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排序方式: 共有195条查询结果,搜索用时 15 毫秒
61.
Fryčák P Jirkovský J Ranc V Bednář P Havlíček V Lemr K 《Journal of mass spectrometry : JMS》2012,47(6):720-726
Atmospheric pressure chemical ionization is known for producing unusual artifacts of the ionization process in some cases. In this work, processes occuring in atmospheric pressure chemical ionization/MS of orotic acid that afforded ions accompanying protonated and deprotonated orotic acid molecules in the spectra were studied. Two processes ran in parallel in the ion source: decarboxylation of neutral orotic acid and collision-induced dissociation of its protonated or deprotonated form. A procedure discerning pre-ionization decomposition and post-ionization dissociation by manipulating ion source parameters was proposed. Experiments with isotopically labeled solvents confirmed ion-molecule reactions of the product of collision-induced dissociation of protonated orotic acid with solvent molecules in the ion source and even under vacuum in the ion trap. 相似文献
62.
Thanks to their ability to bind to specific biological receptors, mannosylated structures are examined in biomedical applications. One of the most common ways of linking a functional moiety to a structure is to use an azide-alkyne click reaction. Therefore, it is necessary to prepare and isolate a propargylated mannose derivative of high purity to maintain its bioactivity. Three known preparations of propargyl-α-mannopyranoside were revisited, and products were analysed by NMR spectroscopy. The preparations were shown to yield by-products that have not been described in the literature yet. Our experiments showed that one-step procedures could not provide pure propargyl-α-mannopyranoside, while a three-step procedure yielded the desired compound of high purity. 相似文献
63.
Jindřich Nejedlý Dr. Michal Šámal Dr. Jiří Rybáček Miroslava Tobrmanová Dr. Florence Szydlo Dr. Christophe Coudret Maria Neumeier Dr. Jaroslav Vacek Dr. Jana Vacek Chocholoušová Miloš Buděšínský Dr. David Šaman Dr. Lucie Bednárová Dr. Ladislav Sieger Dr. Irena G. Stará Dr. Ivo Starý 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(21):5933-5937
A series of oxahelicenes composed of ortho/meta-annulated benzene/pyridine and 2H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)- or nickel(0)-mediated) double, triple, or quadruple [2+2+2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes, thus allowing the construction of 6, 9, or 12 rings in a single operation. The use of a flow reactor was found to be beneficial for the multicyclization reactions. The stereogenic centers present in some of the oligoynes steered the helical folding in such a way that the final oxa[9]-, [13]-, [17]- and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically, the oxa[19]helicenes beat the current record in the length of a helicene backbone. Single-molecule conductivity was studied by the mechanically controllable break-junction method with a pyridooxa[9]helicene. 相似文献
64.
Bohumil Dolensky 《Journal of fluorine chemistry》2004,125(4):501-508
We review in this report the preparation of several side-chain fluorinated analogues of biologically important imidazoles and indoles. The building blocks used should also have applications in other synthetic problems. The addition of “FBr” to vinyl imidazole derivatives was used to prepare β-fluoro- and β,β-difluorohistamine and histidinols, as well as β-fluorourocanic acid. Deoxyfluorination of intermediate acylindoles was used to prepare a series of β,β-difluorotryptamine derivative, including the fluorinated analogue of the important neurotransmitter, serotonin. 相似文献
65.
Andreas Leusch Zdenek R. Holan Bohumil Volesky 《Applied biochemistry and biotechnology》1997,61(3):231-249
Biosorption of cadmium (Cd), cobalt (Co), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) by six fractions of particle sizes, ranging from 0.063 to 1.4 mm of dry marine algal biomass ofSargassum fluitans andAscophyllum nodosum, is examined. Equilibrium metal uptake by larger particles was higher than that by smaller particles in the order of Pb > Cd > Cu > Co > Zn > Ni for both biomass types, withS. fluitans sorbing slightly more thanA. nodosum. Uptakes of metals ranged from the highest, qmax = 369 mg Pb/g (particle size 0.84–1.00 mm), to the low Zn and Ni uptakes, qmax = 77 mg/g (size 0.84–1.00 mm) for S.fluitans. A. nodosum adsorbed metals in the range from qmax = 287 mg Pb/g (particle size 0.84–1.00 mm) to qmax = 73 mg Zn/g (particle size 0.84-1.00mm). Harder stipe fractions of S.fluitans demonstrated generally higher metal uptakes than the softer fractions derived from its blades (leaves). The pH dependence of the Zn uptake byS. fluitans exhibited a S-shaped curve between pH 1.5 and pH 7, with 50% of the maximum (pH 7.0) uptake at pH 3.5. Monovalent Na and K ions at higher concentrations inhibited the biosorption of Zn byS. fluitans. A significant inhibition started at 50 mM potassium chloride or sodium acetate, and at 1M the biosorption was completely blocked. 相似文献
66.
Bohumil Dolenský 《Journal of fluorine chemistry》2005,126(5):745-751
Hemiaminals of methyl 3,3,3-trifluoropyruvate with aromatic amines, benzylic monoamines and diamines were prepared and their interconversions studied using NMR spectrometry. In solution, benzylic hemiaminals were found to be stable in contrast to aromatic hemiaminals, which, in turn, were stable in the solid state. 相似文献
67.
Davis TA Llanes F Volesky B Diaz-Pulido G McCook L Mucci A 《Applied biochemistry and biotechnology》2003,110(2):75-90
The use of a number of species of marine brown algae in the implementation of bioremediation strategies for toxic heavy metals
is being considered and evaluated. The biosorption capacity of these algae for heavy metals resides mainly in a group of linear
polysaccharides known as alginates that occur as a gel in the algal thallus. The potential for selective metal binding by
the biomass of two species of Sargassum was evaluated by 1H-NMR (nuclear magnetic resonance) following a high temperature, alkaline extraction and purification of their alginate polysaccharide.
