Endohedral Complexes Research Articles

Optical and magneto-optical properties of the endohedral complex La@C82 crystal: a Hartree–Fock single-excitation configuration interaction model study

The dielectric constant tensor is used to describe the magneto-optical properties of crystals. Using the Hartree–Fock single-excitation configuration interaction model in conjunction with the local field method, the spin-dependent optical dielectric tensor, refractive index, reflectivity, circular dichroism (CD), and birefringence coefficient (θ) of the simple cubic (sc) phase of the La@C82 crystal are calculated when the La atom is off-center. The calculations show that the spin-dependent optical dielectric tensor of the La@C82 crystal is large. The displacement of the La atom from the center of the cage leads to molecular electric dipole moment which influences the magnetic and optical properties of the La@C82 crystal. The Coulomb interactions and effective magnetic moment cause increasing dielectric tensor, CD, and θ. Our results show that the optical excitation in the region where the energy is lower than 2 eV has the strongest optical absorption bands and refractive index in the La@C82 crystal. Also, our results show that CD and θ of the La@C82 crystal are larger than those of the C60 and C70 crystals. We obtained the Eg of 0.27 eV similar to that of the obtained results of the La@C82 thin film with a semiconducting behavior.

CdS nanoclusters doped with divalent atoms.

ZnS and CdS small nanoclusters have been predicted to trap alkali metals and halogen atoms. However would this kind of nanocompounds be able to encapsulate dianions and dications? This would be very interesting from an experimental point of view, since it would allow the isolation of such divalent ions. Moreover, the resulting endohedral complexes would serve as building blocks for new cluster-assembled materials, with enhanced stability arising from the electrostatic interaction between the incarcerated ions. In this work we have studied the structure and stability of (X@(CdS)i)(±2) with X = Be, Mg, Ca, O, S, Se and i = 9, 12, 15, 16 on the basis of Density Functional Theory and Quantum Molecular Dynamics simulations. Most of the nanoclusters are found to trap both chalcogen and alkaline earth atoms. Furthermore, the chalcogen doped clusters are calculated to be both thermodynamically and thermally stable. However, only a few of alkaline earth metal doped structures are predicted to be thermally stable. Therefore, the charge of the dopant atom appears to be crucial in the endohedral doping. Additionally, the absorption spectra of the title compounds have been simulated by means of Time Dependent Density Functional Theory (TDDFT) calculations. The calculated optical features show a blueshift with respect to the bulk CdS wurtzite. Furthermore, doping modifies notably the optical spectra of nanoclusters, as the absorption spectra shift to lower energies upon encapsulation.

Formation and electronic structure of C20 fullerene transition metal clusters

Using density functional theory, the structure and electronic properties of TM@C20 (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) complexes were investigated. The study revealed that the doping processes of Sc, Ti, V, Cr, Mn, and Fe atoms at the center of C20 fullerene are exothermic and those of Co, Ni, Cu, and Zn atoms are endothermic. In addition, the endohedral complexation with Sc, Ti, V, Cr, Mn, and Fe atoms significantly influence the electronic properties of the C20 clusters. The calculated vibrational frequencies for the Ti, V, and Mn complexes were positive, confirming the complexes are stable and can stabilize the unstable C20 fullerene with I h symmetry. The present results reveal details for the synthesis and structuring of C20 fullerene doped with Ti, V, and Mn and serves for the further developments of C20-based particles to generate new hybrid compounds.

Host-guest complexes of calix[4]tubes - prediction of ion selectivity by quantum chemical calculations VI

The selectivity of the bis(calix[4]arene)tetraethylene abbreviated as calix[4]tube for the endohedral complexation of alkali and alkaline earth metal ions, was predicted on the basis of structures and complex formation energies computed with three different quantum chemical methods: DFT LANL2DZp)/LANL2DZp), PM3/SPASS, and PM6. A comparison with published X-ray structures demonstrated that the most reliable results were achieved applying DFT calculations. The complexation of K⁺ and Ba²⁺ is most favorable, followed by the encapsulation of Rb⁺ and Sr²⁺, respectively. The flexibility of the tube, described by the torsion angles associated with the ethylene linkages between the calix[4]arene units and phenyl rings intersecting the plane of the four methylene carbon atoms, also makes an important contribution to its selectivity. In general, the cavity size is similar to [2.2.2] and [N2N2N2], the cryptands with the largest cavities previously studied in our group applying a similar protocol.

