Commensurate-incommensurate transition of 4He adsorbed on a single C60 molecule

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

The adsorption of helium on charged hexabenzocoronene (Hbc, C₄₂H₁₈), a planar polycyclic aromatic hydrocarbon (PAH) molecule of D_6h symmetry, was 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, the 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 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.

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

The adsorption of helium or hydrogen on cationic triphenylene (TPL, C18H12), a planar polycyclic aromatic hydrocarbon (PAH) molecule, and of helium on cationic 1,3,5-triphenylbenzene (TPB, C24H18), a propeller-shaped PAH, is studied by a combination of high-resolution mass spectrometry and classical and quantum computational methods. Mass spectra indicate that HenTPL+ complexes are particularly stable if n = 2 or 6, in good agreement with the quantum calculations that show that for these sizes, the helium atoms are strongly localized on either side of the central carbon ring for n = 2 and on either side of the three outer rings for n = 6. Theory suggests that He14TPL+ is also particularly stable, with the helium atoms strongly localized on either side of the central and outer rings plus the vacancies between the outer rings. For HenTPB+, the mass spectra hint at enhanced stability for n = 2, 4 and, possibly, 11. Here, the agreement with theory is less satisfactory, probably because TPB+ is a highly fluxional molecule. In the global energy minimum, the phenyl groups are rotated in the same direction, but when the zero-point harmonic correction is included, a structure with one phenyl group being rotated opposite to the other two becomes lower in energy. The energy barrier between the two isomers is very small, and TPB+ could be in a mixture of symmetric and antisymmetric states, or possibly even vibrationally delocalized.

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

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 3 × 3 commensurate pattern common for graphitic surfaces, in contrast to larger carbonaceous molecules like corannulene, coronene, and fullerenes that exhibit a 1 × 1 commensurate phase.

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

Mass spectra of helium nanodroplets doped with H₂ and coronene feature anomalies in the ion abundance that reveal anomalies in the energetics of adsorption sites. The coronene monomer ion strongly adsorbs up to n = 38 H₂ molecules indicating a commensurate solvation shell that preserves the D_6h symmetry of the substrate. No such feature is seen in the abundance of the coronene dimer through tetramer complexed with H₂; this observation rules out a vertical columnar structure. Instead we see evidence for a columnar structure in which adjacent coronenes are displaced in parallel, forming terraces that offer additional strong adsorption sites. The experimental value for the number of adsorption sites per terrace, approximately six, barely depends on the number of coronene molecules. The displacement estimated from this number exceeds the value reported in several theoretical studies of the bare, neutral coronene dimer.

Solvation of carbonaceous molecules by para-H2 and ortho-D2 clusters. II. Fullerenes

The coating of various fullerenes by para-hydrogen and ortho-deuterium molecules has been computationally studied as a function of the solvent amount. Rotationally averaged interaction potentials for structureless hydrogen molecules are employed to model their interaction with neutral or charged carbonaceous dopants containing between 20 and 240 atoms, occasionally comparing different fullerenes having the same size but different shapes. The solvation energy and the size of the first solvation shell obtained from path-integral molecular dynamics simulations at 2 K show only minor influence on the dopant charge and on the possible deuteration of the solvent, although the shell size is largest for ortho-D2 coating cationic fullerenes. Nontrivial finite size effects have been found with the shell size varying non-monotonically close to its completion limit. For fullerenes embedded in large hydrogen clusters, the shell size and solvation energy both follow linear scaling with the fullerene size. The shell sizes obtained for C60 (+) and C70 (+) are close to 49 and 51, respectively, and agree with mass spectrometry experiments.

