Interactions between Ordinary Vibrations and Hindered Internal Rotation. II. Theory of Internal Rotation Fine Structure in Some Perpendicular Bands of Ethane-Type Molecules

A method for determining the barrier to internal rotation in ethane-type molecules from the fine structure of certain perpendicular vibrational bands is presented. This fine structure depends upon the degree of coupling between vibration and internal rotation which, in turn, is a function of the barrier height. By comparing observed and theoretical spectra it is possible to set an upper limit of 100 cal/mole for the barrier to internal rotation in dimethylacetylene. Application to other molecules is discussed.