Symmetric Cage Research Articles

Nuclear spin conversion of water inside fullerene cages detected by low-temperature nuclear magnetic resonance.

The water-endofullerene H2O@C60 provides a unique chemical system in which freely rotating water molecules are confined inside homogeneous and symmetrical carbon cages. The spin conversion between the ortho and para species of the endohedral H2O was studied in the solid phase by low-temperature nuclear magnetic resonance. The experimental data are consistent with a second-order kinetics, indicating a bimolecular spin conversion process. Numerical simulations suggest the simultaneous presence of a spin diffusion process allowing neighbouring ortho and para molecules to exchange their angular momenta. Cross-polarization experiments found no evidence that the spin conversion of the endohedral H2O molecules is catalysed by (13)C nuclei present in the cages.

Interface control in organic electronics using mixed monolayers of carboranethiol isomers.

We employ mixed self-assembled monolayers of carboranethiols to alter the work function of gold and silver systematically. We use isomers of symmetric carboranethiol cage molecules to vary molecular dipole moments and directions, which enable work function tunability over a wide range with minimal alterations in surface energy. Mixed monolayers of carboranethiol isomers provide an ideal platform for the study and fabrication of solution-processed organic field-effect transistors; improved device performance is demonstrated by interface engineering.

Variable framework structures and centricities in alkali metal yttrium selenites, AY(SeO3)2 (A = Na, K, Rb, and Cs).

Pure samples of a series of yttrium selenites, AY(SeO3)2 (A = Na, K, Rb, and Cs), were prepared through hydrothermal reactions using A2CO3, Y(NO3)3·6H2O, and SeO2. All four materials have 3D framework structures; however, they exhibit different channel geometries. The three-dimensional framework of NaY(SeO3)2 consists of zigzag chains of vertex-shared YO7 polyhedra and SeO3 linkers. KY(SeO3)2 and RbY(SeO3)2 reveal channels composed of corner-shared YO6 octahedra and SeO3 polyhedra. CsY(SeO3)2 exhibits a highly symmetric cage structure with radially spreading linkages of YO6 and SeO3 groups. Close structural examinations suggest that the cation size and coordination environment significantly influence the backbone architectures and their centricities. Second-harmonic generation (SHG) measurements on the powder sample of NaY(SeO3)2 reveal that the NCS selenite possesses a similar SHG efficiency to that of NH4H2PO4 (ADP).

Studies of Top-Down Mechanistic Processes of Fullerene and Metallofullerene Formation for Cages Greater Than Ninety Carbons

The elucidation of a mechanistic theory for fullerene formation in electric-arc plasmas, laser ablation, and interstellar space has been a subject of considerable interest over the past two decades. For many years, a “bottom-up” theory of fullerene formation from C2 units was dominant. However, in recent years evidence is emerging suggesting a “top-down mechanism” - that is, fullerenes are formed via shrinkage of giant fullerenes generated from graphene. Previously, we reported molecular structural evidence implying a top-down mechanism for the asymmetric non-IPR metal carbide metallofullerene, M2C2@C1 (51383)-C84 (M=Y,Gd) as a missing link in the mechanism for the formation of smaller, more symmetric metallofullerene cages. In this paper, we extend the study mechanism to include larger fullerenes, namely the C3v (134)C94 and D3 (85)C92 cages.

Alkali Metals-Substituted Adamantanes Lead to Visible Light Absorption: Large First Hyperpolarizability

Adamantane (Ad) is the parent molecule of diamondoids, which has been the subject of much chemical interest because of its highly symmetric cage structure. Many valuable papers demonstrated that the H atom of the methine position in Ad is highly reactive. Compared with highly symmetric carbon structures such as graphene and single-walled carbon nanotubes (SWCNTs), Ad only possesses σ bonds. In this work, we report a quantum chemical calculation on three complexes (Ad-Li, Ad-Na, and Ad-K), which were obtained by substituting the H atom of methine position in Ad with alkali metal (Li, Na, and K). Interestingly, the substitution by alkali metals leads to absorption within the visible region. The maximum absorption wavelengths of the complexes show a red shift trend from the Li to the K complex. This trend indicates that the crucial transition energy becomes smaller, which might lead to a larger nonlinear optical response. Among the three structures, the largest first static hyperpolarizability (βtot) of the ...

