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91.
[reaction: see text] Ligand-free Pd(OAc)(2) can be used as a catalyst in the Heck reaction of aryl bromides as long as the amount of catalyst is kept between 0.01 and 0.1 mol %. At higher concentrations palladium black forms and the reaction stops. The actual catalyst is monomeric. Palladacycles merely serve as a source of ligand-free palladium in Heck reactions of aryl bromides. 相似文献
92.
Travaille AM Kaptijn L Verwer P Hulsken B Elemans JA Nolte RJ van Kempen H 《Journal of the American Chemical Society》2003,125(38):11571-11577
The lateral alignment of [012] habit-modified calcite crystals with respect to a carboxylic acid terminated self-assembled monolayer (SAM) of thiols has been determined. The crystals were grown from a Kitano solution (pH 5.6-6.0), and the samples were investigated with scanning electron microscopy, X-ray diffraction, and polarization microscopy. For the first time, a lattice match in one direction, which is the nearest neighbor direction of the SAM and the calcite <100> direction, has been experimentally shown. The experimental results are in good agreement with the theoretical models proposed in previous work, and it is expected that this method can be applied to similar systems where inorganic crystals nucleate with a preferred orientation to a SAM. 相似文献
93.
Vissers JP Blackburn RK Moseley MA 《Journal of the American Society for Mass Spectrometry》2002,13(7):760-771
A variable flow "peak trapping" liquid chromatography (LC) interface has been developed for the coupling of nanoscale LC to electrospray ionization mass spectrometry (ESI-MS). The presented peak trapping LC interface allows for the extended analysis time of co-eluting compounds and has been employed for the identification of proteins via tandem mass spectrometry (MS/MS). The variable flow process can be controlled either manually or in a completely automated manner where the mass spectrometer status determines the status of the variable flow interface. When the mass spectrometer operates in MS survey mode, the interface is operated in a so-called "high-flow" mode. Alternatively, the interface is operated in a "low-flow" mode during MS/MS analysis. In the "high-flow" mode of the variable flow process the column flow rate is typically around 200 nL/min, whereas in the "low-flow" mode the column effluent is introduced into the source of the mass spectrometer at 25 nL/min. In addition to the flow reduction during MS/MS analysis, the gradient is paused to preserve the peptide separation on the analytical nanoscale LC column. The performance of the variable flow nanoscale LC/MS/MS interface is demonstrated by the automated analysis of standard peptide mixtures and protein digests utilizing variable flow, data-dependent scanning MS/MS techniques, and automated database searching. 相似文献
94.
Krishnan Raghavachari Michael J. Frisch John A. Pople Paul von R. Schleyer 《Chemical physics letters》1982,85(2):145-149
Calculations with an extended polarized basis set and Møller-Plesset perturbation theory including triple substitution correlation corrections in the fourth-order treatment indicate that singlet ethylidene (CH3CH:) is not a local minimum on the C2H4 potential energy surface. Rearrangement to ethylene occurs without actuation. Barriers for hydrogen scrambling and for 1,1-hydrogen elimination are estimated. 相似文献
95.
Johannes J. Voegel Michael M. Altorfer Steven A. Benner 《Helvetica chimica acta》1993,76(5):2061-2069
A new synthesis is reported for 4-aminoimidazo[1,2-a]-1,3,5-triazin-2(1H)-one ( =5-aza-7-deaza-isoguanosine; 8 ), a purine analog that, when incorporated into an oligonucleotide chain, presents a H-bond donor-acceptor-acceptor pattern to a complementary pyrimidine analog. A protected ribose derivative was coupled to 8 to yield 4-amino-8-(β-D -ribofuranosyl)imidazo[1,2-a]-1,3,5-triazin-2(8H)-one ( =5-aza-7-deaza-isoguanosine; 11 ) after deprotection, Alternatively, direct synthesis of both the ribo derivative 11 and the corresponding deoxyribo derivative 17 as the β-D -anomers was achieved using the enzyme purine nucleoside phosphorylase in a one-pot reaction. This adapts a known synthetic approach to yield a new strategy for obtaining diastereoisomerically pure deoxyribonucleoside analogs on 1-gram scales. 相似文献
96.
The synthesis of a chemical compound library using diversity-oriented synthesis (DOS) is discussed. The library is structurally inspired by the Amaryllidaceae alkaloids, a family of natural products which has been known to demonstrate potent antiviral and antineoplastic activity. Highlights of this work include the rapid, high-yielding construction of the octahydroindolinone core and the solid-phase diversification of the lactam using a neutral phosphazene base. 相似文献
97.
Johannes André 《Journal of Geometry》1992,43(1-2):22-29
Configurational conditions (Schließungsaussagen) of a noncommutative space will be developped from pairs (,
) of digraphs where is a partial digraph of
. In this way we obtain an extensive generalization of Pfalzgraf 's q-simplex-conditions Simq.In Memoriam Hans Zassenhaus 相似文献
98.
T. J. Fyvie H. L. Frisch J. A. Semlyen S. J. Clarson J. E. Mark 《Journal of polymer science. Part A, Polymer chemistry》1987,25(9):2503-2509
Poly(2,6-dimethyl-1,4-phenylene oxide) has been crosslinked in the presence of large poly(dimethylsiloxane) cyclics (92 repeating units). Approximately 26% by weight of the cyclics were threaded and permanently captured by the polymer network forming a topological isomeric structure referred to as a polymeric catenane. Nonentrapped cyclics were extracted with chloroform. Chemical analyses and micrographs showed evidence for crosslinking and cyclic entrapment, while physical testing demonstrated distinct differences in physical properties such as the glass transition temperature, ultimate mechanical properties, and dynamic viscoelastic response between the crosslinked control samples, and those containing cyclic poly(dimethylsiloxane). 相似文献
99.
100.
Chinnasamy Thangavel Vijayakumar Johannes Karl Fink 《Journal of polymer science. Part A, Polymer chemistry》1983,21(6):1617-1625
HET acid was condensed with either ethylene glycol or 1,4-butanediol to yield the respective oligoesters. These oligoesters were characterized by molecular weight, measured by vapor-pressure osmometry. The degradation mechanism of oligoesters of HET acid with either ethylene glycol or 1,4-butanediol was elucidated. The thermal properties of these polyesters were studied using differential thermal analysis and thermal volatilization analysis. The mechanism of degradation was proposed on the basis of pyrolysis–gas chromatography studies, the degradation products being subsequently identified by mass spectrometry. It turned out that in the course of decomposition, hexachlorocyclopentadiene was formed, which is believed to be the active substance for flame retardance in these systems. The aspect of tailoring flame retardancy by changing the chemical environment, i.e., by using different diols, is discussed. 相似文献