Endohedral Atoms Research Articles

Which fits better?. Centered and non-Centered coordination modes of small endohedral Fullerenes, insights from Ti@C28 and Zr@C28

The endohedral metallic fullerenes (EMF), exhibit the interplay between stabilizing interaction and steric hindrance effects owing to the internal size provided by the fullerene cage. The small C28 fullerene, offers the smallest cavity realized to encapsulate metal ions. The characterized energy minima structure for Ti@C28 exhibits a non-centered disposition of the Ti atom in a spherical cage, in contrast to Zr@C28, where Zr atom is located at the very center. Here, we explore variations of the 3d-Ti-π-C28 bonding interaction from a centered conformation towards the non-centered disposition in comparison to Zr@C28. Our results show that in such structures, a variation of both electrostatic and orbital character of the metal-cage interaction can be observed. Interestingly, the orbital term is largely favored underlying the more effective bonding in the latter conformation, playing a crucial role in the determination of the coordination mode. In addition, molecular dynamics simulations were employed to determine the endohedral atom variation at 300°K, denoting that Zr@C28 cage exhibits more structural variation in comparison to Ti@C28, owing to the movement of a more fitted metal center induces larger variations to the cage distance.

On the variation of cluster core characteristics by an endohedral atom. Shape variation in 8-ce [EAu4(PPh3)4]2+ (E = N, P, As, Sb) clusters.

Atomically precise gold superatoms have attracted interest owing to their suitable use as building blocks for cluster-assembled materials, favoring ordered structures with advanced properties. In this sense, expanding their versatility is a relevant issue for controlling their properties and retaining a specific nuclearity. Interestingly, the reported structure for isoelectronic [Au4N(PPh3)4]+ and [Au4Sb(PPh3)4]+ clusters denotes two contrasting shapes featuring a tetrahedral and square pyramidal structure, respectively. Herein, we further explore the [Au4E(PPh3)4]+ (E = N, P, As, Sb) series in order to evaluate energetic and structural factors determining the overall shape. Our results show a favorable [Au4(PPh3)4]4+/E3- interaction energy, predicting particular patterns in their UV-vis spectrum. Thus, the use of dopant atoms is enabled to vary the core shape and, in turn, to modify the cluster properties, which serve as a structural control, in addition to ligand-based and size approaches.

Can endohedral transition metals enhance hydrogen storage in carbon nanotubes?

The safe and efficient use of hydrogen energy, which is in high demand worldwide today, requires efficient hydrogen storage. Despite significant advances in hydrogen storage using carbon-based nanomaterials, including carbon nanotubes (CNTs), efforts to substantially increase the storage capacity remain less effective. In this work, we demonstrate the effect of endohedral transition metal atoms on the hydrogen storage capacity of CNTs using reactive molecular dynamics simulations. We find that an increase in the volume fraction of endohedral nickel atoms leads to an increase in the concentration of physisorbed hydrogen molecules around single-walled CNTs (SWNTs) by approximately 1.6 times compared to pure SWNTs. The obtained results provide insight into the underlying mechanisms of how endohedral transition metal atoms enhance the hydrogen storage ability of SWNTs under nearly ambient conditions.

Exploring the parameter space of an endohedral atom in a cylindrical cavity.

