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81.
We present results from density functional theory and computer simulations that unambiguously predict the occurrence of first-order freezing transitions for a large class of ultrasoft model systems into cluster crystals. The clusters consist of fully overlapping particles and arise without the existence of attractive forces. The number of particles participating in a cluster scales linearly with density, therefore the crystals feature density-independent lattice constants. Clustering is accompanied by polymorphic bcc-fcc transitions, with fcc being the stable phase at high densities. 相似文献
82.
Bianca?TeodorescuEmail author Ingrid?Van?Keilegom Ricardo?Cao 《Annals of the Institute of Statistical Mathematics》2010,62(3):465-485
Consider the model f(S(z|X)){\phi(S(z|X))} =
\pmbb(z) [(X)\vec]{\pmb{\beta}(z) {\vec{X}}}, where f{\phi} is a known link function, S(·|X) is the survival function of a response Y given a covariate X, [(X)\vec]{\vec{X}} = (1, X, X
2 , . . . , X
p
) and
\pmbb(z){\pmb{\beta}(z)} is an unknown vector of time-dependent regression coefficients. The response is subject to left truncation and right censoring.
Under this model, which reduces for special choices of f{\phi} to e.g. Cox proportional hazards model or the additive hazards model with time dependent coefficients, we study the estimation
of the vector
\pmbb(z){\pmb{\beta}(z)} . A least squares approach is proposed and the asymptotic properties of the proposed estimator are established. The estimator
is also compared with a competing maximum likelihood based estimator by means of simulations. Finally, the method is applied
to a larynx cancer data set. 相似文献
83.
Antonioli B Bray DJ Clegg JK Gloe K Gloe K Kataeva O Lindoy LF McMurtrie JC Steel PJ Sumby CJ Wenzel M 《Dalton transactions (Cambridge, England : 2003)》2006,(40):4783-4794
Synthesis of the 2,2'-dipyridylamine derivatives di-2-pyridylaminomethylbenzene 1, 1,2-bis(di-2-pyridylaminomethyl)benzene 2, 1,3-bis(di-2-pyridylaminomethyl)benzene 3, 2,6-bis(di-2-pyridylaminomethyl)pyridine 4, 1,4-bis(di-2-pyridylaminomethyl)benzene 5, and 1,3,5-tris(di-2-pyridylaminomethyl)benzene 6 are reported together with the single-crystal X-ray structures of 2, 3, and 5. Reaction of individual salts of the type AgX (where X = NO(3)(-), PF(6)(-), ClO(4)(-), or BF(4)(-)) with the above ligands has led to the isolation of thirteen Ag(I) complexes, nine of which have also been characterised by X-ray diffraction. In part, the inherent flexibility of the respective ligands has resulted in the adoption of a range of coordination arrangements. A series of liquid-liquid (H(2)O/CHCl(3)) extraction experiments of Ag(I) with varying concentrations of 1-6 in the organic phase have been undertaken, with the counter ion in the aqueous phase being respectively picrate, perchlorate and nitrate. In general, extraction efficiencies for a given ionophore followed the Hofmeister order of picrate > perchlorate > nitrate; in each case the tris-dpa derivative 6 acting as the most efficient extractant of the six systems investigated. Competitive seven-metal bulk membrane transport experiments (H(2)O/CHCl(3)/H(2)O) employing the above ligands as the ionophore in the organic phase and equimolar concentrations of Co(II), Ni(II), Zn(II), Cu(II), Cd(II), Pb(II) and Ag(I) in the aqueous source phase were also undertaken, with transport occurring against a pH gradient. Under the conditions employed 1 and 5 yielded negligible transport of any of the metals present in the source phase while sole transport selectivity for Ag(I) was observed for 2-4 and 6. 相似文献
84.
85.
Sándor P.?FeketeEmail author Robert T.?Firla Bianca?Spille 《Mathematical Methods of Operations Research》2003,58(2):319-329
This paper deals with the problem of representing the matching independence system in a graph as the intersection of finitely many matroids. After characterizing the graphs for which the matching independence system is the intersection of two matroids, we study the function (G), which is the minimum number of matroids that need to be intersected in order to obtain the set of matchings on a graph G, and examine the maximal value, (n), for graphs with n vertices. We describe an integer programming formulation for deciding whether (G)k. Using combinatorial arguments, we prove that (n)(log logn). On the other hand, we establish that (n)O(logn/ log logn). Finally, we prove that (n)=4 for n=5,,12, and sketch a proof of (n)=5 for n=13,14,15.An earlier version appears as an extended abstract in the Proceedings of COMB01 [5]. Supported by the Gerhard-Hess-Forschungs-Förderpreis (WE 1462) of the German Science Foundation (DFG) awarded to R. Weismantel. 相似文献
86.
