Electron-Induced Puncturing of Endohedral Metallofullerenes

We demonstrate here the atomic-level detection of the associated migration of individual atoms, occurring in carbon nano-peapods by high-resolution transmission electron microscopy (HR-TEM). Through the atomic defects induced at the fullerene cage, the encapsulated individual metal atoms (Tb and Gd) are transferred from cage to cage through an induced atomic path within a carbon nano-peapod. The present results indicate the possibilities for a method to convey single-atoms by using an “atomic carry-bag” such as fullerenes.     

Surface modification induced shielding effects on electron orbital coupling in metallofullerene

The electronic properties of four metallofullerenol samples (La@C82(OH)32, La@C82(OH)24, La@C82(OH)18, La@C82(OH)12) and La@C82 were investigated by synchrotron radiation photoelectron emission spectra (PES). A new emission peak with binding energy of 13.9 eV, which is attributed to La 5d orbitals coupling with nanocage orbitals, is observed in PES of La@C82 for the first time. This suggestion is also supported by previous DFT calculations. Furthermore, the new emission peak disappears when the surface of La@C82 is modified with specific numbers of OH groups, which means the electron orbital coupling between inner metallic atoms and outer nanocage in metallofullerene could be shielded by surface modifications on the outer nanocage.     

A Review on Endohedral Metallofullerenes: Structures and Properties

The present review deals with some recent results on endohedral metallofullerenes obtained by the Nagoya group. The present structural analyses reveal unique of endohedoral metallofullerenes: (1) the presence of structural isomers with different fullerene cage structures; (2) encaged metal atoms are situated close to the carbon cage and not in the center of the cage; (3) metal atoms in Y@C82 and Sc2@C84 are at a standstill, not moving around, even at room temperature.     

Interaction between fullerenes and single-wall carbon nanotubes: the influence of fullerene size and electronic structure

A series of fullerenes and endohedral metallofullerenes peapods have been synthesized by supercritical method in high filling rate. The interaction between SWNTs and various kinds of fullerenes (C60, C70, C78, C84) and metallofullerenes (Gd@C82, Er@C82, Ho@C82, Y@C82) has been further investigated. The slight blue shift of G-band in Raman spectra with respect to pristine SWNTs was attributed to the charge transfer from SWNTs to fullerenes cage. The obvious RBM shift strongly depended on the distance between the inner wall of the SWNTs and the fullerene cage and also partly associated with the electronic structure of the fullerene. These results indicated that the interaction between fullerenes and SWNTs, which was considered to be the van de walls interaction, can be influenced by the cage size and the kind of fullerenes.     

Isomers of Endohedral Fullerene Sc2@C84

Density functional calculations are carried out to predict the distance between two Sc atoms, that are encapsulated into C₈₄ fullerene cage, and structure of the endohedral fullerene isomers. It is found that the C_s symmetry isomer is originated from cage 21 of C₈₄ fullerene. The new structure of the endohedral fullerene with C_2v symmetry is found in which the distance between two encapsulated Sc atoms is larger than that previously reported.     

Biomedical activities of endohedral metallofullerene optimized for nanopharmaceutics

Endohedral metallofullerenes, a novel form of carbon-related nanomaterials, currently attract wide attention for their potential applications in biomedical fields such as therapeutic medicine. Most endohedral metallofullerenes are synthesized using C60 or higher molecular weight fullerenes because of the limited interior volume of fullerene. It is known that the encapsulated metal atom has strong electronic interactions with the carbon cage in metallofullerenes. Gd@C82 is one of the most important molecules in the metallofullerene family, known as Magnetic Resonance Imaging (MRI) contrast agent candidate for diagnostic imaging. Gadolinium endohedral metallofullerenol (e.g., Gd@C82(OH)22) is a functionalized fullerene with gadolinium trapped inside carbon cage. Our group previously demonstrated that the distinctive chemical and physical properties of Gd@C82(OH)22 are dependent on the number and position of the hydroxyl groups on the fullerene cage. The present article summarizes our latest findings of biomedical effects of Gd@C82(OH)22 and gives rise to a connected flow of the existing knowledge and information from experts in the field. It briefly narrates the synthesis and physico-chemical properties of Gd@C82(OH)22. The polyhydroxylated nanoparticles exhibit the enhanced water solubility and high purity, and were tested as a MRI contrast agent. Gd@C82(OH)22 treatment inhibited tumor growth in tumor-bearing nude mice. Although the precise mechanisms of this action are not well defined, our in vitro data suggest involvements of improved immunity and antioxidation by Gd@C82(OH)22 and its size-based selective targeting to tumor site. The review critically analyzed the relevant data instead of fact-listing, and explained the potential for developing Gd@C82(OH)22 into a diagnostic or therapeutic agent.     

