Stephanie Cazaux, Technical University of Delft, Astrodynamics and Space Missions

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

The understanding of hydrogen attachment on Polycyclic Aromatic Hydrocarbon is essential to the understanding of the formation of cosmic H₂. By using coronene cations as prototypical PAHs, 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 the most stable cations with 5, 11, or 17 hydrogen atoms, exactly the magic numbers found in the experiments. For these stable closed-shell cations, further hydrogenation requires appreciable structural changes associated with a high transition barrier.  The occurrence of stable super hydrogenated PAHs could be important to identify PAHs in space and determine their contribution to the formation of H₂.