Tight-binding Molecular Dynamics Simulation of the Endohedral Complex C4H4@C60

Tight-binding molecular dynamics simulations were carried out to analyze the thermal stability of the tetrahedrane molecule inside the [60]fullerene cage over the temperature range T = 400 − 800 K. The temperature dependence of the endohedral complex lifetime to the moment of tetrahedrane decay was obtained from the numerical experiment. The height of the minimum energy barrier preventing the decomposition of the encapsulated tetrahedrane molecule was found to be similar for the isolated one. Possible channels and final products of its decomposition were also studied.

Theoretical investigation of endohedral complexes of Si and Ge with C60 molecule

The electronic and magnetic properties of Si and Ge doped C60 fullerene are investigated with ab initio density functional theory calculations. In endohedral Sin (n=1–10) and Gen (1–9) doped C60 complexes we have seen that Si and Ge doped complexes are stable up to 9 and 4 atoms, respectively. We have also investigated binding energy and ionization potential of the endohedral fullerene C60. Endohedral fullerenes have been found to be more chemically reactive. Mullikan charge analysis shows the presence of the magnetic moment in these systems.

The molecular and magnetic structure of carbon-enclosed and partially covered Fe55 particles

The structure and magnetic moment distribution are studied for an iron nanoparticle with varying degree of carbon adatom coverage. The limiting models of the study are the clean icosahedral Fe55 particle and the iron particle completely enclosed in carbon cages. Between the two extrema, partially covered particles are considered. The iron cluster with partial coverage of carbon adatoms represents a model of active catalysts in the chemical vapor deposition synthesis of carbon nanotubes. The investigated structures are the bare Fe55 cluster, Fe55N4C(x) (x = 27, 37, 47, 54, 65), and Fe55 encapsulated inside C180 and C240. The two latter are extreme examples of an iron particle completely enclosed in a carbon network. Fe55@C180 and Fe55@C240 present novel structures resembling the endohedral metallofullerenes. Two structural isomers of the Fe55@C180 are considered. Enclosing the Fe55 cluster inside C180 and C240 fullerenes gives rise to changes in the Fe-Fe bond lengths. This alters the magnetic structure of the iron cluster considerably. The interaction between the fullerenes and the enclosed iron cluster is reflected in a charge transfer of 8-13 electrons in the considered endohedral complexes. The localization of atomic charges on the C180 and C240 cages suggests site-selective reactivity of the endohedral complexes. The total magnetic moments of the Fe55N4C(x) nanoparticles vary with the degree of adatom coverage. The magnetic moments of individual Fe atoms depend strongly on the element of the nearest-neighbor atoms and on the coordination number and carry therefore information about the local chemistry.

Laser ablation synthesis of new gold carbides. From gold‐diamond nano‐composite as a precursor to gold‐doped diamonds. Time‐of‐flight mass spectrometric study

Gold carbides can be produced via laser ablation synthesis (LAS) from mixtures of nano-gold (NG) and various carbonaceous materials. The nano-composite of nano-gold (NG) and nano-diamond (ND) might represent a promising precursor for the generation of new gold carbides. Time-of-flight mass spectrometry (TOF MS), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) were used. The stoichiometry of clusters was determined via modelling of the isotopic patterns and MS(n) analysis. A simple procedure for the preparation of ND-NG nano-composite was developed using NG and ND. The formation of AuCn(+) (n = 1-11, 18), Au2Cn(+) (n = 1-16) and Au3Cn(+) (n = 1-10) clusters during LAS of the nano-composite was proved. Structures of gold carbides are proposed and discussed. Diamonds-containing AumCn(+) (m = 1-3, n = 10, 14, 18, 22) clusters might be not carbides but endohedral supramolecular complexes Aum@Cn(+) i.e., 'gold-doped' diamonds. TOF MS was shown to be a useful technique for following the formation of gold carbides in the gas phase. Clusters and 'gold-doped' diamonds generated might inspire synthesis of new Au-C materials with hardly predictable, unusual properties.

