Endohedral Compound Research Articles

Effect of exohedral functionalization on the magnetic properties in the dysprosium-containing endohedral fullerene DySc2N@C80(CF2).

We present a quantum-chemical study of the effect of exohedral functionalization with a CF2 group on the lowest electronic states and the zero-field splitting pattern in a potential single-molecule magnet (SMM) compound DySc2N@C80(CF2). Multiconfiguration perturbational methodology is applied to various spin states of the endohedral compound, comparing different active spaces and state-averaging schemes in order to check for the possible involvement of orbitals other than 4f-Dy in the nondynamical electronic correlation and to suggest the most appropriate computational parameters. Combining the spin-orbit coupling calculations with perturbational corrections, we demonstrate that the interactions within the endohedral cluster and with the fullerene cage exert only a small effect on the non-relativistic approximation to the electronic states of the Dy3+ ion, yet they are significant enough to alter the parameters of zero-field splitting depending on the orientation of the DySc2N cluster inside the fullerene cage.

Intermetallic Compound Re2Ga9Ge with Re- and Ge-Embedded Gallium Clusters: Synthesis, Crystal Structure, Chemical Bonding, and Physical Properties.

Transition metal-based endohedral cluster intermetallic compounds are interesting electron phases, which frequently exhibit superconductivity with a peculiar interplay between the critical temperature and valence electron count. We present a new Re-based endohedral gallium cluster compound, Re2Ga9Ge. Its unique crystal structure (P42/mmc space group, a = 8.0452(3) Å, c = 6.7132(2) Å) is built by two types of gallium polyhedra: monocapped Archimedean antiprisms centered by rhenium atoms and tetrahedra containing a main-group element inside. The analysis of chemical bonding shows the presence of localized pairwise interactions between the p-block elements and the formation of multicenter bonds with the participation of d-orbitals of rhenium. In the electronic band structure, the Fermi level is located in a narrow pseudogap indicating the optimum band filling and thus explaining the virtual absence of a homogeneity range. The compound exhibits Pauli paramagnetism and metallic properties with unexpectedly low thermal conductivity. A sharp anomaly observed on the magnetic susceptibility and resistivity curves presumably indicates the electronic phase transition accompanied by charge ordering at the characteristic temperature of T * = 271 K in zero magnetic field.

Superconductivity in the Endohedral Ga Cluster Compound PdGa5

Superconductivity is observed below Tc = 1.6 K in an endohedral Ga cluster compound PdGa5 using magnetization and heat capacity measurements. Electronic structure calculations show that the density of states (DOS) at the Fermi level is dominated by Ga s and p states and that the overall shape of DOS is similar to what was found in other endohedral Ga cluster superconductors, such as MoxGa5x+1, ReGa5, and T2Ga9 (T = Rh and Ir). Our results provide a more complete picture of the relationship between the valence electron count and superconductivity in the family of endohedral Ga cluster superconductors.

RuAl6—An Endohedral Aluminide Superconductor

Superconductivity is reported in an endohedral aluminide compound, RuAl6, with Tc = 1.21 K. The normalized heat capacity jump at Tc, ΔC/γTc = 1.58, confirms bulk superconductivity. The Ginzburg–Lan...

Localization of the valence electron of endohedrally confined hydrogen, lithium and sodium in fullerene cages

The localization of the valence electron of [Formula: see text], [Formula: see text] and [Formula: see text] atoms enclosed by three different fullerene molecules is studied. The structure of the fullerene molecules is used to calculate the equilibrium position of the endohedrally atom as the minimum of the classical [Formula: see text]-body Lennard–Jones potential. Once the position of the guest atom is determined, the fullerene cavity is modeled by a short range attractive shell according to molecule symmetry, and the enclosed atom is modeled by an effective one-electron potential. In order to examine whether the endohedral compound is formed by a neutral atom inside a neutral fullerene molecule [Formula: see text]@[Formula: see text] or if the valence electron of the encapsulated atom localizes in the fullerene giving rise to a state with the form [Formula: see text]@[Formula: see text], we analyze the electronic density, the projections onto free atomic states and the weights of partial angular waves.

Endohedral gallide cluster superconductors and superconductivity in ReGa5

We present transition metal-embedded (T@Gan) endohedral Ga-clusters as a favorable structural motif for superconductivity and develop empirical, molecule-based, electron counting rules that govern the hierarchical architectures that the clusters assume in binary phases. Among the binary T@Gan endohedral cluster systems, Mo8Ga41, Mo6Ga31, Rh2Ga9, and Ir2Ga9 are all previously known superconductors. The well-known exotic superconductor PuCoGa5 and related phases are also members of this endohedral gallide cluster family. We show that electron-deficient compounds like Mo8Ga41 prefer architectures with vertex-sharing gallium clusters, whereas electron-rich compounds, like PdGa5, prefer edge-sharing cluster architectures. The superconducting transition temperatures are highest for the electron-poor, corner-sharing architectures. Based on this analysis, the previously unknown endohedral cluster compound ReGa5 is postulated to exist at an intermediate electron count and a mix of corner sharing and edge sharing cluster architectures. The empirical prediction is shown to be correct and leads to the discovery of superconductivity in ReGa5. The Fermi levels for endohedral gallide cluster compounds are located in deep pseudogaps in the electronic densities of states, an important factor in determining their chemical stability, while at the same time limiting their superconducting transition temperatures.