Mechanical Ball-Milling Preparation of Fullerene/Cobalt Core/Shell Nanocomposites with High Electrochemical Hydrogen Storage Ability

The design and synthesis of new hydrogen storage nanomaterials with high capacity at low cost is extremely desirable but remains challenging for today's development of hydrogen economy. Because of the special honeycomb structures and excellent physical and chemical characters, fullerenes have been extensively considered as ideal materials for hydrogen storage materials. To take the most advantage of its distinctive symmetrical carbon cage structure, we have uniformly coated C60's surface with metal cobalt in nanoscale to form a core/shell structure through a simple ball-milling process in this work. The X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectra, high-solution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectrometry (EDX) elemental mappings, and X-ray photoelectron spectroscopy (XPS) measurements have been conducted to evaluate the size and the composition of the composites. In addition, the blue shift of C60 pentagonal pinch mode demonstrates the formation of Co-C chemical bond, and which enhances the stability of the as-obtained nanocomposites. And their electrochemical experimental results demonstrate that the as-obtained C60/Co composites have excellent electrochemical hydrogen storage cycle reversibility and considerably high hydrogen storage capacities of 907 mAh/g (3.32 wt % hydrogen) under room temperature and ambient pressure, which is very close to the theoretical hydrogen storage capacities of individual metal Co (3.33 wt % hydrogen). Furthermore, their hydrogen storage processes and the mechanism have also been investigated, in which the quasi-reversible C60/Co↔C60/Co-Hx reaction is the dominant cycle process.

Structure and properties of bimetallic titanium and vanadium oxide clusters

By employing a genetic algorithm together with density functional theory (B3LYP), we investigate the most stable minimum structures of several bimetallic titanium and vanadium oxide clusters that contain four metal atoms. The following compositions are studied: VnTin-4O10(-) (n = 1-4), (TiO2)VOn(-) (n = 1-4), and (TiO2)VOn(+) (n = 1-3). Apart from (TiO2)3VO(-), vanadium oxo groups are always part of the most stable minimum structures when vanadium is present. Anti-ferromagnetic coupling lowers the energy substantially if spin centers are located at neighbored metal atoms rather than at distant oxygen radical sites. Vanadium-rich or oxygen-poor compositions prefer symmetric adamantane-like cage structures, some of which have already been proposed in a previous study. In contrast, vanadium-poor and oxygen-rich compositions show versatile structural motifs that cannot be intuitively derived from the symmetric cage motif. Particularly, for Ti4O10(-) there are several non-symmetric and distorted cages that have an up to 68 kJ mol(-1) lower energy than the symmetric adamantane-like cage structure. Nevertheless, for the adamantane-like cage the simulated infra-red spectrum (within the harmonic approximation) agrees best with the experimental vibrational spectrum. The oxidative power of the (TiO2)3VO3(-) and (TiO2)3VO2(+) clusters as measured by the energy of removing 1/2 O2 (297 and 227 kJ mol(-1), respectively) is less than that of the pure vanadium oxide clusters (V2O5)VO3(-) and (V2O5)VO2(+) (283 and 165 kJ mol(-1), respectively).

A missing link in the transformation from asymmetric to symmetric metallofullerene cages implies a top-down fullerene formation mechanism

Although fullerenes were discovered nearly three decades ago, the mechanism of their formation remains a mystery. Many versions of the classic 'bottom-up' formation mechanism have been advanced, starting with C2 units that build up to form chains and rings of carbon atoms and ultimately form those well-known isolated fullerenes (for example, I(h)-C60). In recent years, evidence from laboratory and interstellar observations has emerged to suggest a 'top-down' mechanism, whereby small isolated fullerenes are formed via shrinkage of giant fullerenes generated from graphene sheets. Here, we present molecular structural evidence for this top-down mechanism based on metal carbide metallofullerenes M2C2@C1(51383)-C84 (M = Y, Gd). We propose that the unique asymmetric C1(51383)-C84 cage with destabilizing fused pentagons is a preserved 'missing link' in the top-down mechanism, and in well-established rearrangement steps can form many well-known, high-symmetry fullerene structures that account for the majority of solvent-extractable metallofullerenes.

