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Nitrogen-Heterocycles in the Interstellar Medium: Experimental and Computational Approaches to an Astrochemical Mystery


Astrochemistry, while a relatively new field, has seen an explosion of growth and scientific interest in the last several decades. Currently, roughly 250 molecules have been identified in space, with the pace of new discoveries increasing each year. The variety of new laboratory techniques, telescopes, and computational technology that have recently become available have helped to fuel this rapid pace of exploration. However, for all we have learned about the chemical environments of the interstellar medium, there remain many unanswered questions. One of these mysteries pertains to nitrogen-containing heterocycles, cyclic hydrocarbons that contain at least one nitrogen within the ring. These species are of fundamental interest to a wide variety of scientific disciplines, due mainly to their critical role in biology as DNA and RNA nucleobases, amino acid side chains, components of vitamins, and building blocks of porphyrins. They have been detected on meteorites with non-terrestrial isotopic abundances, suggesting an interstellar origin. Furthermore, observations of the unidentified infrared bands that are thought to arise from polycyclic aromatic hydrocarbons include one transition with a shift attributed to the presence of a nitrogen atom within the aromatic structure. However, despite the evidence that nitrogen-containing heterocycles are produced in astrochemical environments, not one has been detected in space despite numerous astronomical searches, and they are not included at present in reaction paths within predictive chemical kinetics models. The work presented here serves as the foundation for a broad research program with the goal of drastically increasing our understanding of astrophysical N-heterocycles. This is achieved through spectroscopic characterization of N-heterocycles and their precursors and reaction path discovery with ab initio molecular dynamics. Future work will include kinetics experiments to determine rate constants and branching ratios of formation and destruction pathways, as well as astronomical searches for N-heterocycles in a variety of interstellar and circumstellar environments. Beyond the importance for astrochemistry, the study of gas-phase nitrogen-containing species is particularly relevant for attempts to understand the chemistry within Titan's atmosphere, as well as the terrestrial combustion of biomass.

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