Stereoelectronic influence of the type of bifunctional ansa-monocyclopentadienyldimethylsilylamido ligand on the molecular structures displayed by zirconium dichloride and 1,4-diphenylbutadiene complexes

A series of organozirconium dichloride and 1,4-diphenylbutadiene complexes featuring a dianionic bifunctional ligand with a cyclopentadienyl-type functionality and an appended amido N donor have been prepared and structurally characterized. [(C(5)H(4))SiMe(2)(N-t-Bu)]ZrCl(2), 1, [(C(9)H(6))SiMe(2)(N-t-Bu)]ZrCl(2), 2, and [(C(5)Me(4))SiMe(2)(N-i-Pr)]ZrCl(2), 3, were prepared in two steps, with ligand chelation accomplished by an amine elimination reaction followed by treatment of the diamido Zr intermediate with an excess of SiMe(3)Cl. X-ray structural analyses reveal that in the solid state 2 is monomeric, whereas 1 and 3 are centrosymmetric dimers linked by a pair of bridging chlorides. The level of asymmetry displayed by the central Zr(2)(micro-Cl)(2) moiety is indicated by the variation in the pair of independent bridging Zr-Cl bond distances, which are 2.618(1) and 2.657(1) A in 1 and 2.542(1) and 2.745(1) A in 3, respectively. The metathetical reactions of [Mg(C(4)H(4)Ph(2))(THF)(3)](n)() with 1, 2, 3, and [(C(5)Me(4))SiMe(2)(N-t-Bu)]ZrCl(2) proceed to afford the corresponding 1,4-diphenylbutadiene derivatives 4, 5, 6, and 7, respectively. Solution NMR data show that 6 is obtained exclusively as the supine isomer, whereas compounds 4, 5, and 7 exist as >20:1, 6:1, and 2:1 mixtures of the supine and prone isomers at ambient temperature. The molecular structures of the supine forms of 4, 5, 6, and 7 are appreciably folded (70-80 degrees ) along the line of intersection between the plane containing the Zr and the two terminal butadiene carbons and the plane of the cis-butadiene fragment. An increase in the folding is accompanied by a decrease in the difference between the average Zr-C(terminal) and Zr-C(internal) bond distances and leads to a more pronounced long-short-long C-C bond sequence within the coordinated butadiene.     

Upper limit on beauty lifetime and lower limit on weak mixing angles

Muons from a multihadron sample are used to determine an upper limit τ<1.4 × 10^?12s (95% CL) on the lifetime of beauty particles. The data are obtained with the JADE detector at PETRA. The result is interpreted within the standard model.     

Measurement of average electron densities in Si and Ge using MeV δ-rays produced by channelled high-energy projectiles

For the first time, average electron densities in silicon and germanium were measured using the channeling effect for 5 and 12 GeV/c protons, π⁺ and π^?. In the investigation, the yield of MeV electrons emitted through the back of the target was measured. Such gd-ray yields are proportional to the local electron density averaged along the path of the projectile in the target. For well-aligned, positive particles, the electron yield is reduced to around 15% of normal yield for germanium and 25% for silicon, whereas negatively charged, channeled projectiles give an increase in yield by a a factor of three compared to normal yield. The experimental results have been compared to yield curves calculated using the Lindhard channeling model in connection with special potential models, and very good agreement is obtained for positive particles when the electron density in the middle of the channels is obtained by summing the contributions from many neighbouring strings. For positive projectiles, this channeling method is most sensitive far away from the strings, where other techniques are weak. The results for negative, channeled particles agree fairly well with simplified theoretical calculations, neglecting dechanneling and the lack of equilibrium in angular momenta in the transverse plane.     

Synchrotron radiation excited X-ray fluorescence analysis using a graphite monochromator

Summary X-ray fluorescence analysis with monochromatic synchrotron radiation enables the direct analysis even of samples with a very strong matrix, while other methods require already a chemical pretreatment or preconcentration step. Because of free tunable excitation energy special elements or element groups of interest can be selectively excitated. The use of a graphite crystal for monochromatizing leads to a sufficient fluorescence intensity even for thin samples, because of a larger band width compared to other monochromator crystals. Nevertheless, a sufficient selectivity of the excitation will be achieved. Using a curved crystal focussing the beam in the horizontal plane of the storage ring, the intensity can be improved by a factor of about 20. The limits of detection for realistic samples (in this case aerosol samples are analysed) are in the range of 40 ng/ g. The results of the direct analysis of arsenic in aerosol samples are discussed and compared with an interlaboratory test.

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Röntgenfluorescenzanalyse mit Synchrotronstrahlung unter Verwendung eines Graphitmonochromators

Zusammenfassung Röntgenfluorescenzanalyse mit monochromatisierter Synchrotronstrahlung erlaubt die Direktanalyse auch von stark matrixbeladenen Proben, bei denen bei anderen Methoden schon eine chemische Vorbehandlung oder Anreicherung notwendig wird. Die frei wählbare Monochromatorenergie erlaubt die selektive Anregung bestimmter Elemente oder Elementgruppen. Die Verwendung eines Graphitkristalls zur Monochromatisierung ergibt aufgrund der höheren Bandbreite gegenüber anderen Monochromatorkristallen auch bei dünnen Proben eine ausreichende Fluorescenzintensität bei gleichzeitig genügender Selektivität. Durch Verwendung eines gebogenen Graphitkristalls zur Fokussierung des Strahls in der horizontalen Ebene kann die Intensität noch einmal um den Faktor 20 gesteigert werden. Die Nachweisgrenzen von mit Aerosolen beaufschlagten Filtern liegen im Bereich von 40 ng/g. Die Ergebnisse der Analyse von Arsen in Aerosolproben werden diskutiert und mit den Ergebnissen einer Ringanalyse verglichen.

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Dedicated to Prof. Dr. K. H. Lieser on the occasion of his 65th birthday     

Sorption of heavy metals by untreated red fir sawdust

Applied Biochemistry and Biotechnology - Equilibrium and rate relationships have been determined for the sorption of divalent copper (Cu2+) and hexavalent chromium (Cr6+) onto untreated Red Fir...