Center Of Hexagon Research Articles

QSPR for Physical Properties of cata-Condensed Benzenoids Using Two Simple Dualist-Based Descriptors

Dualists of benzenoids are a special type of graphs that have as vertices the centers of hexagons and as edges the pairs of condensed rings. Catafusenes (cata-condensed benzenoids) have acyclic dualists. Codes of catafusenes with h benzenoid rings start from one end of a dualist and consist of digits 0 (indicating linear annelation, angle of 180 degrees between two adjacent edges) and 1 or 2 for kinks (120 degrees or 240 degrees angles) with the convention of choosing the lowest number formed from these h-2 digits. A shape parameter (n) for the dualist consists of the number of adjacent zeros in the code plus 1. It is shown in the present communication that many molecular properties of catafusenes depend mainly on n, whereas most bulk properties depend mainly on h. Monoparametric or biparametric correlations in terms of the simple integer parameters n and/or h are presented for molecular properties (energy of the p-band in electronic absorption spectra, ionization potentials, electron affinities, hardness and softness) and bulk properties (normal boiling point, chromatographic retention index, the logarithm of the water solubility, and lipophilicity).

Definição da dimensão do indivíduo solo e determinação do número de amostras simples necessário à sua representação

Uma população é composta por indivíduos, e a amostragem correta dos indivíduos estima adequadamente as características da população. Porém, para avaliar a fertilidade do solo, quem seria o indivíduo solo (unidades de amostra) e qual sua dimensão? Com o objetivo de definir a dimensão do indivíduo solo componente de determinada população, sob plantio direto (PD) ou sob plantio convencional antes (PCAA) ou depois da aração (PCDA), visando avaliar a fertilidade do solo e desenvolver um método de amostragem de solos, determinando o número de amostras simples necessário à formação de uma amostra composta que caracterize o indivíduo solo (unidade de amostra), foram coletadas amostras simples de solo (5,4 cm de diâmetro x 10 cm de profundidade) sobre as semidiagonais de cinco hexágonos delimitados sobre cada uma das áreas selecionadas para amostragem (PD, PCAA e PCDA). Os hexágonos de amostragem apresentavam 2 m de lado e, em cada uma das seis semidiagonais dos mesmos, foram coletadas dez amostras simples de solo (55 por hexágono) nas seguintes distâncias, a partir de uma amostra simples central, medidas até o centro do orifício de coleta: 12,5; 25,0; 37,5; 50,0; 75,0; 100,0; 112,5; 150,0 e 200,0 cm, sendo a amostra simples central comum às seis semidiagonais. Foram determinados o pH (H2O), os teores de P e K disponíveis e de Ca2+ e Mg2+, H + Al, P-rem e a matéria orgânica. As amostras simples foram agrupadas sucessivamente a partir do centro do hexágono, formando nove unidades de amostra: A (até 18,75), B (até 31,25), C (até 43,75), D (até 56,25), E (até 81,25), F (até 106,25), G (até 118,75), H (até 156,25) e I (até 206,25 cm). Foi realizada análise de regressão das médias e dos desvios-padrão das características avaliadas, considerando possíveis dimensões da unidade de amostra de solo. A partir dos resultados obtidos, pôde-se concluir que: para a caracterização do indivíduo solo (unidade de amostra), devem-se coletar 25 amostras simples, necessárias à formação de uma amostra composta representativa, num hexágono de 68,75 cm de lado e área de 1,228 m².

Structural and Magnetic Properties of Gd3N@C80

Using relativistic and on-site correlation-corrected density functional theory, we have investigated the structural and magnetic properties of recently synthesized Gd3N@C80. The most stable structure of Gd3N@C80 has the three magnetic Gd ions pointing to the centers of hexagons in C80. The magnetic ground state of this structure has the three coplanar spins (S = 7/2) offset by 120 degrees angles. At the same time, the state with the highest multiplicity, where all the spins are parallel aligned, is found only about 4.5 meV higher in energy. Therefore, at room temperature, we expect Gd3N@C80 to be paramagnetic with the spin fluctuating between different multiplicities. As a result, Gd3N@C80 may exhibit greater proton relaxivity than Gd@C60 and Gd@C82 and serve as a possible candidate for the next generation of commercially available magnetic resonance imaging contrast agents.

