Gas-phase infrared spectra of cationized nitrogen-substituted polycyclic aromatic hydrocarbons

Gas-phase infrared spectra of several ionized nitrogen substituted polycyclic aromatic hydrocarbons (PANHs) have been recorded in the 600-1600 cm-1 region via IR multiple-photon dissociation (IRMPD) spectroscopy. The UV photoionized PANH ions are trapped and isolated in a quadrupole ion trap where they are irradiated with an IR free electron laser. The PANHs were studied in their radical cation (PANH+) and protonated (H+PANH) forms, and include quinoline, isoquinoline, phenanthridine, benzo[h]quinoline, acridine, and dibenzo[f,h]quinoline. Experimental IRMPD spectra were interpreted with the aid of density functional theory methods. The PANH+ IR spectra are found to resemble those of their respective non-nitrogenated PAH cations. The IR spectra of H+PANHs are significantly different owing to the NH inplane bending vibration, which generally couples very well with the aromatic CH bending and CC stretching modes. Implications of the NPAH (+, H+) laboratory spectra are discussed for the astrophysical IR emissions and, in particular, for the band at 6.2 μm.