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31.
Vlekken J D'Olieslaeger M Knuyt G Vandervorst W De Schepper L 《Journal of the American Society for Mass Spectrometry》2000,11(7):650-658
In secondary ion mass spectrometry, the detection of MCs+ clusters (with M an element of the specimen) under a Cs bombardment is frequently used for the quantification of major elements. Despite some very good results obtained by this method, some problems still remain. In order to gain some more insight into these problems, the formation mechanism of the MCs+ clusters is investigated using a Monte Carlo model. It is shown that the majority of the constituent particles of the formed clusters are initially first or second neighbor atoms at the surface and that the velocity distribution of the MCs+ clusters becomes broader and peaked at higher velocities with increasing surface binding energy of the M atom. In addition, it is demonstrated that the interaction potential between the M and Cs+ particle has no influence on the velocity distribution of the MCs+ clusters. On the other hand, the cluster formation probability, defined as the probability that a sputtered M and Cs+ particle will form a MCs+ cluster, is extremely sensitive to this interaction potential. It is also shown that the cluster formation probability decreases with increasing surface binding energy. Finally, a good correspondence is obtained between the calculated and experimental velocity distributions of MCs+ clusters sputtered from different monoatomic materials. As a consequence, the Monte Carlo model and the discussed results can be validated. 相似文献
32.
Artem Malakho Marc Dussauze Bogdan Lazoryak Frederic Adamietz 《Journal of solid state chemistry》2005,178(6):1888-1897
Crystallization of glasses with compositions (1−x)(0.95 NaPO3+0.05 Na2B4O7)+xNb2O5, x=0.4, 0.43, 0.45, 0.48 was investigated by differential scanning calorimetry and X-ray powder diffraction. Crystallization of two phases was observed in the glasses with x=0.43-0.48. First phase is a sodium niobate with the structure of tetragonal tungsten bronze () and second phase is Na4Nb8P4O32 (). The crystallization of sodium niobate is correlated with increasing of nonlinear optical efficiency reported for thermally poled glasses with x>0.4. The results of Raman spectroscopy show the formation of three-dimensional (3D) niobium oxide framework in the glasses with increase of niobium concentration. This framework is supposed to have tetragonal tungsten bronze structure and to be responsible for nonlinear optical properties of the glass. Second harmonic generation signals of as prepared and crystallized glass after thermal poling are compared. The nucleation and crystallization do not improve the NLO properties of the glasses under study. 相似文献
33.
Plumb RS Stumpf CL Gorenstein MV Castro-Perez JM Dear GJ Anthony M Sweatman BC Connor SC Haselden JN 《Rapid communications in mass spectrometry : RCM》2002,16(20):1991-1996
The application of liquid chromatography/mass spectrometry (LC/MS) followed by principal components analysis (PCA) has been successfully applied to the screening of rat urine following the administration of three candidate pharmaceuticals. With this methodology it was possible to differentiate the control samples from the dosed samples and to identify the components of the mass spectrum responsible for the separation. These data clearly show that LC/MS is a viable alternative, or complementary, technique to proton NMR for metabonomics applications in drug discovery and development. 相似文献
34.
The amidinoethylation of alcohols takes place by the addition of sodium alkoxides 2 (R1 = Me, Et) to the CC double bond of a variety of N,N'-substituted-propenamidines 1 (Method A). This illustrates the activation of the CC double bond by the conjugated amidine function and provides a new class of Michael acceptors for alcohols. However, this activation is poorer than with other nucleophiles or Michael acceptors. The amidinoethylation makes available 3-alkoxy-N,N'-substituted-amidines not easily accessible by other classical methods. However, it is demonstrated that the general N,N'-substituted-amidine synthesis via the nitrilium salts can also apply to nitrile compounds having an alkoxygroup present on the molecule (method B). Since the cyanoethylation of alcohols (4) is a very fast and facile reaction the method B is the preferred strategy for the synthesis of 3-alkoxy-N,N'-substituted-propanamidines 3. 相似文献
35.
