Microwave Spectrum, Vibration—Rotation Interaction, and Potential Function for the Ring-Puckering Vibration of Trimethylene Sulfide

Abstract

The microwave spectrum of the ground and first four excited states of the ring‐puckering mode of trimethylene sulfide has been observed and assigned. From an analysis of the perturbations to the rotational spectrum arising from the coupling of the internal angular momentum with the over‐all rotation of the molecule, the separation between the lowest two vibrational levels was determined to be 8229 Mc/sec. Three mixed vibration—rotation transitions have been observed, and give the more accurate value of 8232.5±0.4 Mc/sec (0.27461 cm−1) for this separation. From the vibration—rotation interaction and the relative intensities of certain lines, the potential function governing the vibration has been determined to be 7.0207Q 4−87.7581Q 2 cm−1, where Q is the coordinate describing the vibration. The height of the barrier to the planar configuration is 274.2±2 cm−1. The agreement between the far‐infrared spectrum that was calculated using this potential function derived from microwave data, and that which was recently observed by Borgers and Strauss, is very good. On the basis of a dynamic model for the ring‐puckering motion in which the atoms move along curvilinear paths with no bond stretching, the coupling constant between the over‐all rotation and the internal angular momentum was calculated. It was found to be 170 Mc/sec compared to the experimental value of 176 Mc/sec. The importance of treating the vibrational dependence of the off‐diagonal elements of the inertial tensor as being equivalent to vibrational angular momentum is stressed.