Global stability for convection when the viscosity has a maximum

Until now, an unconditional nonlinear energy stability analysis for thermal convection according to Navier-Stokes theory had not been developed for the case in which the viscosity depends on the temperature in a quadratic manner such that the viscosity has a maximum. We analyse here a model of non-Newtonian fluid behaviour that allows us to develop an unconditional analysis directly when the quadratic viscosity relation is allowed. By unconditional, we mean that the nonlinear stability so obtained holds for arbitrarily large perturbations of the initial data. The nonlinear stability boundaries derived herein are sharp when compared with the linear instability thresholds.Received: 9 April 2003, Accepted: 28 April 2003, Published online: 12 December 2003PACS: 03.50.De, 04.20.-q, 42.65.-kCorrespondence to B. Straughan     

Use of neural networks and diode-array detection to develop an isocratic HPLC method for the analysis of nitrophenol pesticides and related compounds

Summary We have used an artificial neural network to optimize the composition of the mobile phase for an isocratic HPLC method for the analysis of nitrophenol pesticides and related compounds, on the basic of different response functions, and have compared the results with those obtained by application of response-surface methodology. These studies resulted in the selection the mobile phase 10:30:15:45 methanol-acetonitrile-tetrahydrofuran-buffer solution (0.1m acetic acid and 0.1m sodium perchlorate); the flow-rate was 1 mL min⁻¹. Under these conditions a chromatogram showing twelve well-resolved peaks was obtained in 14 min. Although the peaks corresponding to ethylparathion and medinoterb acetate overlapped severely, it was possible, by use, of a diode-array spectrophotometer for detection, and by combining the absorbance measured at different wavelengths as the signal, to separate the peaks corresponding to one or other of the compounds. Calibration plots were constructed for the concentration range 2–10 ppm. Detection limits, calculated by the method of Clayton et al., were approximately 0.32–0.69 ppm. The method has been applied to the analysis of these compounds in fortified river water samples, after previous preliminary preconcentration by solid-liquid extraction on a C₁₈ cartridge.     

New Efficient Synthesis of Methyl Trans-2-Aryl (or Hetaryl)-1-Benzamidocyclopropancarboxylates

Methyl trans-2-aryl(or hetaryl)-1-benzamidocyclopropancarboxylates can be easily obtained by thermal decomposition of trans-4-aryl(or hetaryl)-3-benzamido-3-carbomethoxy-Δ¹-pyrazolines synthesized by 1,3-dipolar cycloaddition of diazomethane with methyl E-3-aryl(or hetaryl)-2-benzamido-2-propenoates.     

A Convenient Method for the Phosphorylation of Phenols with Diethyl Cyanophosphonate

Phosphorylation of phenols with diethyl cyanophosphonate in methylene chloride solution at 0°C is an easy, rapid and good yielding reaction.     

Enantioselective synthesis of alkyl-branched alkanes. Synthesis Of the stereoisomers of 7,11-dimethylheptadecane and 7-methylheptadecane, components of the pheromone of lambdina species

The stereoisomers of 7,11-dimethylheptadecane and 7-methylheptadecane have been synthesized. The key step used has been the intramolecular hydride transfer from a secondary gamma-benzyloxy group with defined absolute stereochemistry to a cation generated by Lewis acid treatment of the suitable tertiary Co(2)(CO)(6)-complexed propargylic alcohol. The application of this method provided stereochemically defined alpha-alkyl-gamma-hydroxy-acetylenes that after hydrogenation and further reductive elimination of the hydroxyl group yielded sec-alkyl hydrocarbons.     

Heronapyrroles A-C: farnesylated 2-nitropyrroles from an Australian marine-derived Streptomyces sp

Chemical analysis of a marine-derived Streptomyces sp. (CMB-M0423) isolated from beach sand off Heron Island, Australia, yielded three new members of the rare pyrroloterpene biosynthetic structure class. Identified by detailed spectroscopic analysis as the first reported examples of naturally occurring 2-nitropyrroles, heronapyrroles A-C (1-3) displayed promising biological activity-with low to submicromolar IC(50) activity against Gram-positive bacteria but no cytotoxicity toward mammalian cell lines.     

Mechanism of multi‐metal(loid) methylation and hydride generation by methylcobalamin and cob(I)alamin: a side reaction of methanogenesis

Metal(loid)s are subject to many transformation processes in the environment, such as oxidation, reduction, methylation and hydride generation, predominantly accomplished by prokaryotes. Since these widespread processes affect the bioavailability and toxicity of metal(loid)s to a large extent, the investigation of their formation is of high relevance. Methanogenic Archaea are capable of methylating and hydrogenating Group 15 and 16 metal(loid)s arsenic, selenium, antimony, tellurium, and bismuth due to side reactions between central methanogenic cofactors, methylcobalamin (CH₃Cob(III)) and cob(I)alamin (Cob(I)). Here, we present systematic mechanistic studies on methylation and hydride generation of Group 15 and 16 metal(loid)s by CH₃Cob(III) and Cob(I). Pentavalent arsenical species showed neither methylation nor reduction as determined by using a newly developed oxidation state specific hydride generation technique, which allows direct determination of tri‐ and pentavalent arsenic species in a single batch. In contrast, efficient methylation of trivalent species without a change in oxidation state indicated that the methyl transfer does not proceed via a Challenger‐like oxidative methylation, but via a non‐oxidative methylation. Our findings also point towards a similar mechanism for antimony, bismuth, selenium, and tellurium. Overall, we suggest that the transfer of a methyl group does not involve a free reactive species, such as a radical, but instead is transferred either in a concerted nucleophilic substitution or in a caged radical mechanism. For hydride generation, we propose the intermediate formation of hydridocobalamin, transferring a hydride ion to the metal(loid)s. Copyright © 2012 John Wiley & Sons, Ltd.     

New oxamidato-bridged Cu(II)-Ni(II) complexes: supramolecular structures with thiocyanate ligands and hydrogen bonds. Magnetostructural studies: DFT calculations

Four new supramolecular compounds of Cu(II)-Ni(II) have been synthesized and characterized: [Cu(Me(2)oxpn)Ni(mu-NCS)(H(2)O)(tmen)](2)(ClO(4))(2) (1), [Cu(Me(2)oxpn)Ni(mu-NCS)(H(2)O)(tmen)](2)(PF(6))(2) (2), [Cu(oxpn)Ni(mu-NCS)(NCS) (tmen)](n) (3), and [Cu(Me(2)oxpn)Ni(mu-NCS)(NCS)(tmen)](n) (4), where oxpn = N,N'-bis(3-aminopropyl)oxamidate, Me(2)oxpn = N,N'-bis(3-amino-2,2'-dimethylpropyl)oxamidate, and tmen = N,N,N',N'-tetramethylethylenediamine. Their crystal structures were solved. Complexes 1 and 2 have the same tetranuclear cationic part but a different counteranion. The cationic part consists of two [Cu(Me(2)oxpn)Ni] moieties linked by SCN(-) bridged ligands and intra-tetranuclear hydrogen bonds. In the case of complex 3, a two-dimensional system was built, the thiocyanate ligand linking the dinuclear units gives a chain, and the chains are linked together by hydrogen bonds; intrachain hydrogen bonds are also present. For complex 4, the thiocyanate ligands produce intermolecular linkages between the dinuclear entities, giving a one-dimensional system; intrachain hydrogen bonds are also present. The magnetic properties of the four complexes were studied by susceptibility measurements vs temperature. DFT calculations were made to study the contribution of the SCN(-) and hydrogen bond bridges in the magnetic coupling.