Lanthanum complexes of spheroidal carbon shells

Ti12C68: A stable T h -symmetry hollow cage

A stable T h -symmetry Ti12C68 cage was systemically investigated using density functional theory. The structure of Ti12C68 is a hollow cage with twelve TiC13 subunit of three pentagons and one hexagon. The calculated frequencies are in the range 95.1 cm-1-1423.9 cm-1. There are no imaginary frequencies, showing its kinetic stability. Ab initio molecular dynamics simulations demonstrate that the topological structure of cage-like Ti12C68 cluster was well maintained when the effective temperature is up to 1139 K. The natural bond orbitals analysis shows that the d orbit of Ti atoms form four σ bonds with the neighboring four carbon atoms in each TiC13 subunit playing an important role in the cluster stability. The molecular frontier orbitals analysis indicates that Ti12C68 cage has a narrow HOMO-LUMO gap with metal-like property. It would be expected to enrich the species of hollow metal carbide clusters.

Penetrating probability and cross section of the Li+-C60 encapsulation process through an ab initio molecular dynamics investigation

The endohedral complex system of Li⁺-C₆₀ has been shown to possess interesting applications in photovoltaics, supramolecular chemistry, and functionalized materials. In this study, we perform a theoretical investigation of Li⁺ encapsulation within a C₆₀ cage by employing an ab initio molecular dynamics approach. The Li⁺ cation is positioned 9 Å away from the C₆₀ center of mass, and fired towards a randomized spot in a six-membered ring with a certain level of inletting energy, which is 7.5 eV, 9 eV, 12 eV, or 15 eV. In total, 2000 samples of MD trajectories are investigated. Our statistical results yielded a penetrating probability in the range of 0.8% to 15.6% with respect to the above inletting energy, while the cross section ranges from 0.006 Ų to 0.123 Ų. Moreover, we observed that the penetrating probability exhibited direct proportionality to the inletting energy. Hence, we can determine that the minimum required inletting energy for reaction occurrence is 6.6 eV. Overall, it seems difficult for Li⁺ to penetrate through the sp²-carbon wall, because a very high inletting energy is required to open the entrance. At the same time, Li⁺ must approach closely to the center of a six-membered ring to enhance the penetration probability.

When metal clusters meet carbon cages: endohedral clusterfullerenes

Fullerenes have the characteristic of a hollow interior, and this unique feature triggers intuitive inspiration to entrap atoms, ions or clusters inside the carbon cage in the form of endohedral fullerenes. In particular, upon entrapping an otherwise unstable metal cluster into a carbon cage, the so-called endohedral clusterfullerenes fulfil the mutual stabilization of the inner metal cluster and the outer fullerene cage with a specific isomeric structure which is often unstable as an empty fullerene. A variety of metal clusters have been reported to form endohedral clusterfullerenes, including metal nitrides, carbides, oxides, sulfides, cyanides and so on, making endohedral clusterfullerenes the most variable and intriguing branch of endohedral fullerenes. In this review article, we present an exhaustive review on all types of endohedral clusterfullerenes reported to date, including their discoveries, syntheses, separations, molecular structures and properties as well as their potential applications in versatile fields such as biomedicine, energy conversion, and so on. At the end, we present an outlook on the prospect of endohedral clusterfullerenes.

Effect of incarcerated HF on the exohedral chemical reactivity of HF@C60

The first chemical modification on the brand new endohedral HF@C₆₀ is reported. In particular, the isomerization from optically pure (2S,5S)-cis-pyrrolidino[3,4:1,2][60]fullerene 2b to (2S,5R)-trans-pyrrolidino[3,4:1,2][60]fullerene 2b has been studied and compared with empty C₆₀ (2a) and endohedral H₂O@C₆₀ (3). The comparative study shows a kinetic order for the isomerization process of H₂O@C₆₀ > HF@C₆₀ > C₆₀, thus confirming the effect of the incarcerated species on the zwitterionic intermediate stability.

Gadolinium based endohedral metallofullerene Gd2@C79N as a relaxation boosting agent for dissolution DNP at high fields

We show increased dynamic nuclear polarization by adding a low dosage of a S = 15/2 Gd based endohedral metallofullerene (EMF) to DNP samples. By adding 60 μM Gd₂@C₇₉N, the nuclear polarization of ¹H and ¹³C spins from 40 mM 4-oxo-TEMPO increases by approximately 40% and 50%, respectively, at 5 T and 1.2 K. Electron-electron double resonance (ELDOR) measurements show that the high spin EMF shortens the electron relaxation times and increases electron spectral diffusion leading to the increased DNP enhancement.