Punching a Carbon Atom of C60 into its Own Cavity to Form an Endohedral Complex CO@C59O6 under Mild Conditions

Punching the ball: By using multistep chemical reactions as a “can opener”, a half circle opening is cut around a pentagon in C60 to form an open-cage fullerene with an 18-membered orifice. During the process, one carbon atom in the fullerene skeleton is eliminated into the cavity as carbon monoxide to form a stable endohedral complex CO@C59O6 (see figure). The single-crystal X-ray structure shows that the trapped carbon monoxide has strong dipole–dipole interaction with the cage.

Endohedral complexes of Polyhedral Oligomeric Silsesquioxane (POSS) cages with transition metal dihydrides

Polyhedral Oligomeric Silsesquioxane (POSS) cages are investigated in terms of their potential to enclose small metal hydrides, with the objective of defining conditions that maximize the number of encapsulated hydrogen atoms. Systems of the form MH2n@Tm, where n=1–3, m=8, 10, and M comprises metal atom species of the groups IV, VI, VIII, X, and XII, are studied by methods of ab initio and density functional theory (DFT). The resulting composites are categorized with respect to their structural and energetic features. For MH2@T8, it is found in all cases considered that including MH2 into the POSS cage is an endothermic process. For MH2@T10 and M=Ti, Ru, Os, Pt, inclusion of the MH2 guest into the cage turns out to be exothermic, and also leaves the cage intact. For MH4@Tm, this behavior is only observed for one system, OsH4@T10.

Infrared spectroscopy of small-molecule endofullerenes

Hydrogen is one of the few molecules that has been incarcerated in the molecular cage of C₆₀ to form the endohedral supramolecular complex H₂@C₆₀. In this confinement, hydrogen acquires new properties. Its translation motion, within the C₆₀ cavity, becomes quantized, is correlated with its rotation and breaks inversion symmetry that induces infrared (IR) activity of H₂. We apply IR spectroscopy to study the dynamics of hydrogen isotopologues H₂, D₂ and HD incarcerated in C₆₀. The translation and rotation modes appear as side bands to the hydrogen vibration mode in the mid-IR part of the absorption spectrum. Because of the large mass difference of hydrogen and C₆₀ and the high symmetry of C₆₀ the problem is almost identical to a vibrating rotor moving in a three-dimensional spherical potential. We derive potential, rotation, vibration and dipole moment parameters from the analysis of the IR absorption spectra. Our results were used to derive the parameters of a pairwise additive five-dimensional potential energy surface for H₂@C₆₀. The same parameters were used to predict H₂ energies inside C₇₀. We compare the predicted energies and the low-temperature IR absorption spectra of H₂@C₇₀.

Endohedral and exohedral complexes of substituted benzenes with carbon nanotubes and graphene

Non-covalent complexes of cyclohexane and a series of substituted benzenes with short carbon nanotube (CNT) models are investigated primarily at the B97-D3∕TZV(2d,2p) level of theory. Understanding non-covalent interactions of arenes with CNTs is vital for the development of next-generation organic electronic materials and for harnessing CNTs as nano-reactors and vehicles for drug delivery. The interaction of benzene and cyclohexane with the interior and exterior of CNTs depends on the nanotube diameter, particularly for endohedral complexes. Both benzene and cyclohexane interact more strongly with the interior of CNTs than the outside, with benzene exhibiting stronger interactions than cyclohexane for CNTs larger than (8,8). Studies of two benzenes inside of CNTs predict the formation of one-dimensional sandwich and parallel-displaced stacks of benzenes within certain sized CNTs, which could have interesting optoelectronic properties. Concerning the impact of substituents on the interaction of benzene with CNTs, we find that electrostatic interactions do not control substituent effects. That is, the electron-donating or -withdrawing character of the substituents is not correlated with the predicted interaction energies. Moreover, substituent effects are the same for both endohedral and exohedral complexes, despite the different electronic character of the interior and exterior CNT walls. Ultimately, substituent effects in π-stacking interactions with CNTs and graphene are explained by differences in dispersion interactions between the substituents and CNT walls or graphene surface.

