Theoretical study of infrared spectra of interstellar PAH molecules with N, NH, and NH2 incorporation

Abstract This work presents theoretical calculations of infrared spectra nitrogen (N)-containing polycyclic aromatic hydrocarbon (PAH) molecules with the incorporation N, NH, and NH2 using density functional theory (DFT). The properties their vibrational modes in 2–15 μm are investigated relation to Unidentified Infrared (UIR) bands. It is found that neutral PAHs, when incorporated N (at inner positions), produce intense bands at 6.2, 7.7, 8.6 have been normally attributed ionized PAHs so far. present results suggest strong 6.2 11.2 can arise from same charge state some N-containing arguing there might be N-abundant astronomical regions where band ratio not a direct indicator PAHs’ ionization. inside carbon structure show UIR features characteristic star-forming as well reflection nebulae (Class A), whereas periphery similar seen planetary post-AGB stars B). presence atoms PAH may attract H or H+ form N–H N–H2 bonds, exhibiting near 2.9–3.0 μm, which yet observationally detected. absence such observations constrains contribution NH substituted could better tested concentrated this range. However, without either do abundance constraint due relevant terms positions Extensive experimental studies required obtain deeper insight.