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排序方式: 共有980条查询结果,搜索用时 31 毫秒
81.
Busacca CA Grossbach D Campbell SJ Dong Y Eriksson MC Harris RE Jones PJ Kim JY Lorenz JC McKellop KB O'Brien EM Qiu F Simpson RD Smith L So RC Spinelli EM Vitous J Zavattaro C 《The Journal of organic chemistry》2004,69(16):5187-5195
The Boehringer-Ingelheim phosphinoimidazoline (BIPI) ligands were applied to the formation of chiral quaternary centers in the asymmetric Heck reaction. Several different substrates were examined in detail, using more than 70 members of this new ligand class. Hammett relationships were determined through systematic variation of the ligand electronics. All substrates showed essentially the same Hammett behavior, where enantioselectivity increased as the ligands were made more electron-deficient. Ligand optimization has led to catalysts which give the highest enantioselectivities reported to date for these difficult systems. 相似文献
82.
S. Eriksson F. Ramirez-Martinez E.A. Curtis B.E. Sauer P.W. Nutter E.W. Hill E.A. Hinds 《Applied physics. B, Lasers and optics》2004,79(7):811-816
We have produced magnetic patterns suitable for trapping and manipulating neutral atoms on a 1-m length scale. The required patterns are made in Co/Pt thin films on a silicon substrate, using the heat from a focused laser beam to induce controlled domain reversal. In this way we draw lines and paint shaped areas of reversed magnetisation with sub-micron resolution. These structures produce magnetic microtraps above the surface that are suitable for holding rubidium atoms with trap frequencies as high as 1 MHz. PACS 39.25.+k; 03.75.Be; 75.50.Ss; 75.70.-i 相似文献
83.
Electron spin qubits in semiconductors are attractive from the viewpoint of long coherence times. However, single spin measurement is challenging. Several promising schemes incorporate ancillary tunnel couplings that may provide unwanted channels for decoherence. Here, we propose a novel spin-charge transduction scheme, converting spin information to orbital information within a single quantum dot by microwave excitation. The same quantum dot can be used for rapid initialization, gating, and readout. We present detailed modeling of such a device in silicon to confirm its feasibility. 相似文献
84.
Kimmo Eriksson Jonas Sjöstrand Pontus Strimling 《International Journal of Game Theory》2008,36(3-4):421-440
A fundamental fact in two-sided matching is that if a market allows several stable outcomes, then one is optimal for all men in the sense that no man would prefer another stable outcome. We study a related phenomenon of asymmetric equilibria in a dynamic market where agents enter and search for a mate for at most n rounds before exiting again. Assuming independent preferences, we find that this game has multiple equilibria, some of which are highly asymmetric between sexes. We also investigate how the set of equilibria depends on a sex difference in the outside option of not being mated at all. 相似文献
85.
86.
J A Meriluoto J E Eriksson K Harada A M Dahlem K Sivonen W W Carmichael 《Journal of chromatography. A》1990,509(2):390-395
87.
M. Bielewski M. Eriksson J. Himbert M. Betti F. Belloni G. Falkenberg 《Journal of Radioanalytical and Nuclear Chemistry》2009,282(2):355-359
XANES has been recently used for the determination of oxidation states of actinides in environmental samples. To obtain reliable
results, however, a sufficiently long counting time at every probing energy and a large number of experimental points per
XANES spectrum are required, due to the complex mathematical model used to fit the measured spectrum. This makes micro-mapping
difficult, since the time required for data collection becomes unacceptably long. A simplified model for data collection and
evaluation is presented. Its effectiveness has been tested by measuring the distribution of Pu oxidation states in a “hot”
particle coming from a nuclear weapon test site. 相似文献
88.
Johan Eriksson Lars Eriksson 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(11):1308-1312
Three flame retardants with very similar molecular structures showing three different packing patterns have been studied. The crystal structure of 2,2′,6,6′-tetrachloro-4,4′-propane-2,2-diyldiphenol, C15H12Cl4O2, can be described as a packing of sheets. The packing shows a very short intermolecular Cl⋯Cl contact distance of 3.094 (2) Å between pairs of molecules inside each sheet. The crystal structure of 2,2′,6-tribromo-4,4′-propane-2,2-diyldiphenol, C15H13Br3O2, can be described as a packing of doubly stranded helical square tubes. These square helices are interconnected through Br⋯Br contacts between different helices. Finally, a previously known structure, 2,2′,6,6′-tetrabromo-4,4′-propane-2,2-diyldiphenol [Simonov, Cheban, Rotaru & Bels'skii (1986). Kristallografiya, 31 , 397–399], C15H12Br4O2, which is the most commonly used flame retardant and which has twofold rotational symmetry, has been refined in the correct absolute configuration. The structure shows large differences from the chloro analogue with regard to packing, van der Waals distances and hydrogen-bond distances. 相似文献
89.
90.
J. A. O. Meriluoto K. Isaksson H. Soini S. E. Nygård J. E. Eriksson 《Chromatographia》1990,30(5-6):301-308
Summary Microcystins-LA,-LR,-RR,-YR and nodularin, cyanobacterial peptide toxins, were separated by internal-surface reversed-phase
(ISRP), high-performance liquid chromatography. The capacity factors of the toxins were measured in the range pH 2–8 using
acetonitrile, isopropanol or tetrahydrofuran in potassium dihydrogenphosphate mobile phase. The main retention mechanism of
the ISRP column was reversed-phase interaction but cation-exchange offered additional selectivity at neutral and slightly
acidic pH. At neutral pH (10% modifier, 0.1 M buffer) the elution order was microcystin-LA (two nonpolar residues leucine
and alanine as the variable amino acids), nodularin, microcystin-LR,-YR and-RR (two basic arginines as the variable amino
acids). The retention times of all toxins except microcystin-RR were substantially longer at acidic pH. At pH 2 (10% modifier,
0.1 M buffer) where the cation-exchange mechanism was inoperative the elution order was changed to microcystin-RR, nodularin,
microcystin-LR,-YR and-LA. The best separation was achieved at pH 2 where even two desmethylated microcystin-RR analogs could
be separated from microcystin-RR. 相似文献