Far-Infrared Absorption Spectrum and Pseudorotation of the Thiacyclopentane Molecule

The far-infrared spectrum of thiacyclopentane vapor has been examined at a resolution of 0.5 cm−1 from 20 to 400 cm−1. The series of pronounced Q branches observed in the range 60–120 cm−1 has been assigned on the basis that the molecule is undergoing a pseudorotational vibration which is hindered by the potential function V(φ) in cm−1 = −12(772.7) (1−cos2φ) + 12(169.1) (1−cos4φ). The entropy of this model has been calculated and found to give excellent agreement with the calorimetric entropy.