Enhanced reactivity of curved PAHs towards atomic hydrogen John Thrower 1 , Rijutha Jaganathan 1 ,Mirko Leccese 2 , Raffaele Cheula 1 , Laura Slumstrup 1 , Rocco Martinazzo 2 , Mie Andersen 1 , Liv Hornekær 1 1 Aarhus University, Denmark, 2 Università degli Studi di Milano, Italy Polycyclic aromatic hydrocarbons (PAHs) are thought to account for a significant portion of the carbon present in the galaxy and give rise to a series of bands in the mid-IR spectra of many interstellar objects [1]. As such, they are thought to play an important role in the chemistry of the interstellar medium.Over the past decade, a combination of experimental and theoretical studies employing coronene (C 24 H 12 ) as a model PAH have revealed that superhydrogenated PAHs can be formed through exposure of PAHs to atomic hydrogen, and that through cycles of addition and abstraction reactions, these so-called HPAHs can act as catalysts for molecular hydrogen (H 2 ) formation [2-4]. The reactivity of PAHs towards atomic hydrogen is sensitive to the molecular structure, with pentacene (C 22 H 14 ) showing a higher reactivity, an effect attributed to the presence of zig-zag edges [5]. Upon reaction with hydrogen atoms, the 6,13-pentacenedione molecule (C 22 H 12 O 2 ) loses its oxygen atoms, potentially releasing small oxygen bearing species such as OH and H 2 O [6]. These planar molecules all contain 6-membered hexagonal carbon rings, while interstellar PAHs are likely to contain defects. One such defect is the presence of a 5-membered carbon ring which leads to the molecule being curved – furthermore, the C 60 molecule consists of alternating hexagonal and pentagonal rings. We have used the corannulene molecule (C 20 H 10 ) as a representative curved PAH as its similarity to coronene allows direct determination of the effect of the curvature on the molecule’s reactivity [7]. Temperature programmed desorption combined with mass spectrometry was used to determine the products of corannulene hydrogenation, as well as the cross-section for hydrogenation. A thin film of close to one molecular layer of corannulene was grown on a highly oriented pyrolytic graphite (HOPG) surface held at 210 K under ultrahigh vacuum (UHV) conditions. The film was subsequently exposed to a beam of hydrogen atoms for a range of difference fluences. For each fluence, the hydrogenated film was heated in front of a quadrupole mass spectrometer, allowing a determination of the product mass distribution. Similar to coronene and pentacene, a series of dominant species are observed, indicating particularly stable hydrogenation configurations. These are largely consistent with DFT calculations exploring the most energetically favourable hydrogenation sequence. Some discrepancies at high hydrogenation degrees might result from the complex adsorption structure of corannulene molecules on the graphite surface, as revealed through low temperature scanning tunnelling microscopy (LT-STM) measurements performed at 5 K. References 1. A. G. G. M. Tielens, 2008, Astron. Astrophys, 2008, 46 , 289 2. J. D. Thrower et al., Astrophys. J., 2012, 752 , 3 3. P. A. Jensen, et al., Mon. Not. R. Astron. Soc., 2019, 486 , 5492 4. S. Cazaux, et al., Sci. Rep., 2016, 6, 19835
5. D. Campisi et al., Phys. Chem. Chem. Phys., 2020, 22 , 1557 6. R. Jaganathan et al., Astron. Astrophys.,2022, 663 , A136 7. M. Lecesse et al., Mon. Not. R. Astron. Soc., 2023 (in press) DOI:10.1093/mnras/stad054
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