Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Circle actions on 4-manifolds I

Using cohomological methods it is proved that the intersection form of a closed (cohomology-) 4-manifoldM withH ₁(M)=0 which admits a non-trivial circle action is the sum of one- and two-dimensional forms provided every fixed point has a neighbourhood containing at most 4 orbit types.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Recent developments in polymer-based sorbents for solid-phase extraction

A review with 136 references on the principles and recent developments in the solid-phase extraction based on polymer sorbents is presented. New polymer-based materials, chromatographic modes, experimental configurations are described and their advantages for a rapid sample preparation of certain classes of compounds with different functional groups are discussed and compared to silica-based sorbents.     

Analysis of carbohydrates using different quaternized polystyrene-divinylbenzene particles and pulsed amperometric detection

Summary This paper reports on the use of a polymer-based, strong anion-exchange stationary phase for rapid, selective and sensitive analysis of physiological important mono-, di- and oligosaccharides by high-performance anion-exchange chromatography (HPAEC) with pulsed amperometric detection (PAD) under alkaline conditions. The adsorbent was obtained by direct nitration of 3 and 5 μm, spherical non-porous highly cross-linked, styrene-divinylbenzene copolymer (PS-DVB) beads, followed by reduction of superficially introduced nitro groups with nascent hydrogen and quaternization of the resultant amino groups with iodomethane. Extended exposure to high pressure and strong alkaline conditions did not have any untoward effect on mechanical stability and chromatographic performance. A comparison of the 3 and 5 μm beads showed, that the synthesized 3 μm highly cross-linked PS-DVB particles are the preferred phases for the separation of monosaccharides and the 5 μm particles are preferable for the separation of oligosaccharides. To demonstrate the suitability for the analysis of complex samples, the optimized and validated system was used for the determination of glucose, fructose and sucrose in apple juice and other soft drinks such as Coca Cola. Finally, analysis within a few minutes without sample pretreatment down to a lower limit of detection of 0.174–0.504 μg mL⁻¹ at a linearity with R²>0.994 and a repooducibility higher than 98% further confirmed the efficiency of these polymeric sorbents.     

Photochemical deuterium exchange of the m-methyl group of 3-methylbenzophenone and 3-methylacetophenone in acidic D2O

Photolysis of the title compounds in acidic aqueous solution results in "activation" of the distal m-methyl group, resulting in deuterium exchange (Phi approximately 0.1) when D(2)O is used. The reaction is not observed in neutral aqueous solution or in deuterated organic solvents such as CD(3)CN. The reaction is best rationalized by a mechanism involving initial protonation at the carbonyl oxygen of the triplet excited state. [reaction: see text]