Low-frequency vibrations and the determination of structure in non-rigid molecules

Abstract

The gas phase far-infrared spectrum of 3-cyclopenten-1-one was recorded in the range 50-400 cm�¹A progression of five Q branches was observed between 80 and 95 cm�¹. This series was assigned to transitions of the ring-puckering vibration. A potential function of the form V(x) = ax� + bx², where x is the ring-puckering coordinate, was calculated from the observed transition frequencies. The ring atoms of 3-cyclopenten-1-one were shown to have a planar equilibrium structure. The origin of the ring-puckering potential function for 3-cyclopenten-1-one was interpreted as resulting primarily from ring angle strain forces. The nearly harmonic nature of the potential function was attributed to the unusually small values of the angles at the alpha carbon atoms. Mid-infrared spectra of 3-cyclopenten-1-one were recorded for the gas, liquid and solid phases. Raman spectra were recorded for the gas and liquid phases. Assignments were made for the fundamental vibrational modes of the molecule. It was found that the features of the spectra, including gas phase band shapes and Raman depolarization ratios, were consistent with the planar ring structure indicated by the far-infrared results.