共查询到20条相似文献,搜索用时 11 毫秒
1.
László I 《Journal of chemical information and computer sciences》2004,44(2):315-322
Generalizing the folding method to any periodic two-dimensional planar carbon structures we have calculated the corresponding electronic structures in the framework of the one orbital one site tight-binding (Bloch-Hückel) method by solving the eigenvalue problems in a numerical way. We discussed the metallic or the nonmetallic behavior of the nanotubes by applying the folding vectors of parameters (m, n). We extended the topological coordinate method to two-dimensional periodic planar structures as well. Nearly regular hexagonal, pentagonal, and heptagonal polygons were obtained. The curvatures of the final relaxed structures can be read from the sizes of the polygons. Thus relying only on the topological information we could describe the shape of the tubular structures and their conductivity behaviors. 相似文献
2.
We propose a series of carbon nanostructures in the shape of tetrapod as a kind of three-dimensional junction for carbon nanotubes.
The tetrapod junctions are such open networks that are made of sp2 carbon atoms only, have negative Gaussian curvature, and connect four nanotubes together. We define the structure of standard
tetrapod junctions, the simplest one, that have 12 heptagons other than hexagons and have the Td symmetry.Our tight-binding energy-band calculations for the standard tetrapod junctions of smaller sizes show that their
electronic property mainly depends on one particular topological factor: the junctions having a carbon atom in the center
of each triangular face of tetrahedron exhibit metallic band structure while the junctions having a benzene ring in the center
of the faces are semiconductors. We also find that tetrapod junctions connecting (6,0) nanotubes exhibit a flat band near
the Fermi energy in a particular momentum region. The origin of the flat band states can be figured out from the wavefunction
distribution. We also show the possibility to extend the standard tetrapod junctions to some non-standard ones that can connect
nanotubes of different kinds and/or radii. 相似文献
3.
Fister TT Vila FD Seidler GT Svec L Linehan JC Cross JO 《Journal of the American Chemical Society》2008,130(3):925-932
We report nonresonant inelastic X-ray scattering (NRIXS) measurement of core-shell excitations from both B 1s and C 1s initial states in all three isomers of the dicarba-closo-dodecarboranes C2B10H12. First, these data yield an experimental determination of the angular-momentum-projected final local density of states (l-DOS). We find low-energy resonances with distinctive local s- or p-type character, providing a more complete experimental characterization of bond hybridization than is available from dipole-transition limited techniques, such as X-ray absorption spectroscopies. This analysis is supported by independent density functional theory and real-space full multiple scattering calculation of the l-DOS which yield a clear distinction between tangential and radial contributions. Second, we investigate the isomer sensitivity of the NRIXS signal and compare and contrast these results with prior electron energy loss spectroscopy measurements. This work establishes NRIXS as a valuable tool for borane chemistry, not only for the unique spectroscopic capabilities of the technique but also through its compatibility with future studies in solution or in high-pressure environments. In addition, this work also establishes the real-space full multiple scattering approach as a useful alternative to traditional approaches for excited states calculations of aromatic polyhedral boranes and related systems. 相似文献
4.
Häkkinen H 《Chemical Society reviews》2008,37(9):1847-1859
Atomic structure and electronic structure are intimately interrelated properties of nanoclusters and nanoparticles, defining their stability, electronic, optical and chemical properties, in other words, their usability as potential components for nanoscale devices. This tutorial review attempts to describe the development in understanding the structures of bare and ligand-protected gold clusters over the past decade, based on selected density-functional-theory calculations. This review should be of interest both to newcomers in the field and to an interdisciplinary community of researchers working in synthesis, characterization and utilization of ligand-protected gold clusters. 相似文献
5.
