ChemInform Abstract: Computational Studies of Atmospheric Chemistry Species. Part 7. An ab initio Study of the N2×××O2 Heterodimer.

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Ru9Zn7Sb8: a structure with a 2 × 2 × 2 supercell of the half-Heusler phase

The title compound Ru(9)Zn(7)Sb(8) was synthesized via a high-temperature reaction from the elements in a stoichiometric ratio, and its structure was solved by a single-crystal X-ray diffraction method. The structure [cubic, space group Fm3m, Pearson symbol cF96, a = 11.9062(14) ? (293 K), and Z = 4] adopts a unique 2a(hh) × 2a(hh) × 2a(hh) supercell of a normal half-Heusler phase and shows abnormal features of atomic coordination against the Pauling rule. The formation of this superstructure was discussed in light of the valence electron concentration per unit cell. It is a metallic conductor [ρ(300 K) = 16 μΩ·m], and differential scanning calorimetry revealed that Ru(9)Zn(7)Sb(8) undergoes a transformation at 1356(1) K and melts, by all indications, congruently at 1386 K. At room temperature, its thermal conductivity is about 3 W/m·K, which is only one-quarter of that of most normal half-Heusler phases. Ru(9)Zn(7)Sb(8) as well as its analogues of iron-, cobalt-, rhodium-, and iridium-containing compounds are expected to serve as a new structure type for exploring new thermoelectric materials.     

Molecular dynamics simulation of liquid CH2Cl2 with 3×3 and 5×5 site-site interactions

A molecular dynamics simulation of liquid CH₂Cl₂ is compared with the far infrared spectrum at the same state point (293K, 1 bar). Two representations of the force field are used, a 3×3 and 5×5 site-site interaction consisting of Lennard-Jones and charge terms. The far infra-red spectrum shows unambiguously that the 5×5 representation is more realistic in the sense that it reproduces the observed spectrum more closely.     

High-Tc ferromagnetic semiconductor-like behavior and unusual electrical properties in compounds with a 2×2×2 superstructure of the half-Heusler phase

Heusler phases, including the full- and half-Heusler families, represent an outstanding class of multifunctional materials on account of their great tunability in compositions, valence electron counts (VEC), and properties. Here we demonstrate a systematic design of a series of new compounds with a 2×2×2 superstructure of the half-Heusler unit cell in X-Y-Z (X=Fe, Ru, Co, Rh, Ir; Y=Zn, Mn; Z=Sn, Sb) systems. Their structures were solved by using both powder and single-crystal X-ray diffraction, and also directly observed by using high-angle annular dark-field imaging in a scanning transmission electron microscope (HAADF-STEM). The VEC values of these new compounds span a wide and continuous range comparable to those for the full- and half-Heusler families, thereby implying tunability in compositions and physical properties in the superstructure. In fact, we observed abnormal electrical properties and a ferromagnetic semiconductor-like behavior with a high and tunable Curie temperature in these superstructures.     

Self-assembly snapshots of a 2 × 2 copper(I) grid

Self-assembled 2 × 2 grids have been characterised as high-fidelity species produced when the correct stoichiometric ratios are met, but rarely are the individual steps leading to and from their formation characterised. Here, we present such a study using equilibrium-restricted factor analysis to model a set of UV–vis spectra starting from a bis-bidentate ligand to the assembly of a 2 × 2 grid complex upon titration with 1 equiv. of [Cu(MeCN)₄](PF₆) and to disassembly upon further titration. Intermediate species [CuL₂]⁺, [Cu₂L₃]²⁺, [Cu₃L₂]³⁺ and [Cu₂L]²⁺ are evidenced along the assembly and disassembly pathways. Complementary ¹H NMR titrations are consistent with the rich set of complexes and equilibria involved. Given the nature of the assembly process, the assembly is entropy driven and likely enthalpy driven as well. The disassembly process is both enthalpy and entropy driven according to the standard free energy values derived from the modelling of the spectrophotometric titration data.     

Composition,structure, and functional properties of organomineral composite sorbents KU-2×8-ZnS and KU-2×8-PbS

Organomineral composite sorbents KU-2×8-ZnS and KU-2×8-PbS were synthesized on the basis of the matrix of a strongly acidic KU-2×8 cation exchanger. Scanning electron microscopy and elemental analysis were used to study their microstructure and chemical composition. With the use of potentiometric titration, their bifunctional nature was revealed and the dissociation constants and the total exchange capacities of the ionogenic groups were determined. The sorption of copper(II), zinc, and cadmium by KU-2×8-ZnS and KU-2×8-PbS sorbents from 0.005 M solutions of their salts was examined. The total dynamic sorption capacity of the composite sorbents for the above metals, found in the study, exceeds similar values for the individual KU-2×8 cation exchanger by a factor of 1.6–2.0 and that in 0.001–0.050 M solutions of potassium and calcium chlorides, by a factor of 1.3–4.5. An explanation is suggested for the sorption process of heavy nonferrous metals on composite sorbents with an active sulfide component by the mechanism of coordination copolymerization.     

