Rotation spectrum of methyl alcohol

Abstract

The rotation spectrum of methanol vapor has been measured from 50 to 457 cm-1. Combining these data with the work of Borden and Barker provides an accurate map of the spectrum to 625 cm-1 together with some provisional observations between 695 and 860 cm-1. The primary purpose of the theoretical discussion is to identify the observed lines and subsequently, through combination relations, to establish the rotational energy levels of the molecule. These levels may be labeled by the quantum number n, [tau], K, and J. n corresponds roughly to a vibration in the hindering potential field and [tau] designates the three types of levels resulting from the threefold potential. J gives the total angular momentum and K its component along the molecular symmetry axis.The method of analysis consisted first in calculating the rotational levels using the barrier height and the moments of inertia established through earlier investigations of the microwave spectrum. The intensities of the lines occurring within the region of experimental observation were calculated. The resulting predicted spectrum when compared with the observed spectrum allowed a considerable number of identifications to be made but revealed deviations due to centrifugal force effects. These latter were then calculated and ultimately a very satisfactory fit was obtained. The rotational levels were then determined for n = 0, 1, 2, and 3 and for K = 0 through 10. The accuracy is estimated to be of the order of a few tenths of a wave per centimeter. The small discrepancies between these observed levels and the calculated levels (taking account of centrifugal distortion) are discussed and it is concluded that the hindering potential must be sinusoidal in form with an accuracy of better than one percent.