Detailed mapping of the local Ir4+ dimers through the metal-insulator transitions of CuIr2S4 thiospinel by X-ray atomic pair distribution function measurements

The evolution of the short-range structural signature of the Ir4+ dimer state in CuIr2S4 thiospinel has been studied across the metal-insulator phase transitions as the metallic state is induced by temperature, Cr doping, and x-ray fluence. An atomic pair distribution function (PDF) approach reveals that there are no local dimers that survive into the metallic phase when this is invoked by temperature and doping. The PDF shows Ir4+ dimers when they exist, regardless of whether or not they are long-range ordered. At 100 K, exposure to a 98 keV x-ray beam melts the long-range dimer order within a few seconds, though the local dimers remain intact. This shows that the metallic state accessed on warming and doping is qualitatively different from the state obtained under x-ray irradiation.     

Probing the high momentum component of the deuteron at high Q2

The (2)H(e,e'p)n cross section at a momentum transfer of 3.5 (GeV/c)(2) was measured over a kinematical range that made it possible to study this reaction for a set of fixed missing momenta as a function of the neutron recoil angle θ(nq) and to extract missing momentum distributions for fixed values of θ(nq) up to 0.55 GeV/c. In the region of 35°≤θ(nq)≤45° recent calculations, which predict that final-state interactions are small, agree reasonably well with the experimental data. Therefore, these experimental reduced cross sections provide direct access to the high momentum component of the deuteron momentum distribution in exclusive deuteron electrodisintegration.     

Expanded substrate scope and improved reactivity of ether-forming cross-coupling reactions of organotrifluoroborates and acetals

Mixed acetals and organotrifluoroborates undergo BF(3)·OEt(2)-promoted cross-couplings to give dialkyl ethers under simple, mild conditions. A survey of reaction partners identified a hydroxamate leaving group that improves the regioselectivity and product yield in the BF(3)·OEt(2)-promoted coupling reaction of mixed acetals and potassium alkynyl-, alkenyl-, aryl- and heteroaryltrifluoroborates to access substituted dialkyl ethers. This leaving group enables the reaction to proceed rapidly under mild conditions (0 °C, 5-60 min) and permits reactions with electron-deficient potassium aryltrifluoroborates that are less reactive with other acetal substrates. A study of the reaction mechanism and characterization of key intermediates by NMR spectroscopy and X-ray crystallography identified a role for the hydroxamate moiety as a reversible leaving group that serves to stabilize the key oxocarbenium intermediate and the need for a slight excess of organodifluoroborane to serve as a catalyst. A secondary role for the boron nucleophile as an activating ligand was also considered. These studies provide the basis for a general class of reagents that lead to dialkyl ethers by a simple, predictable cross-coupling reaction.     

Structure–Activity Relationships of the Antimalarial Agent Artemisinin 10. Synthesis and Antimalarial Activity of Enantiomers of rac-5β-Hydroxy-d-Secoartemisinin and Analogs: Implications Regarding the Mechanism of Action

An efficient synthesis of rac-6-desmethyl-5β–hydroxy-d-secoartemisinin 2, a tricyclic analog of R-(+)-artemisinin 1, was accomplished and the racemate was resolved into the (+)-2b and (−)-2a enantiomers via their Mosher Ester diastereomers. Antimalarial activity resided with only the artemisinin-like enantiomer R-(−)-2a. Several new compounds 9–16, 19a, 19b, 22 and 29 were synthesized from rac-2 but the C-5 secondary hydroxyl group was surprisingly unreactive. For example, the formation of carbamates and Mitsunobu reactions were unsuccessful. In order to assess the unusual reactivity of 2, a single crystal X-ray crystallographic analysis revealed a close intramolecular hydrogen bond from the C-5 alcohol to the oxepane ether oxygen (O-11). All products were tested in vitro against the W-2 and D-6 strains of Plasmodium falciparum. Several of the analogs had moderate activity in comparison to the natural product 1. Iron (II) bromide-promoted rearrangement of 2 gave, in 50% yield, the ring-contracted tetrahydrofuran 22, while the 5-ketone 15 provided a monocyclic methyl ketone 29 (50%). Neither 22 nor 29 possessed in vitro antimalarial activity. These results have implications in regard to the antimalarial mechanism of action of artemisinin.     

