Microwave Spectra, Structure, and Ring-Puckering Vibration of Octafluorocyclopentene

The rotational spectra of octafluorocyclopentene (C₅F₈) has been measured for the first time using pulsed jet Fourier transform microwave spectroscopy in a frequency range of 6 to 16 GHz. As in the molecule cyclopentene, the carbon ring is nonplanar, and inversion through the plane results in an inversion pair of ground state vibrational energy levels with an inversion splitting of 18.4 MHz. This large amplitude motion leads to the vibration-rotation coupling of energy levels. The symmetric double minimum ring-puckering potential function was calculated, resulting in a barrier of 222 cm^-1. The rotational constants A₀ = 962.9590(1) MHz, B₀ = 885.1643(4) MHz, C₀ = 616.9523(4) MHz, A₁ = 962.9590(1) MHz, B₁ = 885.1643(4) MHz, C₁ = 616.9528(4) MHz, and two centrifugal distortion constants for each state were determined for the parent species and all ¹³C isotopologues. A mixed coordinate molecular structure was determined from a least-squares fit of the ground state rotational constants of the parent and each ¹³C isotopologue combined with the equilibrium bond lengths and angles from quantum chemical calculations.