MNRAS 000,1–8 (2021) Preprint 25 May 2021 Compiled using MNRAS LATEX style file v3.0 Hydrogenation of small aromatic heterocycles at low temperatures April M. Miksch,1 Annalena Riffelt,1 Ricardo Oliveira,2 Johannes Kästner,1 and Germán Molpeceres,1¢ 1Institute for Theoretical Chemistry, University of Stuttgart, Stuttgart, Germany 2Chemistry Institute, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil Accepted XXX. Received YYY; in original form ZZZ 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, silaben- zene, 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. Key words: ISM: molecules – Molecular Data – Astrochemistry – methods: numerical 1 INTRODUCTION (Campbell et al. 2015). The importance of these detections lies in the evident interest of knowing that these molecules populate dense Aromatic chemistry happening in astrophysical environments is puz- molecular clouds and the possible synthetic pathways that account zling for one particular reason: While the beginning of organic chem- for the formation of these molecules in a bottom-up approach from istry as a discipline can be arguably attributed to the work of August smaller unsaturated precursors. We highlight the recent study of the Kekulé on the structure of benzene (the building block of aromatic formation of indene in this context (Doddipatla et al. 2021). The molecules), the study of aromatic chemistry in interstellar environ- bottom-up approach contrasts with the top-bottom one, which con- ments is much less common. This fact is indeed surprising when con- siders aromatic molecules as products of the energetic processing of sidering that aromatic molecules are more stable than their aliphatic large polycyclic aromatic hydrocarbons or soot-like structures (Tie- counterparts due to the electron delocalization in the structure, and, lens 2008; Merino et al. 2014). This scenario presents an alternative thus, the prevalence of these molecules should be high in astronom- explanation for the formation of aromatic molecules. ical environments. The nature of the electronic structure of benzene still receives significant attention in the literature (Liu et al. 2020; Everything that has been presented above establishes an exciting Eriksen et al. 2020). ground to explore aromatic chemistry in cold environments. Further- The detection of circumstellar benzene (Cernicharo et al. 2001; more, the detection of C6H5CN, McGuire et al.(2018), also presents Malek et al. 2011), as well as the reliable detection of benzonitrile an intensive, but unsuccessful, search for other aromatic molecules, (C6H5CN) and cyanonapthalene (McGuire et al. 2018, 2021) in the arXiv:2105.11175v1 [astro-ph.GA] 24 May 2021 including, but not limited to, furan, pyrrole, and pyridine. The detec- TMC-1 molecular cloud has initiated a new wave of detections that tion of cyanocyclopentadiene (McCarthy et al. 2020) also raises the are related to aromatic chemistry. The detection of benzonitrile in question of why aromatic heterocycles, such as pyridine or pyrrole, other sources (Burkhardt et al. 2021), as an example, proves that are seemingly absent from observations. Both McGuire et al.(2018) TMC-1 is not a particular case. The detection of other non-armoatic and McCarthy et al.(2020) hypothesize that chemically active rad- cyclic species that can share part of their chemical formation routes icals such as NH and NH2 might be in lower abundance than the (McCarthy et al. 2020; Lee et al. 2021) with aromatic compounds as inert N2 or NH3. NH and NH2 reaction with butadiene is postu- well as the detection of proxies which are thought to be involved in lated as a possible route for the formation of pyrrole and similarly, the synthesis of aromatic molecules (Agundez et al. 2021; Marcelino, pyridine (McCarthy & McGuire 2021). An alternative explanation N. et al. 2021) also contribute to the growth in knowledge concerning points to the CN radical reacting with aromatic material as a possi- aromatic chemistry. In addition to the detection of these molecules, + ble chemical conversion route (McCarthy et al. 2020; Cooke et al. it is important to mention the detection of C60 in diffuse media 2020). The latter argument on chemical conversion must also hold for other reactive species presenting barrierless pathways or chemi- cal reduction with hydrogen (H) atoms. H atoms possess the unique ¢ E-mail: [email protected] trait of being able to tunnel effectively through potential energy bar- © 2021 The Authors 2 Miksch et al. riers, increasing the probability of interstellar chemical reactions. used for the hydrogenation of benzene (Goumans & Kästner 2010), Hydrogenations via quantum tunneling have been extensively stud- the MPWB1K (Zhao & Truhlar 2004) exchange and correlation func- ied in the literature, both experimentally and theoretically (Goumans tional was selected. This study found that MPWB1K yielded com- & Kästner 2010; Meisner et al. 2017; Lamberts & Kästner 2017; parable results to the more expensive CCSD(T)/CBS level. We have Oba et al. 2018; Álvarez-Barcia et al. 2018; Nguyen et al. 2020; kept this selection, but in this study, we have increased the basis Molpeceres & Kästner 2021) to mention a few. The high abundance set size, employing the def2-TZVP (Weigend & Ahlrichs 2005) in of atomic hydrogen in astronomical environments encouraged us to our work. The combination of exchange and correlation functional study the possible outcomes of the interaction of H atoms with aro- and basis set is abbreviated as MPWB1K/def2-TZVP. Electronic matic molecules. Hydrogenation of benzene mediated by tunneling structure calculations were run using the Gaussian16 code (Frisch has been previously postulated as an effective process under astro- et al. 2016). Hydrogenation reactions were sampled in each pos- nomical conditions (Goumans & Kästner 2010). sible position of every molecule under consideration. Additionally, With this paper, we have two intentions: Firstly, we want to evaluate we repeated the study for benzene, now including the bigger basis the viability of the hydrogenation of simple aromatic archetypes con- set. The protocol we have followed to characterize all the possible taining heteroatoms in their aromatic skeleton. Secondly and related, chemical reactions is the same for all archetypes. First, from each we want to evaluate the influence of the heteroatom on the reactivity relaxed structure of the archetypes, we have performed exploratory of the archetypes. Both questions are addressed from a computational potential energy surface (PES) scans, restraining the reaction coor- standpoint. The answers to these questions will help astronomers dinate of interest. In the case of 6-member ring molecules (i.e. a, identify possible targets in present and future surveys looking for e, and f) depicted in Figure1, we analyze hydrogenations in four aromatic molecules in the interstellar medium. As archetypes for our different positions, namely the heteroatom (1), the ortho-position study, we have selected all aromatic six-member and five-member (2), the meta-position (3), and the para-position (4). In the case of rings containing heteroatoms with significant abundance in astro- the archetypes containing 5-membered rings (b, c, and d) the num- chemical models (Asplund et al. 2006). These include pyridine, ber of positions is reduced to three. Both hydrogen additions and pyrrole, furan, thiophene, silabenzene, and phosporine. The list of H2 abstractions were sampled. We can discern between exothermic molecules can be found in Figure1. Previous studies about the syn- and endothermic processes as well as processes with and without thesis of heterocycles under astrophysical conditions showed that a barrier from this initial exploration. Endothermic processes are furan, tiophene and pyrrol must prevail under astronomical condi- discarded based on the low-temperature conditions of astronomical tions (Lattelais et al. 2010). Gas phase synthetic routes for pyridine environments. All the H2 abstractions were found to be endothermic compatible with astrophysical conditions were also investigated in the with only two exceptions for silabenzene and pyrrole. past (Anders et al. 1974; Parker et al. 2015; Parker & Kaiser 2017), as We optimized the transition state for reactions presenting an acti- well as the stability of pyridine derivatives and other N heterocycles vation barrier using the dimer method (Henkelman