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101.
Albert Padwa 《Helvetica chimica acta》2005,88(6):1357-1374
The transition metal catalyzed reaction of α‐diazo carbonyl compounds has found numerous applications in organic synthesis, and its use in either heterocyclic or carbocyclic ring formation is well‐precedented. In contrast to other catalysts that are suitable for carbenoid reactions of diazo compounds, those constructed with the dirhodium(II) framework are most amenable to ligand modification that, in turn, can influence reaction selectivity. The reaction of rhodium carbenoids with carbonyl groups represents a very efficient method for generating carbonyl ylide dipoles. Rhodium‐mediated carbenoid–carbonyl cyclization reactions have been extensively utilized as a powerful method for the construction of a variety of novel polycyclic ring systems. This article will emphasize some of the more recent synthetic applications of the tandem cyclization/cycloaddition cascade for natural product synthesis. Discussion centers on the chemical behavior of the rhodium metal–carbenoid complex that is often affected by the nature of the ligand groups attached to the metal center. 相似文献
102.
Anna M. Maj Albert Demonceau Alfred F. Noels 《Journal of organometallic chemistry》2007,692(14):3048-3056
A range of new imidazolium and imidazolinium chlorides bearing biphenyl units on their nitrogen atoms was synthesized. They differed by the electron-withdrawing or -donating nature and the steric bulk of the substituents on their aromatic rings. These various N-heterocyclic carbene (NHC) precursors were combined with the [RuCl2(p-cymene)]2 dimer and potassium tert-butoxide to generate the corresponding ruthenium-arene complexes [RuCl2(p-cymene)(NHC)] in situ. The catalytic activity of these species was investigated in the photoinduced ring-opening metathesis polymerization (ROMP) of cyclooctene. The results obtained confirmed the necessity of blocking the ortho-positions of the phenyl rings in the vicinity of the metal center in order to attain high catalytic efficiencies. They also showed that changing the steric and electronic properties of the substituents on the remote phenyl rings of the biphenyl units had no significant influence on the outcome of the polymerization. 相似文献
103.
Shi Liang HUANG Yi LUO Zhi Shu HUANG Xian Zhang BU Pei Qing LIU Lin MA Yue Ming LI Albert S. C. CHAN Lian Quan GU 《中国化学快报》2006,17(6):769-772
Mansonone compounds represent a series of naturally occurring o-quinones mainly isolated from the heartwood of Mansonia Altissima1 and Ulmus Glabra2. Mansonone F contained oxaphenalene skeleton which was a relatively novel structure and rarely existed in … 相似文献
104.
Tutusaus O Viñas C Núñez R Teixidor F Demonceau A Delfosse S Noels AF Mata I Molins E 《Journal of the American Chemical Society》2003,125(39):11830-11831
exo-Cluster dicarbollides substitution has allowed tuning of the E degrees (Ru(II)/Ru(III)) potential to obtain the best-performing Kharasch catalyst. We postulate that this is possible through the to-and-fro electron movement between the boron cluster and the sulfonium moieties. 相似文献
105.
Ccile Pasquier Albert Gossauer Walter Keller Christoph Kratky 《Helvetica chimica acta》1987,70(8):2098-2109
(?)-(4S,16S)-8, 12-bis[de(2-carboxyethyl)]mesourobilin-IIIα hydrochloride ( 8 ) has been synthesized from the enantiomerically pure 1,4,5,10-tetrahydro-1-oxodipyrrin-9-carboxylic-acid precursor 6a whose absolute configuration was determined by X-ray diffraction analysis of the N-[(S)-1-(1-naphthyl)ethyl] carboxamide 7b . The present results prove unequivocally that an (S,S)-configurated urobilin chromophore displays a negative Cotton effect in the VIS absorption range. However, the helicity of the inherently dissymmetric chromophore remains undetermined. 相似文献
106.
