Trimethylene Oxide. III. Far-Infrared Spectrum and Double-Minimum Vibration

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Abstract
The 0–1 transition of the out‐of‐plane bending vibration in trimethylene oxide has been observed at 53.4 cm−1. Previously reported bands by Lord and his co‐workers [J. Chem. Phys. 33, 294 (1960)] at 89.8, 105.2, 118.3, 128.9, 139.0, 147.6, 154.9, and 161.8 cm−1 have also been confirmed. In addition, six higher members of this series as well as three members of the Δv = 3 series have been observed. These transitions are readily interpreted in terms of the hot‐band series of a slightly perturbed quartic oscillator. A double‐minimum potential function of the form az4—bz2 predicted all the observed transitions to within experimental error (0.5 cm−1 or better). The central barrier in the potential‐energy function, while low, is finite and has a height of 15.3±0.5 cm−1. It had previously been concluded, from the variation of the rotational constants with vibrational state obtained from microwave spectroscopy that the potential energy contained a small barrier of the order of zero‐point energy at the planar configuration of the molecule [J. Chem. Phys. 33, 1643 (1960)]. The present far‐infrared results are shown to be consistent with the microwave results. Thus an existing discrepancy between the far‐infrared and microwave results on the nature of the potential function for this unique out‐of‐plane bending vibration is finally resolved.