Single-cluster Monte Carlo dynamics for the Ising model

We present an extensive study of a new Monte Carlo acceleration algorithm introduced by Wolff for the Ising model. It differs from the Swendsen-Wang algorithm by growing and flipping single clusters at a random seed. In general, it is more efficient than Swendsen-Wang dynamics ford>2, giving zero critical slowing down in the upper critical dimension. Monte Carlo simulations give dynamical critical exponentsz _w=0.33±0.05 and 0.44+0.10 ind=2 and 3, respectively, and numbers consistent withz _w=0 ind=4 and mean-field theory. We present scaling arguments which indicate that the Wolff mechanism for decorrelation differs substantially from Swendsen-Wang despite the apparent similarities of the two methods.     

Molecularly imprinted polymers for solid-phase extraction and solid-phase microextraction: recent developments and future trends

Molecularly imprinted polymers (MIPs) are synthetic polymers having a predetermined selectivity for a given analyte, or group of structurally related compounds, that make them ideal materials to be used in separation processes. In this sense, during past years a huge amount of papers have been published dealing with the use of MIPs as sorbents in solid-phase extraction, namely molecularly imprinted solid-phase extraction (MISPE). Although the majority of these papers were restricted to describe the use of different templates for different applications, several attempts proposing new alternatives to minimize the inherent drawbacks of the preparation and use of MIPs (i.e. template bleeding, tedious synthesis procedure, etc.) have been reported. Thus, this paper does not pretend to be a collection of MISPE-related papers but to give an overview on the significant attempts carried out during recent years to improve the performance of MIPs in solid-phase extraction. In addition, the use of MIPs packed in high performance liquid chromatography (HPLC) columns for the direct injection of crude sample extracts and the preparation of imprinted fibres for solid-phase microextraction will be also discussed.     

Diastereoselective synthesis of quaternary alpha-amino acids from diketopiperazine templates

Sequential enolate alkylations of (S)-N(1)-methyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one and (S)-N(1)-p-methoxybenzyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one proceed with excellent levels of diastereoselectivity (>90% de) affording quaternary alpha-amino acids in high enantiomeric excess (>98% ee) after deprotection and hydrolysis.     

Axisymmetric trawl cod-ends made from netting of a generalized mesh shape

The equations governing the geometry of axisymmetric trawl cod-endsmade from netting of meshes of a particular generalized structureare derived. From this, by suitable setting of the initial meshbar lengths, the equations governing the geometry of cod-endsthat are of importance to the fishing industry can be readilydeduced. It is assumed that arbitrary membrane forces act normalto the edges of the mesh elements, that there is no shear forceacting on the edge of a mesh element and that the twine thatmakes up the netting is extensible. The case where there isslackness in the mesh bars in the circumferential directionis dealt with and it is demonstrated how the finite structureof a knot can be taken into account. The case where the membraneforces arise solely as a result of and can be expressed by theappropriate components of the tensions in the mesh bars is alsoexamined and numerical solutions are found for a range of examples.     

Effect of reaction conditions on surface properties of TEOS–TBOT–PDMS hybrid materials

Hybrid materials based on tetraethoxysilane (TEOS), tetrabutyl orthotitanate (TBOT), and hydroxyl terminated polydimethyl siloxane (PDMS) have been prepared and characterized. The effects of HCl concentration and PDMS molecular weight (M_W) have been analysed. The physical and chemical characteristics have been discussed based on the results obtained from the characterization by Raman spectroscopy, nitrogen adsorption, mercury intrusion porosimetry and inverse gas chromatography (IGC). Specific surface areas (SSA) and pore volumes increase with HCl concentration and are independent on the PDMS Mw. SSA values as high as 300 m² g⁻¹ have been obtained for hybrid materials prepared with HCl:(TEOS + TBOT) concentration of 0.3 and PDMS Mw of 550 g mol⁻¹. These materials present the higher pore volumes and lower pore sizes. However, for HCl:(TEOS + TBOT) concentration of 0.5, only macroporous materials are obtained with SSA values as low as 2–4 m² g⁻¹, and pore sizes close to 10 μm. In general the presence of Ti on the SSA and porosities give a little decrease in such values. Surface energies are consistent with those of organic–inorganic materials. Dispersive surface energies are higher for the Si–Ti hybrid materials and, for all of them, they decrease with both HCl concentration and PDMS Mw. This result can be assigned to the presence of a higher concentration of PDMS (or CH₃ groups) on the surface of the hybrid material as well as the presence of disordered anatase-like structures. On the other hand, the acid and base properties are dependent on both HCl concentration and PDMS Mw. It has been observed that the acid and base parameters of the Si–Ti hybrid materials are influenced by the disorder degree of the amorphous titanium oxide present in them. As the disorder decrease both acid and base parameters increase.     

