Pentagonal Carbon Rings Research Articles

Theoretical and Quantitative Structural Relationships Studies of Free Energies of Electron Transfer, Electron Transfer Kinetic, and Electrochemical Properties of Metal Nitride Cluster Fullerenes Y3N@C80Methano Mono Adduct Derivatives in [X-UT-V][Y3N@C80-[6,6]-Methanofullerene-R] (R = DEM, ex-TTF, and OCH2-AQ) Supramolecular Complexes

ABSTRACT Since the discovery of fullerenes as a class of nanostructure compounds, many potential applications have been suggested for their unusual structures and properties. The isolated pentagon rule (IPR) states that all pentagonal carbon rings are isolated in the most stable fullerene. Fullerenes Cn , are a class of spherical carbon allotrope group with unique properties. Electron transfer between fullerenes and other molecules is thought to involve the transfer of electrons between molecules surrounding the fullerene cage. One class of interesting molecules is the mono adduct methanofullerene derivatives (Y3N@C80-[6,6]-Methanofullerene-R). Electron transfer between C80 derivatives such as nitride Yttrium cluster Y3N@C80-[6,6]-Methanofullerene-R (R = DEM, exTTF, and OCH2-AQ) 10–12 and other molecules is thought to involve the transfer of electrons between molecules surrounding the fullerene cage. One class of electron-transfer molecules is the [X-UT-V][Y3N@C80-[6,6]-Methanofullerene-R] (R = DEM, exTTF...

Theoretical and Quantitative Structural Relationships of the Electron Transfer and Electrochemical Properties of Cis-Unsaturated Thiocrown Ethers and Supramolecular Complexes [X-UT-Y]@[La2@C72(Adamantylidene Mono-Adducts)n] (n=0,1)

Endohedral fullerenes belong to a new class of compounds that are technologically and scientifically important owing to their unique structures and optoelectronic properties. The isolated pentagon rule (IPR) states that all pentagonal carbon rings are isolated in the most stable fullerene. Metallofullerenes M@Cn, are a class of spherical carbon allotrope group with unique properties. Electron transfer between metallofullerenes and other molecules is thought to involve the transfer of electrons between molecules surrounding the fullerene cage. One class of electron-transfer molecules is the [La2@C72(Adamantylidene Mono-Adducts)n] (n = 0,1) 10–13. The supramolecular complexes 1–9 and 10–13 are shown to possess a previously unreported host-guest interaction for electron transfer processes. The unsaturated, cis-geometry, thiocrown ethers, (1–9), (described as [X-UT-Y], where X and Y indicate the numbers of carbon and sulfur atoms, respectively), are a group of crown ethers that display interesting physiochemical properties in light of their conformational restriction compared to a corresponding saturated system, as well as the sizes of their cavities. Topological indices have been successfully used to construct mathematical methods that relate structural data to various chemical and physical properties. To establish a good relationship between the structures of 1–9 with derivatives of 10–13, an index is utilized, μ cs . This index is the ratio of the sum of the number of carbon atoms (nc ) and the number of sulfur atoms (ns ) with the product of these two numbers for 1–9. In this study, the relationships between this index and oxidation potential ( ox E 1 ) of 1–9, as well as the first and second free energies of electron transfer (ΔGet(n) , for n = 1, 2, which is given by the Rehm-Weller equation) between 1–9 and [La2@C72(Adamantylidene Mono-Adducts)n] (n = 0,1; n = 0 for “a”, n = 1 for “b-g”) 10–13 and their derivatives as [X-UT-Y]@[La2@C72(Adamantylidene Mono-Adducts)n] (n = 0,1; n = 0 for “a”, n = 1 for “b-g”) (groups 14–17 and their derivatives) supramolecular complexes are presented and investigated.

