The sequence to hydrogenate coronene cations: A journey guided by magic numbers

Abstract

The understanding of hydrogen attachment to carbonaceous surfaces is essential to a wide variety of research fields and technologies such as hydrogen storage for transportation, precise localization of hydrogen in electronic devices and the formation of cosmic H2. For coronene cations as prototypical Polycyclic Aromatic Hydrocarbon (PAH) molecules, the existence of magic numbers upon hydrogenation was uncovered experimentally. Quantum chemistry calculations show that hydrogenation follows a site-specific sequence leading to the appearance of cations having 5, 11, or 17 hydrogen atoms attached, exactly the magic numbers found in the experiments. For these closed-shell cations, further hydrogenation requires appreciable structural changes associated with a high transition barrier. Controlling specific hydrogenation pathways would provide the possibility to tune the location of hydrogen attachment and the stability of the system. The sequence to hydrogenate PAHs, leading to PAHs with magic numbers of H atoms attached, provides clues to understand that carbon in space is mostly aromatic and partially aliphatic in PAHs. PAH hydrogenation is fundamental to assess the contribution of PAHs to the formation of cosmic H2.

Molecular hydrogen formation on interstellar PAHs through Eley-Rideal abstraction reactions

“…A typical mass spectrum consists of dominating peaks at M = 301, 303, 305 etc. On top of that, "magic" stages of superhydrogenation are observed for the attachment of 5, 11, and 17 H atoms, corresponding to hydrogenation stages which have particularly high binding energies (Cazaux et al 2016).…”

“…Recent experimental and theoretical research on trapped coronene radical cations C 24 H + 12 revealed that in the gas-phase and at T = 300 K, hydrogenation proceeds through a specific sequence of well-defined atomic sites. Reaction barriers and binding energies lead to odd -even oscillations in the observed superhydrogenation states, and magic numbers of particularly high intensity for the attachment of n =5, 11, and 17 extra H atoms (Boschman et al 2012;Cazaux et al 2016).…”

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H2formation

“…More recent experiments on the hydrogenation of coronene cation reveal that only coronene cations with an odd number of H-atoms are detected through mass spectrometry (Boschman et al 2012;Reitsma et al 2014). Transition states calculations predict a small (0.01 eV) barrier for the first hydrogenation to a duo ring, and a barrier of 0.03 eV for the second one within the same ring (Cazaux et al 2016). Further H-additions have different barriers depending on both, the radical nature of the reactants and the deformation in the carbon structure of the PAH.…”

Calculations of adsorption energies, coadsorptions, and diffusion barriers of H atoms, and the H2 formation on a nanographene surface (coronene)

“…Because the carbonaceous material is one of the main constituents of interstellar grains, a surface model, such as Cor (C 24 H 12 ), which has been used to simulate graphite, graphene, and PAH interstellar grains, was used as a model. This molecule, shown in Figure A, has been widely used for several authors and for our group as a PAH model of ISM. Cor exhibits three different carbon atoms labeled as a (hydrogenated‐edge sites), b (edge sites) in the border ring ( B ), and c (center sites) in the center ring ( C ), see Figure .…”