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121.
Libraries of monovalent compounds can be reacted with each other to give libraries of bivalent ones. If those reactions are efficient, and if the products do not need to be purified, large numbers of bivalent compounds can be produced rapidly, and one might say there is a "combinatorial advantage" to doing so. However, selective formation of heterobivalent products must be possible otherwise statistical mixtures will form. This tutorial review describes methods that will give heterobivalent compounds almost exclusively. Although there are relatively few methods that will give that desired selectivity, such methods are becoming increasingly important as the potential applications of bi- and multivalent compounds emerge. 相似文献
122.
C. Bentabol A. Reyes J. A. Narvaez M. Morell J. J. Laserna F. Garcia Sanchez 《Microchemical Journal》1985,32(3):317-323
A method for the kinetic determination of tonin activity in rat submaxillary gland extracts is described. The method is based on the fluorogenic reaction of o-phthalaldehyde with the dipeptide histidyl-leucine generated in the enzymatic hydrolysis of angiotensin I. The reaction is monitored by fluorescence at 440 nm. At pH 7.5, the method allows the determination of 2–15 nmol/ml histidyl-leucine with relative standard deviation of 6%. 相似文献
123.
Trino Suárez Bernardo Fontal Gustavo León Marisela Reyes Fernando Bellandi Ricardo R. Contreras Pedro Cancines 《Transition Metal Chemistry》2006,31(7):974-976
Catalysis with water-soluble rhodium complexes, RhCl(CO)(TPPMS)2, [TPPMS = P(C6H5)2(C6H4SO3)] (1), RhCl(CO)(TPPDS)2, [TPPDS = P(C6H5)(C6H4SO3)2] (2) and RhCl(CO)(TPPTS)2, [TPPTS = P(C6H4SO3)3] (3) in hydroformylation of 1-hexene, 2-pentene, 2,3-dimethyl-1-butene, cyclohexene and several mixtures of these olefins have
been studied, under moderate reaction conditions (T: 50–150 °C; pCO/pH2 = 1; total p: 14–68 bar; Substrate/Catalyst: 600/1) in biphasic toluene/water media. The catalytic system shows high activity
but low selectivity. The linear and branched oxygenated products obtained are equally useful in naphtha upgrading, as observed
in the real El Palito naphtha tried. The catalysts can be recycled several times without significant activity loss. 相似文献
124.
Aydeé Fuentes Rafael Martínez-Palou Hugo A. Jiménez-Vázquez Francisco Delgado Alicia Reyes Joaquín Tamariz 《Monatshefte für Chemie / Chemical Monthly》2005,136(2):177-192
Summary. The (Z)-N-substituted 4-methylene-5-propylidene-2-oxazolidinone dienes were prepared by a one-step synthesis, starting from 2,3-hexanedione and isocyanates. Diels-Alder cycloadditions of these dienes were carried out in the presence of the dienophiles methyl vinyl ketone, methyl propiolate, and a captodative olefin, under conditions such as solvents of high polarity, Lewis acid catalysis, and non-conventional energy sources. The reactions carried out either with mixtures of H2O/MeOH or under BF3·Et2O catalysis yielded the highest regio- and stereoselectivities. The use of ionic liquids, microwaves, and ultrasound did not significantly increase the selectivity. 相似文献
125.
126.
E. F. Herrera Peraza O. Díaz Rizo M. E. Montero Cabrera A. T. Hernández R. Contreras Folgar M. Ixquiac Cabrera O. Hernández Aguilar M. V. Manso Guevara I. Alvarez Pellon R. A. López Duménigo G. Padrón González M. C. López Reyes 《Journal of Radioanalytical and Nuclear Chemistry》1999,240(2):437-443
This paper explains chronologically the implementation of thek 0-standardization and other parametric methods in Cuba. Fundamental and applied results in the determination of the most important parameters are obtained, using the IBR-2 at Dubna and Triga Mark III at Mexico reactor facilities. Some modifications in the Stoughton-Halpering convention, and its comparison with the modified Westcott and Høgdahl method are presented. The application of the described methods at the CS-ISCTN low flux facility, using the big samples techniques, is presented. 相似文献
127.
