Ba2[Ni3N2]: a low-valent nitridonickelate-synthesis, crystal structure, and physical properties

The ternary alkaline-earth nitridonickelate Ba2[Ni3N2] (Ba2[NiI2Ni0N2]) was prepared by the reaction of mixtures of Ba2N and Ni in nitrogen gas of ambient back-pressure at 1173 K. The crystal structure determined by X-ray single-crystal and powder diffraction methods as well as from neutron diffraction data at various temperatures between 2 and 298 K is orthorhombic (Cmca (no. 64), 298 K: a=715.27(18) pm, b=1032.99(21) pm, c=740.12(20) pm) and provides the first example of a nitridonickelate with a two-dimensional complex anion. The Ni2 atom is described with a split position and the corresponding superstructure variants are investigated by theoretical full-potential nonorthogonal local-orbital calculations (FPLO). The average oxidation state of Ni in Ba2[Ni3N2] is +0.67, the lowest average value observed in nitridonickelates so far. Investigations of the physical properties demonstrate that Ba2[Ni3N2] acts as a "poor" metal with a large resistivity of approximately 2.7 mOmega cm at 300 K and exhibits low-dimensional magnetism with antiferromagnetic ordering at T approximately 90 K. XAS spectra correspond with low-valent Ni states.     

Avoiding negative populations in explicit Poisson tau-leaping

The explicit tau-leaping procedure attempts to speed up the stochastic simulation of a chemically reacting system by approximating the number of firings of each reaction channel during a chosen time increment tau as a Poisson random variable. Since the Poisson random variable can have arbitrarily large sample values, there is always the possibility that this procedure will cause one or more reaction channels to fire so many times during tau that the population of some reactant species will be driven negative. Two recent papers have shown how that unacceptable occurrence can be avoided by replacing the Poisson random variables with binomial random variables, whose values are naturally bounded. This paper describes a modified Poisson tau-leaping procedure that also avoids negative populations, but is easier to implement than the binomial procedure. The new Poisson procedure also introduces a second control parameter, whose value essentially dials the procedure from the original Poisson tau-leaping at one extreme to the exact stochastic simulation algorithm at the other; therefore, the modified Poisson procedure will generally be more accurate than the original Poisson procedure.     

Cationic Xylan Derivatives with High Degree of Functionalization

Xylan from birch wood was characterized regarding both the supramolecular structure (X-ray, CP/MAS ¹³C-NMR) and the sugar composition. The reaction of the birch wood xylan with 2,3-epoxypropyltrimethylammonium chloride in 1,2-dimethoxyethane as slurry medium yields water-soluble, cationic 2-hydroxypropyltrimethylammonium xylan derivatives with high degree of substitution (DS). The DS values up to 1.6 can be controlled by adjusting the molar ratio in a one step synthesis. The structure of the cationic xylan derivatives was confirmed by means of DEPT(135) NMR spectroscopy. Film forming properties of cationic xylan derivatives were investigated with SEM measurements.     

Phenyl radicals react with dinucleoside phosphates by addition to purine bases and H-atom abstraction from a sugar moiety

Laser-induced acoustic desorption combined with mass spectrometry has been used to demonstrate that phenyl radicals can attack dinucleoside phosphates at both the sugar and base moieties, that purine bases are more susceptible to the attack than pyrimidine bases, and that the more electrophilic the radical, the more efficient the damage to dinucleoside phosphates.     

Polyelectrolyte complex formation between poly(diallyldimethyl-ammonium chloride) and copolymers of acrylamide and sodium-acrylate

