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871.
Alexandre G. Patriota Heleno BolfarineReinaldo B. Arellano-Valle 《Journal of multivariate analysis》2011,102(2):386-392
This paper deals with asymptotic results on a multivariate ultrastructural errors-in-variables regression model with equation errors. Sufficient conditions for attaining consistent estimators for model parameters are presented. Asymptotic distributions for the line regression estimators are derived. Applications to the elliptical class of distributions with two error assumptions are presented. The model generalizes previous results aimed at univariate scenarios. 相似文献
872.
Readily available alkenylalanes, arising from hydroalumination of unprotected terminal alkynes, have been directly employed for the copper-catalyzed asymmetric conjugate addition (ACA) to β-substituted cyclic enones. The desired products, containing a quaternary stereogenic center, are generally obtained in good yields and enantioselectivities. 相似文献
873.
Chatelet B Payet E Perraud O Dimitrov-Raytchev P Chapellet LL Dufaud V Martinez A Dutasta JP 《Organic letters》2011,13(14):3706-3709
Hemicryptophanes are host molecules with many applications as supramolecular catalysts or in ion selective recognition. A very convenient and efficient modular approach for the synthesis of hemicryptophane-tren (tren, tris(2-aminoethyl)-amine) derivatives has been developed. For instance, hemicryptophane 1 was synthesized at the gram scale in four steps from vanillyl alcohol compared to the previous seven-step procedure. The size, shape, and functionalities of the molecular cavity were also easily modified. 相似文献
874.
A subset of vertices in a graph is called a dissociation set if it induces a subgraph with a vertex degree of at most 1. The maximum dissociation set problem, i.e., the problem of finding a dissociation set of maximum size in a given graph is known to be NP-hard for bipartite graphs. We show that the maximum dissociation set problem is NP-hard for planar line graphs of planar bipartite graphs. In addition, we describe several polynomially solvable cases for the problem under consideration. One of them deals with the subclass of the so-called chair-free graphs. Furthermore, the related problem of finding a maximal (by inclusion) dissociation set of minimum size in a given graph is studied, and NP-hardness results for this problem, namely for weakly chordal and bipartite graphs, are derived. Finally, we provide inapproximability results for the dissociation set problems mentioned above. 相似文献
875.
Alexandre ScottFrançois Watier 《Statistics & probability letters》2011,81(8):1021-1026
Li and Zhou (2006) established that an investor, following an unconstrained mean-variance strategy, will achieve its discounted targeted wealth with a probability greater than 80%. Surprisingly, we will show that under short-selling restrictions (i.e without the possibility of borrowing stocks) this lower bound probability still holds. 相似文献
876.
Shvartsburg AA Isaac G Leveque N Smith RD Metz TO 《Journal of the American Society for Mass Spectrometry》2011,22(7):1146-1155
Correlations between the dimensions of a 2-D separation create trend lines that depend on structural or chemical characteristics
of the compound class and thus facilitate classification of unknowns. This broadly applies to conventional ion mobility spectrometry
(IMS)/mass spectrometry (MS), where the major biomolecular classes (e.g., lipids, peptides, nucleotides) occupy different
trend line domains. However, strong correlation between the IMS and MS separations for ions of same charge has impeded finer
distinctions. Differential IMS (or FAIMS) is generally less correlated to MS and thus could separate those domains better.
We report the first observation of chemical class separation by trend lines using FAIMS, here for lipids. For lipids, FAIMS
is indeed more independent of MS than conventional IMS, and subclasses (such as phospho-, glycero-, or sphingolipids) form
distinct, often non-overlapping domains. Even finer categories with different functional groups or degrees of unsaturation
are often separated. As expected, resolution improves in He-rich gases: at 70% He, glycerolipid isomers with different fatty
acid positions can be resolved. These results open the door for application of FAIMS to lipids, particularly in shotgun lipidomics
and targeted analyses of bioactive lipids. 相似文献
877.
