Stable isomeric structures of the pyridine cation (C5H5N center dot+) and protonated pyridine (C5H5NH+) elucidated by cold ion infrared spectroscopy

Cold ion action spectroscopy is applied to record the gas phase infrared fingerprint spectra of mass-selected C5H5N•+ and C5H5NH+ cations using an infrared free electron laser. The structures of the cations are deduced from the experimental spectra using anharmonic vibrational frequencies from density functional theory calculations. A very good agreement between experimental and theoretical infrared frequencies is observed. The dominant structure of the C5H5N•+ cation is assigned to the aromatic pyridine radical cation form. Additionally, a minor contribution of the lower energetic α-distonic isomer is observed. The C5H5NH+ cation is ascribed to the aromatic pyridinium cation where protonation has taken place on the nitrogen atom. The rare-gas tag used in the action spectroscopic method has a negligible effect on the vibrational frequencies. The observed species, with now accurately determined vibrational frequencies, are good candidates for future rotational spectroscopic studies and infrared observations in astronomical sources such as interstellar clouds or Titan’s atmosphere.