Reactions of C702– with Organic Halides Revisited: Unusual Magnetic Equivalence for the Diastereotopic Methylene Protons in 2,5-(PhCH2)2C70

The reactions of C70(2-) with organic halides result in several isomeric products. However, the structures of the isomers have not been identified unambiguously and the reactions still remain elusive even though the first report of the reaction appeared more than ten years ago. Herein, the reactions of C70(2-) with ArCH2Br (Ar = Ph, o-, m-, and p-BrC6H4) are revisited. Two types of isomeric para-adducts of 2,5- and 7,23-(ArCH2)2C70 are obtained and characterized with the X-ray single-crystal diffractions, HRMS, (1)H and (13)C NMR, and UV-vis spectroscopies. An unusual singlet resonance instead of the typical AB quartet resonance is shown for the C2 diastereotopic methylene protons in (1)H NMR spectrum of 2,5-(PhCH2)2C70 recorded at ambient temperature. The unexpected magnetic equivalence is studied with the variable-temperature (VT) NMR and density functional theory (DFT) calculations. The results show that the greater local curvature in the C70 polar region is likely responsible for the unusual magnetic equivalence exhibited by the C2 diastereotopic methylene protons of 2,5-(PhCH2)2C70, indicating that under certain cases, it is improbable to elucidate the spectral data of fullerene derivatives with a less symmetrical carbon cage in analogy with those of the C60 derivatives.

Supramolecular Assemblies with Symmetrical Octahedral Structures – Synthesis, Characterization, and Electrochemical Properties

AbstractFunctional nanosized cage complexes with an octahedral framework have been synthesized by the facile self‐assembly reactions of six 90° PdII‐containing compounds {[Pd(dppf)(OTf)2] or [Pd(PPh3)(OTf)2], dppf = 1,1′‐bis(diphenylphosphanyl)ferrocene, OTf = trifluoromethylsulfonate} and four 120° functional tritopic ligands [2,4,6‐tri(4‐pyridyl)‐1,3,5‐triazine] in acetone. Diffusion‐ordered NMR spectroscopy (DOSY) and AFM images clarify the formation of 3–4 nm nanoparticles. These highly symmetrical cage complexes are discrete, face‐directed, functional nanoparticles with symmetrical octahedral structures. The self‐assembled functional cage complexes exhibit interesting electrochemical activities, and thus will potentially be useful in the catalysis of redox reactions, the encapsulation of sizeable guest molecules and the control of organic synthetic reactions.

Temporary and Permanent Trapping of the Metastable Twisted Conformer of an Overcrowded Chromic Alkene via Encapsulation

An overcrowded alkene with an anti-folded conformation was converted to its twisted conformer, accompanied by a dramatic color change from yellow to deep purple, by inclusion in a self-assembled T(d)-symmetric coordination cage. The shape of the caged cavity was suitable and desirable for trapping of the twisted conformer. The twisted conformation was temporarily memorized in the alkene even after guest ejection. Permanent trapping of the twisted conformation was achieved by bromination of the twisted conformer formed in situ in the cage.

Lowest-energy structures of (WO3)n (2 ≤ n ≤ 12) clusters from first-principles global search

Using genetic algorithm combined with density functional theory calculations, we performed global search for the most stable structures of (WO3)n clusters for n=2–12. Small (WO3)n clusters with n=3 or 4 adopt ring-like configurations with W–O alternating arrangement. Starting from (WO3)8, the tungsten oxide clusters transform to symmetric spherical-like cages. The relative stability, HOMO–LUMO gap, electronic states of these (WO3)n clusters were discussed. Analysis of wavefunctions of frontier orbitals and electron density of states shows that the valence bands are dominated by the 2p electrons from oxygen and the conduction bands are mainly contributed by the 5d states from tungsten.