Engtrakul C Irurzun VM Gjersing EL Holt JM Larsen BA Resasco DE Blackburn JL 《Journal of the American Chemical Society》2012,134(10):4850-4856
The atomic specificity afforded by nuclear magnetic resonance (NMR) spectroscopy could enable detailed mechanistic information about single-walled carbon nanotube (SWCNT) functionalization as well as the noncovalent molecular interactions that dictate ground-state charge transfer and separation by electronic structure and diameter. However, to date, the polydispersity present in as-synthesized SWCNT populations has obscured the dependence of the SWCNT (13)C chemical shift on intrinsic parameters such as diameter and electronic structure, meaning that no information is gleaned for specific SWCNTs with unique chiral indices. In this article, we utilize a combination of (13)C labeling and density gradient ultracentrifugation (DGU) to produce an array of (13)C-labeled SWCNT populations with varying diameter, electronic structure, and chiral angle. We find that the SWCNT isotropic (13)C chemical shift decreases systematically with increasing diameter for semiconducting SWCNTs, in agreement with recent theoretical predictions that have heretofore gone unaddressed. Furthermore, we find that the (13)C chemical shifts for small diameter metallic and semiconducting SWCNTs differ significantly, and that the full-width of the isotropic peak for metallic SWCNTs is much larger than that of semiconducting nanotubes, irrespective of diameter. 相似文献
6.
纳米碳管电子结构和键合特性的第一原理研究 总被引:1,自引:0,他引:1
利用第一原理方法对一系列尺寸变化的单层纳米碳管电子结构进行了研究,得到了总态密度和态密度随碳管半径R的变化情况与实验结果完全一致,Fermi能级处态密度值随着管径R的增大而减小,说明纳米管的化学活性随着管径的增大而增强。碳管中C-C之间的键合为2s和2p价电子混合而成的弯曲的σ,π键,随着管径R的增大,化学键的弯曲度逐渐减小,C-C之间的键合作用和结合能逐渐增强,电荷密度和对应的势场也逐渐减弱。这些结果表明管径较小的纳米碳管在复合材料的合成中具有一定的优势。 相似文献
7.
8.
Haertelt M Fielicke A Meijer G Kwapien K Sierka M Sauer J 《Physical chemistry chemical physics : PCCP》2012,14(8):2849-2856
Structural information for neutral magnesium oxide clusters has been obtained by a comparison of their experimental vibrational spectra with predictions from theory. (MgO)(n) clusters with n = 3-16 have been studied in the gas phase with a tunable IR-UV two-color ionization scheme and size-selective infrared spectra have been measured. These IR spectra are compared to the calculated spectra of the global minimum structures predicted by a hybrid ab initio genetic algorithm. The comparison shows clear evidence that clusters of the composition (MgO)(3k) (k = 1-5) form hexagonal tubes, which confirm previous theoretical predictions. For the intermediate sizes (n≠ 3k) cage-like structures containing hexagonal (MgO)(3) rings are identified. Except for the cubic (MgO)(4) no evidence for bulk like structures is found. 相似文献
9.
10.
Pollack L Windus TL de Jong WA Dixon DA 《The journal of physical chemistry. A》2005,109(31):6934-6938
The heats of formation for the n-alkanes C(n)H(n+2) for n = 5, 6, and 8 have been calculated using ab initio molecular orbital theory. Coupled-cluster calculations with perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation-consistent basis sets were used, up through the augmented quadruple zeta, to extrapolate to the complete basis set limit. Geometries were optimized at the B3LYP/TZVP and MP2/aug-cc-pVTZ levels. The MP2 geometries were used in the CCSD(T) calculations. Frequencies were determined at the density functional level (B3LYP/TZVP), and scaled zero point energies were calculated from the B3LYP frequencies. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. The core/valence corrections are not small, (approximately 1.1 kcal/mol per carbon unit) and cannot be neglected for chemical accuracy. The calculated deltaH(298)f values are -35.0, -40.2, and -50.2 kcal/mol for C5H12, C6H14, and C8H18, respectively, in excellent agreement with the respective experimental values of -35.11 +/- 0.19, -39.89 +/- 0.19, and -49.90 +/- 0.31 kcal/mol. Isodesmic reaction energies are presented for some simple reactions involving C8H18 and are shown not to be strongly method dependent. 相似文献
11.
