Oligomerization and structural variation of alkali metal silyloxides

Lithium and potassium silyloxide complexes [Li(OSiPh₃)]_n (1), [K(thf)_0.2 (OSiPh₃)]_n (3) and [K(OSiMe₂^tBu)]_n (6) were prepared by deprotonation of HOSiPh₃ or HOSiMe₂^tBu with [Li(ⁿBu)] in hexane or KH in THF, respectively. Crystalline DME adducts [Li(μ-OSiPh₃)(η²-DME)]₂ (2) and [K₄(μ₃-OSiPh₃)₃(μ₃-OSiPh₂(η¹-Ph))(η²-DME)]₂ (μ-DME) (4) were prepared by dissolving 1 or 3, respectively, in dimethoxyethane followed by precipitation with alkane. The potassium-sequestered complexes [K(18-crown-6) (OSiPh₃)]₂ (5) and [K(18-crown-6)(OSiMe₂^tBu)]_n (7) were prepared from 3 or 5, respectively, and one equiv. of 18-crown-6 ether. The complexes were characterized by single-crystal X-ray diffraction: [Li(μ-OSiPh₃)(η²-DME)]₂ (2): a dimer featuring tetrahedral lithium centres linked by bridging —OSiPh₃ ligands. [Crystal data ( − 156°C): space group P , a = 14.238(6), b = 15.182(7), c = 11.796(5) Å, α = 110.57(2), β = 112.02(2), γ = 63.02(1) Å, V = 2055.33 ų, Z = 2.] [K₄(μ₃-OSiPh₃)₃{μ₃-OSiPh₂(η¹-Ph)}(η²-DME)]₂(μ-DME) (4): (1) two cubanes each having every potassium vertex chemically distinct; (2) one chelating DME ligand, one DME ligand bridging between two cubanes; and (3) a K-ipso-phenyl carbon contact. [Crystal data ( − 133°C): a = 14.246(4), b = 30.939(9), c = 17.981(5) Å, β = 112.33(1)° with Z = 2 in space group P2₁/c.] [K(18-crown-6)OSiPh₃]₂ (5): A dimer with slipped face-to-face stacking of the quasi-planar K(18-crown-6)⁺ part of the two Ph₃SiOK(18-crown-6) molecules; these are linked by a dative bond from one ether oxygen of a given crown to potassium contained in the other crown. [Crystal data ( − 155°C): a = 9.324(2), b = 17.640(5), c = 18.148(15) Å, β = 91.60(1)° with Z = 4 in space group P2₁/c.]     

Determination of desoxycorticosterone acetate using tetrazoliumsalt

Summary The colorimetric procedure for the determination of desoxycorticosterone acetate described byMader andBuck was investigated. In order to obtain reproducible results the determinations should be carried out at a determined alcohol concentration and in the dark and the measurements of the colors should be made a determined interval of time after the mixing of the solutions. A procedure is proposed.

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Zusammenfassung Das vonMader undBuck beschriebene Verfahren zur kolorimetrischen Bestimmung von Desoxycortioosteronacetat wurde nachgeprüft. Um reproduzierbare Ergebnisse zu erhalten, muß eine bestimmte Alkoholkonzentration eingehalten, die Farbreaktion bei Lichtabschluß durchgeführt und die Farbmessung nach Ablauf einer bestimmten Zeit nach dem Mischen der Lösungen vorgenommen werden. Eine genaue Arbeitsvorschrift wird mitgeteilt.

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Résumé On étudie le dosage colorimétrique de l'acétate de désoxycorticostérone décrit parMader etBuck. Afin d'obtenir des résultats reproductibles, les dosages doivent être effectués à une concentration déterminée de l'alcool, à l'obscurité et les mesures des colorations à des intervalles de temps déterminés après le mélange des solutions. On propose un mode opératoire.

     

Heat Content Asymptotics for Riemannian Manifolds with Zaremba Boundary Conditions

The existence of a full asymptotic expansion for the heat content asymptotics of an operator of Laplace type with classical Zaremba boundary conditions on a smooth manifold is established. The first three coefficients in this asymptotic expansion are determined in terms of geometric invariants; partial information is obtained about the fourth coefficient.      

Rapid monitoring of intermediate states and mass balance of nitrogen during denitrification by means of cavity enhanced Raman multi-gas sensing

The comprehensive investigation of changes in N cycling has been challenging so far due to difficulties with measuring gases such as N₂ and N₂O simultaneously. In this study we introduce cavity enhanced Raman gas spectroscopy as a new analytical methodology for tracing the stepwise reduction of ¹⁵N-labelled nitrate by the denitrifying bacteria Pseudomonas stutzeri. The unique capabilities of Raman multi-gas analysis enabled real-time, continuous, and non-consumptive quantification of the relevant gases (¹⁴N₂, ¹⁴N₂O, O₂, and CO₂) and to trace the fate of ¹⁵N-labeled nitrate substrate (¹⁵N₂, ¹⁵N₂O) added to a P. stutzeri culture with one single measurement. Using this new methodology, we could quantify the kinetics of the formation and degradation for all gaseous compounds (educts and products) and thus study the reaction orders. The gas quantification was complemented with the analysis of nitrate and nitrite concentrations for the online monitoring of the total nitrogen element budget. The simultaneous quantification of all gases also enabled the contactless and sterile online acquisition of the pH changes in the P. stutzeri culture by the stoichiometry of the redox reactions during denitrification and the CO₂-bicarbonate equilibrium. Continuous pH monitoring – without the need to insert an electrode into solution – elucidated e.g. an increase in the slope of the pH value coinciding with an accumulation of nitrite, which in turn led to a temporary accumulation of N₂O, due to an inhibition of nitrous oxide reductase. Cavity enhanced Raman gas spectroscopy has a high potential for the assessment of denitrification processes and can contribute substantially to our understanding of nitrogen cycling in both natural and agricultural systems.     

Evaluation of quantum dots applied as switchable layer in a light-controlled electrochemical sensor

Gold electrodes with switchable conductance are created by coating the gold surface with different colloidal quantum dots. For the quantum dot immobilization, a dithiol compound was used. By polarizing the electrode and applying a light pointer, local photocurrents were generated. The performance of this setup was characterized for a variety of different nanoparticle materials regarding drift and signal-to-noise ratio. We varied the following parameters: quantum dot materials and immobilization protocol. The results indicate that the performance of the sensor strongly depends on how the quantum dots are bound to the gold electrode. The best results were obtained by inclusion of an additional polyelectrolyte film, which had been fabricated using layer-by-layer assembly.