Carbon nanospheres: synthesis, physicochemical properties and applications

The discovery of carbon nanostructures, essentially carbon nanotubes (CNT) and carbon nanofibres (CNF) has led to a big effort devoted to their synthesis, characterization, surface modification and use. Indeed, these structures have encountered application in a wide range of technological fields, such as adsorption, catalysis, hydrogen storage or electronics. Apart from the filamentous arrange of graphene sheets conducting to CNT or CNF, carbon can bond in other different ways to create structures with dissimilar properties. The pairing of pentagonal and heptagonal carbon rings can result in the formation of carbon nanospheres (CNS). This novel nanostructure has only now started to attract significant research activity. In its spherical arrangement, the graphite sheets are not closed shells but rather waving flakes that follow the curvature of the sphere, creating many open edges at the surface. Contrary to the chemically inert C60, the unclosed graphitic flakes provide reactive “dangling bonds” that are proposed to enhance surface reactions, establishing CNS as good candidates for catalytic and adsorption applications. Despite the embryonic stage of the field and the existing data being too scattered, this work is aimed to provide a comprehensive review of the existing literature related to CNS, exploring the different preparation routes employed, the critical characterization results as well as the applications studied so far.

Graphene nanoengineering and the inverse Stone-Thrower-Wales defect

We analyze a new fundamental building block for monolithic nanoengineering on graphene: the Inverse-Stone-Thrower-Wales (ISTW) defect. The ISTW is formed from a pair of joined pentagonal carbon rings placed between a pair of heptagonal rings; the well-known Stone-Thrower-Wales (STW) defect is the same arrangement, but with the heptagonal rather than pentagonal rings joined. When removed and passivated with hydrogen, the structure constitutes a new molecule, diazulene, which may be viewed as the result of an ad-dimer defect on anthracene. Embedding diazulene in the honeycomb lattice, we study the effect of ad-dimers on planar graphene. Because the ISTW defect has yet to be experimentally identified, we examine several synthesis routes and find one for which the barrier is only slightly higher than that associated with adatom hopping on graphene. ISTW and STW defects may be viewed as fundamental building blocks for monolithic structures on graphene. We show how to construct extended defect domains on the surface of graphene in the form of blisters, bubbles, and ridges on a length scale as small as 2 angstroms by 7 angstroms. Our primary tool in these studies is density functional theory.

Substrate-free synthesis of large area, continuous multi-layer graphene film

Large area (∼cm2) multi-layer graphene film is synthesized in the substrate-free vapor phase. The scanning electron microscopy observation reveals that the graphene nanoflakes are either six- or five-sided, and the internal angles between adjacent sides are nearly 120° and 105°, respectively. The nucleation of hexagonal and pentagonal carbon rings leads to carbonaceous nuclei, which grow into six- or five-sided nanoflakes, respectively. A well-ordered and large-area graphene monolayer is formed by the interconnection of these flakes, and stacking of these monolayers results in the formation of a multi-layer graphene film. The creation of hexagonal and pentagonal carbon rings is considered to be important for the easy production of large area multi-layer graphene at large scale.

Spin Polarized Conductance in Hybrid Graphene Nanoribbons Using 5−7 Defects

We present a class of intramolecular graphene heterojunctions and use first-principles density functional calculations to describe their electronic, magnetic, and transport properties. The hybrid graphene and hybrid graphene nanoribbons have both armchair and zigzag features that are separated by an interface made up of pentagonal and heptagonal carbon rings. Contrary to conventional graphene sheets, the computed electronic density of states indicates that all hybrid graphene and nanoribbon systems are metallic. Hybrid nanoribbons are found to exhibit a remarkable width-dependent magnetic behavior and behave as spin polarized conductors.

Magnetic properties of carbon nanostructures

We investigate the magnetic response and the ring currents induced by the presence of an external magnetic field in different carbon nanostructures using a π-electron tight binding model in conjunction with the London approximation. We consider fullerenes, corrugated and non-corrugated carbon nanotori, and finite graphene sheets. For corrugated carbon nanotori, we have constructed the structures in two different ways: by joining the ends of carbon nanotubes containing pentagonal, hexagonal and heptagonal carbon rings (rectangular and hexagonal Haeckelites); and by coalescing C60 molecules along the three different axes of symmetry (C2, C3 and C5). The non-corrugated carbon nanotori have been constructed by joining the ends of standard carbon nanotubes containing hexagonal rings only. For the graphene ribbons, we considered those exhibiting armchair or zig-zag edges. Our results reveal strong paramagnetic signals in structures containing non-hexagonal carbon rings. For carbon nanotori, the presence of strong paramagnetism causes the electrons to flow around the torus. We also observed that the ring currents of graphene ribbons are small and localised at the edges. Finally, the magnetic moment of nanotori constructed from Haeckelite structures is also described as a function of doping (adding additional electrons or holes).