Endohedral fullerenes, or endofullerenes, are chemical systems of fullerene cages encapsulating single atoms or small molecules. These species provide an interesting challenge of Potential Energy Surface determination as examples of non-covalently bonded, bound systems. While the majority of studies focus on C60 as the encapsulating cage, introducing some anisotropy by using a different fullerene, e.g., C70 can unveil a double well potential along the unique axis. By approximating the potential as a pairwise Lennard-Jones (LJ) summation over the fixed C cage atoms, the parameter space of the Hamiltonian includes three tunable variables: (M, ɛ, σ) representing the mass of the trapped species, the LJ energy, and length scales respectively. Fixing the mass and allowing the others to vary can imitate the potentials of endohedral species trapped in more elongated fullerenes. We choose to explore the LJ parameter space of an endohedral atom in C70 with ɛ ∈ [20, 150 cm-1], and σ ∈ [2.85, 3.05 Å]. As the barrier height and positions of these wells vary between [1, 264 cm-1] and [0.35, 0.85 Å] respectively, using a 3D direct product basis of 1D harmonic oscillator (HO) wavefunctions centred at the origin where there is a local maximum is unphysical. Instead we propose the use of a non-orthogonal basis set, using 1D HO wavefunctions centred in each minimum and compare this to other choices. The ground state energy of the X@C70 is tracked across the LJ parameter space, along with its corresponding nuclear translational wavefunctions. A classification of the wavefunction characteristics, namely the prolateness and "peanut-likeness" based on its statistical moments is also proposed. Excited states of longer fullerenes are assigned quantum numbers, and the fundamental transitions of Ne@C70 are tracked across the parameter space.

Effect of endohedral nickel atoms on the hydrophilicity of carbon nanotubes

ABSTRACT Carbon nanotubes (CNTs) have been successfully used in biomedicine, including cancer therapy, due to their unique physico-chemical properties. Because pristine CNTs exhibit hydrophobic behaviour, they can have a cytotoxic effect on cells, which limits their practical use in biomedicine. The toxicity of CNTs can be reduced by adding water-soluble functional radicals to their surface, i.e. by increasing their hydrophilicity. Another possibility for increasing the hydrophilicity of CNTs is probably filling them with endohedral metal atoms, which has not yet been studied. Thus, in this study, we use computer simulations to investigate the combined effect of endohedral nickel atoms and functional groups on the hydrophilicity of CNTs. Our simulation results show that the introduction of endohedral nickel atoms into CNTs increases their binding energy with functional groups. We also find that the addition of functional groups to the surface of CNT, along with filling it with endohedral nickel atoms, leads to an increase in the dipole moment of the CNT as well as its interaction energy with water, thereby increasing the hydrophilicity of the CNT and, consequently, its solubility in water. This, in turn, can lead to a decrease in CNT toxicity.

Ne, Ar, and Kr oscillators in the molecular cavity of fullerene C60.

We used THz (terahertz) and INS (inelastic neutron scattering) spectroscopies to study the interaction between an endohedral noble gas atom and the C60 molecular cage. The THz absorption spectra of powdered A@C60 samples (A = Ar, Ne, Kr) were measured in the energy range from 0.6 to 75meV for a series of temperatures between 5 and 300K. The INS measurements were carried out at liquid helium temperature in the energy transfer range from 0.78 to 54.6meV. The THz spectra are dominated by one line, between 7 and 12meV, at low temperatures for three noble gas atoms studied. The line shifts to higher energy and broadens as the temperature is increased. Using a spherical oscillator model, with a temperature-independent parameterized potential function and an atom-displacement-induced dipole moment, we show that the change of the THz spectrum shape with temperature is caused by the anharmonicity of the potential function. We find good agreement between experimentally determined potential energy functions and functions calculated with Lennard-Jones additive pair-wise potentials with parameters taken from the work of Pang and Brisse, J. Chem. Phys. 97, 8562 (1993).

Th@C2(8)-C84 and Th@Cs(15)-C84: impact of actinide metal ions on the electronic structures of actinide endohedral metallofullerenes

Two novel endohedral metallofullerenes, Th@C2(8)-C84 and Th@Cs(15)-C84, have been successfully synthesized and fully characterized. Spectroscopic studies suggest that endohedral actinide metal atoms have an important impact on the electronic structures of actinide metallofullerenes.

Hydroxylated buckminsterfullerene complexes with endohedral europium atom

Hydroxylated buckminsterfullerene complexes with endohedral europium atom

Metallofullertube: From Tubular Endohedral Structures to Properties.