Yaima Henry García Orlando Reyes Zamora Rosalba Troncoso-Rojas Martín Ernesto Tiznado-Hernndez María Elena Bez-Flores Elizabeth Carvajal-Millan Agustín Rascn-Chu 《Molecules (Basel, Switzerland)》2021,26(21)
Large volumes of fruit and vegetable production are lost during postharvest handling due to attacks by necrotrophic fungi. One of the promising alternatives proposed for the control of postharvest diseases is the induction of natural defense responses, which can be activated by recognizing molecules present in pathogens, such as chitin. Chitin is one of the most important components of the fungal cell wall and is recognized through plant membrane receptors. These receptors belong to the receptor-like kinase (RLK) family, which possesses a transmembrane domain and/or receptor-like protein (RLP) that requires binding to another RLK receptor to recognize chitin. In addition, these receptors have extracellular LysM motifs that participate in the perception of chitin oligosaccharides. These receptors have been widely studied in Arabidopsis thaliana (A. thaliana) and Oryza sativa (O. sativa); however, it is not clear how the molecular recognition and plant defense mechanisms of chitin oligosaccharides occur in other plant species or fruits. This review includes recent findings on the molecular recognition of chitin oligosaccharides and how they activate defense mechanisms in plants. In addition, we highlight some of the current advances in chitin perception in horticultural crops. 相似文献
128.
Jessica Berrones Reyes Peter S. Sherin Amrita Sarkar Marina K. Kuimova Ramon Vilar 《Angewandte Chemie (International ed. in English)》2023,62(42):e202310402
G-quadruplex DNA is a non-canonical structure that forms in guanine-rich regions of the genome. There is increasing evidence showing that G-quadruplexes have important biological functions, and therefore molecular tools to visualise these structures are important. Herein we report on a series of new cyclometallated platinum(II) complexes which, upon binding to G-quadruplex DNA, display an increase in their phosphorescence, acting as switch-on probes. More importantly, upon binding to G-quadruplexes they display a selective and distinct lengthening of their emission lifetime. We show that this effect can be used to selectively visualise these structures in cells using Phosphorescence Lifetime Imaging Microscopy (PLIM). 相似文献
129.
A. Fratiello V. Kubo-Anderson E. Bolanos O. Chavez J. Ortega R. D. Perrigan F. Reyes L. Saenz S. M. Stoll T. Thompson 《Journal of solution chemistry》1996,25(4):345-367
A direct, low-temperature nuclear magnetic resonance spectroscopic study of europium(III)-nitrate contact ion-pairing has been completed, and preliminary results for europium(III)-isothiocyanate have been obtained. In water-acetone-Freon mixtures, at –110°C to –120°C, four15N NMR signals are observed for coordinated nitrate ion. Area evaluations of the signals and their concentration dependence indicate the formation of Eu(NO3)2+, Eu(NO3)
2
1+
, and two higher complexes, possibly the tetra-, with either the penta-or hexanitrato. This correlates well with similar15N NMR results obtained for Ce(III), Pr(III), Nd(III), and Sm(III). As a result of a higher dielectric constant, complex formation is significantly less in water-methanol mixtures, wheein only three complexes form with Eu(NO3)
2
1+
dominating at the highest anion concentrations. Competitive complexing experiments in water-methanol also were made by35Cl NMR chemical shift and linewidth measurements, as well as15N NMR. Initial experiments with the Eu3+-NCS– system show four coordinated anion signals, displaced from the bulk anion peak by about –250 ppm and –2,500 ppm in the13C and15N NMR spectra, respectively. Area evaluations are consistent with the presence of Eu(NCS)2+ through Eu(NCS)
4
1-
in these solutions. A consideration of the chemical shifts identified the nitrogen atom as the site of binding in the NCS–. A discussion of these preliminary results, as well as those for several other metal-ions, will be presented. 相似文献
130.
Norman Marin-Astorga Gina Pecchi Patricio Reyes 《Reaction Kinetics and Catalysis Letters》2005,87(1):121-128
Summary Platinum catalysts (1 wt.%) supported on MCM-41 type and SiO2have been prepared, characterized and evaluated in the enantioselective hydrogenation of 1-phenyl-1,2-propanedione at 298
K and 20 bar of hydrogen pressure, using cinchonidine (CD) as chiral modifier. Chemisorption and TEM results revealed that
both catalysts posses similar metal dispersion, however, significant differences in the catalytic behavior were observed.
With dichloromethane as solvent, high hydrogenation rates and ee values around 47% were obtained for the Pt/MCM-41 catalyst.
This fact is attributed to a confinement effect. The initial reaction rate is strongly dependent on the CD concentration,
and the reaction rate (or ee) vsCD concentration plot exhibits bell-type curves. The main products were (R) -1-hydroxy-1-phenylpropanone and (S) -1-hydroxy-1-phenylpropanone.</o:p> 相似文献