A density functional study of silicon fullerene endohedral X@Si20F20 and exohedral X-Si20F20 (X=O2−, S2−, Se2−) complexes

This study reports the structure and electronic properties of three stable endohedral X@Si20F20 and exohedral X-Si20F20 (X=O2−, S2−, Se2−) complexes. The study revealed that the endohedral complexes with S2− and Se2− result as energy minimum structures, with the guest anion located in the cage center. In the case of endohedral O2−-complexes, the pattern is quite different whereas the O2− complexes dramatically deviated from the architecture of S2− and Se2−, by having O2− located toward one of silicon atoms in the cage. With respect to the exohedral form, the energy minimized structure is obtained by positioning the anion between two silicon atoms and forming two Si–X bonds. For both cases, the strength of the interactions is calculated to increase accordingly to the pattern: Se2−@Si20F20 (Se2−-Si20F20)<S2−@Si20F20 (S2−-Si20F20)<O2−@Si20F20 (O2−-Si20F20). Additionally, the calculated HOMO–LUMO energy gaps of the exohedral complexes show lower values than endohedral forms, indicating higher chemical reactivity. This study reports relevant details for the synthesis and structuring of silica based endohedral complexes and serves for the further developments of silica-based particles for energy and microengineering purposes.

Inelastic neutron scattering spectrum of H2@C60 and its temperature dependence decoded using rigorous quantum calculations and a new selection rule

In the supramolecular complex H2@C60, the lightest of molecules, H2, is encapsulated inside the most highly symmetric molecule C60. The elegance and apparent simplicity of H2@C60 conceal highly intricate quantum dynamics of the coupled translational and rotational motions of the guest molecule in a nearly spherical nanoscale cavity, which embodies some of the most fundamental concepts of quantum mechanics. Here we present the first rigorous and highly accurate quantum calculations of the inelastic neutron scattering (INS) spectra of this prototypical endohedral fullerene complex and their temperature dependence. The calculations enable complete assignment of the recently reported experimental INS spectra of H2@C60 measured at several temperatures. We also derive a new and unexpected selection rule for the INS spectroscopy of H2 in a near-spherical confinement, which explains why the INS transitions between certain translation-rotation eigenstates of H2 in C60 have zero intensity and do not appear in the spectra.

Host-guest complexes of mixed glycol-bipyridine cryptands: prediction of ion selectivity by quantum chemical calculations, part V.

The selectivity of the cryptands [2.2.bpy] and [2.bpy.bpy] for the endohedral complexation of alkali, alkaline-earth and earth metal ions was predicted on the basis of the DFT (B3LYP/LANL2DZp) calculated structures and complex-formation energies. The cavity size in both cryptands lay between that for [2.2.2] and [bpy.bpy.bpy], such that the complexation of K+, Sr2+ and Tl3+ is most favorable. While the [2.2.bpy] is moderately larger, preferring Rb+ complexation and demonstrating equal priority for Sr2+ and Ba2+, the slightly smaller [2.bpy.bpy] yields more stable cryptates with Na+ and Ca2+. Although the CH2-units containing molecular bars fixed at the bridgehead nitrogen atoms determine the flexibility of the cryptands, the twist angles associated with the bipyridine and glycol building blocks also contribute considerably.

Forced Bonding and QTAIM Deficiencies: A Case Study of the Nature of Interactions in He@Adamantane and the Origin of the High Metastability

Calculations within the framework of the interacting quantum atoms (IQA) approach have shown that the interactions of the helium atom with both tertiary, tC, and secondary, sC, carbon atoms in the metastable He@adamantane (He@adam) endohedral complex are bonding in nature, whereas the earlier study performed within the framework of Bader's quantum theory of atoms in molecules (QTAIM) revealed that only He---tC interactions are bonding. The He---tC and He---sC bonding interactions are shown to be forced by the high pressure that the helium and carbon atoms exert upon each other in He@adam. The occurrence of a bonding interaction between the helium and sC atoms, which are not linked by a bond path, clearly shows that the lack of a bond path between two atoms does not necessarily indicate the lack of a bonding interaction, as is asserted by QTAIM. IQA calculations showed that not only the destabilization of the adamantane cage, but also a huge internal destabilization of the helium atom, contribute to the metastability of He@adam, these contributions being roughly equal. This result disproves previous opinions based on QTAIM analysis that only the destabilization of the adamantane cage accounts for the endothermicity of He@adam. Also, it was found that there is no homeomorphism of the ρ(r) and -v(r) fields of He@adam. Comparison of the IQA and QTAIM results on the interactions in He@adam exposes other deficiencies of the QTAIM approach. The reasons for the deficiencies in the QTAIM approach are analyzed.