Measurement and Modeling of Low-Frequency Current From Hybrid AC/DC Corona

This paper aims at investigating the mechanism of hybrid ac/dc corona which is usually accompanied with the hybrid HVAC/HVDC power transmission lines. A coaxial wire-cylinder corona cage is designed in this paper, and the combined ac/dc voltage is applied to the wire and cage. The low-frequency component of corona current from the hybrid ac/dc corona in this corona cage with five types of wires is measured. To obtain the detailed characteristic of the interaction between the space charge and ionized electric field, a 1-D method is developed to simulate the hybrid ac/dc corona in the symmetrical corona cage. This method is validated through the comparison with measurement results. Based on this simulation method, analysis of the evolution of space charge and ionized electric field in one time cycle is presented. The regular pattern of charge injection from the discharge electrode is summarized, where the charge injection is found not linearly dependent on the applied voltage. Finally, the corona loss of hybrid ac/dc corona is calculated, and the similar result with the quadratic law in ac corona is found from the calculation results. This research provides useful knowledge for the engineering computation of the effects of hybrid ac/dc corona.

Evolution of atomic and electronic structure of magnetic Gd-doped gold clusters

The evolution of atomic and electronic structure of small Aun (n = 1–16, and 55) clusters doped with a Gd atom has been investigated using density functional theory within generalized gradient approximation for the exchange–correlation energy. Pure gold neutral clusters with n up to 15 are planar. However, with the doping of a Gd atom, the atomic structure of gold clusters changes, and there is a transition from planar-like structures to three dimensional structures at n = 10. The electronic structure of Gd-doped gold clusters shows a sharp increase in the highest occupied–lowest unoccupied molecular orbital (HOMO–LUMO) gap for certain sizes giving rise to their magic behavior. All clusters are magnetic with large magnetic moments ranging from 6 to 8 μB primarily due to the localized 4f electrons on Gd. This makes such clusters with large HOMO–LUMO gaps magnetic superatoms. The main interaction between gold and gadolinium atoms in the clusters is due to hybridization between Au-6s and Gd-5d6s orbitals. Our results indicate the emergence of a wheel structure for Gd@Au7, a symmetric cage structure at n = 15 for Gd@Au15 and n = 16 for Gd@Au16+ and Eu@Au16 corresponding to an electronic shell closing at 18 valence electrons leaving aside the f electrons on Gd while for Gd-doped Au55 corresponding to 58 valence electrons, a Au9Gd@Au46 core–shell structure is obtained in which the Gd atom connects the core of Au9 with the Au46 shell. The binding energy shows odd–even oscillations with enhancement due to Gd doping compared with pure gold clusters. Such magnetic clusters of gold could have multifunctional biological applications in drug delivery, sensor, imaging, and cancer treatment.

ChemInform Abstract: Photochemistry of Tetrasulfur Tetranitride: Laser Flash Photolysis and Quantum Chemical Study.

AbstractThe photochemistry of S4N4 with a highly symmetric cage structure is studied by laser flash photolysis and B3LYP quantum chemical computations.

Computing optimal electronic and mathematical properties of Buckyball nanoparticle using graph algorithms

Purpose – One of the significant underlying principles of nanorobotic systems deals with the understanding and conceptualization of their respective complex nanocomponents. This paper introduces a new methodology to compute a set of optimal electronic and mathematical properties of Buckyball nanoparticle using graph algorithms based on dynamic programming and greedy algorithm.Design/methodology/approach – Buckyball, C60, is composed of sixty equivalent carbon atoms arranged as a highly symmetric hollow spherical cage in the form of a soccer ball. At first, Wiener, hyper‐Wiener, Harary and reciprocal Wiener indices were computed using dynamic programming and presented them as: W(Buckyball)=11870.4, WW(Buckyball)=52570.9, Ha(Buckyball)=102.2 and RW(Buckyball)=346.9. The polynomials of Buckyball, Hosoya and hyper‐Hosoya, which are in relationship with Buckyball's indices, have also been computed. The relationships between Buckyball's indices and polynomials were then computed and demonstrated a good agreemen...