Metallofullertubes are endohedral metallofullerenes with tubular fullerene cage possessing the segment of carbon nanotubes. Metallofullertubes have endohedral metal atom, fullerene cap and nanotube segment. Therefore, it is conceivable that this new kind of molecular materials would bring on many unexpected properties. In recent years, several pioneer metallofullertubes have been successfully reported, such as La2 @D5 (450)-C100 , Ce2 @D5 (450)-C100 , Sm2 @D3d (822)-C104 . Apart from the great effort to synthesize molecules and determine their structures, the physical and chemical properties of metallofullertubes are still waiting to be explored. In this minireview, we revisit the structures of reported metallofullertubes, and then we highlight their electronic and supramolecular properties. Finally, some perspectives for the development of metallofullertubes are also discussed.

Theoretical prediction for growth behavior and electronic properties of monoanionic Ru2Gen− (n = 3–20) clusters

We present a detailed theoretical study on the structural and electronic properties of monoanionic Ru2Gen− (n = 3–20) clusters using comprehensive genetic algorithm combined with density functional theory (CGA-DFT). It is shown that the geometrical growth patterns of the ground state Ru2Gen− clusters are polyhedral pyramidal structures with n ≤ 9 and gradually to form pentagonal prism-based structures with two endohedral Ru atoms from 10 ≤ n ≤ 15, then the structures evolve into a oblate cage-like configuration with two Ru atoms in center from cluster sizes n ≥ 16. In general, one extra electron will influence the geometric structures of Ru2Gen clusters to compare with their neutral ones we have studied. Based on the obtained ground state structures, we find that the clusters of Ru2Ge10−, Ru2Ge12− and Ru2Ge14− have relatively higher structural and chemical stability in which Ru2Ge14− is more prominent through the analysis of binding energy, second order of energy difference and highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap. Furthermore, we analyzed natural charge, natural electronic configuration, the frontier molecular orbitals and orbital composition to verify the clusters of Ru2Gen− with n = 10, 12, 14 and 16 have relatively higher chemical stability owning to the less contribution of Ru-d orbitals. In addition, the clusters of Ru2Ge3−, Ru2Ge4−, Ru2Ge6−, Ru2Ge10− and Ru2Ge16− have magnetic moment of 3 μB, while others are 1 μB. And also the vertical detachment energy (VDE) and adiabatic detachment energy (ADE) for all ground state clusters are computed to support future experiment.

Multiple locations of boron atoms in the exohedral and endohedral C60 fullerene

We study theoretically the bound states of one and two boron atoms in the exohedral and endohedral ${\mathrm{C}}_{60}$ fullerene. The optimal position of one boron atom is found above the midpoint of the ${\mathrm{C}}_{60}$ double bond in the exohedral complex, and at the center of ${\mathrm{C}}_{60}$ or below a carbon atom in the endohedral complex. However, the optimal position of a boron atom is often altered when the second boron atom is added to the molecular complex. For two boron atoms outside the cage the optimal arrangement is realized when the ${\mathrm{B}}_{2}$ molecule is attached by one boron atom to a double bond midpoint of ${\mathrm{C}}_{60}$ or when two boron atoms are above two double bond midpoints on opposite sides of ${\mathrm{C}}_{60}$. Two endohedral boron atoms can lie on the line joining either two opposite double bond midpoints or two centers of opposite pentagons. The latter case corresponds to the optimal geometry of the ${\mathrm{B}}_{2}$@${\mathrm{C}}_{60}$ complex provided that two boron atoms are close enough to form the ${\mathrm{B}}_{2}$ molecule. In case one boron atom is inside while the other is outside the cage, the optimal locations of atoms are above and below two neighboring carbon atoms belonging to the same hexagon of ${\mathrm{C}}_{60}$. Remarkably, all these optimal arrangements have different spin states: in the exohedral complex ${\mathrm{B}}_{2}{\mathrm{C}}_{60}\phantom{\rule{4pt}{0ex}}S=1$, in the endohedral ${\mathrm{B}}_{2}$@${\mathrm{C}}_{60}\phantom{\rule{4pt}{0ex}}S=2$ (as in the isolated ${\mathrm{B}}_{2}$ molecule), whereas for one boron atom inside and the other outside the cage $S=0$. The effective (Bader) charge of boron in these configurations varies appreciably---from $0.06e$ at the center of ${\mathrm{C}}_{60}$ to $2e$ in the ${\mathrm{B}}_{2}{\mathrm{C}}_{58}$ molecule with two boron atoms substituting for two carbon atoms in ${\mathrm{C}}_{60}$. We also discuss various conformations in the exohedral and endohedral molecular complexes.

