Vibrational analysis and ring-puckering inversion barrier of perfluorocyclopentene

Perfluorocyclopentene, like cyclopentene, is expected to have a nonplanar structure with a low barrier to inversion. However, in contrast to cyclopentene, no cluster of bands that could be assigned to the ringpuckering vibration has been reported in the infrared and Raman spectra. To address this problem, ab initio molecular orbital calculations have been carried out to determine the structure and harmonic vibrational frequencies of the puckered molecule (C_s), and the molecule constrained to planarity (C_2v). The molecule is nonplanar at all levels of theory, with a dihedral angle of 24°. The calculated barrier to inversion (320 ± 50 cm^-1), and puckering frequency (42 cm^-1), have been used in a one-dimensional, quadratic, and quartic potential function to calculate the puckering spectrum. The predicted spectrum is characterized mainly by a number of close-lying transitions extending over the range 40 to 56 cm^-1. In light of the calculations, some assignments in the vibrational spectra have been revised.