Anisotropic superfluidity of (4)He on a C36 fullerene molecule

We have performed path-integral Monte Carlo calculations to study the adsorption of (4)He atoms on two different C36 isomers with the D6h and the D2d symmetries. The radial (4)He density distributions reveal layer-by-layer growth with the first layer being located at a distance of ∼5.5 Å from the C36 molecular center and the second layer at ∼8.3 Å. From the angular density profiles of (4)He, we find different quantum states as the number of (4)He adatoms N varies. For N = 20, we observe commensurate solid structures on both D6h and D2d isomers, where each of 8 hexagon and 12 pentagon centers of the fullerene surfaces is occupied by a single (4)He atom. The second-layer promotion starts beyond N = 38 on both isomers, where a compressible incommensurate structure is observed on the D6h isomer and another commensurate structure on D2d. Between N = 20 and N = 38, the (4)He monolayer on D6h shows several distinct rings of delocalized (4)He atoms along with strongly anisotropic superfluid responses at low temperatures, while isotropic but weak superfluid responses are observed in the (4)He layer on D2d.

On the size and structure of helium snowballs formed around charged atoms and clusters of noble gases

Helium nanodroplets doped with argon, krypton, or xenon are ionized by electrons and analyzed in a mass spectrometer. HenNgx(+) ions containing up to seven noble gas (Ng) atoms and dozens of helium atoms are identified; the high resolution of the mass spectrometer combined with advanced data analysis make it possible to unscramble contributions from isotopologues that have the same nominal mass but different numbers of helium or Ng atoms, such as the magic He20(84)Kr2(+) and the isobaric, nonmagic He41(84)Kr(+). Anomalies in these ion abundances reveal particularly stable ions; several intriguing patterns emerge. Perhaps most astounding are the results for HenAr(+), which show evidence for three distinct, solid-like solvation shells containing 12, 20, and 12 helium atoms. This observation runs counter to the common notion that only the first solvation shell is solid-like but agrees with calculations by Galli et al. for HenNa(+) [J. Phys. Chem. A 2011, 115, 7300] that reveal three shells of icosahedral symmetry. HenArx(+) (2 ≤ x ≤ 7) ions appear to be especially stable if they contain a total of n + x = 19 atoms. A sequence of anomalies in the abundance distribution of HenKrx(+) suggests that rings of six helium atoms are inserted into the solvation shell each time a krypton atom is added to the ionic core, from Kr(+) to Kr3(+). Previously reported strong anomalies at He12Kr2(+) and He12Kr3(+) [Kim , J. H.; et al. J. Chem. Phys. 2006, 124, 214301] are attributed to a contamination. Only minor local anomalies appear in the distributions of HenXex(+) (x ≤ 3). The distributions of HenKr(+) and HenXe(+) show strikingly similar, broad features that are absent from the distribution of HenAr(+); differences are tentatively ascribed to the very different fragmentation dynamics of these ions.

4He adsorption on a H(2)-plated C20 molecular surface: the formation of helium buckyballs

We perform path-integral Monte Carlo calculations to study the adsorption of 4He atoms on a H2-plated C20 molecular surface. It is found that 32 H2 molecules form a complete solid layer on C20, where each H2 molecule is located either above one of the 12 pentagon centers or above one of the 20 carbon atoms. The angular density profiles of the first 4He layer on the (H2)32-C20 surface reveal different quantum states as the number of 4He atoms N varies. Especially, the helium layer exhibits an icosidodecahedron structure for N=30, where each 4He atom is located at one of the vertices of 20 corner-sharing triangles. While the 4He density peaks for N=60 constitute a truncated icosahedron with 12 pentagonal and 20 hexagonal faces, the additional atoms beyond N=60 are found to be placed at the hexagon centers of the truncated icosahedron to form a hexakis truncated icosahedron for N=80. The superfluid response of the 4He layer at a temperature of T=0.6 K is found to be completely quenched for N=30 and to be significantly suppressed for N=60 and 80, reflecting the formation of compact buckyball structures.

Adsorption of hydrogen on neutral and charged fullerene: experiment and theory

Helium droplets are doped with fullerenes (either C60 or C70) and hydrogen (H2 or D2) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H2)(n)HC(m)(+) where m = 60 or 70. Another series of even-numbered ions, (H2)(n)C(m)(+), is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H2)(n)(+) is barely detectable. The ion series (H2)(n)HC(m)(+) and (H2)(n)C(m)(+) exhibit abrupt drops in ion abundance at n = 32 for C60 and 37 for C70, indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H2 are adsorbed on C60(+); the corresponding value for C70(+) is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H2/D2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C60 and C70, respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C60-hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H2 is 57 meV for H2C60(+) and (H2)2C60(+), and slightly above 70 meV for H2HC60(+) and (H2)2HC60(+). The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H2 molecules. Calculations for neutral and doubly charged complexes are presented as well.