Metal-Coated Fullerenes C60Mn: Calculations for M = Be,Mg, Al AND n = 12, 20, 32

Abstract

Semiempirical quantum-chemical PM3 calculations are reported for a relatively new class of exohedral metallo-fullerenes - metal-coated or metal-covered fullerenes: C₆₀M_n. The exohedral species were recently observed, however, their geometrical and electronic structure is not known yet. In this paper, relatively-even metal-atom distributions over the fullerene rings are considered - such regular forms are computed for M= Be, Mg, Al. Three selected stoichiometrics are treated: C₆₀M₁₂, C₆₀M₂₀, and C₆₀M₃₂. The stoichiometrics correspond to the location of the metal atoms above the twelve pentagons, above the twenty hexagons, and above each of the thirty two rings of C₆₀ This interesting arrangement over the rings is possible only for some types of atoms, while other elements are localized above bonds or atoms, or inside the cage, or even react and destroy the cage. Other limitation comes from the parametrization of the computational methods - the computations are performed with the PM3 semiempirical method and metal-layer atomization heats are used as a stability measure. Structural characteristics are presented, too. Considerable reductions of the cage symmetry are reported and their relationships to Jahn-Teller effect are discussed, too (no case of the icosahedral symmetry is found).     

Electronic charge transfer in cobalt doped fullerene thin films and effect of energetic ion impacts by x-ray absorption spectroscopy

We report on the electronic charge transfer in cobalt doped fullerene thin films by means of near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy measurement. Co-doped fullerene films were prepared by co-deposition technique and subjected to energetic ion irradiation (120 MeV Au) for possibly alignment or interconnect of randomly distributed metal particles. Polarization dependent NEXAFS spectra revealed the alignment of Co and C atoms along the irradiated ionic path. The structural changes in Co-doped as-deposited and ion irradiated fullerene films were investigated by means of Raman spectroscopy measurements. Downshift of pentagonal pinch mode A_g(2) in Raman spectroscopy indicated the electronic charge transfer from Co atom to fullerene molecules, which is further confirmed by NEXAFS at C K-edge for Co-doped fullerene films.     

Computations of metal-covered C60 and C70

Semiempirical quantum-chemical PM3 calculations are reported for a new class of exohedral metallo-fullerenes – metal-coated or metal-covered fullerenes: C₆₀M_n and C₇₀M_n. The exohedral species have been observed in gas phase, however, their geometrical and electronic structures are not known yet. Relatively-even metal-atom distributions over the fullerene rings are considered – such regular forms are computed for M = Be, Mg, Al. Three selected types of stoichiometries are treated in particular: C₆₀M₁₂/C₇₀M₁₂ (metal atoms above all pentagons), C₆₀M₂₀/C₇₀M₂₅ (metal atoms above all hexagons), and C₆₀M₃₂/C₇₀M₃₇ (metal atoms above all rings). If an odd number of electrons should result (Al), the related cation is computed, or one metal atom added or removed. This interesting arrangement above the rings is possible only for some types of atoms, while other elements are localized above bonds or atoms, or inside the cage, or even react and destroy the cage. Other limitation comes from the parametrization of the computational technique used – the PM3 semiempirical method is parametrized only for some selected metals. Metal-layer atomization heats are suggested as a stability measure. Structural characteristics are also presented and interesting reductions of the cage symmetry are found. Their relationship to Jahn–Teller effect is discussed. The metal covered fullerenes can represent models for metal catalysis in the nanotube synthesis and could eventually lead to new interesting materials.     

Formation Process of Empty and Metal-Containing Fullerenes—Molecular Dynamics and FT-ICR Studies

Abstract

The formation mechanism of empty and metal-containing fullerene was studied through MD (molecular dynamics) simulations and FT-ICR (Fourier transform ion cyclotron resonance) mass spectroscopy of laser vaporized carbon cluster. Multi-body classical potential functions for metal-carbon and metal-metal interactions were constructed based on DFT (density functional theory) calculations of various forms of small clusters MC_n and M_n (M = La, Sc, Ni). Using the modified Brenner potential for carbon-carbon interaction, the clustering process starting from 500 isolated carbon atoms and 5 metal atoms in gas phase was simulated under the controlled temperature condition at 3000K. The difference of clustering process of La@C_n, Sc@C_n and NiC_n were compared with empty fullerene formation simulation.

