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Betson Mark Barker John Barnes Paul Atkinson Tim Jupe Andrew 《Transport in Porous Media》2004,57(2):203-214
This paper describes novel uses of synchrotron radiation in examining porosity distributions within porous media. Tomographic energy dispersive diffraction imaging and Tomographic X-ray fluorescence have been combined within one measurement method and used to highlight the porosity distribution in a typical sample of English Chalk. 相似文献
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Atropisomeric aromatic amides bearing 2-sulfanyl groups are oxidised by m-CPBA to the corresponding sulfoxides apparently with very high diastereoselectivity. NMR studies and oxidations of chiral benzamides however indicate that the kinetic selectivity of the oxidation is in fact relatively poor, and that the final diastereoisomeric ratio (typically >99:1) is under thermodynamic control, with relatively unhindered Ar-CO rotation readily converting the less stable to the more stable product diastereoisomer. Molecular mechanics indicates that the thermodynamic diastereoselectivity results principally from electrostatic repulsion between the C=O and S-O dipoles. 相似文献
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The bis[dimethyl(phenyl)silyl]cuprate reagent introduces a silyl group to the beta-position of three alpha,beta-unsaturated esters: methyl Z-4-dimethyl(phenyl)silylpent-2-enoate 11, and methyl Z- and E-(1'-dimethylphenylsilylbenzyl)but-2-enoates 14 and 15, diastereoselectively in the unexpected sense, syn to the silyl group in the conformation in which the hydrogen atom is 'inside'. The selectivity is low (58:42) in the first case 11, where the nucleophilic attack is adjacent to the stereogenic centre carrying the silyl group, and moderate (72:28) for both Z- and E-alpha,beta-unsaturated esters 14 and 15, where the nucleophilic attack is at the other end of the double bond from the stereogenic centre. It is conceivable that nucleophilic attack actually takes place in a conformation in which the donor substituent, the silicon-carbon bond, is out of conjugation with the double bond. 相似文献
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Betson MS Bracegirdle A Clayden J Helliwell M Lund A Pickworth M Snape TJ Worrall CP 《Chemical communications (Cambridge, England)》2007,(7):754-756
The orientation of Ar-C, Ar-N and Ar-O bonds in biaryls, N,N'-diarylureas and diaryl ethers (whose conformers are distinguishable by NMR) may be controlled with a selectivity up to >95 : 5 by an adjacent stereogenic centre; the selectivity may be greater when a second stereogenic axis is inserted between the controlling centre and the slowly rotating bond. 相似文献
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The microwave heating of two-dimensional slabs in a long rectangularwaveguide propagating the TE10 mode is examined. The temperaturedependency of the electrical conductivity and the thermal absorptivityis assumed to be governed by the Arrhenius law, while both theelectrical permittivity and the magnetic permeability are assumedconstant. The governing equations are the forced heat equationand the steady-state version of Maxwell's equation while theboundary conditions take into account both convective and radiativeheat loss. Approximate analytical solutions, valid for smallthermal absorptivity, are found for the temperature and theelectric-field amplitude using the Galerkin method. As the Arrheniuslaw is not amenable analytically, it is approximated by a rational-cubicfunction. At the steady state the temperature versus power relationshipis found to be multivalued; at the critical power level thermalrunaway occurs when the temperature jumps from the lower (cool)temperature branch to the upper (hot) temperature branch ofthe solution. In the steady-state limit the approximate analyticalsolutions are compared with the numerical solutions of the governingequations for various special cases. These are the limits ofsmall and large heat loss and an intermediate case involvingradiative heat loss. Results are also presented for a case wheredifferential cooling occurs on the different sides on the slab.An alternative heating scenario, where one end of the waveguideis blocked by a short, is also considered. The approximate solutionsare found for this geometry and compared in the small Biot-numberlimit to Kriegsmann (1997). Also, a control process is presented,which allows thermal runaway to be avoided and the desired finalsteady state to be reached. Various special cases of the feedbackparameters associated with the control process are examined. 相似文献
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Andreas?Lüthi Martin?A?Wikstr?m Mary?J?Palmer Paul?Matthews Tim?A?Benke John?TR?Isaac Graham?L?CollingridgeEmail author 《BMC neuroscience》2004,5(1):44