Internal Barrier of Propylene Oxide from the Microwave Spectrum. II

Abstract

Several long progressions of perpendicular transitions have been measured in the microwave spectrum of propylene oxide. These have provided an opportunity to test the theory of internal rotation over a very large range of rotational quantum numbers, J=1 to 48 and K=1 to 25, in the ground torsional state. For low K, the K doubling which arises from the asymmetry of the molecule is much larger than the splittings due to tunneling through the potential barrier. The spectrum resembles that of an ordinary asymmetric rotor except that each line is split into a close doublet by the tunneling. At high K, the K doubling becomes negligible. A triplet fine structure is observed in which the spacings show a periodic dependence on K, essentially the same as that calculated for the case of two coaxial symmetric tops. (For a true symmetric rotor, the selection rules would prevent the observation of this pattern.) At intermediate K, the K doubling and the tunneling perturbations become comparable. The mixing of the rotational wave‐functions which results permits ``forbidden'' lines to appear. Very good agreement with theory is obtained over the whole range of K, with the barrier height V₃=895 cm^—1 (2560 cal/M). Rotational transitions have also been assigned for molecules in the first and second excited torsional states, in which the tunnel effect splittings are greatly magnified. For both the excited states, the barrier height is found to be identical with the ground state result. This supports the usual assumption that perturbations from vibrational interactions with internal rotation and from the sixfold V₆ term in the potential barrier are small enough to be safely neglected.