Binary Clusters AuPt and Au6Pt: Structure and Reactivity within Density Functional Theory

Within density functional theory with the general gradient approximation for the exchange and correlation, the bimetallic clusters AuPt and Au(6)Pt have been studied for their structure and reactivity. The bond strength of AuPt lies between those of Au(2) and Pt(2), and it is closer to that of Au(2). The Pt atom is the reactive center in both AuPt and AuPt(+) according to electronic structure analysis. AuPt(+) is more stable than AuPt. Au(6)Pt prefers electronic states with low multiplicity. The most stable conformation of Au(6)Pt is a singlet and has quasi-planar hexagonal frame with Pt lying at the hexagonal center. The doping of Pt in Au cluster enhances the chemical regioselectivity of the Au cluster. The Pt atom essentially serves as electron donor and the Au atoms bonded to the Pt atom acts as electron acceptor in Au(6)Pt. The lowest triplet of edge-capped rhombus Au(6)Pt clusters is readily accessible with very small singlet-triplet energy gap (0.32 eV). O(2) prefers to adsorb on Au and CO prefers to adsorb on Pt. O(2) and CO have stronger adsorption on AuPt than they do on Au(6)Pt. CO has a much stronger adsorption on AuPt bimetallic cluster than O(2) does. The adsorption of CO on Pt modifies the geometry of AuPt bimetallic clusters.

The role of lithium in hydrogen storage in aromatic carbon materials

Three representative aromatic carbon compounds, C 6H 6, C 10H 8 and C 13H 9, are chosen to simulate H 2 adsorptions in aromatic carbon materials. The calculations of H 2 locating on top of the hexagon center, middle of C–C bond and a C atom of these compounds using density functional and the second order Møller–Plesset perturbation theories indicate that the bindings of H 2 with these compounds are very weak. However, the binding is significantly enhanced when lithium is introduced between the H 2 and the aromatic carbon compound. In particular, the binding energy of H 2 at Li adsorbed on top of the C 13H 9 is as large as 2.5 kcal mol −1, one order of magnitude enhanced. Such a stable H 2 adsorption and the moderate adsorption energy facilitate good H 2 storage. According to the similarity of surface features and chemical reactivities among the various aromatic carbon compounds and carbon nanotubes, we conjecture that the H 2 storage in these materials would all be enhanced by introducing Li.

Fe and Mn atoms interacting with carbon nanotubes

We study the structural, electronic and magnetic properties of Fe and Mn monomers and dimers adsorbed on a semiconductor (8,0) single-wall carbon nanotube. Using spin-polarized total-energy ab initio calculations based on the density-functional theory, we find for both Fe and Mn monomers the hexagonal center as the most stable adsorption site. For Fe dimers in the most stable configuration, the atoms are on opposite C–C bond-centered positions (BC–BC), with an Fe–Fe distance of 2.23Å. For Mn dimers, on the other hand, the most stable configurations are found to be with the Mn atoms adsorbed on top of C atoms, for a high-spin state (Mn–Mn distance of 2.6Å), and a slightly lower-energy BC–BC configuration for the low-spin state (Mn–Mn distance of 2.53Å). These results show that the dimer configurations are not simply related to the isolated monomer structures. The resulting magnetic moments for the adsorbed dimers are found to be similar to their corresponding values in the free diatomic molecules.

Calculation of scanning tunnelling microscopy images for Kr/graphite system

The scanning-tunnelling-microscopy (STM) images of Kr atoms adsorbed on a monolayer graphite sheet (Kr/graphite system) are calculated using the first-principle total-energy electronic structure calculations within the density functional theory in the local density approximation. The results obtained agree well with the observations. It is found that the optimal site of the adsorbed Kr atom is at the top of the centre of the carbon hexagon and its equilibrium distance from monolayer graphite surface is about 0.335nm. It is shown that the hybridization of C 2p electronic states (π-electronic states) and Kr 4p and 5s electronic states is the main origin of the Fermi-level local density of state.