Adsorption of Polar and Nonpolar Molecules on Isolated Cationic C60 , C70 , and Their Aggregates

Physisorption on graphite, graphene, nanotubes, and other graphitic structures has been the subject of numerous studies, partly driven by interest in the nature of order in two-dimensional systems, their phase transitions, and the use of graphitic scaffolds for reversible storage of hydrogen at high volumetric density and low mass. In contrast, physisorption on individual fullerenes or small aggregates of fullerenes has remained largely unexplored, last but not least, because of technical challenges. A summary of recent progress in identifying specific adsorption sites on positively charged C₆₀ , C₇₀ , and their aggregates is given in this Minireview. Adsorption energies and storage capacities for helium, hydrogen, methane, oxygen, nitrogen, water, and ammonia are determined. Mass spectrometric data reveal the formation of a commensurate phase in which all hollow sites of C₆₀ or C₇₀ are occupied. This phase is identified for all nonpolar molecules, including oxygen, which does not form a commensurate phase on planar graphite. The polar molecules, on the other hand, do not wet fullerenes and they do not form this commensurate phase. A hierarchy of other distinct adsorption sites are identified for nonpolar molecules, namely, groove sites for fullerene dimers and beyond, and dimple sites for fullerene trimers and beyond. Furthermore, evidence is presented for the preferential adsorption of hydrogen and methane in registered sites on fullerene dimers. The interpretation of experimental data that merely count the number of preferred adsorption sites is aided by molecular dynamics simulations, which utilize interaction potentials derived from ab initio calculations to determine adsorption energies.

Effects of 3He impurities on the superfluid response of the 4He monolayer on a C20 molecule

The path-integral Monte Carlo calculations have been performed to investigate the effects of (3)He impurities on structural and superfluid properties of the (4)He monolayer on a single C(20) molecule. According to our previous study, the helium monolayer exhibits different quantum states for different numbers of (4)He adatoms and is completed to form a commensurate solid where nanoscale supersolidity can be realized through the activation of mobile vacancy states. We first observe that different structures for different numbers of helium atoms are mostly preserved with the replacement of a few (4)He atoms with the same number of (3)He atoms, whether the helium layer is a fluid or a solid. However, the substitution of (3)He impurities is found to have different effects on the superfluid response of the helium layer, depending on its quantum state. For a partially-filled fluid layer the superfluid fraction decreases monotonically with the increasing (3)He concentration, which can be understood in terms of the suppression of exchange couplings among (4)He atoms due to the presence of (3)He impurities. On the other hand, the substitution of a few (3)He impurity atoms may increase the superfluid fraction of a near-complete monolayer that is in a crystalline solid state. The enhancement of superfluidity in a solid layer is interpreted to be due to interstitial and vacancy defects promoted by larger quantum fluctuations of lighter (3)He atoms. This provides strong evidence that the (4)He monolayer on C(20) shows the vacancy-based supersolidity near its completion.

Optimal covering of C60 fullerene by rare gases

Putative global energy minima of clusters formed by the adsorption of rare gases on a C(60) fullerene molecule, C(60)X(N) (X=Ne, Ar, Kr, Xe; N ≤ 70), are found using basin-hopping global optimization in an empirical potential energy surface. The association energies per rare gas atom as a function of N present two noticeable minima for Ne and Ar and just one for Kr and Xe. The minimum with the smallest N is the deepest one and corresponds to an optimal packing monolayer structure; the other one gives a monolayer with maximum packing. For Kr and Xe, optimal and maximum packing structures coincide. By using an isotropic average form of the X-C(60) interaction, we have established the relevance of the C(60) surface corrugation on the cluster structures. Quantum effects are relevant for Ne clusters. The adsorption of these rare gases on C(60) follows patterns that differ significantly from the ones found recently for He by means of experimental and theoretical methods.