A local mode study of ring puckering effects in the infrared spectra of cyclopentane

We report and interpret recently recorded high-resolution infrared spectra for the fundamentals of the CH2 scissors and CH stretches of gas phase cyclopentane at -26.1 and -50 degrees C, respectively. We extend previous theoretical studies of this molecule, which is known to undergo barrierless pseudorotation due to ring puckering, by constructing local mode Hamiltonians of the stretching and scissor vibrations for which the frequencies, couplings, and linear dipoles are calculated as functions of the pseudorotation angle using B3LYP/6-311++(d,p) and MP2/cc-pVTZ levels of theory. Symmetrization (D-5h) of the vibrational basis sets leads to simple vibration/pseudorotation Hamiltonians whose solutions lead to good agreement with the experiment at medium resolution, but which miss interesting line fractionation when compared to the high-resolution spectra. In contrast to the scissor motion, pseudorotation leads to significant state mixing of the CH stretches, which themselves are Fermi coupled to the scissor overtones. Published under an exclusive license by AIP Publishing.

Automated summary

We report and interpret high-resolution infrared spectra for the fundamentals of the CH2 scissors and CH stretches of gas phase cyclopentane. The frequencies, couplings, and linear dipoles of the stretching and scissor vibrations are calculated as functions of the pseudorotation angle. The simple vibration/pseudorotation Hamiltonians based on symmetrization of the vibrational basis sets show good agreement with the experiment at medium resolution but miss interesting line fractionation compared to the high-resolution spectra.