IR spectra of cationic 1,5,9-triazacoronene and two of its cationic derivatives.

Abstract

Infrared emission features are observed towards diverse astronomical objects in the interstellar medium (ISM). Generally, the consensus is that these IR features originate from polycyclic aromatic hydrocarbons (PAHs) and are hence named aromatic infrared bands (AIBs). More recently, it has been suggested that nitrogen substituted PAHs (PANHs) contribute to the AIBs as well and it has even been shown that nitrogen inclusion in PAHs can improve the match with the AIBs, specifically around the 6.2 μm feature. In order to determine which specific molecules or functional groups are at the origin of the AIBs, IR spectra of various PA(N)Hs are experimentally and computationally studied. In this work we expand on the spectroscopic investigation of PANHs by presenting the gas-phase mid-IR spectra of 1,5,9-triazacoronene˙+ (TAC˙+, m/z 303), a threefold nitrogenated congener of coronene˙+, its protonated derivative TACH+ (m/z 304) and the product that forms when water adds to dehydrogenated TAC˙+[TAC-H + H2O]+ (m/z 320). We analyze the mid-IR spectra by comparing them with vibrational modes calculated at the B3LYP/6-311++G(d,p) level a theory and we present a possible geometry for [TAC-H + H2O]+. The TAC˙+ mid-IR spectrum is compared to that of coronene˙+ and is demonstrated to be remarkably similar. We put TAC˙+ and TACH+ into astronomical context by comparing their recorded mid-IR spectra to observed ISM spectra of four spectral classes. From this we conclude that TAC˙+ and TACH+ could both contribute to the AIBs, with TACH+ being a more likely contributor than TAC˙+.