Structural and magnetic properties of a complete halide series of Ni(II) complexes with a pyridine-containing 14-membered macrocycle

The complete halide series of Ni(II) complexes containing the tetradentate macrocyclic ligand 3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L), was fully characterized by X-ray diffraction. The fluoro, chloro, and bromo complexes are dinuclear species with formula [{Ni(L)}2(mu-X)2]2+ (X = halide), whereas only mononuclear species with formula [Ni(Y)(solv)(L)]n+ (Y = halide or solvent molecule), were obtained with I. To date, the fluoro derivative is the first nonorganometallic coordination compound containing the Ni(mu-F)2Ni core. The magnetic properties of these halo complexes have been studied. Intramolecular interactions were observed in the three dinuclear complexes, being antiferromagnetic in the fluoro derivative and ferromagnetic in both the chloro and bromo ones. The two iodo derivatives are paramagnetic species, as would be expected for mononuclear octahedral Ni(II) complexes. Density functional theory calculations led us to relate the magnetic behaviors of these compounds to the values of the corresponding Ni-X-Ni(i) angle. The analysis of the singly occupied molecular orbitals gave a clear comprehension of the different magnetic exchange interactions found in these Ni(II) complexes.     

Color tuning of a nickel complex with a novel N2S2 pyridine-containing macrocyclic ligand

The novel pyridine-containing 14-membered macrocycle 3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L), which contains an N2S2 donor set, was synthesized, and its protonation behavior was studied by absorption titration with CH3SO3H. The reaction of L with Pd(II) was studied spectroscopically, and the square-planar complex [Pd(L)](BF4) was isolated and characterized. The reactions between L and NiX2 x 6 H2O (X = BF4, ClO4) in ethanol or acetonitrile afforded the octahedral complexes [Ni(CH3CN)(H2O)(L)](X)2 and [Ni(H2O)2(L)](X)2, respectively. The square-planar complexes [Ni(L)](X)2 were obtained by heating these octahedral complexes. Spectrophotometric titrations of [Ni(L)](BF4)2 were performed with neutral and negatively charged ligands. The color of nitromethane solutions of this square-planar complex turns from red to cyan, purple, blue, yellow-green, and pink following addition of halides, acetonitrile, water, pyridine, and 2,2'-bipyridine, respectively. X-ray structural analyses were carried out on the {[Ni(ClO4)(H2O)(L)][Ni(H2O)2(L)]}(ClO4)3, [Ni(CH3CN)(H2O)(L)](ClO4)2, [{Ni(L)}2(mu-Cl)2](ClO4)2, and [{Ni(L)}2(mu-Br)2]Br2 x 2 CH3NO2 complexes.     

Effect of humidity in charge formation and transport in LDPE

Charge distribution and transport have been investigated in LDPE films with different humidity content under electric fields up to 130 MV/m. Pulsed electroacoustic measurements showed that, as water content increases, positive charge packets formation in the anode is enhanced and they propagate toward the cathode with higher transit speeds. Fits of surface potential decay measurements showed that charges in dry samples are injected directly into the volume, but the presence of moisture generates new trap centers in the surface of the material. This new trap level causes a charge accumulation on the surface, that gradually passes into the bulk. The observed behavior in development and propagation of charge packets are explained according to these results. Thermally stimulated depolarization current measurements showed a non-distributed relaxation associated to the new trap levels on the surface of the wet samples.     

Kupfer

Ohne Zusammenfassung     

Rubbing effect on surface morphology and thermoelectric properties of TTF-TCNQ thin films

Thin films of tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ), a typical organic material of charge transfer salts, were prepared on glass substrates by evaporation using TTF-TCNQ powder. The rubbing effect on the surface morphology and thermoelectric properties was studied. TTF-TCNQ films exhibited a bush-like disordered growth on the as-received glass substrate, whereas those on the rubbed glass substrate had extremely flat surfaces tiled with small rectangular TTF-TCNQ single crystals. Due to the ordered alignment of TTF-TCNQ tiles, improvement in the electrical conductivity and enhancement of the Seebeck coefficient were achieved.