Xenon atoms adsorption on graphite sheet: first-principles calculations

The scanning-tunneling-microscopy (STM) images of xenon atoms adsorbed on a monolayer graphite sheet are calculated using the first-principles total-energy electronic structure calculations within the density functional theory (DFT) in the local density approximation (LDA). The results obtained agree well with the observations. The optimal site of adsorbed Xe atom is at the top of the center of the carbon hexagon and its equilibrium distance from monolayer graphite surface is about 3.60 Å. The hybridization of C 2 p electronic states ( π-electronic states) and Xe 5 p and 6 s electronic states is the main origin of the Fermi-level local density of state (LDOS) rendering Xe atoms visible in the STM experiment.

A molecular dynamics study of interstitial boron in diamond

The stable configuration of an interstitial boron atom in diamond has been investigated by molecular dynamics simulation. The Tersoff potential is used to model the boron-carbon binary system. The results show that the interstitial boron, with its initial location at the tetrahedral or hexagonal center, intends to substitute for the carbon atom in the lattice, and it preferably forms a 〈0 0 1〉 split interstitial configuration. The total energy of the system with a stable substitutional-B-self-interstitial (B s–C i) complex is 0.14 eV higher than that of the system with the boron 〈0 0 1〉 split interstitial. It is concluded that the stable structure for an interstitial boron atom in diamond is the 〈0 0 1〉 split interstitial configuration, while neither the tetrahedral nor the hexagonal interstitial structure is stable.

Structural and Electronic Properties of 240° Nanocones

An original nanocone with 240° disclination arises naturally from a planar hexagonal sheet when four 60° segments originating from adjacent hexagonal centers are cut out of the plane and the severed edges of the segments are rejoined. The comparison among the calculated total energies of several pure carbon nanocones with this same 240° disclination but built from different growth modes shows that the cone with four 60° segments is the most stable one. The geometric structures are optimized and the density-functional theory is employed to perform total energy calculations. Defined the designability and the stability of this carbon structure, with four pentagons sharing two atoms at the apex, we simulate a similar BN nanocone by clusters containing 58 B plus N atoms and additional 12 H atoms to saturate the dangling bonds at the edge. Depending on the choice of the sharing atoms (BN, BB, or NN), the local densities of states near the apexes are investigated and sharp resonant states are found to dominate the electronic structure in the region close to the Fermi energy.

A New Multilevel Ground Water Monitoring System Using Multichannel Tubing

AbstractA new multilevel ground water monitoring system has been developed that uses custom‐extruded flexible 1.6‐inch (4.1 cm) outside‐diameter (O.D.) multichannel HOPE tubing (referred to as Continuous Multichannel Tubing or CMT) to monitor as many as seven discrete zones within a single borehole in either unconsolidated sediments or bedrock. Prior to inserting the tubing in the borehole, ports are created that allow ground water to enter six outer pie‐shaped channels (nominal diameter = 0.5 inch [1.3 cm]) and a central hexagonal center channel (nominal diameter = 0.4 inch [1 cm]) at different depths, facilitating the measurement of depth‐discrete piezometric heads and the collection of depth‐discrete ground water samples. Sand packs and annular seals between the various monitored zones can be installed using conventional tremie methods. Alternatively, bentonite packers and prepacked sand packs have been developed that are attached to the tubing at the ground surface, facilitating precise positioning of annular seals and sand packs. Inflatable rubber packers for permanent or temporary installations in bedrock aquifers are currently undergoing site trials. Hydraulic heads are measured with conventional water‐level meters or electronic pressure transducers to generate vertical profiles of hydraulic head. Ground water samples are collected using peristaltic pumps, small‐diameter bailers, inertial lift pumps, or small‐diameter canister samplers.For monitoring hydrophobic organic compounds, the CMT tubing is susceptible to both positive and negative biases caused by sorption, desorption, and diffusion. These biases can be minimized by: (1) purging the channels prior to sampling, (2) collecting samples from separate 0.25‐inch (0.64 cm) O.D. Teflon sampling tubing inserted to the bottom of each sampling channel, or (3) collecting the samples downhole using sampling devices positioned next to the intake ports. More than 1000 CMT multilevel wells have been installed in North America and Europe to depths up to 260 feet (79 m) below ground surface. These wells have been installed in boreholes created in unconsolidated sediments and bedrock using a wide range of drilling equipment, including sonic, air rotary, diamond‐bit coring, hollow‐stem auger, and direct push. This paper presents a discussion of three field trials of the system, demonstrating its versatility and illustrating the type of depth‐discrete data that can be collected with the system.

