THE FAR INFRA-RED SPECTRUM OF METHYL ALCOHOL
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Date
1956
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Ohio State University
Abstract
In 1939 Borden and Barker mapped the infra-red spectrum of methanol and observed a considerable number of irregularly spaced lines in the region from 860 to $380 cm^{-1}$ which they correctly interpreted as arising from hindered rotation transitions. Twelve years later Burkhard measured the rotation spectrum out to $50 cm^{-1}$. Recently Palik and Oetjen have reexamined the region and have refined many of the details. The goal of the present work is the identification of the observed lines and hence the evaluation of the energy levels of the molecule. Since the lines are crowded together with an apparently irregular distribution, both of spacings and of intensities, it is necessary to start with a calculated spectrum which is quite accurate if one hopes to make an unambiguous correlation. The following steps were employed. 1) The microwave spectrum of methanol furnishes reliable values for the barrier height and the molecular dimensions. From these the energy levels and transition probabilities were calculated for n= O, 1 and 2 and for K = O up to 10. 2) The centrifugal stretching is quite large for methyl alcohol and gives a correction to the energy levels which, while it is negligible for the lower levels of n = O, becomes as large as $10 cm^{-1}$ for the levels of n = 2. Knowing the molecular force constants from the near infra-red spectrum it is possible to calculate these corrections without introducing any additional empirical constants. 3) With the aid of the calculated spectrum, including the corrections, it was possible to identify the more intense lines and consequently to find the actual energy levels. The weaker lines now furnished self-consistency checks. 4) The observed levels were examined to see whether they would fit into the structure of levels demanded by the theory of hindered rotation. This last step furnished a strongcheck on the wholeprocess and constitutes convincing evidence for the correctness of the identifications.
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Author Institution: Randall Laboratory of Physics, University of Michigan