Marc Henry 《Journal of Cluster Science》2003,14(3):267-287
Among Achim Müller's prolific crystal structure database, we have selected two crystalline phases in order to perform a whole and complete characterization of water structure at the nanometer scale. The first chosen example involves the Na3(NH4)12[Mo57Fe6(NO)6O174(OH)3(H2O)24] 76H2O compound synthesized and characterized in 1994. Some very interesting yet unnoticed water clusters may be evidenced in the voids generated by the stacking of the polyanionic units. Among them, we have been able to characterized a pure water crown (H2O)18, a loose association of three strongly solvated ammonium ions [H3N–HOH2]+ mediated by two water dimers and one water molecule, a perfectly planar alternating six-member ring [(NH4)3(H2O)3]3+, a puckered chair-shaped hexagonal ring [(NH4)2(H2O)4]3+ and two triangular pyramidal water tetramers (H2O)4. It was also shown that the crown and the chair ring was involved through further hydrogen bonding into the formation of a quite novel supramolecular layer modeling the structure of water in contact with a polyelectrolyte. The second example involves the (gua)32[Mo132O372(H2O)72(SO4)10(H2PO2)20(gua)20]nH2O compound synthesized and characterized in 2002. Here, we provide for the first time a complete structural analysis of all the various hydrogen bond patterns encountered in this system. Among them we may cite, an intramolecular web covering the internal cavity, an intramolecular finite system involving the guanidinium cations and the nine-member ring pores of the Mo132 shell and a central pure water cluster of one hundred water molecules. In this last case, the evolution of the hydrogen bond strengths on a per H-bond basis or within supramolecular aggregates ([H2O]20, [H2O]40, and [H2O]100) is quantitatively studied from standpoints involving both geometry (H–OO bond angles distribution) and energy (partition functions). A survey of other crystalline phases involving water clusters is also presented. It is hoped that the study of these new clusters in a very next future will allow us to solve the well-known water puzzling behaviors. 相似文献
36.
Evelina Liarou Yisong Han Ana M. Sanchez Marc Walker David M. Haddleton 《Chemical science》2020,11(20):5257
Rapidly self-deoxygenating Cu-RDRP in aqueous media is investigated. The disproportionation of Cu(i)/Me6Tren in water towards Cu(ii) and highly reactive Cu(0) leads to O2-free reaction environments within the first seconds of the reaction, even when the reaction takes place in the open-air. By leveraging this significantly fast O2-reducing activity of the disproportionation reaction, a range of well-defined water-soluble polymers with narrow dispersity are attained in a few minutes or less. This methodology provides the ability to prepare block copolymers via sequential monomer addition with little evidence for chain termination over the lifetime of the polymerization and allows for the synthesis of star-shaped polymers with the use of multi-functional initiators. The mechanism of self-deoxygenation is elucidated with the use of various characterization tools, and the species that participate in the rapid oxygen consumption is identified and discussed in detail.The rapidly self-deoxygenating Cu-RDRP in aqueous media is investigated. 相似文献
37.
Marc Wende 《Journal of fluorine chemistry》2003,124(1):45-54
Most compounds designed for immobilization in fluorous media feature linear pony tails of the formula (CH2)m(CF2)n−1CF3 [(CH2)mRfn]. This paper presents a first-generation approach to compounds with branched or “split” pony tails of the formula (CH2)lCH[(CH2)mRfn]2. Allyl tri(n-butyl)tin is reacted twice with perfluorooctyl iodide (Rf8I; first, photochemical, 78-81%; second, thermal with radical initiator, 71%; 13-18 g scales) to give the secondary alkyl iodide ICH(CH2Rf8)2 (3). A subsequent Ni(Cl)2(PPh3)2-catalyzed reaction with allyl tri(n-butyl)tin yields the branched alkene H2CCHCH2CH(CH2Rf8)2 (74%). A palladium-catalyzed Heck coupling with OP(p-C6H4Br)3 gives the fluorous phosphine oxide OP(p-C6H4CHCHCH2CH(CH2Rf8)2)3 (84%), and Pd/C-catalyzed hydrogenation affords OP(p-C6H4(CH2)3CH(CH2Rf8)2)3 (>99%). Reduction with SiHCl3 gives P(p-C6H4(CH2)3CH(CH2Rf8)2)3, which is protected as the air-stable borane adduct H3B·P(p-C6H4(CH2)3CH(CH2Rf8)2)3 (9, 64%). The CF3C6F11/toluene partition coefficient of 9 is much higher than that of the analog with p-(CH2)3Rf8 groups (96.6:3.4 versus 37.3:62.7). The iodide 3 is unreactive towards PAr3 at 175-250 °C. However, a CuBr-catalyzed reaction with C6H5MgBr gives C6H5CH(CH2Rf8)2, which also exhibits a high partition coefficient (97.9:2.1). 相似文献
38.