Insights into fullerene polymerization under the high pressure: The role of endohedral Sc dimer

In this work, through the technology of producing endohedral metallofullerene in macroscopic quantities, the process of pressure polymerization of Sc2C2@C82 and their mechanical properties have been investigated in detail using a set of experimental and theoretical methods. The crucial role of endohedral atoms is demonstrated by comparison with pristine fullerenes. It is shown that the embedding of Sc2C2 complex inside a fullerene significantly facilitates the polymerization process which finally leads to the highly rigid material at high pressures.

Иттрий в фуллеренах

The study of the literature data showed that yttrium is encapsulated in fullerene molecules in the form of metal atoms, clusters of nitrides, carbides, sulfides and other compounds. Endohedral metallofullerenes are capable of encapsulating up to four metal atoms. In the molecules of these compounds, the metal atoms are positively charged due to the transfer of an electron from the endohedral metal atom to the fullerene carbon cage. First of all, the main experimental and theoretical achievements described in early (up to 2000) works are considered. Achievements in the production, separation (isolation) and various spectroscopic characteristics of endohedral metallofullerenes have been thoroughly studied in an attempt to elucidate their structural, electronic, and solid-state properties. It is shown that endohedral metallofullerenes in electrical conductivity can be metals, semiconductors with small gaps, or insulators, depending on the size of the fullerene, the type and number of encapsulated metal atoms. Other electronic and magnetic properties of metallofullerenes are also interesting. Also, some promising applications of metallofullerenes are considered. In addition, when analyzing the literature on the synthesis and properties of metalloendofullerenes, a very large number of publications related to the exohedral functionalization of metallofullerenes attracts attention. First of all, it should be noted that the main problem hindering the development of science, technology and the use of fullerenes, endohedral metallofullerenes and nanotubes was the difficulty of obtaining high-purity samples. When metals are introduced into the electric arc process, the situation becomes more complicated due to the presence of many isomers of both fullerenes and endohedral metallofullerenes. Exohedral functionalization helps to solve the problem of separation of synthesis products, on the one hand, and leads to the production of substances with new useful properties and potential applications in materials science and medicine. It is noted that currently the most productive and widespread method for the production of endohedral fullerenes is the electric arc process. The quantitative and qualitative output of metallofullerenes is significantly influenced by the conditions of the process in the reactor.

Atomically Precise Insights into Metal–Metal Bonds Using Comparable Endo-Units of Sc 2 and Sc 2 C 2

The precise identification of metal–metal bonds is critical to fully understanding the nature of metal–metal bonding but remains a fundamental challenge. Herein, we show the essence of Sc–Sc bonds ...

Favorite Orientation of the Carbon Cage and a Unique Two-Dimensional-Layered Packing Model in the Cocrystals of Nd@C82(I,II) Isomers with Decapyrrylcorannulene.

