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排序方式: 共有321条查询结果,搜索用时 31 毫秒
61.
62.
Greg Hughes Paul O'Shea Julie Goll Danny Gauvreau Jennifer Steele 《Tetrahedron》2009,65(16):3189-5650
Effective protocols for the introduction of chiral α-trifluoromethyl benzyl moieties by nucleophilic displacement of enantiomerically enriched α-trifluoromethyl benzylic triflates are presented. The effects of substrate electronics, solvent polarity, temperature, and base are studied by measuring the diastereomeric or enantiomeric excesses of the displacement products formed by coupling a variety of α-trifluoromethyl benzylic triflates with a range of nucleophiles including amines, carboxylates, thiols, and malonates. Preliminary investigations to elucidate the mechanism(s) involved in the loss of stereochemical integrity at the benzylic center in the nucleophilic displacement reactions are also reported. 相似文献
63.
Vladimir Pozdnyakov J. Michael Steele 《Methodology and Computing in Applied Probability》2009,11(4):551-560
In the theory of interest rate futures, the difference between the futures rate and forward rate is called the “convexity
bias,” and there are several widely offered reasons why the convexity bias should be positive. Nevertheless, it is not infrequent
that the empirical the bias is observed to be negative. Moreover, in its most general form, the benchmark Heath–Jarrow–Morton
(HJM) term structure model is agnostic on the question of the sign of the bias; it allows for models where the convexity bias
can be positive or negative. In partial support of the practitioner’s arguments, we develop a simple scalar condition within
the HJM framework that suffices to guarantee that the convexity bias is positive. Moreover, when we check this condition on
the LIBOR futures data, we find strong empirical support for the new condition. The empirical validity of the sufficient condition
and the periodic observation of negative bias, therefore leads one to a paradoxical situation where either (1) there are arbitrage
possibilities or (2) a large subclass of HJM models provide interest rate dynamics that fail to capture a fundamental feature
of LIBOR futures. 相似文献
64.
Crowley JD Steele IM Bosnich B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(35):8935-8951
Ferrocene has been investigated as a platform for developing protonmotive electrostatic drivers for molecular motors. When two 3-pyridine groups are substituted to the (rapidly rotating) cyclopentadienyl (Cp) rings of ferrocene, one on each Cp, it is shown that the (Cp) eclipsed, pi-stacked rotameric conformation is preferred both in solution and in the solid state. Upon quaternization of both of the pyridines substituents, either by protonation or by alkylation, it is shown that the preferred rotameric conformation is one where the pyridinium groups are rotated away from the fully pi-stacked conformation. Electrostatic calculations indicate that the rotation is caused by the electrostatic repulsion between the charges. Consistently, when the pi-stacking energy is increased pi-stacked population increases, and conversely when the electrostatic repulsion is increased pi-stacked population is decreased. This work serves to provide an approximate estimate of the amount of torque that the electrostatically driven ferrocene platform can generate when incorporated into a molecular motor. The overall conclusion is that the electrostatic interaction energy between dicationic ferrocene dipyridyl systems is similar to the pi-stacking interaction energy and, consequently, at least tricationic systems are required to fully uncouple the pi-stacked pyridine substituents. 相似文献
65.
Tang PH Miles MV Steele P Davidson BS Geraghty SR Morrow AL 《Biomedical chromatography : BMC》2006,20(12):1336-1343
An isocratic HPLC method was developed for the determination of coenzyme Q(10) (CoQ(10)) in human breast milk. After a single-step liquid-liquid extraction, the milk extract was injected directly into the HPLC system. The analytical method is based on pre-column inline treatment of CoQ(10). Chromatographic separation of CoQ(10) and coenzyme Q(9) (CoQ(9)) internal standard was achieved using a reversed-phase Microsorb-MV C(18) analytical column. CoQ(10) and CoQ(9) were monitored by an electrochemical detector (ECD). An excellent linearity (r = 0.999) was observed for CoQ(10) in the concentration range 0.06-2.5 micromol L(-1) in breast milk. The limit of quantitation (LOQ) was 60 nmol L(-1). Coefficients of variations (CVs) for intra-day and inter-day assay precisions were less than 5%. A total of 194 breast milk samples were analyzed for the CoQ(10) concentration; the mean value was 0.32 +/- 0.21 micromol L(-1). 相似文献
66.
A network of excitable nodes based on the photosensitive Belousov-Zhabotinsky reaction is studied in experiments and simulations. The addressable medium allows both local and nonlocal links between the nodes. The initial spread of excitation across the network as well as the asymptotic oscillatory behavior are described. Synchronization of the spatiotemporal dynamics occurs by entrainment to high-frequency network pacemakers formed by excitation loops. Analysis of the asymptotic behavior reveals that the dynamics of the network is governed by a subnetwork selected during the initial transient period. 相似文献
67.
68.
O. V. Yakubovich I. M. Steele V. S. Rusakov V. S. Urusov 《Crystallography Reports》2006,51(3):401-411
The crystal structure of a synthetic analog of the mineral lipscombite (Fe 2.3 2+ Fe 4.7 3+ )[PO4]4O2.7(OH)1.3 obtained under hydrothermal conditions in the LiF-Fe2O3-(NH4)2HPO4-H2O system is resolved (R = 0.040) by X-ray diffraction analysis (Bruker Smart diffractometer with a highly sensitive CCD detector, MoK α radiation): a = 14.776(3) Å, b = 14.959(3) Å, c = 7.394(1) Å, β = 119.188(4)°, sp. gr. C2/c, Z = 4, ρexp = 3.8 g/cm3, ρcalcd = 3.9 g/cm3. Fe2+ and Fe3+ cations are statistically distributed in each of four crystallographically independent positions, while occupying the corresponding octahedra with probabilities of 60, 90, 100, and 91%. The ratio Fe2+/Fe3+ in the composition of the crystals was established by Mössbauer spectroscopy. Lipscombite is interpreted as a mineral of variable composition described by the formula (Fe x 2+ Fe n?x 3+ )[PO4]4Oy(OH)4?y . The field of stability is determined as a function of the iron content and the ratio Fe2+/Fe3+. It is shown that at n = 6 iron cations are ordered in octahedra and barbosalite structure is formed. An interpretation of genetically and structurally related members of the lipscombite family within a unified polysomatic series is proposed. 相似文献
69.
Consolidation of the results of structural, spectroscopic and kinetic studies of a range of intrazeolite guests and their reactions have led to the development of the “crown ether-zeolite ligand analogy” and the concept of the “zeolate” as a macrospheroidal or macrocylindrical multidentate anionic ligand. This view of a zeolite cavity or channel as a collection of interconnected aluminosilicate crown ether-like building blocks, permits a coordination chemistry approach to the understanding of metal-ligand bonding (capping, anchoring, complexing, stabilizing and structure-directing phenomena) in a variety of zeolite guest-host inclusion systems. In this paper we focus attention on the role that the zeolate ligand plays in the organization of semiconductor precursors and their assembly to semiconductor nanoclusters with particular reference to II-VI nanomaterials. 相似文献
70.
John D. Steele 《General Relativity and Gravitation》1991,23(7):811-825
In this paper the spin-coefficient formalism of Penrose and Rindler is applied to the problem of finding space-times that admit a simply-transitive group of homotheties. The method is illustrated by an application to algebraically special vacuum space-times. Vacuum metrics with a fivedimensional homothety group are also considered. 相似文献