The title compounds have been prepared, and their structures have been studied using molecular mechanics calculations, together with dipole moment measurements, and thermodynamic and spectroscopic data reported in the literature. It is concluded that the parent hydrocarbon exists to a very large extent in a “square” diamondoid conformation (D2d symmetry). Other conformations (rectangular, nearly D2, and puckered, (D2d) are considerably higher in energy. The ketone consists of a mixture of both square and rectangular conformations with the CO group in several different positions. The monoethylene ketal exists primarily in a conformation which is square, with the ketal group on a corner. The diketone, diketal, and monoketal of the dione are considered in the light of the above, and conclusions are drawn regarding their conformations. The dipole moments of the difunctional compounds are in agreement with the conformational conclusions reached by the calculations. 相似文献
12.
Examined in this paper is the role of the metal electrode influencing the structure and electronic properties of semiconducting carbon nanotubes near the interface at low bias. Specifically, we present quantum-chemical calculations of finite sections of a (8,0) semiconducting single wall nanotube contacted with gold and palladium clusters. The calculations at the density functional level of theory, which included full geometry optimizations, indicate the formation of bonds between the metal atoms of the electrode and the carbon atoms of the nanotube. The local work function of the metal electrode can be expected to exhibit significant variations as a result of this bond formation. Compared to the gold-contacted nanotubes, the palladium-contacted nanotubes have a small but interesting increase in both length and diameter. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the gold-contacted nanotube are shown localized at the edges. In contrast, the HOMO and LUMO of the palladium-contacted nanotube are extended over the entire nanotube and the metal cluster contacted to it, providing thereby a better conduction path in the contact region of the electrode and the nanotube. The involvement of the highly directional d orbitals in the interactions involving the palladium cluster leads to an enhanced pi electron density in the nanotube. This enhanced pi electron density is synonymous with an improved electron transmission. 相似文献
13.
Balasubramanian K Burghard M Kern K 《Physical chemistry chemical physics : PCCP》2008,10(16):2256-2262
The functionalization of carbon nanotubes through electrochemical routes is gaining importance due to the high degree of control achievable and the ability to render the tubes with a variety of chemical and biological species. In this article, we report systematic investigations on the grafting of phenyl groups through diazonium coupling onto individual metallic and semiconducting carbon nanotubes both experimentally and theoretically. The results show clearly that by optimizing the electrochemical conditions it is possible to obtain a high degree of selectivity for the coupling of phenyl radicals onto metallic nanotubes. The outlined conclusions have strong implications for the design of strategies for the controlled functionalization of individual single-wall carbon nanotubes. 相似文献
14.
E. P. D’yachkov L. O. Khoroshavin I. A. Bochkov E. M. Kol’tsova P. N. D’yachkov 《Russian Journal of Inorganic Chemistry》2014,59(7):683-688
The electronic structure of 3d-metal-intercalated metallic (5,5) and semiconducting (10,0) nanotubes has been studied by quantum-chemical methods. The total and partial densities of states of nanotubes as a function of metal concentration and nature and the carbon-shell structure have been calculated by the linear augmented-cylindrical-wave method. Metalized nanowires based on armchair (5,5) and zigzag (10,0) nanotubes with one, two, three, and four metal atoms in the cross-section have been calculated. The introduction of the metal is accompanied by a sharp increase in the density of states at the Fermi level of the nanowire, which determines the concentration of free electrons involved in charge transfer in the nanotube. The 3d electrons of the metal and the carbon shell are nearly equally involved in electron transport in intercalated wires. Both the 3d electrons of a metal and the carbon shell should be nearly equally involved in electron transport in intercalated wires. The introduction of metals not only affects the conductive state of the carbon nanotube but also changes the entire pattern of its valence band, in particular, increases the valence band width of the nanotube by 5–10 eV owing to the low-energy shift of the 2s(C) states. 相似文献
15.