Perruchas S Tard C Le Goff XF Fargues A Garcia A Kahlal S Saillard JY Gacoin T Boilot JP 《Inorganic chemistry》2011,50(21):10682-10692
Three copper(I) iodide clusters coordinated by different phosphine ligands formulated [Cu(4)I(4)(PPh(3))(4)] (1), [Cu(4)I(4)(Pcpent(3))(4)] (2), and [Cu(4)I(4)(PPh(2)Pr)(4)] (3) (PPh(3) = triphenylphosphine, Pcpent(3) = tricyclopentylphosphine, and PPh(2)Pr = diphenylpropylphosphine) have been synthesized and characterized by (1)H and (31)P NMR, elemental analysis and single crystal X-ray diffraction analysis. They crystallize in different space groups, namely, monoclinic P21/c, cubic Pa ?3, and tetragonal I ?42m for 1, 2, and 3, respectively. The photoluminescence properties of clusters 1 and 3 show reversible luminescence thermochromism with two highly intense emission bands whose intensities are temperature dependent. In accordance to Density Functional Theory (DFT) calculations, these two emission bands have been attributed to two different transitions, a cluster centered (CC) one and a mixed XMCT/XLCT one. Cluster 2 does not exhibit luminescence variation in temperature because of the lack of the latter transition. The absorption spectra of the three clusters have been also rationalized by time dependent DFT (TDDFT) calculations. A simplified model is suggested to represent the luminescence thermochromism attributed to the two different excited states in thermal equilibrium. In contrast with the pyridine derivatives, similar excitation profiles and low activation energy for these phosphine-based clusters reflect high coupling of the two emissive states. The effect of the Cu-Cu interactions on the emission properties of these clusters is also discussed. Especially, cluster 3 with long Cu-Cu contacts exhibits a controlled thermochromic luminescence which is to our knowledge, unknown for this family of copper iodide clusters. These phosphine-based clusters appear particularly interesting for the synthesis of original emissive materials. 相似文献
878.
879.
Rocha AB 《The Journal of chemical physics》2011,134(2):024107
A general strategy to calculate potential curves at multiconfigurational self-consistent field (MCSCF) level for inner-shell states is reported in this paper. Convergence is commonly very tough for inner-shell states, especially at this level of calculation, due to the problem of variational collapse of the inner-shell wave function to the ground or to a low-lying excited state. The present method allows to avoid this drawback by a sequence of constrained optimization in the orbital mixing step. The specific states studied are that resulting from transitions X (1)Σ(+) → (C 1s(-1) π(?)) (1,3)Π of CO. Accurate values are achieved for transition energies and vibrational splittings. A comparison is made with other approach, i.e., inner-shell CI based on a MCSCF wave function optimized for ground or low-lying excited states. This last approach is shown to fail in describing the whole potential curve. 相似文献
880.
Alexandre M. de Bettencourt Maria Filomena Duarte Maria Helena Florêncio Fernando F. Henriques Paulo A. Madeira Maria Inês Portela Luis Filipe Vilas-Boas 《Microchemical Journal》2011,99(2):218-234
Arsenic is a type 1 carcinogen and its toxicity is critically dependent on chemical speciation. However, after decades of research, the biogenesis of at least fifty naturally occurring arsenic species is still not well understood.Here, based on experimental work, it is proposed a set of pathways for the formation of multiple arsenic species that might help to clarify the present situation.These are focused on the thiol protein arsenic bond and on its interaction with reactive metabolites. In fact, arsenic bound to glutathione interacting with sulfur adenosyl methionine (SAM), MethylCB12 and AdoCB12, forms a number of complexes that might be key intermediates in arsenic biochemistry. These include dimethylarsino glutathione (DMAG) m/z 412 [M + H]+, synthesized non-enzymatically from glutathione and cacodylate. Trimethylarsonio glutathione (TMAG) m/z 426 [M]+ synthesized from DMA, GSH and SAM, apparently by a classical Challenger methylcarbonium attack. Tetramethyl arsonium ion m/z 135 [M]+ is formed in a third step, apparently by carbanion methylation. The presence of trimethylarsine oxide (TMAO) m/z 137 [M + H]+ is attributed to the hydrolysis of TMAG or TMA, or to carbanion methylation of dimethylarsinoyl glutathione (m/z 428 [M]+) formed from cacodylate and GSH. Cantoni type attacks of DMAG on SAM were unsuccessful, eventually due to competition of the trivalent S+ atom of SAM for the AsIII atom attack. The presence of dimethylarsonio diglutathione (DMADG m/z 717 [M]+), is suggested to result from a GS- attack on dimethylarsenoyl glutathione (m/z 428 [M + H]+). The presence of dimethylarsenoyladenosine (m/z 372 [M + H]+), trimethylarsenosugar adenine (m/z 370 [M]+), and dimethylthioarsenosugar adenine (m/z 388 [M + H]+), is explained by the synthesis of the pecursor dimethylarsonio-adenosine glutathione DMAAG (m/z 661 [M]+), a likely source of oxo-and trimethylated arsenosugars, as well as of thio-arsenosugars by the cleavage of its S-C bond. The results gathered suggest that cell vacuoles might play a major role in arsenic metabolism, and that the dominance of oxo-As sugars, in algae extracts, may be supported by a mechanism of synthesis independent of DMAAG (m/z 661).They also offer an explanation for the reason why arsenobetaine, and tetramethylarsonium are loosely bound to biotic tissues. Four arsenic species new to science, to the best of our knowledge, and a number of known arsenic compounds were synthesized in this work, identified by HPLC–ESI-MSn and FTICR–ESI-MS, and suggestions regarding their mechanisms of synthesis were advanced. These results provide a framework for arsenic biochemistry which may explain the origin of a significant part of arsenic known metabolites. 相似文献