Trimethylene Oxide. I. Microwave Spectrum, Dipole Moment, and Double Minimum Vibration

Abstract

The microwave spectra of four isotopic species of trimethylene oxide have been investigated. Analysis of the spectra indicates that the four-membered ring is essentially planar. From the intensity measurements of the rotational transitions and their vibrational satellites, it was concluded that the energy levels in the ring-puckering vibration are single levels. The Stark effect also provided independent evidence that the ring is not highly puckered. The dipole moment of the molecule was found to lie solely on the C–O molecular axis. A value of 1.93±0.01 Debye was obtained. The existence of a small barrier in the potential function, however, was established by a detailed analysis of the observed vibration-rotation interaction. Quantitative considerations have led to an accurate determination of the barrier height and the general shape of the potential function. The barrier restricting the ring-puckering motion has been found to be 35±5 cm—1. The ground vibrational level is 8±4 cm—1 above the top of the barrier.