FT-ICR mass spectrometer directly connected to the laser vaporization cluster beam source was implemented in order to experimentally study the clustering process. The increase of cluster nozzle pressure roughly corresponded to the later stage of the molecular dynamics simulation. The FT-ICR mass spectra of metal-carbon composite clusters were compared for various sample materials used for arc-discharge generation of metal-containing fullerene and SWNT (single-wall carbon nanotube); La, Y., Sc, Gd, Ce, Ca, and Ni-Y. Positive La-C, Y-C, Sc-C, Gd-C, Ce-C binary clusters commonly showed strong MC_2n+ signal in the range of 36 < 2n with intense magic numbers at MC₄₄ ⁺, MC₅₀ ⁺ and MC₆₀ ⁺. It was speculated that the even-numbered clusters corresponded to the annealed random caged clusters observed in the MD simulation.     

Thin fullerene-containing films synthesized by ion beam sputtering of fullerene mixtures with doping additives in vacuum

A new approach to the synthesis of films containing fullerenes and doping elements is described. It is suggested that a cluster mechanism of the target sputtering by accelerated ions makes possible the deposition of fullerenes on a substrate with a certain probability for dopant atoms being introduced into the cavities of fullerene molecules and a higher probability of their occurrence between fullerene molecules. The proposed method has been experimentally implemented by using an Ar⁺ ion beam to sputter C₆₀/C₇₀ fullerene mixtures (synthesized in a plasmachemical reactor at a pressure of 10⁵ Pa) pressed into disk targets with a doping element (Fe, Na, B, Gd, or Se). The ion beam sputtering of dopant-containing fullerene mixtures in a vacuum of ～10^?2 Pa allowed micron-thick films containing C₆₀ and C₇₀ fullerenes and the corresponding dopant element (Fe, Na, B, Gd, or Se) to be grown on quartz substrates.     

Complexation of free-base and 3d transition metal(II) phthalocyanines with fullerene C60: A dispersion-corrected DFT study

We performed a systematic comparative theoretical study of noncovalent interactions of free-base H₂Pc and a series of 3d transition metal(II) phthalocyanines (where the transition metals included manganese, iron, cobalt, nickel, copper and zinc) with fullerene C₆₀, by employing a DFT technique accounting for vdW interactions (PBE GGA functional with a dispersion correction by Grimme). We observed four different types of Pc interaction with fullerene cage, depending on central metal atom. Upon complexation with C₆₀, the macrocyclic plane undergoes distortion in all cases, to a different degree. Some correlation was observed between the calculated formation energies for Pc–fullerene complexes and intermolecular separations in them, where stronger binding is generally associated with shorter M^…C_C60 and N^…C_C60 distances. Despite of considerable differences in the structure of Pc–fullerene complexes, the latter do not exhibit notable variations in the distribution of electrostatic potential, contrary to spin density plots for open-shell species. Similarly, HOMO and LUMO distribution can vary within some limits.     

CHROMATOGRAPHIC PURIFICATION OF Kr@C60

Unusual compounds have been reported to form by the incorporation of atoms into fullerene cages. The compounds in which a noble gas atom is trapped into a C₆₀ cage are very stable and only van der Waals interactions occur between the noble gas and the fullerene cage. The study of these compounds requires efficient chromatographic (or other) separations. We performed the isolation of Kr@C₆₀ and a study of its chromatographic separations on several HPLC columns using toluene : hexane mixtures as solvent. We found that the separation factor is small (1.09 to 1.12) with the highest separation factor on a Buckyprep column.     

CHROMATOGRAPHIC PURIFICATION OF Kr@C60

Unusual compounds have been reported to form by the incorporation of atoms into fullerene cages. The compounds in which a noble gas atom is trapped into a C₆₀ cage are very stable and only van der Waals interactions occur between the noble gas and the fullerene cage. The study of these compounds requires efficient chromatographic (or other) separations. We performed the isolation of Kr@C₆₀ and a study of its chromatographic separations on several HPLC columns using toluene : hexane mixtures as solvent. We found that the separation factor is small (1.09 to 1.12) with the highest separation factor on a Buckyprep column.     

On Some Complexes of Allyl Derivatives of C60 Fullerene: Simulation of Molecular and Electron Structure by DFT

Abstract

The problem of existence of η³–π‐complexes of transition metal atoms with the allyl type derivatives C₆₀X₃ of C₆₀ fullerene is discussed. It is shown that complexes C₆₀X₃Co(CO)₃ (X = H, F, Cl, Br), C₆₀H₃NiC₅H₅, C₆₀H₃Fe(CO)C₅H₅, where three atoms X are bound to the C atoms of fullerene in the α‐positions relative to the same five‐membered ring in the C₆₀ fullerene, must be sufficiently stable. In these complexes the metal atoms are η³–π‐bound to the fullerene cage. In contrast to this, the metal atoms with the same allyl type C₆₀H₃ derivative of C₆₀ fullerene in the C₆₀H₃Li and C₆₀H₃FeC₅H₅ complexes are η⁵–π‐coordinated to the carbon cage. Calculations were carried out by the DFT with the exchange‐correlation potential by Perdew–Burke–Ernzerhow.     