 The interaction between oppositely charged polyelectrolytes, in this study poly(diallyldimethylammonium chloride) (PDADMAC) and copolymers of acrylamide and sodium-acrylate differing in their chain length and charge density parameter (ξ) was investigated in relation to the molar charge ratio of anionic to cationic charges (n ₋/n ₊). The molecular weights of the polyelectrolytes used were 2.9·10⁵g/mol for PDADMAC and for the polyacrylamide copolymers 14 ·10⁶g/mol as well as 5·10⁵g/mol obtained by ultrasonic degradation of the high molecular weight copolymers. The charge density parameters of the polyanions used (ξ ^PR ) varied between 0.14 and 0.64. Complexation between PDADMAC and high molecular weight polyanions leads mainly to macroscopic phase separation whereas the degraded polyanions and PDADMAC formed soluble complexes as well as stable dispersions, if charge excess was available. Precipitates and dispersions were characterized by several methods such as element analysis, thermogravimetry, pyrolysis-GC/MS, PEL titration, ζ-potential measurements, determination of turbidity, particle size measurements and determination of carbon content (TOC).  All precipitated complexes include about 20% water and are of 1:1 stoichiometry concerning ionic binding. Investigations of dispersions confirm 1:1 stoichiometry of complex particles stabilized by excess polyelectrolyte and soluble complexes. It was also found that the particle size can be varied via the charge density parameter of the polyanions used in the range of negative charge excess. Received: 21 June 2001 Accepted: 9 October 2001     

Crystal structure investigations of ZrAsxSey (x>y, x+y≤2) by single crystal neutron diffraction at 300 K, 25 K and 2.3 K

Large single crystals of ZrAs_xSe_y (x>y, x+y≤2, PbFCl type of structure, space group P4/nmm) were grown by Chemical Transport. Structural details were studied by single crystal neutron diffraction techniques at various temperatures. One single crystal specimen with chemical composition ZrAs_1.595(3)Se_0.393(1) was studied at ambient temperature (R1=5.10 %, wR2=13.18 %), and a second crystal with composition ZrAs_1.420(3)Se_0.560(1) was investigated at 25 K (R1=2.70%, wR2=5.70 %) and 2.3 K (R1=2.30 %, wR2=4.70 %), respectively. The chemical compositions of the crystals under investigation were determined by wavelength dispersive X-ray spectroscopy. The quantification of trace elements was carried out by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry. According to the crystal structure refinements the crystallographic 2a site is occupied by As, together with a significant amount of vacancies. One of the 2c sites is fully occupied by As and Se (random distribution). With respect to the fractional coordinates of the atoms, the crystal structure determinations based on the data obtained at 25.0 K and 2.3 K did not show significant deviations from ambient temperature results. The temperature dependence of the displacement parameters indicates a static displacement of As on the 2a sites (located on the (0 0 1) planes) for all temperatures. No indications for any occupation of interstitial sites or the presence of vacancies on the Zr (2a) site were found.     

Automated one-loop calculations with GoSam

We present the program package GoSam which is designed for the automated calculation of one-loop amplitudes for multi-particle processes in renormalisable quantum field theories. The amplitudes, which are generated in terms of Feynman diagrams, can be reduced using either D-dimensional integrand-level decomposition or tensor reduction. GoSam can be used to calculate one-loop QCD and/or electroweak corrections to Standard Model processes and offers the flexibility to link model files for theories Beyond the Standard Model. A standard interface to programs calculating real radiation is also implemented. We demonstrate the flexibility of the program by presenting examples of processes with up to six external legs attached to the?loop.     

Superdeformation in142Eu

A rotational band with 15 transitions has been observed in¹⁴²Eu in an experiment with the GASP Ge detector array by the¹¹⁰Pd(³⁷Cl,5_n) reaction. The average energy spacing between the neighbouring transitions is 60 keV, which is similar to the energy spacings in the superdeformed bands in the mass 140 region. The band has a constant dynamic moment of inertia as suggested by Cranked Shell Model calculations.     

Direct Imaging of Octahedral Distortion in a Complex Molybdenum Vanadium Mixed Oxide

Complex Mo,V‐based mixed oxides that crystallize in the orthorhombic M1‐type structure are promising candidates for the selective oxidation of small alkanes. The oxygen sublattice of such a complex oxide has been studied by annular bright field scanning transmission electron microscopy. The recorded micrographs directly display the local distortion in the metal oxygen octahedra. From the degree of distortion we are able to draw conclusions on the distribution of oxidation states in the cation columns at different sites. The results are supported by X‐ray diffraction and electron paramagnetic resonance measurements that provide integral details about the crystal structure and spin coupling, respectively.