Aromatic compounds are an important class of molecules for astrochemistry and prebiotic chemistry. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous and stable in astrophysical environments (1-3) and account for a significant fraction of the known Universe’s molecular carbon inventory. Heterocyclic aromatic compounds are a related class of molecules in which one or more of the carbon atoms in the aromatic skeleton are replaced with a heteroatom such as N or O (N- and O-heterocycles, respectively). Small aromatic molecules have been sought in interstellar and circumstellar environments and although benzene has been detected (4), to date no single ring heterocyclic molecules have been observed, despite the plausibility of their formation (5,6). Here we show that simple O- and N-heterocycles could be formed from the UV-photoirradiation of benzene and naphthalene in H2O- and NH3-containing ices (7). In these experiments, ices were deposited on a cryocooled substrate in a vacuum chamber and exposed to UV irradiation. This resulted in the formation of a refractory organic residue that remains at room temperature that could be analyzed with gas chromatography coupled with mass spectrometry. Nitrogen and oxygen were shown to replace carbon atoms within aromatic rings while leaving the ring structure intact. Specifically, pyridine was produced from benzene-containing ices while phthalide, coumarin, isocoumarin, quinolone, and possibly isoquinoline, were produced from naphthalene-containing ices. This process is an alternative route to the formation of small heterocyclic molecules and may explain the presence of some N-heterocycles in meteorites regardless of their existence in space in the gas phase. 1. Allamandola, L.J., Tielens, A.G.G.M., & Barker, J.R. (1989) Astrophys. J. Suppl. Ser., 71, 733 2. Puget, J.L., & Léger, A. (1989) Ann. Rev. Astron. Astrophys., 27, 161 3. Galliano, F., Madden, S.C., Tielens, A.G.G.M., et al. (2008) Astrophys. J., 679, 310 4. Cernicharo J., Heras, A.M., Tielens, A.G.G.M., et al. (2001) Astrophys. J., 546, 123 5. Kuan, Y-.J., Yan, C-.H., Charnley, S.B., et al. (2003) Month. Not. R. Astron. Soc., 345, 650 6. Kuan, Y.-J., Charnley, S.B., Huang, H.-C., et al. (2004) Adv. Space Res., 33, 31 7. Materese, C.K., Nuevo, M., Sandford, S.A. (2014) Astrophys. J. (submitted)

Formation of aromatic heterocycles from the UV-photoirradiation of aromatic hydrocarbons in ices

249th National Meeting (2015)

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