CH-stretching overtone spectra of cis- and trans-1,3-pentadiene

The room-temperature vapor-phase overtone spectra of cis- and trans-1,3-pentadiene (piperylene) have been recorded in the 5000-17500 cm(-1) region with the use of conventional and intracavity laser photoacoustic spectroscopy. The presence of five nonequivalent olefinic CH bonds and one methyl group in each molecule complicates assignment of the spectra. We have used a harmonically coupled anharmonic oscillator local mode model with one oscillator for each of the nonequivalent CH bonds to calculate the CH-stretching overtone spectra and thus facilitate assignment of the spectra. Our calculated spectra are in good agreement with the observed spectra. The observed high overtone spectra are distinctively different despite the similarity of the two molecules and the numerous and broad transitions.