STM observation of the electro-adsorption of lithium ions onto graphite and of the ensuing solid electrolyte interphase formation

The basal planes of highly oriented pyrolytic graphite (HOPG) have been observed by near field microscopy in the course of their polarization in LiClO 4/propylene carbonate (PC) solutions. At +2.4 V to +2.1 V vs. Li +/Li, i.e. at moderately negative potentials vs. the potential of zero charge of graphite (assumed at 3.0 V vs. Li +/Li), a near square two dimensional (2D) configuration with the symmetry of the 2D space group P2 was observed by electrochemical scanning tunneling microscopy. It was interpreted as the image of the Helmholtz electrical double layer, evidencing Li + ions in-plane solvated by two PC dipoles in a “frozen” state. This state may originate from the choice of Li + ions to settle above the centers of the graphite hexagons, hereafter favoring epitaxy. At lower potentials, formation of a non-homogeneous solid electrolyte interphase film was observed by atomic force microscopy. This film was characterized by FTIR spectroscopy as predominantly composed of lithium alkyl carbonates while Li 2CO 3 generally found appeared here in minority part.

Muon spin relaxation study of the magnetic penetration depth inMgB2

The magnetic vortex lattice (VL) of polycrystalline ${\mathrm{MgB}}_{2}$ has been investigated by transverse-field muon spin relaxation (TF-\ensuremath{\mu}SR). The evolution of the TF-\ensuremath{\mu}SR depolarization rate \ensuremath{\sigma}, which is proportional to the second moment of the field distribution of the VL, has been studied as a function of temperature and applied magnetic field. The low-temperature value \ensuremath{\sigma} exhibits a pronounced peak near ${H}_{\mathrm{ext}}=75\mathrm{mT}.$ This behavior is characteristic of strong-pinning-induced distortions of the VL which put into question the interpretation of the low-field TF-\ensuremath{\mu}SR data in terms of the magnetic penetration depth $\ensuremath{\lambda}(T).$ An approximately constant value of \ensuremath{\sigma}, such as expected for an ideal VL in the London limit, is observed at higher fields of ${H}_{\mathrm{ext}}>0.4\mathrm{T}.$ The TF-\ensuremath{\mu}SR data at ${H}_{\mathrm{ext}}=0.6\mathrm{T}$ are analyzed in terms of a two-gap model. We obtain values for the gap size of ${\ensuremath{\Delta}}_{1}=6.0(3)\mathrm{meV}$ $[2{\ensuremath{\Delta}}_{1}{/k}_{B}{T}_{c}=3.6(2)],$ ${\ensuremath{\Delta}}_{2}=2.6(2)\mathrm{meV}$ $[2{\ensuremath{\Delta}}_{2}{/k}_{B}{T}_{c}=1.6(1)],$ a comparable spectral weight of the two bands, and a zero-temperature value for the magnetic penetration depth of ${\ensuremath{\lambda}}_{\mathrm{ab}}\ensuremath{\approx}100\mathrm{nm}.$ In addition, we performed $\ensuremath{\mu}\mathrm{SR}$ measurements in zero external field. We obtain evidence that the muon site (at low temperature) is located on a ring surrounding the center of the boron hexagon. Muon diffusion sets in already at rather low temperature of $T>10\mathrm{K}.$ The nuclear magnetic moments can account for the observed relaxation rate and no evidence for electronic magnetic moments has been obtained.

The dependence of the chemical potential of WO 3 films on hydrogen insertion

Knowledge of the chemical potential of electrochromic films coloured by hydrogen is important for matching the elements of an electrochromic device, for understanding the colouring mechanism and for obtaining information about the microscopic structure of the film. The dependence of the chemical potential on the hydrogen concentration was measured electrochemically for tungsten oxide films of different crystallinity and water content. A new method for determining the chemical potential by catalytic coloration by hydrogen gas is introduced. It revealed that the increase in electromotive force with increasing crystallinity is due only to different binding energies of the protons. We expect the protons to be located in the centres of hexagons, which are created by WO 6 octahedra. According to our model, amorphous sputtered films show a hexagonal structure which is similar to that of evaporated films, but the hexagons are connected, leading to more hexagon centre sites, which increases the electromotive force.