Reliable charge assessment on encapsulated fragment for endohedral systems

A simple scheme to determine charge distribution in endohedral complexes is suggested. It is based on comparison of inner-shell atomic orbital energies of the encapsulated species to the corresponding energies in reference systems with unambiguously defined charges on X. This robust approach is applied to endohedral borospherenes X@B₃₉, for which the conventional schemes provide in some cases quite different results. Efficiency of proposed scheme also has been proven for typical fullerene based Sc₃N@C₈₀ endohedral complex.

Noble gas encapsulated B40 cage

The efficacy of B₄₀ borospherene to act as a host for noble gas atoms is explored via density functional theory based computations. Although the Ng@B₄₀ complexes are thermochemically unstable with respect to dissociation into free Ng and B₄₀, it does not rule out their viability as all the systems possess a high activation free energy barrier (84.7-206.3 kcal mol^-1). Therefore, once they are formed, it is hard to take out the Ng atom. Two Ng atoms can also be incorporated within B₄₀ for the lighter Ng atoms (He and Ne). In fact, the destabilization offered by the encapsulation of one and two He atoms and one Ne atom inside B₄₀ is significantly less than that in experimentally synthesized He@C₂₀H₂₀, highlighting their greater possibility for synthesis. Although Ar₂ and Kr₂ encapsulated B₄₀ systems are very much destabilized by the repulsive interaction between Ng₂ and B₄₀, an inspection of the bonding situation reveals that the confinement can even induce some degree of covalent interaction between two otherwise non-bonded Ng atoms. Ng atoms transfer electrons towards B₄₀ which is smaller for lighter Ng atoms and gradually increases along He to Rn. Even if the electrostatic interaction between Ng and B₄₀ is the most predominant term in these systems, the extent of the orbital interaction is also considerable. However, the very large Pauli repulsion counterbalances the attractive interaction, eventually turning the interaction repulsive in nature. Ng@B₄₀ also shows dynamical behaviour involving continuous exchange between hexagonal and heptagonal holes, similar to the host cage, as understood from the very little variation in the activation barrier because of the Ng encapsulation. Furthermore, sandwich complexes like [(η⁵-C₅Me₅)Fe(η⁶-B₄₀)]⁺ and [(η⁵-C₅Me₅)Fe(η⁷-B₄₀)]⁺ are noted to be viable with the latter being slightly more stable than the former. The encapsulation of Xe slightly improves the dissociation energy associated with the decomposition into Xe@B₄₀ and [Fe(η⁵-C₅Me₅)]⁺ compared to that in the bare one.

C60 as Electron Acceptor and Donor: A Comparative DFT Study of Li@C60 and F@C60

Fullerene (C60) is a stable prototype system for a special class of nanomaterials. In this work, the smallest alkali metal (Li) and halogen (F) atoms were encapsulated in the C60 cage, and comparative quantum chemical calculations (QCCs) were performed on their various properties using a density functional theory approach. It was noted that the off-centre distance of Li is higher than that of F. The QCCs of the charge transfer to and from C60 were also analysed. Although charge transfer to and from the C60 cage takes place in both cases, Li@C60 becomes more polar than F@C60, suggesting a better electron-accepting nature of C60 than electron-donating behaviour. This fact is consistent with the natural bond orbital (NBO) charge on the trapped atoms and the dipole moment as well as the binding energy values of the encapsulated C60. Although the encapsulation of both atoms reduces the frontier orbital energy gap, the frontier orbital gap of Li@C60 is smaller than that of F@C60. More interestingly, the depression in the polarizability of Li@C60 is significantly large relative to that of F@C60. These findings also support the tendency of C60 to act as electron acceptor. This study provides some insights into the fundamental properties of C60 and should be helpful in designing new endofullerene complexes for a variety of applications.

Ultrastable actinide endohedral borospherenes

DFT-PBE0 calculations on AnB_n (An = U, Th; n = 36, 38, and 40) reveal the feasibility of actinide-doped borospherenes.

Can CF3-Functionalized La@C60 Be Isolated Experimentally and Become Superconducting?

