2026MNRAS.545f1901B

In this study, we report the first high-resolution gas-phase infrared absorption spectra and anharmonic vibrational simulations for three nitrogenated aromatic molecules, namely indole (C$_8$H$_7$N), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN; C$_7$H$_{12}$N$_2$), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU; C$_9$H$_{16}$N$_2$) with excellent agreement between measurement and computation. Anharmonic cascade emission spectra (2-8 eV) of the three systems show that nitrogen substitution strongly modifies the emission spectra: indole reproduces the unidentified 3.3 $\mu$m emission in Titan's atmosphere, while DBN and DBU exhibit redshifted aliphatic C-H stretches. In the 5-10 $\mu$m region, indole shows shifted C-H and C-C modes compared to its non-substituted analogue indene, complicating the estimates of the aliphatic to aromatic ratio. These results demonstrate the sensitivity of infrared emission spectra to nitrogen substitution and provide the first anharmonic spectroscopic benchmarks for singly- and doubly-nitrogenated heterocycles. They establish a foundation for interpreting upcoming astronomical observations and point to the need to extend such studies to larger nitrogenated polycyclic aromatic hydrocarbons to evaluate their astrophysical relevance and potential role as precursors to biorelevant molecules.