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排序方式: 共有803条查询结果,搜索用时 15 毫秒
711.
[reaction: see text] A reusable polymer-supported hydrophosphinylation catalyst is described for the preparation of H-phosphinic acids. The polystyrene-based ligand is prepared in one step from commercially available compounds. The polymeric catalyst generally gives good yields for a variety of substrates and is water- and air-tolerant, although the scope of alkenes and alkynes which can be employed is somewhat narrower than with our original xantphos/Pd(2)dba(3) catalyst. 相似文献
712.
Jacques Boulanger Jean-Luc Chabert 《Transactions of the American Mathematical Society》2004,356(12):5071-5088
Let be a cyclotomic field with ring of integers and let be a polynomial whose values on belong to . If the ideal of generated by the values of on is itself, then every algebraic integer of may be written in the following form:
for some integer , where the 's are roots of unity of . Moreover, there are two effective constants and such that the least integer (for a fixed ) is less than , where
for some integer , where the 's are roots of unity of . Moreover, there are two effective constants and such that the least integer (for a fixed ) is less than , where
713.
Mbaye MD Renaud JL Demerseman B Bruneau C 《Chemical communications (Cambridge, England)》2004,(16):1870-1871
Regio- and enantioselective substitution of cinnamyl chloride by phenols has been achieved with up to 82% enantiomeric excess, using a ruthenium catalyst prepared from [Cp*(CH(3)CN)(3)Ru][PF(6)] and a chiral bisoxazoline ligand. 相似文献
714.
Janzen DE Burand MW Ewbank PC Pappenfus TM Higuchi H da Silva Filho DA Young VG Brédas JL Mann KR 《Journal of the American Chemical Society》2004,126(46):15295-15308
A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers has been investigated for comparison with a previously studied quinoid oligothiophene that has demonstrated high mobilities and ambipolar transport behavior in thin-film transistor devices. Each new quinoidal thiophene derivative shows a reversible one-electron oxidation between 0.85 and 1.32 V, a quasi-reversible one-electron second oxidation between 1.37 and 1.96 V, and a reversible two-electron reduction between -0.05 and -0.23 V. The solution UV-vis-NIR spectrum of each compound is dominated by an intense (epsilon congruent with 100 000 M(-1) cm(-1)) low energy pi-pi transition that has a lambda(max) ranging between 648 and 790 nm. All X-ray crystal structures exhibit very planar quinoidal backbones and short intermolecular pi-stacking distances (3.335-3.492 A). Structures exhibit a single pi-stacking distance with parallel cofacial stacking (sulfur atoms of equivalent rings pointed in the same direction) or with alternating distances and antiparallel cofacial stacking (sulfur atoms of equivalent rings pointed in the opposite direction). Examples of the layered and herringbone-packing motifs are observed for both the parallel and the antiparallel cofacial stacking. Analysis of the X-ray structures and molecular orbital calculations indicates that all of these compounds have one-dimensional electronic band structures as a result of the pi-stacking. For structures with a unique pi-stacking distance, a simple geometric overlap parameter calculated from the shape of the molecule and the slip from perfect registry in the pi-stack correlates well with the transfer integrals (t) calculated using molecular orbital theory. The calculated valence (633 meV) and conduction (834 meV) bandwidths for a quinoid quaterthiophene structure are similar to those calculated for the benchmark pentacene and indicate that both hole and electron mobilities could be significant. 相似文献
715.
Fan FR Lai RY Cornil J Karzazi Y Brédas JL Cai L Cheng L Yao Y Price DW Dirk SM Tour JM Bard AJ 《Journal of the American Chemical Society》2004,126(8):2568-2573
We report excellent correlations between the first negative threshold potentials (V(TH)s) for electric conduction, electrochemical potentials, and computed lowest unoccupied molecular orbital energies in a series of phenylene-ethynylene oligomers bearing a sulfur-based anchoring unit and different electroactive substituents on the central benzene ring. The theoretical and electrochemical results strongly suggest that the peaks observed in the i-V curves have a true molecular origin and are associated with distinct unoccupied molecular levels of the compounds that are strongly localized on the central ring (except for compound I). This localization might account for the existence of a long-lived radical-anion state that permits lateral electron hopping and leads to charge trapping and storage. 相似文献
716.
717.
[reaction: see text] Sulfur ylide methodology has been used to construct the Taxol side chain with a high degree of enantioselectivity via a trans-aziridine followed by stereospecific rearrangement of the trans-benzoylaziridine into a trans-oxazoline. 相似文献
718.
The aim of this work is to analyze the structure of a tracially symmetric Dirichlet form on a -algebra, in terms of a killing weight and a closable derivation taking values in a Hilbert space with a bimodule structure. It is shown that the generator of the associate Markovian semigroup always appears, in a natural way, as the divergence of a closable derivation. Applications are shown to the decomposition of Dirichlet forms and to the construction of differential calculus on metric spaces. 相似文献
719.
Development of a capillary zone electrophoresis method for caseinoglycomacropeptide determination 总被引:1,自引:0,他引:1
Samir Cherkaoui Nathalie Doumenc Pierre Tachon Jean-Richard Neeser Jean-Luc Veuthey 《Journal of chromatography. A》1997,790(1-2):195-205
Caseinoglycomacropeptide (CGMP) is a polypeptide of 64 amino acid residues, derived from the C-terminal part of bovine κ-casein. A sensitive and selective capillary zone electrophoresis method has been developed and validated for the analysis and quantitation of CGMP. Separation is carried out at 30 kV, using an uncoated fused-silica capillary and 20 mM sodium citrate buffer at acidic pH 3.5. The described method allows the separation of various CGMP subcomponents. The validation data proves that the method has the requisite selectivity, sensitivity, reproducibility and linearity for CGMP assay and for quality control during CGMP manufacturing (batch-to-batch reproducibility). 相似文献
720.