The assembly of organic radical anions between metal-borate scaffolds

Metal-organic frameworks based on the Pb[B(Im)(4)](+) unit form layered structures analogous to those observed in clays and double layered hydroxide minerals. These layers can act as scaffolds for the organization of anionic organic guests. In this report, we use this scaffold to assemble TEMPO and PROXYL carboxylates in the interlayer spacings of Pb[B(Im)(4)](4-carboxy-TEMPO) 1 and Pb[B(Im)(4)](3-carboxy-PROXYL)(H(2)O)2, respectively. The resultant materials are paramagnetic, and the organization of the radical units differs between the two compounds. This results in changes in electronic structure of the radical unit, as observed by EPR spectroscopy.     

Magnetically aligned phospholipid bilayers with large q ratios stabilize magnetic alignment with high order in the gel and L(alpha) phases

The magnetic alignment behavior ofbicelles (magnetically alignable phospholipid bilayered membranes) as a function of the q ratio (1,2-dihexanoyl-sn-glycerol phosphatidylcholine/1,2-dimyristoyl-sn-glycerol phosphatidylcholine mole ratio) and temperature was studied by spin-labeled X-band electron paramagnetic resonance (EPR) spectroscopy and solid-state 2H and 31P NMR spectroscopy. Well-aligned bicelle samples were obtained at 45 degrees C for q ratios between 2.5 and 9.5 in both the EPR and NMR spectroscopic studies. The molecular order of the system, S(mol), increased as the q ratio increased and as the temperature decreased. For higher q ratios (> or = 5.5), bicelles maintained magnetic alignment when cooled below the main phase transition temperature (approximately 30 degrees C when in the presence of lanthanide cations), which is the first time, to our knowledge, that bicelles were magnetically aligned in the gel phase. For the 9.5 q ratio sample at 25 degrees C, S(mol) was calculated to be 0.83 (from 2H NMR spectra, utilizing the isotopic label perdeuterated 1,2-dimyristoyl-sn-glycerol phosphatidylcholine) and 0.911 (from EPR spectra utilizing the spin probe 3beta-doxyl-5alpha-cholestane). The molecular ordering of the high q ratio bicelles is comparable to literature values of S(mol) for both multilamellar vesicles and macroscopically aligned phospholipid bilayers on glass plates. The order parameter S(bicelle) revealed that the greatest degree of bicelle alignment was found at higher temperatures and larger q ratios (S(bicelle) = -0.92 for q ratio 8.5 at 50 degrees C).     

The influence of a HF and an annealing treatment on the barrier height of p- and n-type Si MIS structures

In this paper, the influence of two pre-evaporation surface treatments on the electrical characteristics of n- and p-Si/Au, Cr and Ti MIS diodes are studied. A strong dependence of the barrier height on the pre-evaporation treatment is observed and is found to be independent of the metal work function. In order to explain this, it is suggested that the interfacial charges are strongly affected by the pre-evaporation treatment used.     

Separation of sialyl-oligosaccharides by high-performance liquid chromatography. Application to the analysis of mono-, di-, tri- and tetrasialyl-oligosaccharides obtained by hydrazinolysis of alpha 1-acid glycoprotein

High-performance liquid chromatography has been applied to the separation of isomers of mono-, di-, tri- and tetrasialylated oligosaccharides derived from alpha 1-acid glycoprotein by hydrazinolysis. The separation of the sialyl-oligosaccharides on the basis of their negative charges was carried out with quaternary amine-bonded silica. Within each class, the anionic oligosaccharides were fractionated on the basis of their net carbohydrate content on alkylamine-modified silica using a mobile phase consisting of a mixture of acetonitrile and potassium dihydrogen phosphate with 0.01% of 1,4-diaminobutane or 0.01% of tetraethylpentamine.     

Double threshold behaviour of I－V characteristics of CoSi2/Si Schottky contacts

The forward current－voltage (I－V) characteristics of polycrystalline CoSi₂/n-Si(100) Schottky contacts have been measured in a wide temperature range. At low temperatures (≤200K), a plateau-like section is observed in the I－V characteristics around 10^-4A·cm^-2. The current in the small bias region significantly exceeds that expected by the model based on thermionic emission (TE) and a Gaussian distribution of Schottky barrier height (SBH). Such a double threshold behaviour can be explained by the barrier height inhomogeneity, i.e. at low temperatures the current through some patches with low SBH dominates at small bias region. With increasing bias voltage, the Ohmic effect becomes important and the current through the whole junction area exceeds the patch current, thus resulting in a plateau-like section in the I－V curves at moderate bias. For the polycrystalline CoSi₂/Si contacts studied in this paper, the apparent ideality factor of the patch current is much larger than that calculated from the TE model taking the pinch-off effect into account. This suggests that the current flowing through these patches is of the tunnelling type, rather than the thermionic emission type. The experimental I－V characteristics can be fitted reasonably well in the whole temperature region using the model based on tunnelling and pinch-off.     

Polymerisation und Copolymerisation von cyclischen ungesättigten Sulfiden

Ohne Zusammenfassung     

Fourier transforms having only real zeros

Let be a real entire function of order less than with only real zeros. Then we classify certain distribution functions such that the Fourier transform has only real zeros.

     

Insights into base-free OsO4-catalyzed aminohydroxylations employing chiral ligands

Attempts to perform the OsO₄-catalyzed enantioselective base-free aminohydroxylation of β,β-disubstituted enoates are described. Low yields and racemic products were obtained in the presence of standard chiral ligands, suggesting the occurrence of a “Second Cycle” process due to slow hydrolysis of the amino alcohol product from the Os metal center. Support for this hypothesis was provided by the slightly improved enantioselectivity (60:40 er) obtained with an amino alcohol ligand. Based on density functional theory calculations, it is proposed that the lack of significant enantioselectivity is due to a low-energy (3 + 2) oxo/imido cycloaddition transition state without the chiral ligand in the Second Cycle that outcompetes protonolysis in the First Cycle.     

A study of sputter-induced defects in magnetron-sputtered CoSi2 and TiSi2 Schottky barriers on n- and p-type GaP

Magnetron-sputtered CoSi₂ and TiSi₂ Schottky barriers on n- and p-type GaP were investigated. Their hitherto unknown barrier heights were determined to be 0.98 eV (for CoSi₂/n-GaP and CoSi₂/p-GaP), 0.91 eV (for TiSi₂/n-GaP), and 0.90 eV (for TiSi₂/p-GaP). It was found that magnetron-sputtering induced a compensated layer near the surface, both for n- and p-type GaP, with a thickness of about 0.05 m. As the dependence of the shift of the Mott-Schottky intercept with the V-axis on the substrate dopant concentration obeyed some specific law, we proposed that the defects are neutral complexes of dopant ions and sputter-induced native defects. These native defects were assumed to depend on the Fermi level position, namely the P_Ga antisite and the V_P vacancy for p-GaP and the V_Ga vacancy for n-GaP. The conversion between these defects occurs by nearest neighbour hopping of a phosphorus atom. The Schottky barrier heights obtained on p-GaP could be explained by Fermi level pinning at the surface due to the P_Ga defects. This could not be confirmed by n-GaP as the energy level position of the V_Ga was not available. The defects could be annealed out between 200° C and 300° C and the associated change of the Schottky barrier height corroborated the proposed model.On leave from the Université de Burundi, Faculté des Sciences, Bujumbura, Burundi