Medium-sensitivity rotating collimator for high-performance radioisotopic imaging

The high-resolution radioisotopic detector allows the detection of small structures, but its sensitivity is poor. Increased parallel collimator sensitivity is obtained by enlarging the hole diameter. However, a hole diameter greater than detector intrinsic spatial resolution leads to artifacts on acquired images. Linear collimator motions can remove these artifacts. These motions consist in sampling the generic rectangle of the hexagonal collimator structure and theoretically lead to images without artifacts. However, such motions are difficult to perform. To this end, a circular translation motion is studied in this paper. Motion effectiveness was evaluated by an inhomogeneity index based on hexagon center loci. Several rotation radii optimizing image homogeneity were found. Moving collimator transparency was deterministically simulated using a uniform isotropic source and a line source. For every rotation radius that minimized homogeneity, uniform source images showed variations below 1%.. The smallest suitable motion radius is 0.32 times the structure diameter, which leads to a very small decrease in the field of view. A device performing such collimator motions was made for a high-resolution gamma imaging probe based on a photomultiplier position sensitive tube and tested using a low-energy medium-sensitivity collimator. The measured spatial impulse response was the same on the whole field of view for a given source-to-detector distance. An image restoration process allowed the improvement of spatial resolution from 3.2 to 2.6 mm. Moving a medium-sensitivity collimator allows us both to obtain good sensitivity with a high-resolution gamma detector and to restore high resolution in radioisotopic imaging. For thyroid diagnosis, this will increase the ability of cold nodule detection with low-energy isotopes. Furthermore, images obtained with /sup 131/I will be free of artifacts due to the septa size of medium-energy collimators.

STUDY ON STRUCTURAL,ELECTRONIC AND MAGNETIC PROPERTIES OF ALKALI DOPED β-CAGE

The structural and electronic properties of eight and fourteen Na or K atoms adsorbed in the β-cage of zeolite LTA(Linde type A) have been studied by using the first-principles discrete variational method(DVM)based on the framework of local density formalism.The β-cage are used as a cluster to simulate the geometrical structures of alkali metal atoms incorporated into the zeolite cages.It isfound that after Na and K atoms are adsorbed by the β-cage of zeolite LTA,the adsorbed Na or K cations prefer to stay near the hexagonal center site of the β-cage.Since the adsorbed Na or K cations are far away from each other(＞0.4nm),the interaction between them is weak and it is hard to say that they form a cluster.The calculated average moment of adsorbed Na or K is very small.For adsorbed eight and fourteen Na,the moments are 0.01μB and 0.02μB,respectively and the total moments of the doped β-cage are 0.10μB and 0.31μB.For adsorbed eight and fourteen K,the moments are all 0.01μB and the the total moments of the doped β-cage are 0.12μB and 0.22μB.These results are in agreement with the experimental observation.

Ternary cobalt germanium pnictides CoGexPn1-x (Pn = P, As, Sb) and the structure of Co3Ge2Sb, an intermetallic compound with stuffed Sb2 pairs

The ternary cobalt germanium pnictide systems CoGexPn1-x (Pn = P, As, Sb; 0 <x <1) were investigated. The ternary phosphides CoGexP1-x and arsenides CoGexAs1-x adopt the MnP structure (Pnma) in the range 0 <x <0.8, with Ge and Pn (Pn = P, As) disordering over the anion sites. Reactions in the antimonide system CoGexSb1-x result in a mixture of ternary phases, one with the NiAs structure (P63/mmm) in which Ge and Sb disorder over the anion sites, and another that is a new intermetallic compound, Co3Ge2Sb. Single-crystal X-ray diffraction reveals that Co3Ge2Sb crystallizes in the hexagonal space group P6/mmm with a = 8.9128(13) Å, c = 7.6312(8) Å, and Z = 6 (T = 22°C). The structure of Co3Ge2Sb comprises alternating kagomé nets of Co atoms and 63 nets of Ge atoms, with Sb2 pairs stuffed along the centres of the concentric hexagons of these nets. Co3Ge2Sb may be regarded as an intermediate structure that is distorted from the limiting CoSn and CaCu5 structures. Key words: cobalt, germanium, antimony, pnictide, crystal structure