Regular high-nuclearity species from square building blocks: a triangular 3 × [2 × 2] Ni12 complex generated by the self-assembly of three [2 × 2] Ni4 molecular grids

A dodecanuclear 3 × [2 × 2] nickel(II) complex has been obtained via the self-association of three tetranuclear [2 × 2] molecular grids. X-ray diffraction shows a "propeller-like" structure of the [Ni(4)](3) scaffold with a central μ(3)-hydroxide. The dodecanuclear species remains stable in solution and can be deposited without decomposition on highly ordered pyrolytic graphite surfaces.     

Disulfide passivation of the Ge(100)-2 × 1 surface

Understanding the bonding of sulfur at the germanium surface is important to developing good passivation routes for germanium-based electronic devices. The adsorption behavior of ethyl disulfide (EDS) and 1,8-naphthalene disulfide (NDS) at the Ge(100)-2 × 1 surface has been studied under ultrahigh vacuum conditions to investigate both their fundamental reactivity and their effectiveness as passivants of this surface. X-ray photoelectron spectroscopy, multiple internal reflection-infrared spectroscopy, and density functional theory results indicate that both molecules adsorb via S-S dissociation at room temperature. Upon exposure to ambient air, the thiolate adlayer remains intact for both EDS- and NDS-functionalized surfaces, indicating the stability of this surface attachment. Although both systems resist oxidation compared to the bare Ge(100)-2 × 1 surface, the Ge substrate is significantly oxidized in all cases (17-57% relative to the control), with the NDS-passivated surface undergoing up to two times more oxidation than the EDS-passivated surface at the longest air exposure times studied. The difference in passivation capability is attributed to the difference in surface coverage on Ge(100)-2 × 1, where EDS adsorption leads to a saturation coverage 17% higher than that for NDS/Ge(100)-2 × 1.     

Spin symmetry and gel'fand patterns of higher SU2×ℓ n groups

The nature of? ₅ spin algebra is considered together with? ₅↓? ₅↓C _4v mapping in order to specify the dual spin symmetry for MQ-NMR ofnido-¹¹B₅H₉. The forms of Gel'fand shapes for? ₅ spin symmetry are presented to show how they specify thefull range of multiplicities found in higher-n ? _n -partitions. Tuples, or number-partitions and their? _n ↓G invariance sets provide models for both the five-foldI _i =3/2 component, and the two distinct types of spin-1/2 subsystems. The full spin symmetry is derived in terms of the direct product ((? ₅↓C _4v )?(? ₄↓C _4v ))^1/2?(? ₅↓C _4v )^3/2. The concepts used are implicit in the substructure ofp-tuple model invariances over the subduced symmetry, or derive from the inner tensor product algebras under the? _n group. Both as a check on the combinatorially derived multiplicities of [λ]s and for insight into (non-simply-reducible) substructure of number-partitions, the study of mapping from :hrr'.:-tuples onto the? _n -partitional set for higherI _i is invaluable. The motivation for this work lies in its pertinence to the MQ-NMR spin dynamics of clusterlike molecules. The accessible information content of a spin algebra over either form of spin space is bound up with a suitable symmetry partitioning of the problem, as implied by the use of {T ^kq (v:[λ])} bases within higherq subspaces of the Liouville formalism.     

Self-assembly of [2×2] grids and a hexagon using bis(dipyrrin)s

Neutral Zn(II) [2×2] grids (M(4)L(4)) and a hexagon structure (M(6)L(6)) were, for the first time, synthesized using bis(dipyrrin)s and confirmed by X-ray diffraction analysis. The channel structures in the solid state result from several intermolecular noncovalent interactions.     

Theoretical analysis of spin crossover in iron(II) [2×2] molecular grids

We use quantum-chemical density functional theory calculations to elucidate the origin of spin-crossover pathways in two iron(II) [2×2] molecular grids with carbohydrazide-based bridging ligands. The complexes are characterized energetically and structurally in five available spin states. Special attention is paid to analysis of the structural distortion induced on each iron center by spin transition on any of its neighbors. The evolution of coordination polyhedra is monitored using the Continuous Shape Measures. It is demonstrated that a succession of spin transitions on different centers depends on the character of the induced distortion, either approaching or getting them away from a more regular low-spin geometry. These effects, resulting from the elasticity of bridging ligands, can be modulated by weak perturbations such as a change of the positions of the hydrogen atoms.     

