Experimental and Anharmonic Theoretical Spectroscopic Benchmarks of Nitrogenated and Doubly-Nitrogenated Polycyclic Heterocycles

Authors

Selma Bejaoui, NASA Ames Research CenterFollow
Brant E. Billinghurst, Canadian Light Source Inc.
Christiaan Boersma, NASA Ames Research Center
Vincent J. Esposito, Chapman UniversityFollow

Document Type

Article

Publication Date

11-3-2025

Abstract

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 (C₈H₇N), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN; C₇H₁₂N₂), and 1,8- diazabicyclo[5.4.0]undec-7-ene (DBU; C₉H₁₆N₂) 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 μm emission in Titan’s atmosphere, while DBN and DBU exhibit redshifted aliphatic C–H stretches. In the 5–10 μm 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.

Comments

This article was originally published in Monthly Notices of the Royal Astronomical Society, volume 545, issue 2, in 2026. https://doi.org/10.1093/mnras/staf1901 

Recommended Citation

Salma Bejaoui, Brant E Billinghurst, Christiaan Boersma, Vincent J Esposito, Experimental and anharmonic theoretical spectroscopic benchmarks of nitrogenated and doubly-nitrogenated polycyclic heterocycles, Monthly Notices of the Royal Astronomical Society, Volume 545, Issue 2, January 2026, staf1901, https://doi.org/10.1093/mnras/staf1901

Copyright

This work is co-written by US Government employees and is in the public domain in the US.