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

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Authors

Miriam Kappe, University of Innsbruck
Florent Calvo, Université Grenoble Alpes
Johannes Schöntag, University of Tübingen
Holger F. Bettinger, University of Tübingen
Serge Krasnokutski, Friedrich Schiller University of Jena
Martin Kuhn, University of Innsbruck
Elisabeth Gruber, University of Innsbruck
Fabio Zappa, University of Innsbruck
Paul Scheier, University of Innsbruck
Olof E. Echt, olof.echt@unh.eduFollow

Abstract

The adsorption of helium on charged hexabenzocoronene (Hbc, C₄₂H₁₈), a planar polycyclic aromatic hydrocarbon (PAH) molecule of D_6h symmetry, is investigated by a combination of high-resolution mass spectrometry and classical and quantum computational methods. The ion abundance of He_nHbc⁺ 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, mass spectra of anionic He_nHbc^– 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_6h symmetry of the substrate, but helium atoms in partially filled rings are rather delocalized. The first three rings contain 6 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.

Department

Physics

Publication Date

10-10-2022

Journal Title

Molecules

Language

English

Publisher

MDPI AG, Basel

Digital Object Identifier (DOI)

https://dx.doi.org/10.3390/molecules27196764

Document Type

Article

Recommended Citation

M. Kappe, F. Calvo, J. Schöntag, H. F. Bettinger, S. Krasnokutski, M. Kuhn, E. Gruber, F. Zappa, P. Scheier, O. Echt, Molecules 27 (2022) 6764

Rights

© 2022 by the authors. Licensee MDPI, Basel, Switzerland.

Comments

This is an Open Access article published by MDPI in Molecules in 2022, available online: https://doi.org/10.3390/molecules27196764