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排序方式: 共有446条查询结果,搜索用时 343 毫秒
341.
Posner GH Dai H Bull DS Lee JK Eydoux F Ishihara Y Welsh W Pryor N Petr S 《The Journal of organic chemistry》1996,61(2):671-676
(R)-(+)-Binol-titanium coordinates with commercial methyl 2-pyrone-3-carboxylate and promotes mild, highly enantiocontrolled Diels-Alder cycloadditions with electron-rich vinyl ether CH(2)=CHOCH(2)-1-naphthyl and vinyl silyl ether CH(2)=CHOSiMe(2)Bu-t leading to valuable 1alpha,25-dihydroxyvitamin D(3) (calcitriol) intermediate (-)-2. Unexpectedly, two subtle variables were found to be critical for obtaining reproducibly over 90% enantioselectivities in gram scale cycloadditions: (1) the moisture content (15-17% is best) of the molecular sieves used to prepare the binol-titanium complex according to the Mikami protocol and (2) the temperature (50 degrees C is best) at which the pyrone ester is mixed with the binol-titanium complex. Unsubstituted 2-pyrone undergoes ytterbium-promoted, high-pressure, regioselective, and stereoselective Diels-Alder cycloaddition with benzyl vinyl ether to form versatile bicyclic lactone (+/-)-4 on gram scale. 相似文献
342.
This paper describes the design and operation of a liquid-core liquid-cladding (L(2)) lens formed by the laminar flow of three streams of liquids in a microchannel whose width expands laterally in the region where the lens forms. Two streams of liquid with a lower refractive index (the cladding) sandwich a stream of liquid with a higher refractive index (the core). As the core stream enters the expansion chamber, it widens and becomes biconvex in shape, for some rates of flow. This biconvex fluidic element focuses light. Manipulating the relative rates of flow of the streams reconfigures the shape, and therefore the focal distance, of the L(2) lens. This paper also describes a technique for beam tracing, and for characterization of a lens in an enclosed micro-scale optical system. The use of a cladding liquid with refractive index matched to that of the material used in the fabrication of the microfluidic system (here, poly(dimethylsiloxane)) improves the quality of the focused beam. 相似文献
343.
Maxwell CI Shah K Samuleev PV Neverov AA Brown RS 《Organic & biomolecular chemistry》2008,6(15):2796-2803
Some chiral lanthanide complexes of the Schiff base adducts of: a) bis(2-pyridylcarboxaldehyde) and (1R),(2R)-trans-1,2-diaminocyclohexane (Pyr-R,R'-chxn: 3); b) 6-methyl-2-pyridylcarboxaldehyde and (1R),(2R)-trans 1,2-diaminocyclohexane (MePyr-chxn, 4); and c) 2,6-pyridyldicarboxaldehyde and (1R),(2R)-trans-1,2-diaminocyclohexane ((Pyr-R,R'-chxn)(2), 5) have been screened for their utility to promote kinetic resolution via metal catalyzed alcoholyses of the p-nitrophenyl esters of chiral D- and L-Boc-protected glutamine and phenylalanine. Solvents were varied to optimize the kinetic selectivity values, defined as k(2)(L)/k(2)(D) or k(2)(D)/k(2)(L), for the methanolysis and in some cases, ethanolysis of these substrates. At ambient temperature the greatest selectivity was found for the ethanolysis of Boc-Gln-OPNP, catalyzed by 3:Yb(3+):((-)OEt) (k(2)(L)/k(2)(D) = 7.2). The greatest selectivity for Boc-Phe-OPNP is k(2)(D)/k(2)(L) = 3.9 for its methanolysis promoted by 5:La(3+):((-)OMe). A kinetic method is introduced for the determination of both d and l rate constants for catalyzed alcoholysis from a single kinetic experiment. The activation parameters DeltaH(double dagger) and DeltaS(double dagger) were determined for the metal catalyzed methanolysis and ethanolysis of the Boc-Gln-OPNP substrates, and selectivity factors were found to increase at lower temperatures. A low temperature time course for the ethanolysis of racemic Boc-Gln-OPNP catalyzed by 3:Yb(3+):((-)OEt) at -15 degrees C indicated that after 3 hours 60% residual d-enantiomer was observed having an enantiomeric excess of >95% ee. The activation parameters for the ethanolysis of the same substrate catalyzed by (Pyr-R,R'-chxn)(2):La(3+):((-)OEt) predict a k(2)(D)/k(2)(L) = 40.4 at -40 degrees C with a large ee of >99% with approximately 80% of l isomer remaining at that temperature which has been experimentally confirmed. 相似文献
344.
I. N. Stan’kov A. A. Sergeeva V. B. Sitnikov O. T. Morozova I. D. Derevyagina S. N. Mylova V. M. Shchukin 《Journal of Analytical Chemistry》2007,62(6):564-570
A procedure was proposed for the gas-chromatographic determination of trace O-isobutyl-S-2-(N,N-diethylamino)ethyl ester of methylthiophosphonic acid (mixed ester, ME) in burning products formed after the detoxication of elements in building constructions, personal protection equipment, waste degassing solutions, and other production wastes (slime) at a level of (1.0–10.0) × 10?8 mg/g. The procedure is based on the extraction of ME from an analyzed material with a mixture of aqueous solutions of monoethanolamine and NaOH, its back extraction to hexane, the evaporation of the hexane layer (with the addition of HCl) to dryness, the transformation of ME into methyl ester of O-isobutylmethylphosphonic acid with the use of methanol in the presence of AgNO3, and the chromatography of the derivative on an open tubular column with the chemically modified HP-INN OWax stationary phase with the use of a flame-photometric detector. 相似文献
345.
