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Solvation of Large Polycyclic Aromatic Hydrocarbons in Helium: Cationic and Anionic Hexabenzocoronene

Affiliation
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, 6020 Innsbruck, Austria
Kappe, Miriam;
ORCID
0000-0002-3621-3046
Affiliation
Laboratoire Interdisciplinaire de Physique, CNRS, Université Grenoble Alpes, F-38000 Grenoble, France
Calvo, Florent;
Affiliation
Institut für Organische Chemie, Universität Tübingen, 72076 Tübingen, Germany
Schöntag, Johannes;
ORCID
0000-0001-5223-662X
Affiliation
Institut für Organische Chemie, Universität Tübingen, 72076 Tübingen, Germany
Bettinger, Holger F.;
ORCID
0000-0002-9816-3187
Affiliation
Laboratory Astrophysics and Cluster Physics Group of the MPI for Astronomy, University of Jena, Helmholtzweg 3, 07743 Jena, Germany
Krasnokutski, Serge;
Affiliation
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, 6020 Innsbruck, Austria
Kuhn, Martin;
ORCID
0000-0002-1195-3638
Affiliation
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, 6020 Innsbruck, Austria
Gruber, Elisabeth;
ORCID
0000-0003-4452-8520
Affiliation
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, 6020 Innsbruck, Austria
Zappa, Fabio;
ORCID
0000-0002-7480-6205
Affiliation
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, 6020 Innsbruck, Austria
Scheier, Paul;
ORCID
0000-0002-0970-1191
Affiliation
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, 6020 Innsbruck, Austria
Echt, Olof

The adsorption of helium on charged hexabenzocoronene (Hbc, C 42 H 18 ), a planar polycyclic aromatic hydrocarbon (PAH) molecule of D 6 h symmetry, was investigated by a combination of high-resolution mass spectrometry and classical and quantum computational methods. The ion abundance of He n Hbc + complexes versus size n features prominent local anomalies at n = 14, 38, 68, 82, and a weak one at 26, indicating that for these “magic” sizes, the helium evaporation energies are relatively large. Surprisingly, the mass spectra of anionic He n Hbc − complexes feature a different set of anomalies, namely at n = 14, 26, 60, and 62, suggesting that the preferred arrangement of the adsorbate atoms depends on the charge of the substrate. The results of our quantum calculations show that the adsorbate layer grows by successive filling of concentric rings that surround the central benzene ring, which is occupied by one helium atom each on either side of the substrate. The helium atoms are fairly localized in filled rings and they approximately preserve the D 6 h symmetry of the substrate, but helium atoms in partially filled rings are rather delocalized. The first three rings contain six atoms each; they account for magic numbers at n = 14, 26, and 38. The size of the first ring shrinks as atoms are filled into the second ring, and the position of atoms in the second ring changes from hollow sites to bridge sites as atoms are filled into the third ring. Beyond n = 38, however, the arrangement of helium atoms in the first three rings remains essentially frozen. Presumably, another ring is filled at n = 68 for cations and n = 62 for anions. The calculated structures and energies do not account for the difference between charge states, although they agree with the measurements for the cations and show that the first solvation shell of Hbc ± is complete at n = 68. Beyond that size, the adsorbate layer becomes three-dimensional, and the circular arrangement of helium changes to hexagonal.

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