Superconducting behavior even under harsh ambient conditions is expected to occur in La@C₆₀ if it could be isolated from the primary metallofullerene soot when functionalized by CF₃ radicals. We use ab initio density functional theory calculations to compare the stability and electronic structure of C₆₀ and the La@C₆₀ endohedral metallofullerene to their counterparts functionalized by CF₃. We found that CF₃ radicals favor binding to C₆₀ and La@C₆₀ and have identified the most stable isomers. Structures with an even number m of radicals are energetically preferred for C₆₀ and structures with odd m for La@C₆₀ due to the extra charge on the fullerene. This is consistent with a wide HOMO-LUMO gap in La@C₆₀(CF₃)_m with odd m, causing extra stabilization in the closed-shell electronic configuration. CF₃ radicals are both stabilizing agents and molecular separators in a metallic crystal, which could increase the critical temperature for superconductivity.

Er3+ Photoluminescence in Er2@C82 and Er2C2@C82 Metallofullerenes Elucidated by Density Functional Theory

Metallofullerenes with two erbium atoms encapsulated in IPR C₈₂ cage isomers C_s-6 (I), C_2v-9 (II), and C_3v-8 (III) were investigated using density functional theory. The calculations suggest that erbium atoms assume a trivalent state with Er (4f¹¹) valence electronic configuration in Er₂@C₈₂ and Er₂C₂@C₈₂, where two electrons (6s²) per Er atom are transferred to the cage carrying four negative charges (C₈₂^4-), while the third electron is promoted from the 4f to the 5d shell, becoming involved in covalent bonding to near atoms. Experimentally, Er³⁺-like emission from ⁴I_13/2 to ⁴I_15/2 was observed, and our calculations indicate that the Er-Er covalent metal bond in Er₂@C₈₂, and the Er-C/C₂ covalent bonds in Er₂C₂@C₈₂, can account for the observed photoluminescence despite the cage with C₈₂^4-. Such existence is the reason that the C₂ unit was found to be neutral on the basis of MEM-Rietveld X-ray measurements, although formally it should be described as C₂^2-. Our prediction for isomer photoluminescence intensity agrees with the experimentally determined order (III > I > II), where the most pronounced activity of isomer III in Er₂C₂@C₈₂ stems from its higher charge of formal Er³⁺ and its largest HOMO-LUMO gap.

Formation of endohedral metallofullerene (EMF) ions of MnC2m+ (M = La, Y, n ≤ 6, 50 ≤ 2m ≤ 194) in the laser ablation process with graphene as precursor

RATIONALE

Graphene has proved to be a versatile precursor for the generation of mono-endohedral metallofullerenes (EMFs) of alkali and alkali earth metal atoms. Thus, it is important to investigate whether graphene is also a good precursor for the generation of EMFs including multi-metal atoms or some transition metal atoms.

METHODS

Experiments were performed on a 7.0 T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Laser ablation mass spectra were obtained by laser irradiation on mixtures of graphene and LaCl₃ , YCl₃ or HAuCl₄ on a stainless steel plate. A 355 nm Nd:YAG laser with a typical energy of 2.5 mJ/pulse was employed in these experiments. Tandem mass spectrometry was performed by on-resonance excitation collision-induced dissociation (CID) for selected single isotopic peaks.

RESULTS

Metallofullerene La1-6C2m+/ Y1-6C2m+ ions were observed in the laser ablation mass spectra of mixtures of graphene and LaCl₃ /YCl₃ . CID and reactant gas experiments on selected ions showed that these ions had endohedral structures. Considering the experimental condition applied here, a mechanism based on a top-down process for the generation of the EMF ions is suggested. Further experiments on HAuCl₄ showed some weak signals of AuC2m+ for the first time.

CONCLUSIONS

Mono- and multi-EMF ions of MnC2m+ (M = La, Y, n = 1-6, 50 ≤ 2 m ≤ 194) were generated by laser ablation of graphene and the corresponding metal salts and detected by FT-ICR mass spectrometry. The results have shed light on the formation and generation of some new EMF species. Copyright © 2017 John Wiley & Sons, Ltd.

Adsorption characteristics of Er3N@C80on W(110) and Au(111) studied via scanning tunneling microscopy and spectroscopy

We performed a study on the fundamental adsorption characteristics of Er₃N@C₈₀ deposited on W(110) and Au(111) via room temperature scanning tunneling microscopy and spectroscopy. Adsorbed on W(110), a comparatively strong bond to the endohedral fullerenes inhibited the formation of ordered monolayer islands. In contrast, the Au(111)-surface provides a sufficiently high mobility for the molecules to arrange in monolayer islands after annealing. Interestingly, the fullerenes modify the herringbone reconstruction indicating that the molecule-substrate interaction is of considerable extent. Investigations concerning the electronic structure of Er₃N@C₈₀/Au(111) reveals spatial variations dependent on the termination of the Au(111) at the interface.

