Vibrational spectra, ab initio calculations, and ring-puckering potential energy function for γ-crotonolactone

The infrared and Raman spectra of liquid and vapor γ-crotonolactone have been collected. Both the experimental data and ab initio calculations show that the molecule is rigidly planar in its electronic ground state. This conclusion agrees with the previously reported microwave studies and is attributed to the conjugation between the C=C and C=O double bonds of the ring. The ring-puckering potential energy function was generated from ab initio calculations and was confirmed by the vapor-phase Raman spectra to be nearly harmonic. Density functional theory (DFT) calculations predict a harmonic ring-puckering frequency of 203 cm^-1 as compared to the observed vapor-phase Raman value of 208 cm^-1. The DFT calculations were also used to compute the infrared and Raman spectra of γ-crotonolactone, and these agree very well with the experimental spectra.