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Nucleophiles attack the methiodide of the 3,4-benzo derivative of the title system to displace the P-methyl group in peference to the benzylic moiety. The trigonal bipyramidal intermediates (as well as a stable dioxyphosphorane) appear to undergo rapid isomerization. 相似文献
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N. Severijns R. Prieels J. Deutsch F. Gimeno-Nogues I. Pepe P. A. Quin P. De Moor P. Schuurmans R. Siebelink W. Vanderpoorten J. Vanhaverbeke L. Vanneste J. Wouters M. Allet O. Naviliat-Cuncic 《Hyperfine Interactions》1992,75(1-4):181-188
We report on the first measurements of the longitudinal polarization of positrons emitted by polarized nuclei, using cryogenically
polarized107In. This so-called polarization-asymmetry correlation is very sensitive to the mass of a possible right-handed gauge boson,
which is invoked by parity symmetric extensions of the standard V-A electroweak model to explain the experimentally observed
strong violation of parity which, however, may not be complete. The positron polarization is deduced from the time-resolved
decay spectrum of positronium hyperfine states. Our preliminary result points to a lower limit of about 210 GeV for the mass
of an eventual right-handed charged W gauge boson. This result can still be improved. 相似文献
47.
Enzymes are increasingly being used in an industrial setting as a cheap and environmentally-friendly alternative to chemical catalysts. In order to produce the ideal biocatalyst, natural enzymes often require optimization to increase their catalytic efficiencies and specificities under a particular range of reaction conditions. A number of enzyme engineering strategies are currently employed to modify biocatalysts, improving their suitability for large-scale industrial applications. These include various directed evolution techniques, semi-rational design techniques, and more recently, the de novo design of novel enzymes. Advances in mutant library design, high-throughput selection processes, and the introduction of powerful computer algorithms have all contributed to the current exponential growth of the field of enzyme engineering. This review article aims to present some of the currently employed strategies for enzyme engineering and attempts to highlight the most recent advances in methodology. 相似文献
48.
The 13C—31P coupling constant is much larger for a 2-methyl substituent (about 20 Hz) than for a 3-methyl substituent (about 5 Hz) in phospholes and 2-phospholenes, and appears useful for assigning structures in these compounds. 相似文献
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Poolman HR Boersma DJ Harvey M Higinbotham DW Passchier I Six E Alarcon R van Amersfoort PW Bauer TS Boer Rookhuizen H van Den Brand JF van Buuren LD Bulten HJ Ent R Ferro-Luzzi M Geurts DG Heimberg P de Jager CW Klimin P Koop I Kroes F van Der Laan J Luijckx G Lysenko A Militsyn B Nesterenko I 《Physical review letters》2000,84(17):3855-3858
We report on first measurements with polarized electrons stored in a medium-energy ring and with a polarized internal target. Polarized electrons were injected at 442 MeV (653 MeV), and a partial (full) Siberian snake was employed to preserve the polarization. Longitudinal polarization at the interaction point and polarization lifetime of the stored electrons were determined with laser backscattering. Spin observables were measured for electrodisintegration of polarized 3He, with simultaneous detection of scattered electrons, protons, neutrons, deuterons, and 3He nuclei, over a large phase space. 相似文献