Bis-trifluoromethyl Effect: Doubled Transitions in the Rotational Spectra of Hexafluoroisobutene, (CF₃) ₂C=CH ₂

Rotational spectra for hexafluoroisobutene, and its ¹³ C isotopologues, have been recorded between 8 and 16 GHz using a chirped pulse, Fourier transform microwave spectrometer. Notably, all spectra observed are doubled with separations between the doublets being between 1 and 60 MHz. We propose that the bis-trifluoromethyl groups of the target molecule are staggered in the equilibrium configuration, and that a novel, out-of-phase rotation through a F-CCC-F planar configuration with low barrier (<100 cm ^-1 ), leads to the observed doubled rotational spectra.