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