Vibrational Spectra and Structure of Small-Ring Compounds. X. Spectroscopic Evidence for Pseudorotation in Cyclopentane

The infrared spectrum of cyclopentane has been examined from 33 to 4000 cm−1. A series of pronounced Q branches with a 5-cm−1 spacing was observed on top of the CH2 deformation. The Q branch frequencies could be reproduced by assuming that the cyclopentane ring was undergoing pseudorotation. The spectroscopically derived parameter mq02 was found to be (11.0 ± 0.2) × 10−40 g·cm2.