Gas-Phase Vibrational Spectroscopy of the Hydrocarbon Cations l-C(3)H(+), HC(3)H(+), and c-C(3)H(2)(+): Structures, Isomers, and the Influence of Ne-Tagging

[Image: see text] We report the first gas-phase vibrational spectra of the hydrocarbon ions C(3)H(+) and C(3)H(2)(+). The ions were produced by electron impact ionization of allene. Vibrational spectra of the mass-selected ions tagged with Ne were recorded using infrared predissociation spectroscopy in a cryogenic ion trap instrument using the intense and widely tunable radiation of a free electron laser. Comparison of high-level quantum chemical calculations and resonant depletion measurements revealed that the C(3)H(+) ion is exclusively formed in its most stable linear isomeric form, whereas two isomers were observed for C(3)H(2)(+). Bands of the energetically favored cyclic c-C(3)H(2)(+) are in excellent agreement with calculated anharmonic frequencies, whereas for the linear open-shell HCCCH(+) ((2)Π(g)) a detailed theoretical description of the spectrum remains challenging because of Renner–Teller and spin–orbit interactions. Good agreement between theory and experiment, however, is observed for the frequencies of the stretching modes for which an anharmonic treatment was possible. In the case of linear l-C(3)H(+), small but non-negligible effects of the attached Ne on the ion fundamental band positions and the overall spectrum were found.