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171.
An efficient synthesis of (-)-kainic acid, through a high-pressure-promoted Diels-Alder cycloaddition of a vinylogous malonate derived from 4-hydroxyproline, is described. The bicyclic adduct could be converted into the natural product with complete stereocontrol.  相似文献   
172.
Four different skeletal structural arrangements with very different connectivities are known for 6-vertex/68-electron of M4E2 core (M = transition metal; E = main-group atom or ligand). DFT calculations on a large number of title model compounds allow to rationalize the preferences between these structural shapes with respect to the nature of the metal and main-group elements constituting the cluster cage. In particular, the electronegativity of M and the “size” (first-row vs. second-row element) of E play an important role in the stability preference of a particular isomer. For several compounds, although only one type of structure is known, other low-energy isomeric forms are also likely to exist. Moreover, two structural types, so far unreported, are predicted to be stable enough for being synthesized.  相似文献   
173.
In this paper, we show how concentration inequalities for Gaussian quadratic form can be used to propose confidence intervals of the Hurst index parametrizing a fractional Brownian motion. Both cases where the scaling parameter of the fractional Brownian motion is known or unknown are investigated. These intervals are obtained by observing a single discretized sample path of a fractional Brownian motion and without any assumption on the Hurst parameter H.  相似文献   
174.
This work aims at developing an efficient method to compute the compliance due to a crack modeled as a flat ellipsoid of any shape in an infinite elastic matrix of arbitrary anisotropy (Eshelby problem) when no closed-form solution seems currently available. Whereas the solution of this problem usually requires the calculation of the so-called fourth-order Hill polarization tensor if the ellipsoid is not singular, it is shown that the crack compliance can be derived from the first-order term in the Taylor expansion of the Hill tensor with respect to the smallest aspect ratio of the ellipsoidal inclusion. For a 3D ellipsoidal crack model, this first-order term is expressed as a simple integral thanks to the Cauchy residue theorem. A similar method allows to express the same term in the case of a cylindrical crack model without any integral. A numerical example is finally treated.  相似文献   
175.
176.
This paper develops a class of consistent estimators of the parameters of a fractional Brownian motion based on the asymptotic behavior of the k-th absolute moment of discrete variations of its sampled paths over a discrete grid of the interval [0,1]. We derive explicit convergence rates for these types of estimators, valid through the whole range 0 < H < 1 of the self-similarity parameter. We also establish the asymptotic normality of our estimators. The effectiveness of our procedure is investigated in a simulation study. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
177.
We establish the fundamental results of genus theory for finite (non necessary Galois) extensions of global fields by using narrow S-class groups, when S is an arbitrary finite set of places. This exposition, which involves both the number fields and the functions fields cases, generalizes most classical results on this subject. Received: 8 February 1999 / Revised version: 17 December 1999  相似文献   
178.
The association within one molecule ofcalix[4]arene and crown ether moieties leads toligands with new complexing properties. In particular,calix[4]arene bis(crown-6) and some of itsderivatives have been shown to be highly selectiveextractants for caesium ions. This review presents thebackground of the study and the results of crystalstructure determinations and molecular modellingcalculations performed during the investigation of twomolecular families, the bis(crown ether) and theazobenzocrown derivatives of calix[4]arene.  相似文献   
179.
The daily returns of financial market indices of nineteen Eastern European countries are modelled, linear trend or ARMA(p, q) for the levels and GARCH(p ′, q ′) for the residuals. For the non Gaussian residuals -stable distributions are proposed. Then, risk measures, like the STARR and the R-ratio are used to analyse the risk in the cases of Gaussian and -stable distribution models of the residuals. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
180.
Two very rare cases of barium boryloxides, the homoleptic [Ba(OB{CH(SiMe3)2}2)2⋅C7H8] and the heteroleptic [{LONO4}BaOB{CH(SiMe3)2}2] stabilised by the multidentate aminoetherphenolate {LONO4}, are presented, and their structural properties are discussed. The electron-deficient [Ba(OB{CH(SiMe3)2}2)2⋅C7H8] shows, in particular, resilient η6-coordination of the toluene molecule. Together with its amido parents [Ba{N(SiMe3)2}2⋅thf2] and [Ba{N(SiMe3)2}2]2, this complex catalyses the fast and chemoselective dehydrocoupling of borinic acids R2BOH and hydrosilanes HSiR′3, yielding borasiloxanes R2BOSiR′3 in a controlled fashion. The assessment of substrate scope indicates that, for now, the reaction is limited to bulky borinic acids. Kinetic analysis shows that the rate-limiting step of the catalytic manifold traverses a dinuclear transition state. A detailed mechanistic scenario is proposed on the basis of DFT computations, the results of which are fully consistent with experimental data. It consists of a stepwise process with rate-determining nucleophilic attack of a metal-bound O-atom onto the incoming hydrosilane, involving throughout dinuclear catalytically active species.  相似文献   
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