Sabine Strohschein Matthias Pursch Heidrun Händel K. Albert 《Analytical and bioanalytical chemistry》1997,357(5):498-502
The separation of cis/trans isomers of β-carotene has been performed with a C30 stationary phase employing 1H NMR spectroscopy as an on-line detection technique. 1D as well as 2D NMR spectra have been recorded in the stopped-flow
mode for the predominant chromatographic peaks. Structural assignment of the five identified isomers was performed via comparison
of simulated 1D 1H NMR spectra on the basis of the structures of β-carotene cis/trans isomers with the experimental data, and also by the analysis of the proton-proton connectivities in the 2D NMR spectra of
three isomers with the highest concentration. The chromatographic retention behaviour of the isomers agreed well with previously
reported data. The advantage of the applied hyphenated coupling technique compared to conventional off-line techniques lies
in the fact that chromatographic separation and NMR detection are performed in a closed system, so that reisomerization of
the separated compounds is inhibited.
Received: 29 May 1996 / Revised: 1 July 1996 / Accepted: 4 July 1996 相似文献
107.
Ph. Albert 《Journal of Radioanalytical and Nuclear Chemistry》1980,55(2):453-462
In this article the research topics dealt with at Research Group for Applications of Nuclear Reactions to Chemical Analysis
(GARNAC) are described. Particular emphasis is laid on the main contribution of the GARNAC to the development and the evolution
of charged particle activation analysis, evolving with the analytical studies performed and with the scientific and technical
progress realized in material and semiconductors science.
相似文献
108.
Albert Fischli 《Helvetica chimica acta》1982,65(8):2697-2708
The olefinic system in 3β-methoxy-4-cholesten-6 a-ol ( 2 ) is reduced using cob (I)alamin ( 1 ( I ); see Scheme 1) as catalyst, aqueous acetic acid as solvent and metallic zinc as electron source (cf. Schemes 2 and 3). Experimental evidence for an attack of 1 ( I ) on both faces of the double bond is presented. By the same catalyst (1 R)-10, 10-dimethyl-2-pinene- 10-carbonitrile ( 9 ) is first transformed to the menthene derivative 11 (see Schemes 4 and 5). The ring opening is then followed by a fast saturation of the disubstituted olefinic system in 11 , and ultimately the remaining double bond is reduced in a slow reaction. The cis-configurated saturated menthane derivative 16 is the main final product ( 16 / 17 ≈ 10:1). 相似文献
109.
The first part of the paper deals with a critical discussion of the methodical basis of essential work of fracture (EWF) concept with respect to the specimen geometry (especially the notch depth) and application to polymers. In the second part, an in situ testing device, which combines a tensile testing machine with an optical strain-field measuring system, has successfully demonstrated possibility of characterization of fracture behaviour of polystyrene-polybutadiene block copolymers and block copolymer/homopolymer blends as examples of nanostructured polymer materials. It has been shown that knowledge of the time evolution of the strain field close to the crack tips leads to a simple verification of the basic precondition for the applicability of the EWF concept, the precondition “plastic zone coalescence-before-stable crack propagation”. 相似文献
110.
A single-crystal neutron diffraction analysis of the cluster complex [H(4)Co(4)(C(5)Me(4)Et)(4)] was carried out on the new quasi-Laue diffractometer VIVALDI at the Institut Laue-Langevin. The structure consists of four face-bridging hydrides attached to a tetrahedral cobalt metal core. Average distances and angles in the core of the molecule are as follows: Co-Co = 2.571(8), Co-C = 2.158(6), Co-H = 1.749(7), H.H = 2.366(9) A; Co-H-Co = 94.6(3), H-Co-H = 85.1(3) degrees. The hydride ligands are located off the Co-Co-Co planes by an average distance of 0.923(8) A. It is suggested that the dimensions of the HCo(3) fragments found in this molecule provide reasonable estimates for analogous distances and angles associated with chemisorbed H atoms situated on the 3-fold hollows of a cobalt surface. Crystallographic details: space group P2(1)/a (monoclinic); a = 21.979(2), b = 10.924(1), c = 34.406(2) A; beta = 90.81(1) degrees; Z = 8. Final agreement factor: R(F) = 0.099 for 3779 reflections [I > 2sigma(I)] collected at 20 K. 相似文献