Endohedral complexes of fullerene-like silica molecules with H2O, CH4, and CH3NH2 molecules

The possibility of formation of (SiO2)60@H2O, (SiO2)60@CH4, and (SiO2)60@CH3NH2 endohedral complexes was studied by the density functional (DFT) method (B3LYP exchange correlation functional, 6-31G** basis). The penetration of these molecules into the cavity of fullerene-like silica molecules is hindered by high activation barriers, which ensures the stability of the complexes formed during the synthesis of these molecules.

DFT study of [Fe@B36N36]n+ (n=2, 3) endohedral nanocages: Chemical reactivity, NBO analysis and thermochemistry

In this study, the structures, vibrational stabilities, energy gaps of B36N36 nanocage and its two endohedral derivatives, [Fe@B36N36]n+ (n=2, 3) were investigated via B3LYP hybrid functional of DFT. The transfer of charges and the inclusion energies of Fen+-encapsulated endohedrals were discussed as well. The detailed natural bond orbital (NBO) analysis revealed the nature of the hyperconjugative interactions of each nanocage. The NBO analysis specified an endohedral complex of Fe2+ with three nitrogen atoms of B36N36 while Fe3+ did not form such an endohedral complex. Also the global reactivity of the endohedrals based on NBO analysis was interpreted, and possible redox reactions of the studied nanocages were explained qualitatively.

Theoretical Insight for the Metal Insertion Pathway of Endohedral Alkali Metal Fullerenes

We have investigated the mechanism of alkali metal incorporation into C60 fullerene by density functional theory (DFT) at the UB3LYP/6-31G* level of theory. Calculations were performed to study the insertion pathways of Li(+), Na(+), and K(+) through six- or five-membered rings of fullerene, and the computed energy barriers of metal ion insertion are compared with the available experimental data. Between the two possible insertion pathways, metal ion insertion through [2 + 2 + 2] ring opening of the six-membered ring is found to be more favored than the insertion through the ring opening of the five-membered ring. The size of the ring openings generated by the three metal ions is likely to be correlated with their ionic size, which shows the smallest opening for Li(+) and the largest for K(+) cation. The insertion energy barriers of the ions are found to be increased in the order of Li(+) < Na(+) < K(+) in line with the experimental results. The ring opening made by breaking of C-C bonds during the metal ion insertion in six- or five-membered rings can cause the ring to be rearranged and convert back into a closed fullerene cage to form a stable endohedral metal-fullerene complex.

Gas storage of simple molecules in boron oxide nanocapsules

The capability of the B20O30 nanocapsule to store H2, N2, CO, CO2, NH3, CH4, and Cl2 molecules on the outer surface and inside of the cage was investigated using Monte Carlo simulations, long-range and dispersion corrected density functional theory, and Moller–Plesset second-order perturbation theory. Also, Monte Carlo simulations were employed to investigate the adsorption behavior of larger number of guest molecules inserted into and onto the larger B80O120 and B20O30@B80O120 cages. Absolute localized molecular orbitals energy decomposition analysis was used to describe the nature of intermolecular interactions in these endohedral and exohedral complexes. It is found that the hydrogen and ammonia gases are diffused to the inside of spherical B20O30 capsule, while other guest molecules prefer to interact with the outer surface of spherical and pyramidal capsules. For B80O120, up to 26 H2 and 11–14 N2, CO, CO2, NH3, and CH4 molecules are stored inside the capsule. The residual molecules are adsorbed on the outer surface of nanocapsule. © 2013 Wiley Periodicals, Inc.