INTERACTION BETWEEN ORDINARY VIBRATIONS AND HINDERED INTERNAL ROTATION CORIOLIS COUPLING IN ETHANE-TYPE $MOLECULES^{*}$

Abstract

“It has been $shown^{1}$ that molecules with the same symmetry as ethane may exhibit a fine structure in certain perpendicular vibrational bands which is due to internal rotation. A theoretical analysis of this fine structure, which is caused by coroilis coupling between the ordinary internal angular momentum $P_{\sigma}$, and the vibrational internal angular momentum $P_{\sigma}$, will be presented. $P_{\sigma}$ is generated by combination of the two different types of perpendicular vibrations. When these vibrations are nearly degenerate, as they often will be, coriolis coupling becomes important. In that event, to a good approximation, the observed tine structure depends only on the barrier to internal rotation, the frequency difference between the two vibrations, and the coriolis $\zeta$ of either vibration. Detailed appearance of the theoretical spectrum as a function of these three parameters and the temperature will be shown. From the high resolution infrared $spectrum^{2}$ of dimethyl acetylene, it is possible to conclude that the barrier to internal rotation is less than 100 cal/mole. Other molecules will be discussed.”