Internal Barrier of Propylene Oxide from the Microwave Spectrum. I

Abstract

In order to study the effects of hindered internal rotation of a methyl group for a high barrier case, the microwave spectrum of propylene oxide has been observed. Rotational transitions have been assigned up to J=30 in the ground torsional state and to J=10 in the first excited torsional state. The structure of the molecule was partially determined by combining the rotational constants derived from the spectrum with the known structure of ethylene oxide. From Stark effect measurements, the dipole moment was determined to be 2.00±0.02 debye. The theory of the interaction of internal rotation with over‐all rotation is discussed, including the necessary extension of the theory of Wilson, Lin, and Lide for a molecule having no symmetry. Because the tunnel effect splits each torsional state into two levels, the rotational lines appear as doublets. The threefold barrier hindering internal rotation of the methyl group is fitted to the doublet separations. For the ground state, the splitting of the lines was observed only for high J rotational transitions. A barrier of 2710 cal/mole was obtained. For the first excited torsional state, the splittings are about fifty times larger, and are observable for low J transitions. These yield a barrier of 2560 cal/mole. The causes of this apparent difference in barrier height and the errors in the method are discussed.