Synthesis and electronic properties of coalesced graphitic nanocones

Thermal annealing of graphitic conical nanofibers, produced by pyrolysing Pd precursors under an Ar atmosphere at 850–1000 °C, results in the coalescence of adjacent cones. These novel cone-like arrangements exhibit heptagonal and pentagonal carbon rings located in the cauterized rims between adjacent cones. We propose molecular models that resemble such structures and calculate their electronic properties using a self-consistent local approach based on a realistic tight-binding Hamiltonian. The calculations demonstrate that coalesced cones exhibit a conducting behaviour and a local analysis of the density of states (DOS) reveals that curvature is crucial in modifying the electronic structure.

The dynamic observation of the field emission site of electrons on a carbon nanotube tip

A carbon nanotube (CNT) tip in an electric field was dynamically observed at an atomic scale. The field emission site of electrons was investigated by using a field ion microscope (FIM) and a field emission microscope (FEM). In-situ transmission electron microscope observations demonstrated that the CNT tip was deformed by bending the outer layers during field emission. The deformation process resulted in the formation of a nanoscale protrusion as an emission site at the CNT tip. The deformation mechanism can be explained by the reconstruction of carbon atoms. The high electric field that was concentrated in the vicinity of the pentagonal carbon rings on the CNT tip and the electrical attraction caused by applying the bias voltage between electrodes may have contributed to the formation of the protrusion. The atomic structural observations by FIM and FEM indicated that electrons were emitted from the nanoscale protrusion on the carbon network, rather than from the pentagonal carbon ring. The formation of the emission site contributed to the excellent field emission characteristics of the CNT. However, the structural change can be considered as damage caused to the CNT by the high electric field.

Growth Energetics of Single-Wall Carbon Nanotubes with Carbon Monoxide

The microscopic growth energetics of single-wall carbon nanotubes (SWNTs) with gas-phase CO molecules is investigated. Our density functional calculations show that CO molecules can form carbon networks directly at an open edge of SWNT by adsorption and subsequent desorption procedure. CO molecules adsorb to the carbon atom in an open edge of the SWNT, not through the oxygen atom but through the carbon atom, because the frontier orbital of the CO molecule has more carbon character. Such adsorption results in the formation of a carbon-carbon bond. Formation of a hexagonal carbon ring is thermodynamically more favorable than that of a pentagonal carbon ring and becomes a driving force for the growth of SWNTs. A possible growth mechanism through formation of hexagonal ring structure is suggested from the results of this study. Our results of energy calculations suggest that growth in the direction of a zigzag wall is more favorable. The roles of a catalyst, which nucleates the growth of SWNT and stabilizes an open edge with dangling bonds, are further discussed.

A major milestone in nanoscale material science: the 2002 Benjamin Franklin Medal in Physics presented to Sumio Iijima

The Franklin Institute, Philadelphia, Pennsylvania, awarded the 2002 Benjamin Franklin Medal in physics to Sumio Iijima for his discovery and elucidation of the atomic structure and helical character of multi- and single-wall carbon nanotubes. His pioneering work created a new, tremendously active and expanding area in the field of nanoscience and technology that involves condensed matter and material scientists, chemists and computer scientists. Iijima has also made key contributions to the mechanisms that are involved in the growth of carbon nanotubes, to the role of pentagonal and heptagonal carbon rings in the formation of caps that form at the ends of the nanotubes and to the encapsulation of molecules within the nanotubes.

Direct Dynamics Studies of CO-Assisted Carbon Nanotube Growth

Semiempirical direct dynamics simulations are used to study the potential role of CO in the gas-phase catalytic growth of carbon nanotubes. Calculations employing a fully self-consistent AM1 semiempirical electronic Hamiltonian predict that CO molecules can chemically adsorb to the open edge of partially grown zigzag and armchair nanotubes. The adsorbed CO molecules can form either pentagonal or hexagonal carbon rings by an electrocyclic reaction with neighboring adsorbed CO groups. Formation of hexagonal carbon rings in zigzag nanotubes is kinetically and thermodynamically favored by a CO insertion reaction with CO-saturated ends. These results provide some computational evidence that gas-phase CO can adsorb and contribute to the intermediate and latter stages of carbon nanotube growth. This study assumes that growth proceeds by the addition of carbon to the nanotube edge held open by metal catalyst particles, as opposed to a root growth or capped end growth mechanism.

