Mise en evidence de la formation de R2MMR2 a partir de disila-1,2 et digerma-1,2 cyclohexenes-4

Intermediates with π-bonded Si or Ge are generated during gas phase pyrolysis of 1,2-disila- or 1,2-digerma-cyclohex-4-enes.Compounds such as R₂MMR₂ are successfully trapped by addition or [2 + 2] and [4 + 2] cycloadditions. These results were proved by mass spectrometry.     

A New Route to Crystalline Salts of the Hydrolytic Dimer of Chromium(III)

Crystalline salts of the hydrolytic dimer of Cr(III), [Cr₂(μ-OH)₂(H₂O)₈]X₄·n H₂O (X = p-toluenesulfonate (tos) or mesitylene-2-sulfonate (dmtos)) have been prepared in good yields via a simple two-step procedure: H⁺ oxidation of Cr metal to give Cr²⁺ (T ≈? 70°) followed by O₂ oxidation, of Cr²⁺ to give the dimer (T ≈? 25°). The mechanism of conversion of Cr²⁺ into the dimer is discussed.     

Matrix effects in immunobiosensor determination of clenbuterol in urine and serum

The construction of immunochemical inhibition assays for beta-agonist and hormone residues have previously been described. In the present work the beta-agonist assay was further optimised for application to biological samples, using urine as the main model matrix. Matrix interferences with the antigen-antibody interaction and non-specific binding (NSB) of matrix components to the sensor surface were systematically studied. A full factorial design experiment was employed for evaluating the effects of assay buffer composition. In addition, the influence of antibody concentration and sample dilution on the matrix background was investigated. NSB from urine was highly affected by buffer pH and salt concentration, while buffer composition had little effect on matrix interferences with the antigen-antibody interaction. Ultra-filtration efficiently prevented NSB from urine and serum samples. Increased antibody dilution reduced the matrix background while sample dilution had an opposite effect.     

Koordinative Verankerung von Nickelkomplexen an Hectorit-Schichtsilicaten mit Hilfe langkettiger Diphenylphosphinogruppen

Coordinative Anchoring of Nickel Complexes on Hectorite-Layer Silicates through long-chain Diphenylphosphine Groups The treatment of sodium and calcium hectorites with 2-diphenylphosphinoethyltriethoxysilane leads, by condensation of free hydroxyl groups, to functionalized layer silicates with long-chain diphenylphosphine groups onto which nickel chloride can be coordinated. In these modified hectorites the fourth coordination site of the tetrahedrally coordinated nickel is occupied by a labile solvent molecule. On the other hand, the complex bis(2-diphenylphosphinoethyltriethoxysilane)nickel(II) chloride prepared beforehand also reacts with sodium hectorite under condensation; but the square-planar coordination of the four stable ligands (two chloride and two phosphor atoms) is maintained. This finds its expression in a significant activity decrease in catalytic reactions as compared to the aforementioned products.     

Rapid ion-exchange technique for the separation and preconcentration of chromium(VI) and chromium(III) in fresh waters

A technique for the separation and preconcentration of Cr(VI) and Cr(III) in fresh waters is presented. The analytical procedure involves the use of anion- and cation-exchange columns. The columns are eluted and the eluate is analysed for chromium using a graphite furnace atomic absorption spectrometer. The recovery of Cr(VI) and Cr(III) is 97.86 ± 1.31% and 102.36 ± 1.25% (95% confidence), respectively. The detection limits are 0.019 and 0.020 μg 1^?1 for 200 ml of sample (twice the standard deviation of eleven replicate blanks). The method is rapid and the need for minimum sample handling avoids contamination problems.     

Mechanistic investigations of the ethylene tetramerisation reaction

The unprecedented selective tetramerisation of ethylene to 1-octene was recently reported. In the present study various mechanistic aspects of this novel transformation were investigated. The unusually high 1-octene selectivity in chromium-catalyzed ethylene tetramerisation reactions is caused by the unique extended metallacyclic mechanism in operation. Both 1-octene and higher 1-alkenes are formed by further ethylene insertion into a metallacycloheptane intermediate, whereas 1-hexene is formed by elimination from this species as in other reported trimerisation reactions. This is supported by deuterium labeling studies, analysis of the molar distribution of 1-alkene products, and identification of secondary co-oligomerization reaction products. In addition, the formation of two C6 cyclic products, methylenecyclopentane and methylcyclopentane, is discussed, and a bimetallic disproportionation mechanism to account for the available data is proposed.     

Serial potentiometric and conductometric detection in fast non-suppressed ion chromatography applied to the analysis of filter collected airborne particulates

Non-suppressed ion chromatography (NSIC) has been optimized to permit the determination of chloride, nitrate and sulfate in the low mg/l concentration range within 3 min. Using conductometric detection the extraneous (positive) injection peak was found to adversely affect the early eluting chloride signal when samples with high amounts of total cations are analyzed. The serial arrangement of a downstream potentiometric detector with a chloride ion selective electrode, which does not respond to other alterations of the composition of the eluent than the change in chloride concentrations, is shown to be suitable for interference-free, sensitive and reliable chloride evaluation. Application of the proposed system to the determination of chloride in extracts of filter collected airborne particulates revealed (for those samples that could be evaluated by both detection methods) high precision and no systematic deviations between conductometric and potentiometric detection.Dedicated to Professor Dr. Peter Brätter on the occasion of his 60th birthday