Réaction du n-bromosuccinimide sur les aryl-2 dioxolannes-1,3 d'eroulement de la réaction

Reaction of NBS with 1,3-dioxolanes fused to a bicyclo[2.2.1] skeleton involves two intermediates, the dioxolanylium ion and the 2-succinimido-1,3-dioxolane. Evidence is presented that the unusual stability of the former ionic species is due to the steric hindrance towards endo attack by bromide ion, thus leading to the succinimido compound which by action of hydrobromic acid formed insitu, yields the former ionic species which finally gives the dioxolane ring-opening products. Some factors influencing the efficiency of the NBS reaction with hindered dioxolanes are pointed out.     

The matrix-isolated molecular complexes CO/XF (X=Cl,Br,I) and the molecular structure of FC(O)Br

FC(O)Br has been synthesized, and its IR spectrum in the gas phase and isolated in an Ar matrix, as well as, its Raman spectrum in the solid state at -196 degrees C has been analyzed. Its molecular structure has been determined and its UV has been measured. FC(O)Br and FC(O)Cl has been photodissociated in an argon matrix at 17 K with a 193 nm laser. The photolysis produces CO and XF which recombine to form CO/XF complexes. The formation of complexes are followed by the shift of the normal vibration modes with respect to CO and XF isolated in argon matrix. In the case of FC(O)Br, three isomers are identified, OC...BrF, OC...FBr, and CO...BrF, whereas for FC(O)Cl only one isomer is observed, OC...ClF. High level quantum chemical calculations are used to help the assignment of the different isomers.     

One-step synthesis and structure of an oligo(spiro-orthocarbonate)

The reaction of pentaerythritol and tetraethylorthocarbonate at 260 degrees C for 12 h yields a white crystalline material that was characterized by 13C CPMAS NMR, CHN analysis, FT-IR, electron and X-ray powder diffraction, and Rietveld analysis. The white crystalline material was found to have the formula C6H8O4 and a crystal structure with a monoclinic cell [a = 9.167 A, b = 5.681 A, c = 5.880 A, beta = 90.0 degrees , space group I2] of hexagonally arranged spiro-oligomeric chains.     

Production and characterization of nitrided CVD-SiC films

Silicon Carbide (SiC) and SiC with free silicon [SiC(Si)] thin films were prepared by chemical vapor deposition (CVD) using a CH₃SiCl₃-H₂-Ar gas mixture at a temperature of 1223 K. Afterwards these layers were gas nitrided in an ammonia-hydrogen-argon mixture at 1273 K. The solid product is an extremely thin film of silicon nitride on SiC or SiC(Si)-basic layers. These ultra thin silicon nitride films were investigated by glow discharge optical spectroscopy (GDOS) and x-ray photoelectron spectroscopy (XPS). The thickness of the layers was determined to a maximum value of 30 nm.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Synthesis and Stereochemistry of Some New 1,3-Dioxane Derivatives of 1,4-Diacetylbenzene

Summary.  The synthesis and stereochemistry of new 1,3-dioxane derivatives of 1,4-diacetylbenzene are reported. The anancomeric structure of these compounds, the axial orientation of the aryl group for both 1,3-dioxane rings, and the cis and trans isomerism of some of these compounds is discussed considering data of conformational analysis, NMR investigations, and single crystal X-ray diffractometry. Received September 12, 2001. Accepted (revised) November 5, 2001     

Electrochemical deposition of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> for thermoelectric microdevices

The electrolyses of solutions of bismuth oxide and tellurium oxide in nitric acid with molar ratios of Bi:Te=3:3–4:3 lead to cathodic deposits of films of bismuth telluride (Bi₂Te₃), an n-type semiconductor. Current densities of 2–5 mA/cm² were applied. Voltammetric investigations showed that Bi₂Te₃ deposition occurred at potentials more negative than −0.125 V (Ag/AgCl, 3 M KCl). The deposit was identified as bismuth telluride (γ-phase) by X-ray analysis. Hall-effect measurements verified the n-type semiconducting behaviour. The films can be deposited in microstructures for thermoelectric microdevices like thermoelectric batteries or thermoelectric sensors.     

Effects of some admixtures on the decomposition of calcium sulphate

The by-product phosphogypsum contains CaHPO₄, H₃PO₄, Ca₅(PO₄)₃F, Al₂O₃, Fe₂O₃, SiO₂ etc. besides the main product CaSO₄. By means of TG-DTA the thermochemical decomposition of pure phosphogypsum and phosphogypsum containing CaO, Al₂O₃, Fe₂O₃ and SiO₂ in a reductive atmosphere was studied. The solid phase was examined by high-temperature thermomicroscopy, scanning electronmicroscopy, X-ray diffraction and other methods. It could be confirmed, that most of the admixtures undergo secondary reactions with CaO and CaS, modifying the decomposition rate of CaSO₄.

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Zusammenfassung Das Abfallprodukt Phosphatgips enthält neben dem Hauptbestandteil CaSO₄ noch CaHPO₄, H₃PO₄, Ca₅(PO₄)₃F, Al₂O₃, Fe₂O₃, SiO₂ und anderes. Die thermochemische Zersetzung von Phosphatgips bzw. Phosphatgips mit Zusätzen von CaO, Al₂O₃, Fe₂O₃ und SiO₂ wurde in reduzierender Atmosphäre mittels TG-DTA untersucht. Die feste Phase wurde durch Hochtemperatur-Thermomikroskopie, Rasterelektronenmikroskopie, Röntgenbeugung und andere Methoden untersucht. Dabei wurde bestätigt, dass die meisten Beimengungen mit dem Calciumoxid bzw. -sulfid Sekundärreaktionen eingehen, wodurch auch die Zersetzungsgeschwindigkeit des CaSO₄ beeinflusst wird.

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, , ₄, Ca₅(PO₄)₃F, Al₂O₃, Fe₂O₃, SiO₂ . - , , , . , , . , , .

     

Ion transfer processes at ionic liquid based redox active drop deposited on an electrode surface

Ion transfer across the boundary formed at an ionic liquid drop deposited on an electrode immersed in aqueous solution, generated by electrochemical redox reaction at the electrode-ionic liquid interface, is studied to obtain information about the ability of anions to be transferred into a room temperature ionic liquid.