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排序方式: 共有599条查询结果,搜索用时 93 毫秒
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P. R. Hageman J. te Nijenhuis M. J. Anders L. J. Giling 《Journal of Crystal Growth》1997,170(1-4):270-275
Doping studies of the incorporation behaviour of three different dopants (Zn, In and Si) versus the misorientation of the (100) surface during MOVPE growth of GaAs have been carried out with diethylzinc, trimethylindium and disilane as precursors. The incorporation of the dopants has been studied as function of the input mole fraction dopant, growth temperature, degree and direction of misorientation. In order to explain the results we discuss the BCF theory and the nature of the steps as function of above mentioned parameters. It appears that the BCF theory alone cannot explain the results, a counteracting mechanism has been introduced based on preferential arsenic desorption from the step edges. 相似文献
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Dupieux P Alard JP Augerat J Babinet R Bastid N Brochard F Charmensat P De Marco N Fanet H Fodor Z Fraysse L Girard J Gorodetzky P Gosset J Laspalles C Lemaire MC L'Hôte D Lucas B Marroncle J Montarou G Parizet MJ Poitou J Qassoud D Racca C Schimmerling W 《Physics letters. [Part B]》1988,200(1-2):17-21
Proton-proton small angle correlations have been measured in neon-nucleus collisions, using the 4 pi detector Diogene, at 400 and 800 MeV per nucleon incident energies. Values of the size of the emitting region are obtained by comparison with the Koonin formula, taking into account the biases of the apparatus. The dependence of the density on target mass and incident energy is also analysed. 相似文献
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The fluorescence spectrum of crystals grown from newly synthesized 4-(dimethylamino)benzonitrile (DMABN), measured from 25 down to −112 °C, consists of a single emission band originating from a locally excited (LE) state. The fluorescence decay of the DMABN crystals is single exponential at all temperatures investigated. These results show that intramolecular charge transfer (ICT) does not occur in crystalline DMABN. The additional red-shifted emission bands and multiexponential fluorescence decays previously reported for DMABN crystals are attributed to a minor amount of the impurity 4-(dimethylamino)benzaldehyde, the synthetic precursor of commercial DMABN. 相似文献
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The isomerization of 1,2-epcxycyclopentane ( 1 ) to enantiomerically enriched (R)-cyclopent-2-enol ( 2 ) in protic solvents is catalyzed by cob(I)alamin. The enantiomeric excess (e.e.) of (R)- 2 is usually ca. 60%; it is only slightly dependent on the temperature, but increases with decreasing dielectric constant ε of the solvent. Standard kinetic methods show the reaction to be first order in vitamin B12 and zero order in 1 . The rate constant increases exponentially with increasing ε of the solvent. An Arrhenius plot at ε = 40 gives activation parameters ΔH≠ = 78 ± 4 kJ·mol?1 and ΔS≠ = ?49 ± 1 J·mol?1·K?1. The isomerization 1 → 2 proceeds in two steps (Schemes 2 and 7): (i) The epoxide ring is first opened by the proton-assisted fast and irreversible nucleophilic attack of the chiral CoI catalyst to form diastereoisomeric (1R,2R)- and (1S,2S)-(2-hydroxycyclo-pentyl)cob(III)alamins 6 in a ratio of ca. 4:1 which are the dominant species in the steady state; (ii) The intermediates 6 then decompose in the rate-limiting step to form 2 and recycled catalyst. Experiments with specifically 2H-labeled 1 showed the hydro-cobalt elimination 6 → 2 to be non-stereoselective. It proceeds via reversible Co? C bond homolysis to a free 2-hydroxycyclopentyl radical from which stereoelectronically controlled H-abstraction by Co11 takes place. 相似文献
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Wender PA Baryza JL Bennett CE Bi FC Brenner SE Clarke MO Horan JC Kan C Lacôte E Lippa B Nell PG Turner TM 《Journal of the American Chemical Society》2002,124(46):13648-13649
Macrocycle 1 is a new highly potent analogue of bryostatin 1, a promising anti-cancer agent currently in human clinical trials. In vitro, 1 displays picomolar affinity for PKC and exhibits over 100-fold greater potency than bryostatin 1 when tested against various human cancer cell lines. Macrocycle 1 can be generated in clinically required amounts by chemical synthesis in only 19 steps (LLS) and represents a new clinical lead for the treatment of cancer. 相似文献
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Yoshihara T Druzhinin SI Zachariasse KA 《Journal of the American Chemical Society》2004,126(27):8535-8539
The planar rigidized molecule fluorazene (FPP) undergoes fast reversible intramolecular charge transfer (ICT) in the excited state, with a reaction time of 12 ps in the polar solvent ethyl cyanide at -45 degrees C. The ICT state of FPP has a dipole moment mu(e)(ICT) of 13 D, much larger than that of the locally excited state LE (1 D). The ICT behavior of FPP is similar to that of its flexible counterpart N-phenylpyrrole (PP), for which mu(e)(ICT) = 12 D. These results show that intramolecular charge transfer to a planar ICT state can occur efficiently. In designing ICT systems capable of rapid switching, it is therefore important to realize that large amplitude motions such as those necessary for the formation of a twisted intramolecular charge transfer (TICT) state are not required. 相似文献
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