Orientation of a Water Molecule: Effects on Electronic Nature of the C59N Cage

A hydrogen-bonding network is a key impelling force for an assembly in bulk water. The fullerene cage can incarcerate a water molecule without hydrogen-bonding. Herein, we focused on spin system H₂O@C₅₉N^·. The ¹H NMR relaxation time of entrapped H₂O was significantly reduced by the paramagnetic effect. Interestingly, the electron affinity and ionization energy were suggested to vary depending on the orientation of entrapped H₂O owing to the degree of the partial charge transfer from entrapped H₂O to C₅₉N^·.

Rationalizing the relative abundances of trimetallic nitride template-based endohedral metallofullerenes from aromaticity measures

The synthesis of endohedral metallofullerenes (EMFs) from a carbon soot sample of an arc discharge leads to a variety of EMFs that are obtained in different relative abundances. In the present work, we show that these abundances can be predicted from aromaticity calculations. In particular, we use the normalized Additive Local Aromaticity (ALAN) index. Our results show that the most abundant Sc3N-based and Y3N-based EMFs in fullerene soot are the most aromatic. This study reinforces the idea that aromaticity plays a key role in determining the stability of EMFs.

Quantum mechanical calculation of nanomaterial-ligand interaction energies by molecular fractionation with conjugated caps method

Molecular fractionation with conjugate caps (MFCC) method is introduced for the efficient estimation of quantum mechanical (QM) interaction energies between nanomaterial (carbon nanotube, fullerene, and graphene surface) and ligand (charged and neutral). In the calculations, nanomaterials are partitioned into small fragments and conjugated caps that are properly capped, and the interaction energies can be obtained through the summation of QM calculations of the fragments from which the contribution of the conjugated caps is removed. All the calculations were performed by density functional theory (DFT) and dispersion contributions for the attractive interactions were investigated by dispersion corrected DFT method. The predicted interaction energies by MFCC at each computational level are found to give excellent agreement with full system (FS) calculations with the mean energy deviation just a fractional kcal/mol. The accurate determination of nanomaterial-ligand interaction energies by MFCC suggests that it is an effective method for performing QM calculations on nanomaterial-ligand systems.

The Role of Super-Atom Molecular Orbitals in Doped Fullerenes in a Femtosecond Intense Laser Field

The interaction of gas phase endohedral fullerene Ho₃N@C₈₀ with intense (0.1–5 × 10¹⁴ W/cm²), short (30 fs), 800 nm laser pulses was investigated. The power law dependence of Ho₃N@C₈₀^q+, q = 1–2, was found to be different from that of C₆₀. Time-dependent density functional theory computations revealed different light-induced ionization mechanisms. Unlike in C_60, in doped fullerenes, the breaking of the cage spherical symmetry makes super atomic molecular orbital (SAMO) states optically active. Theoretical calculations suggest that the fast ionization of the SAMO states in Ho₃N@C₈₀ is responsible for the n = 3 power law for singly charged parent molecules at intensities lower than 1.2 × 10¹⁴ W/cm².

Au20Si12: A hollow Catalan pentakis dodecahedron

A stable hollow Au₂₀Si₁₂ cage with I_h symmetry has been predicted using first-principles density functional theory. The stability of the cage-like Au₂₀Si₁₂ structure is verified by vibrational frequency analysis and molecular dynamics simulations. A relatively large highest occupied molecular orbital-lowest unoccupied molecular orbital gap of 1.057 eV is found. Electronic structure analysis shows that clearly p-d hybridizations between Si atoms and Au atoms are of great importance for the stability of Au₂₀Si₁₂ cage. The cage-like Au₂₀Si₁₂ structure may have potential applications in semiconductor industry and microelectronics.

Does the endohedral borospherene supersalt FLi2@B39maintain the “super” properties of its subunits?

The behavior of the entirely unique system represented by superalkaline species incorporated into a superhalogen cage has been studied using density functional theory. The calculations revealed that superhalogen and superalkaline properties inherent in the separated fragments are lost in FLi₂@B₃₉complexes.

