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

Abstract

This work presents theoretical calculations of infrared spectra of nitrogen (N)-containing polycyclic aromatic hydrocarbon (PAH) molecules with the incorporation of N, NH, and NH2 using density functional theory (DFT). The properties of their vibrational modes in 2–15 μm are investigated in relation to the Unidentified Infrared (UIR) bands. It is found that neutral PAHs, when incorporated with NH2 and N (at inner positions), produce intense infrared bands at 6.2, 7.7, and 8.6 μm that have been normally attributed to ionized PAHs so far. The present results suggest that strong bands at 6.2 and 11.2 μm 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 μm band ratio is not a direct indicator of the PAHs’ ionization. PAHs with NH2 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 atoms at the periphery of a PAH may attract H or H+ to form N–H and N–H2 bonds, exhibiting features near 2.9–3.0 μm, which are not yet observationally detected. The absence of such features in the observations constrains the contribution of NH and NH2 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 μm features and are relevant in terms of positions of the UIR bands. Extensive theoretical and experimental studies are required to obtain deeper insight.

Rotational spectra of interstellar N- and CN-PAHs: pyrene and coronene

“…There are two most appropriate forms of PANHs-(i) PAHs having N within the PAH skeleton or N-substituted PAHs (N-PAHs, Fig. 1) (Hudgins et al 2005;Ricca et al 2021;Vats et al 2022) and, (ii) PAH nitriles or CN-attached PAHs (CN-PAHs, Fig. 1).…”

“…The GAUSSIAN 09 (Frisch et al 2009) suite of programs was used to compute each molecule's geometry, rotational constants, quadrupole coupling constants, quartic centrifugal distortion constants and dipole moments using Density Functional Theory (DFT) with anharmonic vibrational frequency calculations. DFT has been widely utilized in rotational spectroscopy of interstellar Ncontaining PAHs to determine the molecular or spectroscopic constants (Kisiel et al 2003;McNaughton et al 2008;Pirali et al 2015;McNaughton et al 2018), as well as to compute their vibrational modes (Hudgins et al 2005;Vats et al 2022;Ricca et al 2021). The use of B3LYP functional with the 6-311+G(d,p) basis set or the correlation-consistent polarized triple valence basis set (cc-pVTZ) are found effective for studying the molecular or spectroscopic constants of CN-PAHs (McNaughton et al 2018), whereas the MP2/6-31G(d,p) combination is found better for quinoline (Kisiel et al 2003).…”

“…Based on the above, other readily detectable PAH species have been studied, out of which the most prominent and suitable PAH candidates are nitrogen containing PAHs or polycyclic aromatic nitrogen heterocycles (PANHs) (Hudgins et al 2005). PANHs are strongly polar in nature, likely widespread in the ISM and possible carriers of the UIR band at 6.2 µm (Hudgins et al 2005;Ricca et al 2021;Vats et al 2022). PANHs have been detected in carbonaceous meteorites (Sephton 2002) and in the form of PAH nitriles, such as benzonitrile (C 6 H 5 CN), 1-cyano-1, 3-cyclopentadiene (C 5 H 6 CN), and 1-and 2-cyano-naphthalene (C 11 H 7 CN) in the TMC-1, a cold, dark molecular cloud (McGuire et al 2018;McCarthy et al 2021;McGuire et al 2021).…”

Time-Dependent Density Functional Study of Nitrogen-Substituted Polycyclic Aromatic Hydrocarbons and Diffuse Interstellar Bands

“…In general, N−H stretching features fall near 2.9 μm (2900 nm) 47,58 and the nondetection of N−H vibrational modes in observations restricts the contribution of N−H-containing PAHs. 47 Ricca et al 58 derived the upper limit for their possible contribution to the interstellar, 3.3 (3300 nm) and 6.2 μm bands (6200 nm) as 0.36 and 2.5%, respectively. The PANH species selected for our computational analysis are Nsubstituted coronene (C 24 H 12 ) and N-substituted circumcoronene (C 54 H 18 ).…”

“…Several investigations have found that a N atom can be included in the PAH structure. ,,− The effect of nitrogen on the vibrational and electronic spectra of PAHs has also been investigated in various theoretical and experimental studies. − , While for PANHs, the time-dependent density functional theory (TD-DFT) calculation technique is useful in interstellar cloud modeling, as the knowledge of oscillator strength enhances the understanding of cosmic abundances.…”