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31.
Prélot B Poinsignon C Thomas F Schouller E Villiéras F 《Journal of colloid and interface science》2003,257(1):77-84
Relationships between lattice parameters of manganese dioxides and their surface properties at the solid-aqueous solution interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered MD samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth) and Tw (rate of microtwinning). Water adsorption isotherms showed that the cross sectional area of water molecules adsorbed in the first monolayer is positively correlated to Pr. Titration of the surface charge of the MD series evidenced a positive linear relationship between the PZC and Pr (Pr=0, Tw=0, PZC=1 for ramsdellite; Pr=1, Tw=0, PZC=7.3 for pyrolusite; gamma-MD with intermediate values of Pr (0.2 to 0.45) have increasing PZC values). The rate of microtwinning appeared as a secondary factor for the increase of the PZC. The above correlations are explained by the chemical defects at the origin of the structural disorder, respectively Mn(3+)/Mn4+ substitution for Pr and Mn vacancies for Tw, which result in proton affinity and thus in increased PZC. The experimental results are compared with data collected in the literature for manganese dioxides as well as for dioxides of transition elements with tetragonal structure. 相似文献
32.
A simple and rapid method for the substoichiometric determination of traces of manganese in various materials by neutron-activation analysis has been developed. After dissolution of the irradiated test sample, manganese(II) carrier is added and subsequently oxidised by peroxodisulphate to the heptavalent state. The permanganate thus formed is finally extracted into chloroform as tetraphenylarsonium permanganate using a substoichiometric amount of tetraphenylarsonium chloride. This single separation step isolates radiochemically pure manganese-56 in the analysis of a relatively simple material in which interfering elements (gold, rhenium, etc.) are absent. When this is not true, a preliminary separation of manganese from the irradiated sample, based on the extraction of manganese diethyldithio-carbamate into chloroform and followed by stripping of the manganese with dilute sulphuric acid, must be used. A simultaneously irradiated standard containing manganese must be treated in exactly the same way as a test sample. In the materials analysed by the new method 10(-5) to 10(-3)% of manganese has been determined. 相似文献
33.
V. Ondrus L' Fisera K. Polborn P. Ertl N. Prónayová 《Monatshefte für Chemie / Chemical Monthly》1995,126(8-9):961-969
Summary N-(3,5-Dichlorophenyl)-maleimide reacts smoothly with a variety of substituted fulvenes (1) to give onlyendo adducts (3) independent of the nature of fulvene substituent,Lewis acid catalyst, and reaction solvent and temperature. The structure of theDiels-Alder adduct3f was determined by X-ray crystallography. Semi-empirical quantum methods (AM1) were used to rationalize theendo stereoselectivity.Dedicated to Professor Fritz Sauter on the occasion of his 65th birthday 相似文献
34.
[reaction: see text] Reported herein is a strategy employing a Mukaiyama reaction in tandem with a hydrogen transfer reaction for the elaboration of 2,3-anti-3,4-anti and 2,3-anti-3,4-syn propionate motifs. The mode of complexation is controlled through monodentate or chelate pathways for the Mukaiyama reaction to give access to either syn or anti aldol products, precursors of the free-radical reduction reaction. Boron Lewis acid is used to control the free-radical reaction through the exocyclic pathway. 相似文献
35.
It was found that the first step of thermal decomposition of the pyridinium salt of 1,3-bis(trimethylsilyl)-2, 4-dimercapto-2,4-dithioxo-1,3-diaza-25,45-diphosphetidine (I) most probably involves breakdown to the acid form HS(S)P(NHSiMe3)2P(S)SH (II). The latter is very unstable and decomposes further, the end-product being a polymer (PNS)x. In this work, the mechanism of this process is investigated.
Zusammenfassung Man fand, daß der erste Schritt der thermischen Zersetzung des Pyridiniumsalzes von 1,3-Bis(trimethylsilyl)-2,4-dimercapto-2,4-dithioxo-1,3-diaza-25,45-diphosphetidin (I) sehr wahrscheinlich einen Abbau zur Säureform HS(S)P(NHSiMe3)2P(S)SH (II) beinhaltet. Letztere ist sehr unbeständig und zersetzt sich, wobei als Endprodukt ein (PNS)x-Polymer entsteht. In vorliegendem Manuskript wird der Mechanismus dieses Prozesses näher untersucht.相似文献
36.
