Sch?dlinosbek?mpfung

Ohne Zusammenfassung     

Laminar compressible flow in a tube

A two-dimensional solution for the velocity and pressure distributions in steady, laminar, isothermal flow of an ideal gas in a long tube is obtained as a double perturbation expension in β, the radius to length ratio, and ε, the relative pressure drop. It is found that simple approximations estimate the exact flow rate-pressure drop relationship accurately.     

Nachweis von Arsen

Ohne Zusammenfassung     

Zum Nachweis der Bors?ure

Ohne Zusammenfassung     

Tabakuntersuchung und Nicotinbestimmung

Ohne Zusammenfassung     

Intramolecular Heck reactions of aryl chlorides with alkynes

We have developed a reaction that affords the selective preparation of hexahydro-2H-pyrido[2,1-a]isoquinoline dienes, allenes, or alkenes via an intramolecular Heck cyclization of an aryl chloride with an alkyne. Tricyclic isoquinoline core structures of this nature are difficult to access by alternative methods. The unsaturated product formed can be partially controlled by choice of ligand and reaction solvent.     

Pronounced negative thermal expansion from a simple structure: cubic ScF(3)

Scandium trifluoride maintains a cubic ReO(3) type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ～300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, α(l) ≈ -14 ppm K(-1)). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW(2)O(8) and positive thermal expansion above ～1100 K. While the cubic ReO(3) structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF(3) is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.     

Pt dendrimer nanocomposites for oxygen reduction reaction in direct methanol fuel cells

Dendrimer-encapsulated Pt nanoparticles (G4OHPt) were prepared by chemical reduction at room temperature. The G4OHPt, with average diameters of ca. 2.7 nm, were characterized by X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. Electrocatalytic behavior for oxygen reduction reaction was investigated using a rotating disk electrode configuration in an acidic medium, with and without the presence of methanol (0.01, 0.1, and 1 M). Kinetic studies showed that electrodes based on Pt nanoparticles encapsulated inside the dendrimer display a higher selectivity for ORR in the presence of methanol than electrodes based on commercial Pt black catalysts. Also, the dendritic polymer confers a protective effect on the Pt in the presence of methanol, which allows its use as a cathode in a direct methanol fuel cell operating at different temperatures. Good performance was obtained at 90 °C and 2 bar of pressure with a low platinum loading on the electrode surface.     

Engineering enzyme specificity using computational design of a defined-sequence library

Engineered biosynthetic pathways have the potential to produce high-value molecules from inexpensive feedstocks, but a key limitation is engineering enzymes with high activity and specificity for new reactions. Here, we developed a method for combining structure-based computational protein design with library-based enzyme screening, in which inter-residue correlations favored by the design are encoded into a defined-sequence library. We validated this approach by engineering a glucose 6-oxidase enzyme for use in a proposed pathway to convert D-glucose into D-glucaric acid. The most active variant, identified after only one round of diversification and screening of only 10,000 wells, is approximately 400-fold more active on glucose than is the wild-type enzyme. We anticipate that this strategy will be broadly applicable to the discovery of new enzymes for engineered biological pathways.