Chemical synthesis of oriented ferromagnetic LaSr-2 × 4 manganese oxide molecular sieve nanowires

We report a chemical solution based method using nanoporous track-etched polymer templates for producing long and oriented LaSr-2 × 4 manganese oxide molecular sieve nanowires. Scanning transmission electron microscopy and electron energy loss spectroscopy analyses show that the nanowires are ferromagnetic at room temperature, single crystalline, epitaxially grown and self-aligned.     

Observation of photocatalytic dissociation of water on terminal Ti sites of TiO2(110)-1 × 1 surface

The water splitting reaction based on the promising TiO(2) photocatalyst is one of the fundamental processes that bears significant implication in hydrogen energy technology and has been extensively studied. However, a long-standing puzzling question in understanding the reaction sequence of the water splitting is whether the initial reaction step is a photocatalytic process and how it happens. Here, using the low temperature scanning tunneling microscopy (STM) performed at 80 K, we observed the dissociation of individually adsorbed water molecules at the 5-fold coordinated Ti (Ti(5c)) sites of the reduced TiO(2) (110)-1 × 1 surface under the irradiation of UV lights with the wavelength shorter than 400 nm, or to say its energy larger than the band gap of 3.1 eV for the rutile TiO(2). This finding thus clearly suggests the involvement of a photocatalytic dissociation process that produces two kinds of hydroxyl species. One is always present at the adjacent bridging oxygen sites, that is, OH(br), and the other either occurs as OH(t) at Ti(5c) sites away from the original ones or even desorbs from the surface. In comparison, the tip-induced dissociation of the water can only produce OH(t) or oxygen adatoms exactly at the original Ti(5c) sites, without the trace of OH(br). Such a difference clearly indicates that the photocatalytic dissociation of the water undergoes a process that differs significantly from the attachment of electrons injected by the tip. Our results imply that the initial step of the water dissociation under the UV light irradiation may not be reduced by the electrons, but most likely oxidized by the holes generated by the photons.     

Dynamics of H2 dissociation on the 1/2 ML c(2 × 2)-Ti/Al(100) surface

The dissociation of H(2) on Ti-covered Al surfaces is relevant to the rehydrogenation and dehydrogenation of the NaAlH(4) hydrogen storage material. The energetically most stable structure for a 1/2 monolayer of Ti deposited on the Al(100) surface has the Ti atoms in the second layer with a c(2 × 2) structure, as has been confirmed by both low-energy electron diffraction and low-energy ion scattering experiments and density functional theory studies. In this work, we investigate the dynamics of H(2) dissociation on a slab model of this Ti/Al(100) surface. Two six-dimensional potential energy surfaces (PESs) have been built for this H(2) + Ti/Al(100) system, based on the density functional theory PW91 and RPBE exchange-correlation functionals. In the PW91 (RPBE) PES, the lowest H(2) dissociation barrier is found to be 0.65 (0.84) eV, with the minimum energy path occurring for H(2) dissociating above the bridge to top sites. Using both PESs, H(2) dissociation probabilities are calculated using the classical trajectory (CT), the quasi-classical trajectory (QCT), and the time-dependent wave-packet methods. We find that the QCT H(2) dissociation probabilities are in good agreement with the quantum dynamics results in the collision energy range studied up to 1.0 eV. We have also performed molecular beam simulations and present predictions for molecular beam experiments. Our molecular beam simulations show that H(2) dissociation on the 1/2 ML Ti/Al(100) surface is an activated process, and the reaction probability is found to be 6.9% for the PW91 functional and 1.8% for the RPBE at a nozzle temperature of 1700 K. Finally, we have also calculated H(2) dissociation rate constants by applying transition state theory and the QCT method, which could be relevant to modeling Ti-catalyzed rehydrogenation and dehydrogenation of NaAlH(4).     

Nature of the binding of a c(2×2)-CO overlayer on Ag(001) and surface mediated intermolecular coupling

We present a first-principles study of the nature of the binding of a c(2×2)-CO overlayer on Ag(001) and of the origin of CO-CO interactions upon adsorption. Electronic structural changes induced by molecular adsorption provide an interpretation for earlier X-ray photoemission valence band spectra of CO/Ag(001). Our results establish that CO chemisorbs on clean Ag(001) and follows the Blyholder model of donation and back-donation between CO and metal orbitals. We analyze the origin of the dispersion of the C-O stretch mode and attest that it is caused by the metal-CO coupling. Specifically, the coupling of CO to Ag, although the weakest of those between it and transition and other noble metals, greatly enhances the intermolecular force constants. We also find that the response of the charge density around CO is much stronger and of longer range when the molecule stretches than when it rigidly vibrates against the surface. This difference explains why the C-O stretch mode disperses while the Ag-CO stretch mode does not.     