Sculimbrene BR Morgan AJ Miller SJ 《Chemical communications (Cambridge, England)》2003,(15):1781-1785
The ability to use small peptides as catalysts for asymmetric phosphorylation provides opportunities for rapid syntheses of phosphate-containing compounds and natural products. This short review outlines the genesis of this concept and its reduction to practice in the context of concise syntheses of the enantiomeric D-I-1P and D-I-3P targets. The implications for the development of additional site-selective catalysts are discussed. 相似文献
87.
Kerckhoffs JM ten Cate MG Mateos-Timoneda MA van Leeuwen FW Snellink-Ruël B Spek AL Kooijman H Crego-Calama M Reinhoudt DN 《Journal of the American Chemical Society》2005,127(36):12697-12708
This article describes the synthesis and binding properties of highly selective noncovalent molecular receptors 1(3).(DEB)6 and 3(3).(DEB)6 for different hydroxyl functionalized anthraquinones 2. These receptors are formed by the self-assembly of three calix[4]arene dimelamine derivative molecules (1 or 3) and six diethylbarbiturate (DEB) molecules to give 1(3).(DEB)6 or 3(3).(DEB)6. Encapsulation of 2 occurs in a highly organized manner; that is, a noncovalent hydrogen-bonded trimer of 2 is formed within the hydrogen-bonded receptors 1(3).(DEB)6 and 3(3).(DEB)6. Both receptors 1(3).(DEB)6 and 3(3).(DEB)6 change conformation from staggered to eclipsed upon complexation to afford a better fit for the 2(3) trimer. The receptor selectivity toward different anthraquinone derivatives 2 has been studied using 1H NMR spectroscopy, X-ray crystallography, UV spectroscopy, and isothermal microcalorimetry (ITC). The pi-pi stacking between the electron-deficient center ring of the anthraquinone derivatives 2a-c and 2e-g and the relatively electron-poor melamine units of the receptor is the driving force for the encapsulation of the guest molecules. The selectivity of the hydrogen-bonded host for the anthraquinone derivatives is the result of steric interactions between the guest molecules and the calix[4]arene aromatic rings of the host. 相似文献
88.
89.
Buccolieri A Buccolieri G Cardellicchio N Dell'Atti A Di Leo A Maci A Petronio BM 《Annali di chimica》2004,94(7-8):469-478
Concentrations of metals (Al, Fe, Ti, Ni, Mn, Cr, Zn, Cu, Pb) were determined in surface sediments from Taranto gulf (Ionian Sea, Southern Italy) in order to assess the distribution and the bioavailability of metals. Total metal content and speciation of Fe, Ni, Mn, Zn, Cu and Pb were determined. Sediment cores were also sampled in some stations of particular interest. The analytical results show that metals distribution does not show significant variations in relation to depth. Metal distribution in the Taranto gulf is mainly influenced by industrial wastes from Taranto town, by rivers of the Basilicata region and by prevailing anticlockwise marine currents. These factors cause an accumulation of metals in the sediments near to the coasts of Calabria. The statistical analysis, effected by HCA and PCA methods, can be considered a good starting point in order to classify sediment samples and to locate accumulation areas. Metal speciation shows that Fe, Ni and Zn are present in sediments mainly as sulphides, Mn is present principally in an ion-exchange form or bound to carbonate, while Cu and Pb are mainly present in the fraction bounds to humic acids. 相似文献
90.
Fiorella Bachechi Alfredo Burini Rossana Galassi Bianca Rosa Pietroni 《Journal of Molecular Structure》2005,740(1-3):119-123
The Hg(II) complex [Hg(TFP)2(OTFP)3][ClO4]2 with TFP=tri-2-furyl-phosphine and OTFP=tri-2-furylphosphinoxide has been prepared and characterised. It crystallises in the hexagonal P63/m space group with Z=2, a=13.308(3), c=21.092 (4) Å, V=3235(1) Å3. The structure of the complex cation consists of independent molecules with Hg pentacoordinated in exact trigonal bipyramidal geometry. 相似文献