Adsorption of Helium on Cationic or Anionic Tetracene and Pentacene

Adsorption of helium on tetracene (Tet) and pentacene (Pen) ions is investigated by high‐resolution mass spectrometry of doped helium nanodroplets and a combination of classical global optimization techniques and path‐integral molecular dynamics (PIMD) simulations. The ion abundances of He n Tet ± and He n Pen ± versus size n feature several pronounced local anomalies; they signal evaporation energies that are locally enhanced compared to the next larger complex. Evaporation energies computed with PIMD feature several local anomalies that correlate with experimental anomalies, but miss other anomalies. Remarkably, adding just two helium atoms (one on each side of the molecule) may drastically change the atomic arrangement or destroy the localization; even the number of atoms in the innermost shell may change. The adsorbate layer remains one‐dimensional up to n = 6; it changes from two‐ to three‐dimensional at approximately n = 20. Completion of the first solvation shell occurs between n = 43 and 60; anions tend to accommodate a smaller number of atoms in the first shell than cations. Nonmonotonic size effects are also found in the simulations. Our calculations reveal that, even under cryogenic conditions, some low‐frequency vibrational modes could be activated enough due to zero‐point motion alone and induce significant torsional deformations.