Pneumatically tunable optofluidic 2 × 2 switch for reconfigurable optical circuit

We presented a pneumatically tunable 2 × 2 optofluidic switch for on-chip light routing that was controlled by compressed air. The device was fabricated with an optically clear elastomer-polydimethylsiloxane (PDMS)-by soft-lithography. The optical switching is realized with a tunable air-gap mirror by which the light is deflected due to total internal reflection in the bypass state. When the device is subjected to high pressure, the air gap collapses and hence the light will be switched to the crossover state. The device had a switching speed of more than 5 Hz and an extinction ratio of 8 dB. This switch can be readily integrated with other microfluidic circuits. We demonstrated a simple reconfigurable optical waveguide circuit for dual-channel microfluidic spectroscopy measurement on a chip.     

Si(001)-(2×2×1):H表面O2吸附的密度泛函理论研究

A novel model was developed to theoretically evaluate the O2 adsorption on H-terminated Si(001)-(2*2*1) surface. The periodic boundary condition, the ultrasoft pseudopotentials technique based on density functional theory (DFT) with generalized gradient approximation (GGA) functional were applied in our ab initio calculations. By analyzing bonding energy on site, the favourable adsorption site was determined. The calculations also predicted that the adsorption products should be Si=O and H2O. This theoretical study supported the reaction mechanism provided by Kovalev et al. The results were also a base for further investigation of some more complex systems such as the oxidation on porous silicon surface.     

Taxoids from the Needles of Taxus×media Hicksii

Taxol@,ataxanediterpenoidisolatedfromthePacificyewtreebyWantetal.',hasattractedmuchattentionandiscurrentlyconsideredasoneofthemostimportantchemotherapeuticagents'forcancer.Inordertomeettherequirementsoftaxolforclinicalusageandtoprotectthenaturalresources,greatinterestisconcentratedontheisolationoftaxolanditsrelatedcompoundsfromrenewableresources,includingThousxmediaHicksii3.Inapreviouspaper,wereportedtheisolationofanewtaxoid,taxamedinA4,fromtheneedlesofthisplant.Oncontinuousinvestigationanot…     

Atomic imaging of atomic layer deposition oxide nucleation with trimethylaluminum on As-rich InGaAs(001) 2 × 4 vs Ga/In-rich InGaAs(001) 4 × 2

Formation of a contaminant free, flat, electrically passive interface to a gate oxide such as a-Al(2)O(3) is the critical step in fabricating III-V metal oxide semiconductor field effect transistors; while the bulk oxide is amorphous, the interface may need to be ordered to prevent electrical defect formation. A two temperature in situ cleaning process is shown to produce a clean, flat group III or group V rich InGaAs surface. The dependence of initial surface reconstruction and dosing temperature of the seeding of aluminum with trimethylaluminum dosing is observed to produce an ordered unpinned passivation layer on InGaAs(001)-(4 × 2) surface at sample temperatures below 190 °C. Conversely, the InGaAs(001)-(2 × 4) surface is shown to generate an unpinned passivation layer with a seeding temperature up to 280 °C. For both reconstructions, the chemical drive force is consistent with formation of As-Al-As bonds. The optimal seed layer protects the surface from background contamination.     

Determination of elaidic and vaccenic acids in foods using GC×GC-FID and GC×GC-TOFMS

Trans fatty acids (TFAs) are present in meat and dairy products as m ruminant animals and in vegetable fats due to partial hydrogenation. This study aimed to discriminate between natural (N-TFA) and hydrogenated trans fatty (H-TFA) acids by GC × GC-flame ionization detection (GC × GC-FID) and comprehensive GC × GC-time-of-flight mass spectrometry (GC × GC-TOFMS). The separation of two kinds of trans fats, vaccenic acid (18:1 trans-11) and elaidic acid (18:1 trans-9), was performed using GC × GC-FID and GC × GC-TOFMS. A 100 m × 0.25 mm I.D. × 0.2 μm (film thickness) SP-2560 (bis-cyanopropyl polysiloxane) fused capillary column (first separation dimension, 1D) was coupled to a 1.5 m × 0.18 mm I.D. × 0.18 μm (film thickness) RTX-5 (5% diphenyl/95% dimethyl polysiloxane) fused capillary column (second separation dimension, 2D). The RSD of the intra-day repeatability by both GC × GC-FID and GC × GC-TOFMS for elaidic and vaccenic acids was ≤9.56% and ≤9.97%, and the RSD of the inter-day repeatability was ≤8.49 and ≤9.06%, respectively. It was found that the V/E value (vaccenic acid to elaidic acid ratio) could be used to distinguish H-TFA from N-TFA and to evaluate the quality of the fatty foods.