Coating Polycyclic Aromatic Hydrocarbon Cations with Helium Clusters: Snowballs and Slush

Abstract

The classical and quantum structures of cationic polycyclic aromatic hydrocarbon molecules (benzene, pyrene, coronene, and circumcoronene) coated by helium atoms have been theoretically investigated using a variety of computational methods. Classical shell filling, as determined from global optimization, is generally found to proceed by epitaxial additions on the graphitic surfaces before peripheral closure. From the quantum mechanical perspective provided by path-integral molecular dynamics simulations, vibrational delocalization is found to generally decrease the size of this first solvation shell, but also to give rise to a variety of situations in which the helium atoms are more or less localized depending on their environment, with strong finite size effects depending both on the hydrocarbon cation and the number of coating helium atoms. While the graphitic planes tend to bind helium sufficiently to give rise to snowball precursors, the peripheral regions are less dense and more delocalized, not as liquid as the outer layers but within an intermediate slushy character.

Adsorption of molecular hydrogen on coronene with a new potential energy surface

“…Thus, locations of H 2 on the coronene plane are comparable to those found in previous studies on these systems [34][35][36]85 .…”

The structure of coronene cluster ions inferred from H2 uptake in the gas phase

“…Guided by previous experimental data 36 for He n Cor + and theoretical work involving complexes of coronene with hydrogen, 37 helium, [38][39][40] and heavier noble gases 41,42 we attribute the anomaly at n = 38 to an abrupt drop in the adsorption energy once all hollow adsorption sites are occupied, completing a solvation shell. As shown in Fig.…”

“…1). [36][37][38][39][40] Particles at i-and o-sites are pushed radially outwards because their size exceeds the separation (0.246 nm) between the centers of adjacent carbon rings. For helium, 3 or 6 additional atoms can be accommodated at or close to o-sites resulting in anomalies at n = 41 and 44.…”

Isomeric Broadening of C60+ Electronic Excitation in Helium Droplets: Experiments Meet Theory

“…The PIMD methodology has been described in detail elsewhere 39,40 and will not be repeated here. A Trotter number of 128 was chosen at all temperatures with time steps of 5 fs (1 and 5 K) and 1 fs (10 K) and simulation times of 100 ps for equilibration and 500 ps for production runs.…”