isotopic molecules. This theory, which contains a number of approximations, involves only 21 constants. These were evaluated from a nonlinear least squares analysis and the spectrum was recomputed. The agreement with the observed spectrum is good, but not as good as in the former case.The barrier height V3 was found to decrease upon deuteration and a mechanism is suggested based upon the zero point vibrations of the atoms. The barrier potential is shown to be highly sinusoidal in form with V6/V3 = -0.0002 +/- 0.0006.