Rotational dynamics of endohedral C 60 fullerene complexes

The structure of the rotational low-energy eigenvalue spectrum and eigenfunction of endohedral C 60 fullerene complexes is investigated. The selected systems are Li +@C 60, Na +@C 60, CO@C 60, and LiH@C 60. The eigenvalue spectra may be classified basically into two types. Below some critical degree of anisotropy in the guest-cage interaction the spectrum looks rather simple, and similar to that of an effective diatomic molecule. For high enough anisotropic interactions, the low-energy eigenvalue spectrum is characterized by a double-band structure: a ground band localized around the fullerene hexagon centers, and an excited band embedded in a rotational quasi-continuum and localized at the unstable equilibrium regions around the fullerene pentagon centers. The cage rotation makes the transition from one type to the other much sharper type. Rotation dipole and Raman spectra are assigned in terms of these two types of structure in the eigenvalue spectrum.

ATP synthase: a tentative structural model

Adenosine triphosphate (ATP) synthase produces ATP from ADP and inorganic phosphate at the expense of proton- or sodium-motive force across the respective coupling membrane in Archaea, Bacteria and Eucarya. Cation flow through the intrinsic membrane portion of this enzyme (F O, subunits ab 2 c 9–12) and substrate turnover in the headpiece (F 1, subunits α 3 β 3 γ δ ϵ) are mechanically coupled by the rotation of subunit γ in the center of the catalytic hexagon of subunits ( α β) 3 in F 1. ATP synthase is the smallest rotatory engine in nature. With respect to the headpiece alone, it probably operates with three steps. Partial structures of six out of its at least eight different subunits have been published and a 3-dimensional structure is available for the assembly ( α β) 3 γ. In this article, we review the available structural data and build a tentative topological model of the holoenzyme. The rotor portion is proposed to consist of a wheel of at least nine copies of subunits c, ϵ and a portion of γ as a spoke, and another portion of γ as a crankshaft. The stator is made up from a, the transmembrane portion of b 2, δ and the catalytic hexagon of ( α β) 3. As an educated guess, the model may be of heuristic value for ongoing studies on this fascinating electrochemical-to-mechanical-to-chemical transducer.

Stochastic Simulations of Two-Dimensional Composite Packings

In recent years, dense packings of two- and three-dimensional objects have been studied intensely in the context of computational physics and material sciences. For example, computer simulations of disordered solids usually employ a two-dimensional model which is based on hexagonal networks of elastic and rigid bonds or arrangements of mixed soft and hard disks, respectively. Both types of bonds/disks are distributed randomly. Large systems of equations have to be solved at any simulation step for the calculation of local displacements or particle velocities. The simulations start from equidistant nodes of the hexagonal network or centers of disks, respectively, which, in general, may not be in an equilibrium state. We suggest an extension of the model where first a near-equilibrium packing of randomly distributed bonds/disks is calculated. Then, we can compute the displacement caused by external forces from this near-equilibrium initial packing of the elementary units. To this end, we propose a stochastic simulation of the external impact by incorporating the computation of near-equilibrium states as well as specific boundary conditions. Our methodology is based on a two-step approach consisting of a preprocessing stage, where physical properties of different types of particles are analyzed by numerical methods, and a second stage of stochastic (annealing-based) simulations which exploits approximate formulas for local interactions. We have implemented two types of cooling schedules with an expected serial run-timen· ln2nandn3/2· ln5/2n, respectively, to reach near-equilibrium states forndisks. The algorithms were parallelized on a 20-processor machine, and for a sufficiently large number of objects the speedup is close to the number of processors. For example, the parallel run-time for computing near-equilibrium states is about 212h for 449 disks, using the first cooling schedule, and about 37 h for 1068 disks, using the second cooling schedule. We performed a number of computer simulations calculating the average displacement in near-equilibrium states from regular, equidistant initial packings. The underlying physical model for our implementations is very similar to the model used for the analysis of granular composites which involves arrangements of mixed soft and hard disks. However, our emphasis is on the computational aspects rather than on particular systems of physical interactions, because substituting a system of physical interactions by another one does not affect significantly the run-time or the overall approach.