The rates and products of cleavage of methyl (2-chloro-4-nitrophenyl) phosphate (2) promoted by a dinuclear Zn(II) complex (3) of 1,3-bis-N,N'(1,5,9-triazacyclododecyl)propane along with 1 equiv of ethoxide were investigated in ethanol solution containing small amounts of water (8 mM or=1.6 x 10(17) times relative to the background hydroxide reaction, suggesting that complex 3 promotes the hydrolysis at least 1000 times more effectively than ethanolysis. 相似文献
346.
I. N. Stan’kov I. D. Derevyagina V. B. Condrat’ev V. B. Sitnikov S. V. Sadovnikov S. N. Mylova 《Journal of Analytical Chemistry》2008,63(7):668-673
A procedure is proposed for the gas-chromatographic determination of S-[2-(N,N)-diethylamino) ethyl]methylphosphonothioic acid (monothiol) in water at a level of 5 × 10?5%. The procedure is based on the extraction of monothiol from water by liquid-liquid extraction, treatment of the extract with isopropanol in the presence of AgNO3 and with diazomethane to obtain O-isopropyl-O-methyl methylphosphonate, and the chromatographic detection of the derivative obtained with a flame-photometric detector. The relative error of determining monothiol in water does not exceed 35%; the time of analysis is 60 min. 相似文献
347.
We analyze a nonstandard isoperimetric problem in the plane associated with a metric having degenerate conformal factor at two points. Under certain assumptions on the conformal factor, we establish the existence of curves of least length under a constraint associated with enclosed euclidean area. As a motivation for and application of this isoperimetric problem, we identify these isoperimetric curves, appropriately parametrized, as traveling wave solutions to a bistable Hamiltonian system of PDEs. We also determine the existence of a maximal propagation speed for these traveling waves through an explicit upper bound depending on the conformal factor.© 2015 Wiley Periodicals, Inc. 相似文献
348.
Adriana Corina Hangan Roxana Liana Stan Alexandru Turza Luminiţa Simona Oprean Emöke Páll Sînziana Gheorghe-Cetean Bogdan Sevastre 《Transition Metal Chemistry》2017,42(2):153-164
An N-sulfonamide (HL = N-(5-(4-methoxyph-enyl)-[1, 3, 4]–thiadiazole–2-yl)-naphtalenesulfonamide) and two of its Cu(II) complexes, [Cu(L)2(py)2] (C1) and [Cu(L)(phen)2](L)1.25(MeOH) (C2), were synthesized. The X-ray crystal structures of the complexes have been determined. In complex C1, the copper atom is four-coordinated, forming a CuN4 chromophore, while in complex C2, the copper atom is five-coordinated, forming a CuN5 chromophore. The ligand acts as monodentate, coordinating the metal through a single thiadiazole N atom. The molecules from the reaction medium (pyridine and phenanthroline) are also involved in coordination to the copper atoms. The complexes have a square planar (C1) and slightly distorted square pyramidal (C2) geometries. The compounds were characterized by physicochemical and spectroscopic methods. Nuclease activity studies of the complexes confirm their capacity to cleave DNA. Both complexes also have SOD-like activity, but weaker than the native Cu2Zn2SOD activity. Cytotoxicity studies were carried out on melanoma cell line B16F10 and on normal retinal epithelial cell line (D407) and confirmed that C2 inhibits the growth of B16F10 cells, in a dose-dependent manner. Also, C2 displays a cytoselective profile, since it is not toxic to the D407 cell line. The results of the cell cytotoxicity studies are in concordance with the DNA cleavage and SOD mimetic activity studies and indicate that complex C2 has a high biological activity. 相似文献
349.
A Versatile Approach for the Site‐Specific Modification of Recombinant Antibodies Using a Combination of Enzyme‐Mediated Bioconjugation and Click Chemistry 下载免费PDF全文
Dr. Karen Alt Dr. Brett M. Paterson Dr. Erik Westein Stacey E. Rudd Stan S. Poniger Shweta Jagdale Katie Ardipradja Timothy U. Connell Dr. Guy Y. Krippner Ashish K. N. Nair Dr. Xiaowei Wang Prof. Henri J. Tochon‐Danguy Prof. Paul S. Donnelly Prof. Karlheinz Peter Prof. Christoph E. Hagemeyer 《Angewandte Chemie (International ed. in English)》2015,54(26):7515-7519
A unique two‐step modular system for site‐specific antibody modification and conjugation is reported. The first step of this approach uses enzymatic bioconjugation with the transpeptidase Sortase A for incorporation of strained cyclooctyne functional groups. The second step of this modular approach involves the azide–alkyne cycloaddition click reaction. The versatility of the two‐step approach has been exemplified by the selective incorporation of fluorescent dyes and a positron‐emitting copper‐64 radiotracer for fluorescence and positron‐emission tomography imaging of activated platelets, platelet aggregates, and thrombi, respectively. This flexible and versatile approach could be readily adapted to incorporate a large array of tailor‐made functional groups using reliable click chemistry whilst preserving the activity of the antibody or other sensitive biological macromolecules. 相似文献
350.
Stan Chiriţă Alexandre Danescu 《International Journal of Solids and Structures》2008,45(17):4850-4859
The present paper studies the strong ellipticity for all crystal classes of tetragonal system in a linearly elastic material. Explicit conditions characterizing the strong ellipticity of the elasticity tensor are established for the tetragonal system with six elasticities (that is tetragonal–scalenohedral, ditetragonal–pyramidal, tetragonal–trapezohedral, ditetragonal–dipyramidal crystal classes) as well as for the tetragonal system with seven elasticities (tetragonal–disphenoidal, tetragonal–pyramidal and tetragonal–dipyramidal crystal classes). 相似文献