Carbon Cage Vibrations of M@C82 and M2@C2 n (M = La,Ce; 2n = 72, 78, 80): The Role of the Metal Atoms

Infrared spectra of La₂@C₈₀ and a series of Ce-based endohedral metallofullerenes (EMFs), including Ce@C₈₂, Ce₂@C₇₂, Ce₂@C₇₈, and Ce₂@C₈₀ are reported. DFT calculations are used for their thorough analysis and assignment. The vibrations of the fullerene cages in all studied EMFs differ from those of their empty, charged cage analogues. Furthermore, EMFs with the same carbon cage but different type of encapsulated species also show significant differences in their cage vibrational patterns. These phenomena are explained by a different coupling of the vibrational modes as well as by the different charge distributions in EMFs and empty, isostructural fullerene anions.     

Spin-Transparency of the Fullerene Shell of Endometallofullerene La@C82

For La@C₈₂ embedded into the polycarbonate film, an intensive ¹H-electron-nuclear double resonance (ENDOR) has been revealed. Since La-EMF does not contain hydrogen atoms, it evidences for the electron spin density on the matrix protons. Furthermore, for the solutions of La-EMF in hexamethylphosphoramide (HMPA), the paramagnetic shift of the ³¹P NMR signal of bulk solvent molecules has been revealed. These findings testify the partial localization of unpaired electron outside the fullerene cage. The “transparency” of the fullerene shell to the electron spin density may serve the important factor in designing the fullerene-based materials.     

Alkali-metal-induced enhancement of hydrogen adsorption in C60 fullerene: an ab Initio study

It is demonstrated that the doping of alkali metal atoms on fullerene, C60, remarkably enhances the molecular hydrogen adsorption capacity of fullerenes, which is higher than that of conventionally known other fullerene complexes. This effect is observed to be more pronounced for sodium than lithium atom. The formation of stable complex forms of a sodium-doped fullerene molecule, Na8C60, and the corresponding hydrogenated species, [Na(H2)6]8C60, with 48 hydrogen molecules has been demonstrated to lead to a hydrogen adsorption density of approximately 9.5 wt %. One of the main factors favoring the interactions involved is attributed to the pronounced charge transfer from the sodium atom to the C60 molecule and electrostatic interaction between the ion and the dihydrogen. The suitability of these complexes for developing fullerene-based hydrogen storage materials is discussed.     

Fe/C interactions during SWNT growth with C2 feedstock molecules: A quantum chemical molecular dynamics study

We are presenting the first quantum chemical molecular dynamics (QM/MD) model simulations for iron catalyzed single-walled carbon nanotube (SWNT) growth based on the density functional tight binding (DFTB) quantum chemical potential. As model systems, open-ended (10,10) armchair tube fragments were selected with 0, 10, and 20 Fe atoms attached in 1,4-positions on the open rims, and ensembles of randomly oriented C2 molecules were included to simulate carbon plasma feedstock molecules. Isokinetic trajectories at 1500 K to 3000 K show that divalent Fe increases the number of coordination partners with carbon and/or Fe, depending on the Fe concentration. Fe/C interactions weaken the tube sidewall due to electron transfer from Fe into antibonding carbon orbitals, and C2 addition occurs mainly in an Fe-C2-Fe bridge addition mechanism, while growth of polyyne chains characteristic for high-temperature carbon systems is suppressed in the presence of Fe on the rims of the growing SWNT. Our findings are the first quantum chemical evidence for the importance of intermetallic interactions during SWNT growth.     

Spin‐Transparency of the Fullerene Shell of Endometallofullerene La@C82

Abstract

For La@C₈₂ embedded into the polycarbonate film, an intensive ¹H‐electron‐nuclear double resonance (ENDOR) has been revealed. Since La‐EMF does not contain hydrogen atoms, it evidences for the electron spin density on the matrix protons. Furthermore, for the solutions of La‐EMF in hexamethylphosphoramide (HMPA), the paramagnetic shift of the ³¹P NMR signal of bulk solvent molecules has been revealed. These findings testify the partial localization of unpaired electron outside the fullerene cage. The “transparency” of the fullerene shell to the electron spin density may serve the important factor in designing the fullerene‐based materials.