Spectra and Structure of Small Ring Compounds. XX. Fluorocyclobutane

The infrared spectrum of fluorocyclobutane vapor has been recorded from 4000–33 cm⁻¹. The infrared spectrum of the solid at − 196°C has been recorded from 4000–200 cm⁻¹. The Raman spectra of the liquid at room temperature and the solid have been recorded and depolarization values have been measured. The spectra have been interpreted on the basis of a puckered model ring with only one conformation which has a plane of symmetry and belongs to the $C_s$ point group. Assignment of the 30 normal vibrations has been based on depolarization values, band contours, group‐frequency correlations, and relative intensities. A series of four $Q$ branches was observed in the far‐infrared spectrum and has been interpreted as arising from the fundamental and “upper‐state” transitions of the anharmonic ring‐puckering vibration. The series could be fit equally well by two different double‐minima potential functions, one with a relatively high barrier of 803 cm⁻¹ and the other with a 485‐cm⁻¹ barrier. On the basis of previous microwave data and the midinfrared and Raman spectra of the solid and fluid states, the higher barrier potential is believed to more nearly describe the actual potential function governing the ring‐puckering vibration; the energy difference between the two conformations was calculated to be 722 cm⁻¹.