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151.
Dennis Jansen Johannes Gramüller Felix Niemeyer Torsten Schaller Matthias C. Letzel Stefan Grimme Hui Zhu Ruth M. Gschwind Jochen Niemeyer 《Chemical science》2020,11(17):4381
Organocatalysis has revolutionized asymmetric synthesis. However, the supramolecular interactions of organocatalysts in solution are often neglected, although the formation of catalyst aggregates can have a strong impact on the catalytic reaction. For phosphoric acid based organocatalysts, we have now established that catalyst–catalyst interactions can be suppressed by using macrocyclic catalysts, which react predominantly in a monomeric fashion, while they can be favored by integration into a bifunctional catenane, which reacts mainly as phosphoric acid dimers. For acyclic phosphoric acids, we found a strongly concentration dependent behavior, involving both monomeric and dimeric catalytic pathways. Based on a detailed experimental analysis, DFT-calculations and direct NMR-based observation of the catalyst aggregates, we could demonstrate that intermolecular acid–acid interactions have a drastic influence on the reaction rate and stereoselectivity of asymmetric transfer-hydrogenation catalyzed by chiral phosphoric acids.Supramolecular acid–acid interactions lead to competing monomeric and dimeric pathways in phosphoric acid catalysis – so that stereoselectivities depend on catalyst concentration. 相似文献
152.
153.
Dr. Werner Fudickar Prof. Dr. Torsten Linker 《Angewandte Chemie (International ed. in English)》2018,57(39):12971-12975
The generation of reactive singlet oxygen under mild conditions is of current interest in chemistry, biology, and medicine. We were able to release oxygen from dipyridylanthracene endoperoxides (EPOs) by using a simple chemical trigger at low temperature. Protonation and methylation of such EPOs strongly accelerated these reactions. Furthermore, the methyl pyridinium derivatives are water soluble and therefore serve as oxygen carriers in aqueous media. Methylation of the EPO of the ortho isomer affords the parent form directly without increasing the temperature under very mild conditions. This exceptional behavior is ascribed to the close contact between the nitrogen atom and the peroxo group. Singlet oxygen is released upon this reaction, and can be used to oxygenate an acceptor such as tetramethylethylene in the dark with no heating. Thus, a new chemical source of singlet oxygen has been found, which is triggered by a simple stimulus. 相似文献
154.
Manganese‐Catalyzed Dehydrogenative Alkylation or α‐Olefination of Alkyl‐Substituted N‐Heteroarenes with Alcohols
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Dr. Guoying Zhang Dr. Torsten Irrgang Thomas Dietel Fabian Kallmeier Prof. Dr. Rhett Kempe 《Angewandte Chemie (International ed. in English)》2018,57(29):9131-9135
Catalysis with earth‐abundant transition metals is an option to help save our rare noble‐metal resources and is especially interesting when novel reactivity or selectivity patterns are observed. We report here on a novel reaction, namely the dehydrogenative alkylation or α‐olefination of alkyl‐substituted N‐heteroarenes with alcohols. Manganese complexes developed in our laboratory catalyze the reaction with high efficiency whereas iron and cobalt complexes stabilized by the same ligands are essentially inactive. Hydrogen is liberated during the reaction, and bromine and iodine functional groups as well as olefins are tolerated. A variety of alkyl‐substituted N‐heteroarenes can be functionalized, and benzylic and aliphatic alcohols undergo the reaction. 相似文献
155.
We present a mild and efficient method for the completely regioselective alcoholysis of styrene oxides utilizing a cooperative Br?nsted acid-type organocatalytic system comprised of mandelic acid (1 mol %) and N,N'-bis-[3,5-bis-(trifluoromethyl)phenyl]-thiourea (1 mol %). Various styrene oxides are readily transformed into their corresponding beta-alkoxy alcohols in good to excellent yields at full conversion. Simple aliphatic and sterically demanding, as well as unsaturated and acid-sensitive alcohols can be employed. 相似文献
156.
Spitzer T 《Analytical and bioanalytical chemistry》2008,391(8):2923-2929
Young lignite from two locations in Sendai City, Japan, were analyzed for polynuclear aromatic compounds (PACs). Lignite samples
were extracted with toluene and PACs were isolated by a compound-class-selective, reactive matrix clean-up. This clean-up
separates all compounds of polynuclear aromatic structure, for example hydrocarbons and ketones and their primary metabolites,
from interfering organic compounds. The result of this isolation procedure is, therefore, a group of a large number of polycyclic
compounds with different functional groups. Further analysis is done by glass capillary gas chromatography. Perylene was identified
as the only compound obtained by the reactive matrix clean-up of lignite samples from both locations. Concentrations were
in the range of 1–10 mg kg−1. Conversely, PACs resulting from particulate emissions from fossil fuel combustion always contain isomeric PAHs (polynuclear
aromatic hydrocarbons) with some polycyclic aromatic ketones and thousands of primary and secondary metabolites at very low
concentrations. This was demonstrated for a sample of urban air particulate matter, which is the source of PAC contamination
of surface soil. The absence of accompanying PAHs and polynuclear aromatic ketones in the lignite samples confirms that perylene
did not originate from a combustion process. It is assumed that the high concentrations of perylene are the result of a reduction
of perylene quinone. Thus, the high perylene content of the lignite samples investigated is of biogenic origin. 相似文献
157.
Two different reaction pathways in the photooxygenation of 9,10-diarylanthracenes are identified, with strong evidence for polar (forward, singlet oxygen addition) and radical (backward, thermolysis) intermediates. 相似文献
158.
159.
Widely used regression approaches in modeling quantitative structure-property relationships, such as PLS regression, are highly susceptible to outlying observations that will impair the prognostic value of a model. Our aim is to compile homogeneous datasets as the basis for regression modeling by removing outlying compounds and applying variable selection. We investigate different approaches to create robust, outlier-resistant regression models in the field of prediction of drug molecules' permeability. The objective is to join the strength of outlier detection and variable elimination increasing the predictive power of prognostic regression models. In conclusion, outlier detection is employed to identify multiple, homogeneous data subsets for regression modeling. 相似文献
160.
Koch R Finnerty JJ Bruhn T Borget F Wentrup C 《The journal of physical chemistry. A》2008,112(38):8999-9004
The complex reaction of thermally generated iminopropadienones with amines in the gas phase and upon matrix deposition and its varying product composition is investigated using density functional theory. In the high energy gas phase addition a single amine molecule reacts readily with iminopropadienone with the decisive step being a 1,3-hydrogen shift and activation barriers of at least 100 kJ/mol. In accordance with the experiment, the formation of ketenes is favored. In the condensed phase of an amine matrix, the utilization of amine dimers both as reagents and as explicit solvents lowers the activation energy required to a feasible 20-30 kJ/mol and predicts ketenimines as the main products, as observed experimentally. 相似文献