全文获取类型
收费全文 | 5959篇 |
免费 | 153篇 |
国内免费 | 46篇 |
专业分类
化学 | 3720篇 |
晶体学 | 35篇 |
力学 | 208篇 |
数学 | 1281篇 |
物理学 | 914篇 |
出版年
2022年 | 54篇 |
2021年 | 57篇 |
2020年 | 65篇 |
2019年 | 67篇 |
2018年 | 58篇 |
2017年 | 49篇 |
2016年 | 96篇 |
2015年 | 116篇 |
2014年 | 136篇 |
2013年 | 310篇 |
2012年 | 294篇 |
2011年 | 326篇 |
2010年 | 222篇 |
2009年 | 207篇 |
2008年 | 305篇 |
2007年 | 316篇 |
2006年 | 306篇 |
2005年 | 272篇 |
2004年 | 267篇 |
2003年 | 216篇 |
2002年 | 210篇 |
2001年 | 99篇 |
2000年 | 92篇 |
1999年 | 98篇 |
1998年 | 73篇 |
1997年 | 67篇 |
1996年 | 91篇 |
1995年 | 66篇 |
1994年 | 73篇 |
1993年 | 81篇 |
1992年 | 65篇 |
1991年 | 43篇 |
1990年 | 75篇 |
1989年 | 59篇 |
1988年 | 48篇 |
1987年 | 53篇 |
1986年 | 47篇 |
1985年 | 68篇 |
1984年 | 81篇 |
1983年 | 51篇 |
1982年 | 78篇 |
1981年 | 79篇 |
1980年 | 94篇 |
1979年 | 76篇 |
1978年 | 80篇 |
1977年 | 76篇 |
1976年 | 71篇 |
1975年 | 51篇 |
1974年 | 38篇 |
1973年 | 58篇 |
排序方式: 共有6158条查询结果,搜索用时 15 毫秒
101.
102.
[reaction: see text] A short synthesis of intermediates possessing the tricyclic core of natural madangamines, bioactive alkaloids found in marine sponges, is described. The key reaction entails the condensation of the sodium salt of diethylacetonedicarboxylate with a dihydropyridinium salt derivative. This new approach is modeled on a biogenetic proposal linking madangamines to ircinals, related alkaloids occurring in sponges of the same order. 相似文献
103.
Roger Azria Yvonnick Le Coat Jean-Pierre Ziesel Jean-Pierre Guillotin Brahim Mharzi Michel Tronc 《Chemical physics letters》1994,220(6):417-422
Absolute kinetic energy distributions and yields associated with ground state 3P and excited state 1D oxygen atoms have been obtained for O− anion electron stimulated desorption from condensed O2 in the electron energy range 6–15 eV. The observed yields are understood as resulting essentially from dissociative electron attachment reactions via the two lowest 2Σ+g O−2 resonance states through adiabatic and non-adiabatic transitions to the limits O−(2P) + O(3P) and O−(2P) + O(1D). The kinetic energy distributions show the prominent role of electron multiple collision processes and post-dissociation interactions of the O− anions in the condensed phase. 相似文献
104.
The crystal structure of the fluorozirconate SmZrF7 has been determined from single crystal X-ray data; the final R value is 0.028. In this compound, isotypic with all the fluorozirconates LnZrF7 (Ln = rare earth), the symmetry of the cell is monoclinic (space group: P21). The Zr atoms are surrounded by six fluorine atoms forming an almost perfect octahedron: the average distance ZrF is 2.006 Å. The Sm atoms are surrounded by a 8 fluorine atoms polyhedron, the distances SmF lie from 2.221 to 2.411 Å. The association of these two polyhedra by sharing corner forms slabs with an approximately ReO3 type arrangement: the thickness of the slabs is two octahedra.These slabs with the composition M2X7 are held together so that the top fluorine atoms at the surface of a slab increase the coordination of the Sm atom of an adjacent slab. This MX3.5 phase corresponds to a new structural type between MX3 (ReO3 type) and MX4 (SnF4 type). 相似文献
105.