To date, the experimental studies on Nd-based metallofullerenes are only limited to spectroscopic characterizations. In this work, the molecular structures of Nd@C82(I,II) isomers, including the isomeric symmetry of the C cage and the position of endohedral Nd atom, as well as their unique two-dimensional (2D)-layered crystallographic packing structures were initially and unambiguously elucidated, based on the X-ray structural analyses of the cocrystals of Nd@C82(I) or Nd@C82(II) with cocrystallizing agent decapyrrylcorannulene (DPC). In the V-shaped unit cell, the endohedral Nd atom prefers a site as far away from the DPC molecules as possible because of the unevenly distributed charge on the C cage mainly related to the charge transfers from the endohedral Nd atom, cocrystallizing agent DPC, and solvent toluene molecules to the C82 cage. Apart from charge transfers, multiple C-H···π intermolecular interactions are also confirmed to play important roles both for the orientation of the C cage correlated with the preferential sites of the endohedral Nd atom and for the 2D-layered packing structures within the cocrystals. Density functional theory computations offered theoretical support for the molecular structures of Nd@C82(I,II) isomers, the valence of the endohedral Nd atom (between II+ and III+), and the global ground state, i.e., the Nd@C2v(9)-C82 isomer in the quintet state.

Endohedrally Doped Cage Clusters.

The discovery of carbon fullerene cages and their solids opened a new avenue to build materials from stable cage clusters as "artificial atoms" or "superatoms" instead of atoms. However, cage clusters of other elements are generally not stable. In 2001, ab initio calculations showed that endohedral doping of Zr and Ti atoms leads to highly stable Zr@Si16 fullerene and Ti@Si16 Frank-Kasper polyhedral clusters with large HOMO-LUMO gaps. In 2002, Zr@Ge16 was shown to form a Frank-Kasper polyhedron, suggesting the possibility of designing novel clusters by tuning endohedral and cage atoms. These results were subsequently confirmed from experiments. In the past nearly two decades, many experimental and theoretical studies have been carried out on different clusters, and many very stable cage clusters with possibly high abundance have been found by endohedral doping. Indeed in 2017, Ta@Si16 and Ti@Si16 cage clusters have been synthesized in bulk quantity of about 100 mg using a dry-chemistry method, giving rise to a new hope of developing cluster-based materials in macroscopic quantity besides the well-known C60 fullerene solid. Also, wet-chemistry methods have been used to synthesize endohedrally doped clusters as well as ligated clusters and their solids, which auger well for the development of novel nanostructured materials using atomically precise clusters with unique properties. In this comprehensive review, we present results of many such developments in this fast-growing field including (i) endohedrally doped Al, Ga, and In clusters, (ii) small endohedral carbon fullerene cages with ≤ 28 carbon atoms, (iii) metal doped boron cages, (iv) endohedrally doped cages of group 14 elements (Si, Ge, Sn, and Pb), (v) coinage metal (Cu, Ag, Au) cages doped with a transition metal atom as well as their ligated clusters and crystals, (vi) endohedrally doped cages of compound semiconductors, and (vii) multilayer Matryoshka cages and core-shell structures. In a large number of cases, we have performed ab initio calculations to present updated results of the most stable atomic structures and fundamental electronic properties of the endohedrally doped cage clusters. We discuss electronic, magnetic, optical, and catalytic properties in order to shed light on their potential applications. The stability of the doped cage clusters has been correlated to the concept of filling the electronic shells for superatoms such as within a spherical potential model and also using various electron counting rules including Wade-Mingos rules, systems with 18 and 32 electrons, and the spherical aromaticity rule. We also discuss cluster-cluster interaction in cluster dimers and assemblies of some of the promising doped cage clusters in different dimensions. Finally, we give a perspective of this field with a bright future.

Non-Functionalized Fullerenes and Endofullerenes in Aqueous Dispersions as Superoxide Scavengers.