Structural change at the carbon-nanotube tip by field emission

Carbon-nanotube tips are plastically deformed during field emission. High-resolution transmission electron microscopy and structural simulations suggest that the deformed structure of the closed nanotube is explained by heterogeneous nucleation of the pentagonal and heptagonal carbon ring pairs, and that of the opened one is represented by sp3-like line defects in the hexagonal carbon network. It is considered that the changing of the inclination of the Fowler–Nordheim plots corresponds to the structural change in which a tip becomes sharp. The field ion microscope image and the corresponding field-emission pattern suggest that the electron emission from a closed nanotube is not necessarily from pentagonal carbon rings, but from the protrudent carbon network sites on the tip.

Growth of spiral carbon tubes by a mixed-valent oxide-catalytic carbonization process

Abstract The synthesis of carbon tubes is vitally important in carbon research and applications. In this paper, spiral carbon tubes are grown for the first time by a mixed-valent oxide-catalytic carbonization process. The carbon tubes are highly twisted and have many growth-induced nodes. The carbon tubes are nucleated on spherical carbon cores, created by unclosed polyhedron carbon shells containing pentagonal carbon rings. The conical growth of graphitic layers along the tube is the result of a temporary shortage of the supply of pentagons. The pairing of pentagons and heptagons is the key to nucleating the nodes. Our experiments prove that the fraction and nucleation rates of pentagons, hexagons and heptagons determine the surface geometry of the product.

Mixed-valent oxide-catalytic carbonization for synthesis of monodispersed nano sized carbon spheres

Abstract A mixed-valent oxide-catalytic carbonization process for synthesizing monodispersed carbon spheres at very low cost is reported for the first time. The carbon spheres are formed by catalytic carbonization of natural gas (primarily methane) with the assistance of mixed-valent metal oxides. The catalyst is reusable and the entire synthesis process produces no environmental waste. The product can be controlled by temperature to produce macroscopic quantities and a high percentage (>95%) of nano sized carbon spheres; thus, measurements of their physical properties can be performed easily. The carbon spheres are solid and comprise layered graphitic flakes. The sphere is nucleated from a pentagonal carbon ring, followed by a spiral shell growth. When the sphere grows larger, graphitic flakes of atomic thickness are nucleated on the surface owing to the nucleation of the paired pentagonal-heptagonal (P-H) carbon rings. The combination of the P-H carbon rings with the hexagonal networks produces eight ba...

Pairing of Pentagonal and Heptagonal Carbon Rings in the Growth of Nanosize Carbon Spheres Synthesized by a Mixed-Valent Oxide-Catalytic Carbonization Process

Carbon spheres have been synthesized using a mixed-valent oxide-catalytic carbonization process in macroscopic quantities and at low cost. The technique uses natural gas and reusable catalysts and produces no environmental waste or pollution. The product is all solid spheres (∼210 nm) comprised of layered graphitic flakes. Microstructures of the spheres are studied by transmission electron microscopy. The sphere is believed to be nucleated from a pentagon carbon ring followed by a spiral shell growth. When the sphere grows larger, graphitic flakes are nucleated on the surface due to the formation of paired pentagonal−heptagonal (P−H) carbon rings. A combination of the P−H pairs with the hexagonal networks produces eight basic graphitic configurations for forming the sphere, and they have been observed experimentally.

Prostaglandins and thrombosis formation

As a group, postaglandins (PG) are composed of a pentagonal carbonring with oxygensubstitutes and two long parallel carbonchains. According to the precise composition of the pentane ring, they are called by the letters A until I (for instance: prostaglandin F), the subscript numeral points to the number of double bonds in the side chain (for instance: F1) and a Grecian letter shows there exist isomers (for instance: Prostaglandin F1), All prostaglandins proceed from essential fatty acids with 20 carbon atoms.