The Infra-Red Absorption Spectrum of Methyl Alcohol

Abstract

Using a grating spectrometer having a KBr foreprism the spectrum of methyl alcohol vapor was studied in the region from 2.5 to 26μ. This molecule has bands at 3683, 2978, 2845, 2054, 1477, 1455, 1340, 1034.18 cm—1 and a very broad band extending from 860 to beyond 380 cm—1. These bands are typical perpendicular and parallel bands and indicate that the molecule is only slightly asymmetric.From the fine structure of the 1034.18 cm—1 parallel band and from certain assumptions about the structure of the molecule the two largest moments of inertia, A and B, were found to be 35.18 and 33.83×10—40 g cm2, respectively. It was not possible to measure C directly but it was estimated to be approximately 6.8×10—40 g cm2.The low frequency band is probably due to the vibration of the hydroxyl hydrogen atom perpendicular to the O☒H bond and perpendicular to the figure axis of the molecule. The presence of this band indicates that the hydroxyl group is not free to rotate, at least in the ground state. There is evidence of rotation in the structure of the band near the high frequency edge. The spacing increases toward high frequencies and finally there is a group of lines having a spacing of about 40 cm—1 between 600 and 860 cm—1. Since these lines are very weak and occur at such high frequencies it appears that free rotation exists only in states of high excitation. A quantitative study of free rotation in a symmetric molecule has been made. This investigation indicates that in a first approximation free rotation should give rise to groups of lines having a spacing of about 40 cm—1 between each group