R. R. Muslukhov L. M. Khalilov I. R. Ramazanov A. Z. Sharipova A. G. Ibragimov U. M. Dzhemilev 《Russian Chemical Bulletin》1997,46(12):2082-2085
The parameters of13C NMR spectra of linear and cyclic alkenylalanes synthesized from mono- and disubstituted acetylene and the simplest alkylalanes have been obtained. A strong paramagnetic effect of the aluminum atom on shielding of α- and β-carbon atoms at the double bond has been observed for the dimeric form of organoaluminum compounds (OAC) in inert solvents, unlike that for the monomeric form solvated in electron-donor solvents (Et2O, THF, and Et3N). The results were interpreted in terms of the model of the electron density redistribution on going from the dimeric structure of OAC to the monomeric one. The PM3 method describes most adequately (as compared to MNDO and AM1) the equilibrium geometry of cyclic dimers of OAC. 相似文献
16.
It is still a challenge to accurately calculate the polarizabilities of large fullerene cages and nanotubes. In this paper, a simple bond order bond polarizability relationship for carbon was found, which allowed us to apply the bond polarizability model to any pentagon isolation rule (PIR) fullerene (cage or nanotube). Following this approach, the following simple equation, alpha=1.262n, was obtained relating the static dipole polarizability (alpha) of PIR fullerenes (cages or closed nanotubes) to their number (n) of carbon atoms. Furthermore, it was shown that the polarizabilities of C60 and C70, calculated on the basis of this model, are in excellent agreement with those obtained experimentally and by density-functional theory calculations. 相似文献
17.
Arne Rosén 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,25(2):175-180
The electronic structure of C60 doped with thep-elements B-F, Al-Cl and Ga-Br located at the centre of the cage has been analyzed within the local density approximation using the von Barth-Hedin exchange correlation potential. The calculations show the existence ofn- andp-type doped Buckminsterfullerene, where a partly occupied level occurs in the band gap, similar to the donor or acceptor levels in traditionally doped semiconductors. 相似文献
18.
Hexaazaisowurtzitane or cage molecules have attracted attention concerning their synthesis because hexanitrohexaazaisowurtzitane (HNIW or CL20) is presently the most powerful energetic compound. The synthesis of hexaazaisowurtzitanes was considered to be limited solely to the condensation of certain benzylamines with glyoxal. Here, we present the synthesis and characterization of seven novel non-benzylic hexaazaisowurtzitanes, such as hexapropargylhexaazaisowurtzitane. The substituents on the six nitrogen atoms are different to those of the benzyl or substituted benzyl groups to which previous syntheses were limited. X-ray structures are given for the hexapropargyl and hexa-2-thienylmethylene derivatives. Steric strains limit the synthesis with alpha-substituted benzyl and allyl derivatives. The reaction mechanism and the role of the intermediate diimines are discussed. Some of the novel hexaazaisowurtzitanes are potential precursors of hexanitrohexaazaisowurtzitane. 相似文献
19.
The electronic band structures of the rhombohedral-based boron compounds -B12, B12P2 and B12 As2 have been investigated along all symmetry directions. The calculations show that the band gap, in all cases, is of the order of 2 eV, which correlates with the known color of -rhombohedral boron. The materials should be intrinsic semi-conductors, as has recently been shown experimentally. The states around the band gap in -B12 are dominated by the boron 2p atomic states. The bonding in the icosahedra, as illuminated by cluster calculations, is shown to be rather similar to that in the isolated B12 icosahedron. Of the intericosahedron interactions, those between B(2) and B(2) atoms are the strongest and have a bond index just above unity. In B12P2 the orbitals of the P2 moiety make a significant contribution to the valence band edge states and the conduction band edge states also incorporate considerable (55%) phosphorus 3d orbital character. In B12As2 the arsenic 4d orbitals do not have as much effect in that crystal as do the 3d orbitals in B12P2. 相似文献