Detection of Rotor Fault in an Induction Motor Under Standstill Condition – A Power Quality Perspective

The purpose of this paper is to present a study on the power quality issue based on the harmonics present in the air gap flux due to broken bar fault in induction motors. The numerical magnetic field analysis of squirrel cage induction motor based on finite element method is performed to carry out this study. Finite element analysis can be used for predicting the performance of an induction motor having different faults implemented in their structure. Magnetic field is calculated for the case of symmetrical rotor cage and for several cases of rotor electrical asymmetry due to broken rotor bars. The induction motor torque in healthy and faulty condition is also obtained and compared. It shows the relation between the increasing number of broken bars and unproportional distribution of induced rotor currents, which results as a degradation of steady-state and dynamic performance of induction motor. For this study, a squirrel cage induction motor with thirty six stator slots and twenty eight rotor bars has been investigated. This study provides quality assessment of stator and rotor independent of load operating conditions. In this paper, a two-dimensional FEM is performed for modeling and analysis of an Induction motor with broken bar fault using ANSYS software.

How are small endohedral silicon clusters stabilized?

Clusters in the (Be, B, C)@Si(n)((0,1,2+)) (n = 6-10) series, isoelectronic to Si(n)(2-), present multiple symmetric structures, including rings, cages and open structures, which the doping atom stabilizes using contrasting bonding mechanisms. The most striking feature of these clusters is the absence of electron transfer (for Be) or even the inversion (for B and C) in comparison to classic endohedral metallofullerenes (e.g. from the outer frameworks towards the enclosed atom). The relatively small cavity of the highly symmetric Si(8) cubic cage benefits more strongly from the encapsulation of a boron atom than from the insertion of a too large beryllium atom. Overall, the maximization of multicenter-type bonding, as visualized by the Localized Orbital Locator (LOL), is the key to the stabilization of the small Si(n) cages. Boron offers the best balance between size, electronegativity and delocalized bonding pattern when compared to beryllium and carbon.

Selective formation of a polar incomplete coordination cage induced by remote ligand substituents

Instead of highly symmetrical T-symmetry cages common in self-assembly, the p-NMe(2)-substituted triphosphine CH(3)C{CH(2)P(4-C(6)H(4)NMe(2))(3) gives open, polar C(3) symmetry cages [Ag(6)(triphos)(4)X(3)](3+) which lack one of the expected face-capping anions; despite its subtlety this difference occurs selectively in solution and two examples have been crystallographically characterised.

Syntheses and Structures of Digold Complexes of Macrobicyclic Dibridgehead Diphosphines That Can Turn Themselves Inside Out

When isomers of the dibridgehead diphosphine P((CH2)14)3P (2) are treated with Me2SAuCl, 2·(AuCl)2 is obtained in high yields. With in,in-/out,out-2 (97:3 equilibrium mixture), only a single isomer can be detected by low-temperature NMR, which is assigned as out,out-2·(AuCl)2 on the basis of the crystal structure. With in,out-2, in,out-2·(AuCl)2 is obtained, as confirmed by a crystal structure. Both lattices show hydrogen bonding between the AuCl moieties of one molecule and the PCH2 hydrogen atoms of a neighboring molecule. This requires access to the backside of the P–Au–Cl linkages, distorting the diphosphine cages. When out,out-2·(AuCl)2 is treated with CH3Li, out,out-2·(AuCH3)2 can be isolated (91%). Analogous hydrogen bonding is impossible, and this molecule crystallizes with a much more symmetrical cage, the center of which is occupied by a methylcyclopentane solvate.