Euzenat L Horhant D Ribourdouille Y Duriez C Alcaraz G Vaultier M 《Chemical communications (Cambridge, England)》2003,(18):2280-2281
Thermal decomposition of hindered amine-borane adducts leads in high yields to monomeric (dialkylamino)boranes R1R2N-BH2 (R1 and R2 = alkyl) that are new and efficient boron-sources in the Pd0 catalyzed borylation reaction affording monomeric aryl(dialkylamino)boranes R1R2N-BHR3 (R3 = aryl). 相似文献
106.
Musangu Mbuyi Michel Evers Guy Tihange Andr Luxen Lon Christiaens 《Tetrahedron letters》1983,24(52):5873-5876
We describe the synthesis of a new heterocyclic system : the 1,3-benzotellurazole, and some of its substituted derivatives. 相似文献
107.
Choua S Dutan C Cataldo L Berclaz T Geoffroy M Mézailles N Moores A Ricard L Le Floch P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(16):4080-4090
EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P-P bond (3.479 A), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P-P distance in neutral 1 is large (5.8 A). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P-P single bond (P-P 2.389 A). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C-Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P-P distance. The ability of the phosphinine radical anion to reversibly form weak P-P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P-P bond. 相似文献
108.
Pierre-Anton Aichinger Martin Michel Colin Servais Marie-Lise Dillmann Martine Rouvet Nicola D''Amico Ralf Zink Henning Klostermeyer David S. Horne 《Colloids and surfaces. B, Biointerfaces》2003,31(1-4):243-255
Fermentation of a reconstituted skim milk concentrate (8% protein) was investigated to elucidate the effects of various fermentation parameters on the structural, rheological and visual (wheying-off) properties of the resulting gels (pH 4.60). Fermentation trials were performed with non-exocellular polysaccharide-producing strains of Streptococcus thermophilus at various fermentation temperatures and at various chymosin levels. Oscillatory vane rheometry carried out on the intact gels (at 4 °C) showed that the level of chymosin had a significant impact on the gel strength (storage modulus G′). This can be explained by the arrangement of casein micelles into more compact aggregates and the enhanced fusion of aggregated casein micelles as revealed by transmission electron microscopy for the gels fermented with chymosin. Wheying-off of the stirred gels as measured by a centrifugation test (at 4 °C) and pore size of the intact gel structures investigated by scanning electron microscopy both increased with increasing level of chymosin and increasing fermentation temperature (resulting in an increase in acidification rate). A higher level of syneresis (wheying-off) is explained by the larger pore size, since larger pores present a lower resistance to the outflow of whey from the gel. 相似文献
109.
Odile Meyer Jean-Franois Hoeffler Catherine Grosdemange-Billiard Michel Rohmer 《Tetrahedron》2004,60(52):12153-12162
An optimised gram scale synthesis allows the production of 1-deoxy-d-xylulose and 1-deoxy-d-xylulose 5-phosphate with possible deuterium labelling at C-5. Such substrates are required for investigations on the mevalonate-independent 2-C-methyl-d-erythritol 4-phosphate pathway for isoprenoid biosynthesis in bacteria and chloroplasts of phototrophic eukaryotes and for the biosynthesis of vitamins B1 (thiamine diphosphate) and B6 (pyridoxol phosphate) in bacteria. 相似文献
110.
Philippe Hennig Eric Raimbaud Christophe Thurieau Jean-Paul Volland André Michel Jean-Luc Fauchère 《Journal of computer-aided molecular design》1996,10(1):83-86
Summary The conformation in dimethylsulfoxide of the somatostatin derivative angiopeptin and of three disulfide-free analogs was estimated by two-dimensional nuclear magnetic resonance spectroscopy at room temperature. The resulting 3D molecular graphics were compared and shown to reflect the observed differences in the inhibition of restenosis after rat aorta balloon injury by these octapeptide inhibitors. Angiopeptin and its active analog 2 displayed a relatively rigid conformation of the cyclic hexapeptide backbone due to the presence of two well-defined hydrogen bonds, further stabilized by a third hydrogen bond outside the ring. No such constraints were detected for the two biologically inactive analogs, which, compared to 2, had a two-atom longer or shorter hexapeptide ring. The well-defined structure of compound 2 may serve as an improved pharmacophore for this new class of drugs. 相似文献