Oxidation reactions of an anionic gallium(I) N-heterocyclic carbene analogue with group 16 compounds
The reactivity of an anionic gallium(I) heterocycle, [K(tmeda)][:Ga([N(Ar)C(H)]2)], Ar = C6H3Pr(i)2-2,6, towards sources of elemental chalcogens and diorgano-dichalcogenides has been investigated and comparisons drawn with the reactivity of the valence isoelectronic N-heterocyclic carbene class of ligand. The reactions of the heterocycle with N2O or (Te)PEt3 yielded the dimeric, dianionic gallium(III) complexes, [K(L)]2[(mu-E)Ga([N(Ar)C(H)]2)]2, E = O, L = tmeda; E = Te, L = THF. Treatment of [K(tmeda)][:Ga([N(Ar)C(H)]2)] with the diphenyl dichalcogenides, PhEEPh, E = Se or Te, gave the one dimensional polymer, [K[(PhSe)2Ga([N(Ar)C(H)]2)]]infinity and the monomeric complex, [K(OEt2)3][(PhTe)2Ga([N(Ar)C(H)]2)], respectively. The X-ray crystal structures of the four complexes are reported. 相似文献
39.
A simple procedure for the determination of manganese in different sections of human brain samples by graphite furnace atomic absorption spectrometry has been developed. Brain sections included cerebellum, hypothalamus, frontal cortex, vermix and encephalic trunk. Two sample preparation procedures were evaluated, namely, slurry sampling and microwave-assisted acid digestion. Brain slurries (2% w/v) could be prepared in distilled, de-ionized water, with good stability for up to 30 min. Brain samples were also digested in a domestic microwave oven using 5 ml of concentrated HNO3. A mixed palladium+magnesium nitrate chemical modifier was used for thermal stabilization of the analyte in the electrothermal atomizer up to pyrolysis temperatures of 1300 °C, irrespective of the matrix. Quantitation of manganese was conducted in both cases by means of aqueous standards calibration. The detection limits were 0.3 and 0.4 ng ml−1 for the slurry and the digested samples, respectively. The accuracy of the procedure was checked by comparing the results obtained in the analysis of slurries and digested brain samples, and by analysis of the NIST Bovine Liver standard reference material (SRM 1577a). The ease of slurry preparation, together with the conventional set of analytical and instrumental conditions selected for the determination of manganese make such methodology suitable for routine clinical applications. 相似文献
40.
Hale KJ Frigerio M Manaviazar S Hummersone MG Fillingham IJ Barsukov IG Damblon CF Gescher A Roberts GC 《Organic letters》2003,5(4):499-502
[structure: see text] The synthesis of two truncated bryostatin analogues 2 and 3 is described. High-field NMR measurements on the C-ring analogue 3 in C(2)H(3)CN containing 25% (2)H(2)O have shown that it binds to the CRD2 of human PKC-alpha at virtually the same position as phorbol-13-acetate (PA) and bryostatin 1 (1). NMR titration studies have also revealed that 3 binds to the CRD2 with a potency similar in magnitude to PA but much less potently than 1. 相似文献