Photoisomerization and Infrared Spectra of Allene and Propyne Cations in Solid Argon

Allene and propyne as well as their cationic forms play important roles in combustion and interstellar chemistry and serve as a model system for molecular spectroscopic studies. Both cations show Jahn–Teller (J–T) distortions in their ground states. These J–T distortions make the theoretical and experimental studies of their electronic structures difficult. We produced allene cations upon electron bombardment during matrix deposition of Ar containing a small proportion of allene. The intensities of the absorption features of the allene cation decreased after irradiation with UV light, whereas new bands attributed to propyne cations increased. The observed line wavenumbers, relative intensities, and deuterium-substituted isotopic ratios of the isomers of C 3 H 4 + agree satisfactorily with those predicted by density functional theory at the B3PW91/aug-cc-pVTZ level of theory. This method produced the hydrocarbon cations of interest with few other fragments that enabled the clear identification of the IR spectra of allene and propyne cations.