Interpretation of the vibrational spectra of small ring systems II. Perfluorocyclobutane

The vibrational spectra of gaseous and solid perfluorocyclobutane have been interpreted in terms of a planar molecule with D4h symmetry in view of the observed mutual exclusion between the infrared and Raman frequencies. A vibrational assignment for perfluorocyclobutane is presented and based upon the observed infrared and Raman band contours, depolarization ratios, relative band intensities, and positions. A comparison of the ring-puckering potential functions and skeletal structures of cyclobutane, 1,1-difluorocyclobutane, trimethylene oxide, and 3,3-difluoroxetane suggests the inversion barrier is reduced significantly upon introduction of a CF2 group in the cyclic systems. The apparent trend toward planarity appears to continue in 1,1,2,2-tetrafluorocyclobutane and perfluorocyclobutane. The factors that may dictate the equilibrium structures of small ring molecules are considered in view of the data presently available.