Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a⁵⁸Ni neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the NIST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm× 45 mm onto a focal spot of diameter 0.53 mm (fwhm) wich an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the -count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).     

An Approach to the Core Structure of CP-225,917

The complex natural product CP-225, 917 (1), which is an inhibitor of Ras fanesyl tranferase,[1～3] has at tracted much attention of several groups for its total synthesis. Our own approach is based on siloxy Cope rearrangement, which could be accomplished thermally in very good yield (99%) to from 10 to 11,[4] a model compound lacking the side arms of the natural product. Its synthetic route from 6 is shown.     

Natural products--a simple model to explain chemical diversity

A simple evolutionary model is presented which explains why organisms produce so many natural products, why so many have low biological activity, why enzymes involved in natural product synthesis have the properties they do and why natural product metabolism is shaped as it is.     

Micro-thermal analysis of NiTi shape memory alloy thin films

A novel technique of micro-thermal analysis (micro-TA) has been used to investigate martensitic to austenitic transformations of near equi-atomic NiTi shape memory alloy (SMA) thin films deposited on silicon wafer by a plasma assisted sputter deposition technique. The results demonstrate that both power and sensor deflection signal of the technique, equivalent to micro-differential thermal analysis (μDTA) and micro-thermomechanical analysis (μTMA), respectively, have a capability of locally characterising transformation temperatures of the SMA films. The phase transition temperatures can be identified as an abrupt deviation of power and thermal expansion from linearity. The change in probe deflection reveals a sample contraction of 0.44% following the martensite to austenitic transformation. This dimension change is consistent with the difference in the unit cell volumes of the different phases. The individual films investigated here show a spatial variation on the micron-scale in the martensite to austenite transition temperatures as the surface is probed. A possible reason for this may lie in the inhomogeneous distribution of Ti and Ni in the film structure as the transition temperature is very sensitive to composition, showing typically a 100 K temperature change between 50 and 51 at.% Ni in Ti. Conventional bulk DSC experiments were carried out on the same materials and the results were compared with those from the micro-TA.     

Magnetic circular dichroism of cyclic π-electron systems—25: Oxo derivatives of indazole,benzimidazole and purine

The simple perimeter model combined with PMO theory and semimpirical calculations is used to rationalize the observed MCD spectra of a series of oxo derivatives of azaindoles, such as caffeine and xanthine. The MCD spectra indicate the presence of substantial amounts of the 9-H tautomers in aqueous solutions of xanthines.     

Ballistic impact to access the high-rate behaviour of individual silk fibres

A revision of a classic transverse fibre impact technique is presented, as applied to the problem of obtaining the high strain-rate constitutive behaviour of commercial Bombyx mori silk. Medium tenacity nylon was also studied. Two approaches are presented: firstly a fixed pre-stress, varied impact velocity method that derives stress–strain behaviour by inverse fit; and secondly a fixed impact velocity, varied pre-stress approach, assuming basic elastic jump conditions to obtain a locus of post-impact states. The post-impact stress–strain states obtained using the two approaches converge for silk but diverge for nylon. This we attribute to silk's fine structure being able to homogenise energy dissipation at static and dynamic deformation rates. However, the coarser microstructure of nylon results in a different loading path dependence, thus divergence in the two approaches. It was also noted that silk exhibited a comparatively stable level of impact energy absorption under varying pre-stress, when compared to nylon.     

Synthesis and application of palladium stearates as precursors for the preparation of palladium nanoparticles

This report successfully demonstrates the synthesis and application of palladium stearates. It was found that the branching of the carboxylate anion of metal precursors could influence the size and shape of palladium nanoparticles (PdNPs). Worm‐like nanowires were formed when using the branched isomer palladium isostearate (PdISt₂), while triangular nanoparticles were produced in a majority when using the normal form: palladium stearate (PdSt₂). Furthermore, when applying CO₂ to the system, both types of PdNPs transformed into more spherical shapes with smaller sizes. The formation of carbamates from the amine stabilizer with CO₂ could prevent the further growth and aggregation of PdNPs. The PdNPs were tested as catalysts for the hydrogenation of styrene, and higher catalytic activities were achieved with PdNPs that were prepared with the assistance of CO₂. Copyright © 2012 John Wiley & Sons, Ltd.