Hydrogenation of small aromatic heterocycles at low temperatures

TL;DR

This study theoretically evaluates the hydrogenation reactions of various aromatic molecules relevant to interstellar chemistry, providing insights into their stability and potential as detectable species in space.

Contribution

It offers the first comprehensive quantum chemical analysis of hydrogenation pathways for multiple aromatic heterocycles under astrophysical conditions.

Findings

Most hydrogenation reactions are exothermic with activation barriers overcome by quantum tunneling.

Radicals formed can undergo further hydrogenation, creating potential new interstellar molecules.

Hydrogenated derivatives of furan, pyrrole, and specific dihydro compounds are promising for future detection.

Abstract

The recent wave of detections of interstellar aromatic molecules has sparked interest in the chemical behavior of aromatic molecules under astrophysical conditions. In most cases, these detections have been made through chemically related molecules, called proxies, that implicitly indicate the presence of a parent molecule. In this study, we present the results of the theoretical evaluation of the hydrogenation reactions of different aromatic molecules (benzene, pyridine, pyrrole, furan, thiophene, silabenzene, and phosphorine). The viability of these reactions allows us to evaluate the resilience of these molecules to the most important reducing agent in the interstellar medium, the hydrogen atom (H). All significant reactions are exothermic and most of them present activation barriers, which are, in several cases, overcome by quantum tunneling. Instanton reaction rate constants are provided between 50 K and 500 K. For the most efficiently formed radicals, a second hydrogenation step has been studied. We propose that hydrogenated derivatives of furan, pyrrole, and specially 2,3-dihydropyrrole, 2,5-dihydropyrrole, 2,3-dihydrofuran, and 2,5-dihydrofuran are promising candidates for future interstellar detections.