ChemInform Abstract: Peroxide‐Mediated Selective Cleavage of [60]Fullerene Skeleton Bonds: Towards the Synthesis of Open‐Cage Fulleroid C55O5

AbstractReview: [49 refs.

Peroxide-mediated selective cleavage of [60]fullerene skeleton bonds: towards the synthesis of open-cage fulleroid C55O5.

Replacement of a pentagon in [60]fullerene with five oxygen atoms yields the open-cage compound C55O5 with five carbonyl groups on the rim of the orifice. Our attempts to synthesize such a target molecule starting from C60 have led us to prepare the fullerene-mixed peroxides such as C60(OO-t-Bu)6 with all the peroxo addends surrounding the same pentagon. Further investigations of the peroxide chemistry have generated various open-cage fullerene derivatives, including the carbon monoxide encapsulated endohedral compound CO@C59O6. This Personal Account mainly discusses peroxide-based processes resulting in selective cleavage of the fullerene skeleton bonds.

Structures and nonlinear optical properties of the endohedral metallofullerene-superhalogen compounds Li@C60–BX4 (X = F, Cl, Br)

It has recently been demonstrated that superatoms, which can exhibit behaviors reminiscent of atoms in the periodic table, might have synthetic utility, and represent potential building blocks for the assembly of novel, nanostructured materials [Science 2004, 304, 84-87; Science 2005, 307, 231-235; J. Phys. Chem. C 2009, 113, 2664]. In this work, a new type of endohedral metallofullerene-superhalogen compound, Li@C60-BX4 (X = F, Cl, Br), is proposed and characterized using density functional theory. The electron transfer from Li@C60 to BX4 contributes greatly to the Li@C60-BX4 compound formation. Such compounds exhibit considerable stabilities with large binding energies and ionization potentials, as well as large HOMO-LUMO gaps. The investigation of the nonlinear optical (NLO) properties of Li@C60-BX4 reveals a strong dependence of the static first hyperpolarizability, β0, on the atomic number of the involved halogen atom X. This means that one can enhance the first hyperpolarizabilities of the endohedral metallofullerene by introducing superhalogens. The present investigation may promote the development of novel nanomaterials with unusual properties (i.e. NLO properties), and enrich the knowledge of chemical bonds (for example, long-range interactions between trapped atoms in a C60 cage and the outside superatom motif).

Self-Consistent Hartree–Fock Approach to Electronic Structure of Endohedral Fullerene Ar@C60

Results of the first self-consistent calculation of electronic structure of endohedral fullerenes within the Hartree-Fock approximation are presented. The case study is Ar@C60. A phenomenon of the significant hybridization of the valent shell of the confined atom due to the presence of the fullerene core is observed. It is shown that the exchange interaction between all electrons of the endohedral compound plays a significant role in the process of hybridization. It is also shown that the hybridization of the atomic valent shell leads to the significant charge redistribution inside the endohedral compound.

Diagnostics of atoms encapsulated into fullerenes by the polarization bremsstrahlung from accelerated electrons

It is shown that the polarization bremsstrahlung (PB), which arises during scattering of coherent electromagnetic fields of fast charged particles by atomic electrons and resonantly reacts to changes of the density of a substance, allows one to diagnose the states of atoms that are encapsulated in fullerene. As a result of modeling, by comparing the spectra of an empty fullerene and fullerene with an encapsulated atom and using difference fitting, it is possible to detect compressions or expansions of an electron shell of the atom that is encapsulated in a fullerene. The calculations are fulfilled for the model approximation of the P@C60 endohedral compound.

Role of exchange interaction in self-consistent calculations of endohedral fullerenes

Results of the self-consistent calculation of electronic structure of endohedral fullerene Ar@C60 within the Hartree–Fock and the local density approximations are presented. Hartree–Fock approximation is used for the self-consistent description for the first time. It is shown that the accurate account of the exchange interaction between all electrons of the compound leads to the significant modification of the atomic valent shell which causes the noticeable charge redistribution inside the endohedral compound.

Probing photoelectron multiple interferences via Fourier spectroscopy in energetic photoionization ofXe@C60

Considering the photoionization of the $\mathrm{Xe}@{\mathrm{C}}_{60}$ endohedral compound, we study in detail the ionization cross sections of various levels of the system at energies higher than the plasmon resonance region. Five classes of single-electron levels are identified depending on their spectral character. Each class engenders distinct oscillations in the cross section, emerging from the interference between active ionization modes specific to that class. Analysis of the cross sections based on their Fourier transforms unravels oscillation frequencies that carry unique fingerprints of the emitting level.

Is Fullerene C60 Large Enough to Host a Multiply Bonded Dimetal?