The alkaline extraction protocol applied to Sargassum fluitans and Sargassum siliquosum yielded alginate samples of low viscosity, suitable for direct acquisition of well-resolved spectra. Estimates of both the
ratio of β-d-mannopyranuronosyl (M) and α-l-gulopyranuronosyl (G) residues along the polymer chain and the frequencies of occurrence of diad uronic acid residue pairs
were obtained. Guluronic acid (G) was the major component in all, extracts and the GG diads accounted for more than 49% of
the polymer diads. Whereas the performance of Sargassum spp. in the metal biosorption process is a function of both its alginate content and composition, the occurrence of “G-blocks”
in both purified alginates and in the raw brown seaweed is critical because it results in a well-established selectivity for
divalentions, potentially increasing the commercial effectiveness of targeted biosorption as a means of remediation. 相似文献
68.
Bakardjiev M Holub J Carr MJ Kennedy JD Stibr B 《Dalton transactions (Cambridge, England : 2003)》2005,(5):909-913
The reaction of the Tl+ salt of the [nido-7,8,9-P2CB8H9]- anion (1-) with [CpFe(CO)2I](Cp =eta(5)-C5H5) in refluxing mesitylene for 12 h gives mixed-sandwich [1-Cp-closo-1,2,3,4-FeP2CB8H9] (2) (yield 63%). Reaction of the PPh4+ salt of the isomeric [nido-7,8,10-P2CB8H9]- anion 3- with [CpFe(CO)2I] in refluxing mesitylene gives [1-Cp-closo-1,2,3,5-FeP2CB8H9]4 (yield 56%), isomeric with 2. Compound 4 also results (yield 92%) from the sublimation of 2 under argon at ca. 350 degrees C. The constitution of all compounds is established by mass spectrometry, IR spectroscopy and multinuclear NMR spectroscopy (1H, 11B, 31P, and 13C; two-dimensional [11B-11B]-COSY, and 1H- 11B(selective)), further confirmed in the case of 4 by a single-crystal X-ray diffraction analysis. 相似文献
69.
Bohumil Hájek Pavel Karen Vlastimil Brožek 《Monatshefte für Chemie / Chemical Monthly》1986,117(11):1271-1278
The composition of the products of carbothermal reduction of Sc2O3 is examined by X-ray diffraction and chemical analysis and by the hydrolysis method. At pressures of 10–2-1 Pa, the reaction starts in the temperature region of 1 000–1 200°C. The first product is Sc2OC of NaCl type; at 1 Pa and 1 400–1 500°C this substance is formed quantitatively (according to stoichiometry) within 50–100 h, repeated homogenization, however, is necessary, or else Sc2OC reacts locally with Sc2O3 giving Sc2O1+x
C1–x
. The lattice parameter of Sc2OC in the presence of Sc15C19 is 457.63pm. At temperatures above 1 500°C, Sc15C19 is incompletely formed by subsequent reaction with carbon. The product melts at cca. 1 800°C; carbon dissolves and the final composition approaches ScC2. Carbon separates during solidification. The phase fractions in the products are affected by evaporation, the vapour pressures above both Sc2OC and Sc15C19 being comparable with the pressure requisite for the carboreduction process. The results are discussed with respect to the often ambiguous published data.
Untersuchung hydrolysierbarer Carbide, XXII. Die karbothermische Reduktion von Scandiumoxid
Zusammenfassung Zur Untersuchung von Produkten der karbothermischen Reduktion von Scandiumoxid wurden sowohl röntgenographische und chemische Analyse als auch die hydrolytische Methode verwendet. Bei 10–2 bis 1 Pa beginnt die Reaktion im Temperaturintervall von 1 000–1 200°C. Das erste Reaktionsprodukt ist das Scandiumoxidcarbid Sc2OC von NaCl-Typ, das quantitativ (der Stöchiometrie entsprechend) bei 1 400–1 500°C und 1 Pa in 50–100 Stunden entsteht. Eine wiederholte Homogenisierung ist allerdings notwendig, damit es zu keiner lokalen Reaktion zwischen Sc2OC und Sc2O3 kommt, bei der dann die Phase Sc2O1+x C1–x entsteht. In Gegenwart von Sc15C19 ist der Gitterparameter von Sc2OCa=457.63pm. Über 1 500°C führt eine weitere Reaktion mit Kohlenstoff zu einer nicht ganz vollendeten Bildung von Sc15C19. Bei cca. 1 800°C schmilzt das Reaktionsprodukt bei gleichzeitiger Auflösung von weiterem Kohlenstoff und die Zusammensetzung nähert sich der Formel ScC2, beim Erstarren fällt der Kohlenstoff wieder aus. Die Verteilung der Phasen im Produkt wird von der Verdampfung beeinflußt, da die Dampfdrucke von Sc2OC und Sc15C19 mit dem zum Karboreduktionverlauf notwendigen Druck vergleichbar sind. Die erhaltenen Ergebnisse werden in Relation mit den nicht eindeutigen Angaben in der Literatur diskutiert.相似文献
70.
The method for constructing infinite-dimensional representations of Lie superalgebras proposed by the authors recently is applied to the superalgebraosp (1, 4). Explicit formulae for its generators in terms of two or three pairs of operators fulfilling the canonical commutation relations, at most one pair of operators fulfilling the canonical anticommutation relations and at most one real parameter are obtained. The generators of the Lie subalgebrasp (4, ) osp (1,4) are represented skew-symmetrically and both the Casimir operators are equal to multiples of the unity operator.Dedicated to Professor Ivan Úlehla on the occasion of his sixtieth birthday. 相似文献