Adsorption of Helium on Small Cationic PAHs: Influence of Hydrocarbon Structure on the Microsolvation Pattern

Abstract

The adsorption of up to ∼100 helium atoms on cations of the planar polycyclic aromatic hydrocarbons (PAHs) anthracene, phenanthrene, fluoranthene, and pyrene was studied by combining helium nanodroplet mass spectrometry with classical and quantum computational methods. Recorded time-of-flight mass spectra reveal a unique set of structural features in the ion abundance as a function of the number of attached helium atoms for each of the investigated PAHs. Path-integral molecular dynamics simulations were used with a polarizable potential to determine the underlying adsorption patterns of helium around the studied PAH cations and in good general agreement with the experimental data. The calculated structures of the helium–PAH complexes indicate that the arrangement of adsorbed helium atoms is highly sensitive toward the structure of the solvated PAH cation. Closures of the first solvation shell around the studied PAH cations are suggested to lie between 29 and 37 adsorbed helium atoms depending on the specific PAH cation. Helium atoms are found to preferentially adsorb on these PAHs following the commensurate pattern common for graphitic surfaces, in contrast to larger carbonaceous molecules like corannulene, coronene, and fullerenes that exhibit a 1 × 1 commensurate phase.

Adsorption of helium on a charged propeller molecule: hexaphenylbenzene

“…However, as in the case of n = 42, the helium densities again show that the second solvation shell is partly filled at these sizes, no obvious symmetry being discernable in those densities. While the fluxional character of the second solvation shell has already been noticed in earlier investigations [6,20,23,[26][27][28][29], we are thus cautious regarding the interpretation of the enhanced stabilities at sizes 42 and above.…”

“…In contrast, the hypothetical 1 × 1 phase, where each carbon ring of the honeycomb lattice is decorated by one helium atom, is crowded if one considers extended planar systems like graphene. The 1 × 1 phase will readily form on curved (convex) surfaces such as fullerenes and corannulene, while graphene supports the √ 3 × √ 3 phase where all second-nearest carbon rings are covered [12,[21][22][23][24][25][26][27][28].…”

“…The preferred structure of helium layers formed on planar, positively charged PAHs has been explored by experiment and theory for anthracene (C 14 H 10 ), phenanthrene (C 14 H 10 ), fluoranthene (C 16 H 10 ), pyrene (C 16 H 10 ), and coronene (C 24 H 12 , Cor) [25,26,28]. One might surmise that in these systems the crowded 1 × 1 phase is stabilized relative to adsorption on extended graphitic surfaces because the helium atoms adsorbed on the outer rings could relax outwards.…”

Adsorption of Helium and Hydrogen on Triphenylene and 1,3,5-Triphenylbenzene

“…Quantum effects may also hinder the formation of a commensurate adsorption layer. Helium atoms and hydrogen molecules adsorbed on fullerenes [ 1 , 2 , 3 , 4 , 12 ] or polycyclic aromatic hydrocarbons (PAHs) [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ] may exhibit strong vibrational delocalization, depending on the density of the adsorbed layer and the charge of the substrate.…”

“…In our recent published work, we combined experimental techniques (high-resolution mass spectrometry and optical absorption spectroscopy) and path-integral molecular dynamics simulations to study the adsorption of He and H 2 on cationic planar PAHs, including anthracene (C 14 H 10 ), phenanthrene (C 14 H 10 ), fluoranthene (C 16 H 10 ), pyrene (C 16 H 10 ), and coronene (C 24 H 12 ) [ 26 , 29 , 30 , 31 , 32 ]. One might surmise that in these systems, the crowded 1 × 1 phase is stabilized relative to adsorption on extended graphitic surfaces because the helium atoms adsorbed on the outer rings can relax outwards.…”