37.
Alexandra Gaubert Yohann Clement Anne Bonhomme Benjamin Burger Delphine Jouan-Rimbaud Bouveresse Douglas Rutledge Hervé Casabianca Pierre Lanteri Claire Bordes 《Analytica chimica acta》2016
This paper presents the analysis of surfactants in complex mixtures using Raman spectroscopy combined with signal extraction (SE) methods. Surfactants are the most important component in laundry detergents. Both their identification and quantification are required for quality control and regulation purposes. Several synthetic mixtures of four surfactants contained in an Ecolabel laundry detergent were prepared and analyzed by Raman spectroscopy. SE methods, Independent Component Analysis and Multivariate Curve Resolution, were then applied to spectral data for surfactant identification and quantification. The influence of several pre-processing treatments (normalization, baseline correction, scatter correction and smoothing) on SE performances were evaluated by experimental design. By using optimal pre-processing strategy, SE methods allowed satisfactorily both identifying and quantifying the four surfactants. When applied to the pre-processed Raman spectrum of the Ecolabel laundry detergent sample, SE models remained robust enough to predict the surfactant concentrations with sufficient precision for deformulation purpose. Comparatively, a supervised modeling technique (PLS regression) was very efficient to quantify the four surfactants in synthetic mixtures but appeared less effective than SE methods when applied to the Raman spectrum of the detergent sample. PLS seemed too sensitive to the other components contained in the laundry detergent while SE methods were more robust. The results obtained demonstrated the interest of SE methods in the context of deformulation. 相似文献
38.
39.
Dr. Daniel Brodbeck Dr. Sonia Álvarez-Barcia Dr. Jan Meisner Dr. Florian Broghammer Julian Klepp Dr. Delphine Garnier Dr. Wolfgang Frey Prof. Dr. Johannes Kästner Prof. Dr. René Peters 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(6):1515-1524
Asymmetric 1,2-additions of cyanide yield enantioenriched cyanohydrins as versatile chiral building blocks. Next to HCN, volatile organic cyanide sources are usually used. Among them, cyanoformates are more attractive on technical scale than TMSCN for cost reasons, but catalytic productivity is usually lower. Here, the development of a new strategy for cyanations is described, in which this activity disadvantage is overcome. A Lewis acidic Al center cooperates with an aprotic onium moiety within a remarkably robust bifunctional Al–F–salen complex. This allowed for unprecedented turnover numbers of up to 104. DFT studies suggest an unexpected unique trimolecular pathway in which the ammonium bound cyanide attacks the aldehyde, which itself is activated by the carbonyl group of the cyanoformate binding to the Al center. In addition, a novel practical carboxycyanation method was developed that makes use of KCN as the sole cyanide source. The use of a pyrocarbonate as carboxylating reagent provided the best results. 相似文献
40.
Dr. Siddhartha De Dr. Alexandrine Flambard Dr. Delphine Garnier Patrick Herson Dr. Frank H. Köhler Dr. Abhishake Mondal Dr. Karine Costuas Dr. Béatrice Gillon Prof. Rodrigue Lescouëzec Dr. Boris Le Guennic Dr. Frédéric Gendron 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(52):12120-12136
The local magnetic structure in the [FeIII(Tp)(CN)3]− building block was investigated by combining paramagnetic Nuclear Magnetic Resonance (pNMR) spectroscopy and polarized neutron diffraction (PND) with first-principle calculations. The use of the pNMR and PND experimental techniques revealed the extension of spin-density from the metal to the ligands, as well as the different spin mechanisms that take place in the cyanido ligands: Spin-polarization on the carbon atoms and spin-delocalization on the nitrogen atoms. The results of our combined density functional theory (DFT) and multireference calculations were found in good agreement with the PND results and the experimental NMR chemical shifts. Moreover, the ab-initio calculations allowed us to connect the experimental spin-density map characterized by PND and the suggested distribution of the spin-density on the ligands observed by NMR spectroscopy. Interestingly, significant differences were observed between the pseudo-contact contributions of the chemical shifts obtained by theoretical calculations and the values derived from NMR spectroscopy using a simple point-dipole model. These discrepancies underline the limitation of the point-dipole model and the need for more elaborate approaches to break down the experimental pNMR chemical shifts into contact and pseudo-contact contributions. 相似文献