Characterization of in situ mechanically worked PVD Mg–Ti alloys

Mg–Ti alloys with up to 46?wt%?Ti in solid solution were produced by physical vapour deposition (PVD) with in situ mechanical working. All the alloys exhibited compositional inhomogeneity, columnar microstructures and typical PVD defects. The thermal stability of the solid solutions decreased with increasing Ti content, with the most stable solid solution breaking up above 566?K. The in situ mechanical working reduced porosity by closing pores and flattened surface asperities. The oxide film on the surface of the Mg–8?wt%?Ti alloy was Ti-free. The air formed oxide film on the alloys with a Ti content above 8?wt% was identiffied as a mixture of MgO and TiO₂ covered by Mg(OH)₂ and hydromagnesite at the outermost surface. Magnesium oxide and titanium oxide were also formed at the columnar boundaries that were not affected by the in situ mechanical working. The oxide chemistry was studied using Auger parameter analysis.     

Disconnections in simple and complex structures

Disconnections are interfacial defects with dislocation and step character; for example, twinning dislocations in homo-phase and transformation dislocations in hetero-phase materials. They play important structural roles and are classified as either ‘perfect’, separating energetically degenerate regions of interface, or ‘partial’, bounding a faulted region. In kinetic mechanisms, disconnection motion can be glissile, by conservative climb or climb. In the present paper, disconnection mobility is analyzed in terms of shear and shuffle-type atomic displacements and diffusional flux. This is applied to a sequence of hetero-phase materials with increasing structural complexity, namely the β to α transformation in Ti, θ′ precipitation in Al(Cu) and the orthorhombic to monoclinic martensitic transformation in ZrO₂. A disconnection source mechanism in Ti, involving conservative climb, is also described.     

Porous Polymers by Controlling Phase Separation during Vapor Deposition Polymerization

A template‐free method is described to fabricate continuous‐phase, porous polymer films by simultaneous phase separation during vapor deposition polymerization. The technique involves concurrent polymerization, crosslinking, and phase separation of condensed species and reaction products. Deposited films form open‐cell, macroporous structures consisting of crosslinked and glassy poly(glycidyl methacrylate). By limiting phase separation during vapor phase deposition, spatially dependent morphologies, such as layered morphologies, can be grown. Results show that combining vapor deposition polymerization with phase separation establishes morphological control, which may be applied to applications including cellular scaffolds, thin cushions and vibration dampers, and membranes for separations.

     

Comparison of sample preparation methods for stable isotope analysis of dissolved sulphate in forested watersheds

Pretreatment methods for measuring stable sulphur (δ³⁴S) and oxygen (δ¹⁸O) isotope ratios of dissolved sulphate from watersheds have evolved throughout the last few decades. The current study evaluated if there are differences in the measured stable S and O isotope values of dissolved sulphate from forested watersheds when pretreated using three different methods: Method 1 (M1): adsorb sulphate on anion exchange resins and send directly to isotope facility; Method 2 (M2): adsorb sulphate on anion exchange resins, extract sulphate from anion exchange resins, and send the produced BaSO₄ to the isotope facility; and Method 3 (M3): directly precipitate BaSO₄ without anion exchange resins with the precipitates being sent to the isotope facility. We found an excellent agreement of the δ³⁴S_sulphate values among all the three methods. However, some differences were observed in the δ¹⁸O_sulphate values (M1 versus M2:?1.5 ‰; M1 versus M3:?1.2 ‰) associated with possible O contamination before isotope measurement. Several approaches are recommended to improve the pretreatment procedures for δ¹⁸O_sulphate analysis.     

Calculations with the CNDO method on clusters of silver atoms

An attempt has been made to explore the applicability of the complete neglect of differential overlap (CNDO) method for investigating clusters of silver atoms. A new parametrization for silver has been obtained by comparing charge distributions, as well as local and total density of states, from CNDO calculations with those from the X_α scattered wave (X_αSW) method for an Ag₇ cluster which represents a fragment of the silver lattice. These parameters have then been used for making CNDO calculations on four further clusters of the same type, namely Ag₆, Ag₁₀, Ag₁₃ and Ag₁₉, and the results are compared with previous X_αSW calculations. These CNDO calculations give d-band widths in broad agreement with those from the X_αSW method. The most significant difference is that the CNDO method gives less localization on central atoms with high coordination numbers than is found from the X_αSW calculations. It is suggested that this apparent deficiency of the CNDO calculations may be less serious when the clusters are being used for modelling part of a solid metal rather than for specifically investigating the properties of small particles.     

Morphology of a Main Chain Liquid Crystal Polymer Containing Semi-Flexible Coupling Chain

A new procedure for the study of textures or director patterns in liquid crystal polymers is reported. The director patterns of the liquid crystal phase are decorated using a lower temperature crystalline phase, a process which does not disrupt the larger scale molecular organisation representative of the liquid crystal structure. The pattern of the decorating crystal lamellae is revealed using an etching technique; the director pattern lies orthogonal to these “lamellae”. Examples are shown of the texture relating to point and line defects. The potential of a quantitative approach to the director distribution about an inversion wall to provide numerical values of the curvature elastic constants is detailed.