Hard X‐ray phase‐contrast tomography of non‐homogeneous specimens: grating interferometry versus propagation‐based imaging

X‐ray phase‐contrast imaging is an effective approach to drastically increase the contrast and sensitivity of microtomographic techniques. Numerous approaches to depict the real part of the complex‐valued refractive index of a specimen are nowadays available. A comparative study using experimental data from grating‐based interferometry and propagation‐based phase contrast combined with single‐distance phase retrieval applied to a non‐homogeneous sample is presented (acquired at beamline ID19‐ESRF). It is shown that grating‐based interferometry can handle density gradients in a superior manner. The study underlines the complementarity of the two techniques for practical applications.     

Cationic Si−H−Si Bridges in Polysilanes: Their Detection and Targeted Formation in Stable Ion Studies

The ionization of 1,1‐dihydridocyclopentasilane 7 has been found to yield the cyclic polysilanylsilyl cation 8 instead of the expected hydrogen‐substituted silylium ion 6 . The silyl cation 8 is stabilized by the formation of an intramolecular Si?H?Si bridge, which also provides the thermodynamic driving force for its formation. In general, the preference for the formation of Si?H?Si bridges can be used to scavenge and identify transient intermediates in the Lewis acid induced rearrangement of polysilanes. The validity of this concept has been demonstrated for one central step in this chemistry, the ring‐contraction reaction of cyclohexasilanes to form silylcyclopentasilanes.     

Synthesis,crystal structures and magnetic behaviors of two dicyanamide bridged di- and polynuclear complexes of cobalt(II) derived from 2,4,6-tris(2-pyridyl)1,3,5-triazine and imidazole

Two new dicyanamido-bridged di- and polynuclear complexes of Co(II), [Co(dca)(tptz)(H₂O)]₂·2(ClO₄) (1) and [Co(dca)₂(imz)₂]_n (2) [dca, dicyanamide; tptz, 2,4,6-tris(2-pyridyl)1,3,5-triazine; and imz, imidazole] have been synthesized and characterized structurally, as well as magnetically. The X-ray single crystal structure determination of complex 1 shows that two symmetry related octahedral Co(II) ions are separated by dca ligand and other coordination sites are satisfied by tptz and aquo ligands. Each dinuclear unit is associated with each other by intramolecular hydrogen bonding interactions, giving rise to a 1D chain structure. On the other hand complex 2 is a 1D coordination polymer having [Co(II)(imz)₂] units connected by double bridging dca ligands. These 1D chains interact through face-to-face π–π stacking interactions of the imz rings extending the dimensionality to a 2D supramolecular network. The variable temperature (300–2 K) magnetic measurements of both compounds reveal that dicyanamide exhibits a weak antiferromagnetic interaction between the metal centers.     

Dispersion energy enforced dimerization of a cyclic disilylated plumbylene

By reaction of 1,4-dipotassio-1,1,4,4-tetrakis(trimethylsilyl)tetramethyltetrasilane with PbBr(2) in the presence of triethylphosphine a base adduct of a cyclic disilylated plumbylene could be obtained. Phosphine abstraction with B(C(6)F(5))(3) led to formation of a base-free plumbylene dimer, which features an unexpected single donor-acceptor PbPb bond. The results of density functional computations at the M06-2X and B3LYP level of theory indicate that the dominating interactions which hold the plumbylene subunits together and which define its actual molecular structure are attracting van der Waals forces between the two large and polarizable plumbylene subunits.     

Syntheses of Functionalized Oligosilanes

Summary. A number of differently substituted isotetra- and isoheptasilanes were synthesized.     

über Kapillaren und Kapillarsysteme,XXIII1

Zusammenfassung Der normale Luftgehalt filmbildender, organischer Flüssigkeiten (Streich- und Spritzlacke, pigmentierte und nichtpigmentierte Lacke, harzfreie und harzhaltige öllacke, Gie\lösungen für Membranen) wird in einer Vakuumapparatur unter Verwendung einer Sprengelpumpe gemessen und durch den Bunsenschen Absorptionskoeffizienten ausgedrückt.Die bei 20 und 760 mm Hg Gesamtdruck mit Luft gesÄttigten Flüssigkeiten lösen 5 bis 18 Vol.-Proz. Luft, wÄhrend Wasser unter den gleichen Bedingungen nur 2 Vol.-Proz. löst.Die Entgasung durch Ultraschall liefert bei den hochviskosen Lacken recht unbefriedigende Entgasungsgrade.     

The first stable beta-fluorosilylanion

The reaction of 2 equiv of tris(trimethylsilyl)silylfluoride with potassium tert-butoxide in the presence of donor molecules (THF, DME, 18-crown-6) leads to the clean formation of an adduct of 1-potassio-2-fluorotetrakis(trimethylsilyl)disilane. Attempts to transmetalate this compound effect the elimination of metal fluoride accompanied by the formation of tetrakis(trimethylsilyl)disilene. The latter can either be trapped in a cycloaddition reaction or in the absence of trapping reagents dimerizes to octakis(trimethylsilyl)cyclotetrasilane.     

Baumaterialien

Ohne Zusammenfassung