Catching metallic nitride endohedral fullerenes in organic and aqueous media

Two bis-tetraphenylporphyrin tweezers-like conjugates bearing different linkers have been characterized by means of electrochemical and photophysical methods. The thrust of this work was the complexation of these bis-porphyrins with C[Formula: see text] and metallic nitride endohedral fullerenes (MNEF) like Sc3N@C[Formula: see text] in organic and in aqueus media — studied by a combination of steady-state and time-resolved spectroscopies. In the context of charge separation evidence for the formation of the 1a[Formula: see text]-Sc3N@C[Formula: see text] and 1c[Formula: see text]-Sc3N@C[Formula: see text] charge separated states in organic and aqueous media, respectively, came from transient absorption spectroscopy. Multi-wavelength analyses afforded charge separated state lifetimes of around 400 ps.

Carbon nanotube fibers and films: synthesis, applications and perspectives of the direct-spinning method

The direct-spinning method of creation of CNT macroassemblies has received a lot of attention because of its simplicity to produce high-performance material without apparent limits to its size. CNT fibers or films have shown unparalleled properties and opened new areas of research and commercial development. The process designed more than a decade ago has already given interesting information about the basic science of nanomaterials, which in parallel led to the creation of the first prototypes with high potential of implementation in everyday life. Because of this, there has been growing interest in this technique with research articles coming into view from all around the world on a frequent basis. This review aims to summarize all the progress made in the direct-spinning process on a spectrum of fronts ranging from the study of complex synthesis parameters, material properties to its viable applications. The strong and weak points of the "Cambridge process" are carefully evaluated to put forward what challenges are most pressing. The future overlook puts the state of the art into perspective and suggests the prospective research directions.

Another big discovery-metallofullerenes

Several days after the first experimental observation of the 'magic number' soccerball-shaped C60 in a laser-vaporized cluster beam mass spectrum by Kroto and co-workers (Heath et al 1985 J. Am. Chem. Soc. 107, 7779-7780. (doi:10.1021/ja00311a102)) they also found a magic number feature owing to La@C60 in a mass spectrum prepared by laser vaporization of a LaCl3-impregnated graphite rod. With the advent of macroscopic synthesis and the following successful separation and purification of metallofullerenes, both experimental and theoretical studies of metallofullerenes have developed quite rapidly to date so as to elucidate their structural, electronic, magnetic and transport properties. Furthermore, a bottom-up closed network growth mechanism has experimentally been shown to play a crucial role in generating various types of metallofullerenes.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Structural Stability and Deformation of Solvated Sm@C2(42)-C90 under High Pressure

Solvated fullerenes recently have been shown to exhibit novel compression behaviors compared with the pristine fullerenes. However, less attention has been focused on the large cage endohedral metallofullerenes. Here, we have firstly synthesized solvated Sm@C₉₀ microrods by a solution drop-drying method, and then studied the transformations under high pressure. The pressure-induced structural evolutions of Sm@C₉₀ molecules both undergo deformation and collapse. The band gaps of both samples decrease with increasing pressure. The trapped Sm atom plays a role in restraining the compression of the adjacent bonds. The solvent plays a role in protecting Sm@C₉₀ against collapse in the region of 12–20 GPa, decreasing and postponing the change of band gap. Above 30 GPa, the carbon cages collapse. Released from 45 GPa, the compressed solvated Sm@C₉₀ forms a new ordered amorphous carbon cluster (OACC) structure with metal atoms trapped in the units of amorphous carbon clusters, which is different from the OACC structure formed by compressing solvated C₆₀ and C₇₀. This discovery opens the door for the creation of new carbon materials with desirable structural and physical properties when suitable starting materials are selected.

Electrochemical reduction of cationic Li+@C60 to neutral Li+@C60˙-: isolation and characterisation of endohedral [60]fulleride

Li@C₆₀ was synthesised by electrochemical reduction of ionic Li⁺@C₆₀ salt. This is the first report of isolation and unambiguous characterisation of endohedral metallo[60]fullerene.

Lithium-encapsulated [60]fullerene Li@C₆₀, namely, lithium-ion-encapsulated [60]fullerene radical anion Li⁺@C₆₀˙^–, was synthesised by electrochemical reduction of lithium-ion-encapsulated [60]fullerene trifluoromethanesulfonylimide salt [Li⁺@C₆₀](TFSI^–). The product was fully characterised by UV-vis-NIR absorption and ESR spectroscopy as well as single-crystal X-ray analysis for the co-crystal with nickel octaethylporphyrin. In solution Li@C₆₀ exists as a monomer form dominantly, while in the crystal state it forms a dimer (Li@C₆₀–Li@C₆₀) through coupling of the C₆₀ radical anion cage. These structural features were supported by DFT calculations at the M06-2X/6-31G(d) level of theory.