Some dimetal fullerenes M 2@C 60 (M = Cr, Mo, W) have been studied with computational quantum chemistry methods. The transition metal diatomic molecules Cr 2, Mo 2, W 2 form exohedral complexes with C 60, while U 2 forms a highly symmetric endohedral compound and it is placed in the center of the C 60 cavity. This highly symmetric structure is an artifact due to the small size of the C 60 cavity, which constrains U 2 at the center. If a larger cavity is used, like C 70 or C 84, U 2 preferentially binds the internal walls of the cavity and the U-U bond no longer exists.

What is stable structure about Tb 3N@C 84? IPR or IPR-violating

The geometric and electronic structures of fullerenes encapsulating trimetallic nitride cluster Tb 3N@C 84 were studied using the density functional theory. Geometric optimization showed that Tb 3N@C 84 that violates the Isolated-Pentagon Rule (IPR) with a pentalene unit formed by fused pentagon pair is the most kinetically stable isomer with the largest energy gap. Compared to IPR-violating Tb 3N@C 84 isomers, IPR-Tb 3N@C 84 is the most thermodynamically favorable endohedral compound with the lowest energy. Tb 3N@C 84 has a higher density of states (DOS) than empty C 84 molecule at the Fermi energy. This indicated that the embedment of Tb 3N unit can increase the conductance of C 84.

Trimetallic Nitride Endohedral Metallofullerenes: Reactivity Dictated by the Encapsulated Metal Cluster

The first derivatives of Y(3)N@C(80) have been synthesized and fully characterized. 1,3-Dipolar cycloaddition of N-ethylazomethine ylide yielded mainly the pyrrolidine monoadduct of the icosahedral (I(h)()) symmetry cage exclusively at a [6,6] double bond. The same regioselectivity on a [6,6] double bond was observed when the endohedral compound was cyclopropanated with diethyl bromomalonate. These results are in pronounced contrast to those observed for icosahedral symmetry Sc(3)N@C(80), for which all reported derivatives add completely regioselectively to [5,6] double bonds. (1)H NMR, (13)C NMR, and HMQC spectroscopy revealed that the addition pattern on Y(3)N@C(80) resulted in a pyrrolidinofullerene derivative with unsymmetric pyrrolidine carbons and symmetric geminal protons. The cyclopropanated monoadduct exhibited symmetric ethyl groups on the malonate, consistent with regioselective addition at a [6,6] double bond. Attempts to perform the same cyclopropanation reaction on (I(h)()) Sc(3)N@C(80) failed to yield any identifiable products. These observations clearly indicate that the reactivity of trimetallic nitride endohedral metallofullerenes toward exohedral chemical functionalization is profoundly affected and effectively controlled by the nature of the endohedral metal cluster.

Low energy modes of metals in fullerene cages

Vibrations between carbon cage and Sc are discussed in the endohedral compound Sc2@C84. The attention was concentrated to the region below 300 cm-1 in which one expects this type of motions. Comparing Raman and far-infrared spectra for empty and filled cages with the same symmetry we identified 9 modes for the Sc-cage vibrations. This large number of the modes will be discussed from a point of view of different mechanisms of splitting. We present also temperature dependent measurements which suggest a possible order-disorder phase transition between 160 and 180K.

Infrared measurements for two isomers of the endohedral metallofullerene Sc 2@C 84

We present IR transmission mesurements for two isomers of the endohedral compound Sc 2@C 84 with D 2d (No.23) and C s (No.10) symmetry. The measurements were performed im the far infrared region between 80 cm −1 and 500 cm −1 and at temperatures from 80 K to 300 K. We identified several modes and investigated their dependence on the symmetry of the cage and on the temperature.

Endohedral Formation from Neutron Activation of Interstitial Rare Cas C60 Fullerides

Rare gas interstitial fullerenes, produced by hot isostatic pressing solid C60 in the presence of Ar, Kr or Xe, have been neutron irradiated and their behaviour investigated. The activity of the generated radionuclides was found to be in agreement with calculations and this combined with X-ray powder diffraction showed that both the activated radionuclides and the unactivated rare gas remained trapped in the solid after they have been subjected to the harsh conditions encountered in a nuclear reactor. Gamma spectroscopy of the irradiated solids and solutions of them in toluene provided strong evidence for endohedral compound formation. We estimate 1–2% of the activated rare gas atoms, which recoil as a result of prompt gamma emission, end up in the centre of what is most likely too be the C60 molecule or some other fullerene derivative. On this basis, we postulate the formation of RN@C6o where the radionuclide (RN)is 125gXe, 133gXe, I35gXe, 41Ar or85mKr.

Vibrational structure of C 84 and Sc 2@C 84 analyzed by IR spectroscopy

The isomer III of Sc 2@C 84 was separated by multi-cycle HPLC purification. We present temperature dependent IR absorption measurements of Sc 2@C 84 which have been performed between 50 and 300 K and between 400 and 5000 cm −1, respectively. The vibrational structure of the endohedral compound is compared to the structure of unfilled C 84. We find a strong overall broadening of the vibrational modes in Sc 2@C 84. Also some of the vibrational absorption lines are strongly enhanced if compared to the spectrum for the empty cage. With decreasing temperature, a dramatic narrowing of the lines in the spectral range between 700 and 800 cm −1 is observed.