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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.

The Brenner potential is adapted to handle chemical interactions and reactions of H atoms at a graphene surface. The adapted potential reproduces several important features of DFT computed data and reveals an extended puckering of the surface upon its adsorption of an H atom. This potential is used to investigate in a much more realistic way than has been done before, the Eley-Rideal abstraction reaction producing H(2) in H + H-graphene collisions at energies E(col)< or = 0.2 eV. The graphene surface is represented by a slab of 200 carbon atoms and the study is carried out using classical molecular dynamics for vertical incidences in a cylinder centered about the chemisorption axis. A highlight of the present study is that upon the arrival of the gas phase H atom, the adsorbent C atom is attracted and pulls out its surrounding surface atoms. The hillock thus formed remains puckered until the newly formed molecule is released. The range of impact parameters leading to reaction depends on the collision energy and is governed by the shape of the entrance channel potential; the reaction probability in this range is 100%. On average, in the studied E(col) range, the available energy (3.92 eV + E(col)) is shared as: 69-52% for the internal energy, 11-23% for the translation energy and 20-25% for the energy imparted to the surface. Also, the average vibration and rotation energy levels of the nascent H(2) molecule are, respectively, v = 5-4 and j = 2-4.

We study the quasi-classical dynamics of OH formation on a graphitic surface through the Langmuir-Hinshelwood (LH) mechanism when both O and H ground-state atoms are initially physisorbed on the surface. The model proceeds from previous theoretical work on the LH formation of the H 2 molecule on graphite [Morisset, S.; Aguillon, F.; Sizun, M.; Sidis, V. J. Chem. Phys. 2004, 121, 6493; ibid 2005, 122, 194704]. The H-graphite system is first revisited with a view to get a tractable DFT-GGA computational prescription for the determination of atom physisorption onto graphitic surfaces. The DZP-RPBE combination is found to perform well; it is thereafter used along with MP2 calculations to determine the physisorption characteristics of atomic oxygen on graphitic surfaces. We also deal with chemisorption. In accordance with previous work, we find that O chemisorbs on graphite in a singlet spin state epoxy-like conformation. In the triplet state we find only "metastable" chemisorption with an activation barrier of 0.2 eV. The physisorption results are then used in the LH dynamics calculation. We show that in the [0.15 meV, 12 meV] relative collision energy range of the reacting O and H atoms on the surface, the OH molecule is produced with a large amount of internal energy ( approximately = 4eV) and a significant translation energy (>or=100 meV) relative to the surface.

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We present a first-principles computational study of the interaction of an H atom with the (010) surface of forsterite (Mg 2 SiO 4 ). Periodic DFT-GGA calculations (PBE) are carried out using the SIESTA code with core pseudopotentials and TZP localized basis sets. Potential energy curves are determined for the approach of the H atom toward different sites of the surface: atop, near, or in between the O, Mg, and Si atoms. An outer adsorption well is found for all investigated sites; it is deepest (162 meV) at a so-called "displaced Mg− O bridge" position. The binding at this well is of the "weak chemisorption"/"strong physisorption" type. An inner stronger chemisorption well (670 meV deep) exists exclusively near an O site but not strictly atop. Depending on the path, we find activation barriers (25− 170 meV high) against chemisorption, the lowest of these occurs for the top O site. General trends of the computed interaction energies qualitatively agree with the QM/MM results of Goumans et al. [Mon. Not. R. Astron. Soc. 2009, 393, 1403, but adsorption binding energies and barrier heights differ significantly.