Endohedral metal fullerene are potential nanopharmaceuticals for MRI; thus, it is important to study their effect on reactive oxygen species (ROS) homeostasis. Superoxide anion radical is one of the key ROS. The reactivity of aqueous dispersions of pristine (non-functionalized) fullerenes and Gd@C82 endofullerene have been studied with respect to superoxide in the xanthine/xanthine oxidase chemiluminescence system. It was found that C60 and C70 in aqueous dispersions react with superoxide as scavengers by a similar mechanism; differences in activity are determined by cluster parameters, primarily the concentration of available, acting molecules at the surface. Gd endofullerene is characterized by a significantly (one-and-a-half to two orders of magnitude) higher reactivity with respect to C60 and C70 and is likely to exhibit nanozyme (SOD-mimic) properties, which can be accounted for by the nonuniform distribution of electron density of the fullerene cage due to the presence of the endohedral atom; however, in the cell model, Gd@C82 showed the lowest activity compared to C60 and C70, which can be accounted for by its higher affinity for the lipid phase.

Expansion of Magnetic Aromaticity Criteria to Multilayer Structures: Magnetic Response and Spherical Aromaticity of Matryoshka-Like Cluster [Sn@Cu12 @Sn20 ]12.

Structural characterization of the discrete [Sn@Cu12 @Sn20 ]12- cluster exposed a fascinating architecture composed of three concentric structural layers in which an endohedral Sn atom is enclosed in a Cu12 icosahedron, which in turn is embedded in an Sn20 dodecahedron. Herein, the possibility of sustaining aromatic behavior for this prototypical multilayered species was evaluated, in order to extend this concept to more complex clusters on the basis of magnetic response and bonding analysis by the AdNDP approach. This revealed characteristic features of spherical aromatics, given by the ability to sustain the shielding cone property, similar to archetypal aromatics. The favorable bonding pattern in the [Sn@Cu12 @Sn20 ]12- cluster fulfills the 2(N+1)2 Hirsch rule for aromaticity; thus, the cluster could be regarded as a first member of aromatic multilayered structures. The set of four 13c-2e aromatic bonds that was identified in the internal SnCu12 structure results in spherical aromatic character of this multilayered cluster. This insight builds a bridge between the traditional concept of Hückel's aromaticity and the aromaticity of complex and stable 3D systems that may be explored on the basis of magnetic response and bonding analysis. It also may open a way to novel findings in bottled clusters displaying aromatic behavior in multilayer structures, which are of great interest for inorganic nano- and material sciences due to their unprecedented stability.

On the formation of spherical aromatic endohedral buckminsterfullerene. Evaluation of M@C60 (M = Cr, Mo, W) from relativistic DFT calculations.

Endohedral metallofullerenes are key species for expanding the range of viable fullerenes, their versatility, and applications. Here we report our computational evaluation on the formation of spherical aromatic counterparts of the C60 fullerene from relativistic DFT calculations, based on the inclusion of Cr, Mo and W endohedral atoms. The resulting M@C60 endohedral fullerenes are 66-π electron neutral species exhibiting bonding properties and electronic structure mimicking the aromaticity and diamagnetic insulator behavior of alkali-C606- phases. The resulting structures are interesting candidates for further experimental realization as novel neutral building blocks for more flexible nanostructured organic materials, highlighting truly spherical aromatic neutral species retaining the truncated icosahedral structure of the seminal Buckminster fullerene.

Modification of the Endohedral Potential after Instant Ionization of an Inner Atom

We investigate here the variation of endohedral A@CN potential due to instant ionization of an inner atom A. Using a reasonable model to describe the fullerenes shell, we managed to calculate this variation that is a consequence of the monopole polarization of CN shell. We demonstrate that the phenomenological potentials that properly simulate the CN shell should belong to a family of potentials with a non-flat bottom, instead of very often used square well potential. As concrete example, we use the Lorentz-bubble model potential. By varying the shape of this potential, we describe the various degrees of the monopole polarization of the CN shell by positive electric charge at the center of the shell. We calculated the photoionization cross sections of He, Ar, and Xe atoms located at the center of C60 shell with and without taking into account accompanying this process monopole polarization of the fullerenes shell. Unexpectedly, we found that the monopole polarization do not affect the photoionization cross sections of these endohedral atoms, thus presenting a very seldom example of potential conspiracy.