Theoretical study of infrared spectra of interstellar PAH molecules with N, NH & NH${}_2$ incorporation

Abstract

This work presents theoretical calculations of infrared spectra of nitrogen (N)-containing polycyclic aromatic hydrocarbon (PAH) molecules with incorporation of N, NH and NH${}_2$ using density functional theory (DFT). The properties of their vibrational modes in 2--15 $\mu \mathrm{m}$ are investigated in relation to the Unidentified Infrared (UIR) bands. It is found that neutral PAHs, when incorporated with NH${}_2$ and N (at inner positions), produce intense infrared bands at 6.2, 7.7 and 8.6 $\mu \mathrm{m}$ that have been normally attributed to ionized PAHs so far. The present results suggest that strong bands at 6.2 and 11.2 $\mu \mathrm{m}$ can arise from the same charge state of some N-containing PAHs, arguing that there might be some N-abundant astronomical regions where the 6.2 to 11.2 $\mu \mathrm{m}$ band ratio is not a direct indicator of PAHs' ionization. PAHs with NH${}_2$ and N inside the carbon structure show the UIR band features characteristic to star-forming regions as well as reflection nebulae (Class A), whereas PAHs with N at the periphery have similar spectra to the UIR bands seen in planetary nebulae and post-AGB stars (Class B). The presence of N atom at the periphery of a PAH may attract H or H${}^{+}$ to form N-H and N-H${}_2$ bonds, exhibiting features near 2.9--3.0 $\mu \mathrm{m}$, which are not yet observationally detected. The absence of such features in the observations constrains the contribution of NH and NH${}_2$ substituted PAHs that could be better tested with concentrated observations in this range. However, PAHs with N without H either at the periphery or inside the carbon structure do not have the abundance constraint due to the absence of 2.9--3.0 $\mu \mathrm{m}$ features and are relevant in terms of positions of the UIR bands. Extensive theoretical and experimental studies are required to obtain deeper insight.

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