全文获取类型
收费全文 | 10338篇 |
免费 | 477篇 |
国内免费 | 81篇 |
专业分类
化学 | 7267篇 |
晶体学 | 93篇 |
力学 | 386篇 |
综合类 | 1篇 |
数学 | 976篇 |
物理学 | 2173篇 |
出版年
2023年 | 48篇 |
2022年 | 140篇 |
2021年 | 182篇 |
2020年 | 169篇 |
2019年 | 201篇 |
2018年 | 127篇 |
2017年 | 126篇 |
2016年 | 321篇 |
2015年 | 298篇 |
2014年 | 380篇 |
2013年 | 591篇 |
2012年 | 760篇 |
2011年 | 806篇 |
2010年 | 543篇 |
2009年 | 460篇 |
2008年 | 691篇 |
2007年 | 569篇 |
2006年 | 591篇 |
2005年 | 532篇 |
2004年 | 425篇 |
2003年 | 378篇 |
2002年 | 373篇 |
2001年 | 241篇 |
2000年 | 205篇 |
1999年 | 123篇 |
1998年 | 95篇 |
1997年 | 92篇 |
1996年 | 140篇 |
1995年 | 98篇 |
1994年 | 95篇 |
1993年 | 116篇 |
1992年 | 86篇 |
1991年 | 88篇 |
1990年 | 73篇 |
1989年 | 73篇 |
1988年 | 58篇 |
1987年 | 50篇 |
1986年 | 48篇 |
1985年 | 57篇 |
1984年 | 43篇 |
1983年 | 44篇 |
1982年 | 41篇 |
1981年 | 43篇 |
1980年 | 27篇 |
1979年 | 33篇 |
1978年 | 25篇 |
1977年 | 38篇 |
1976年 | 31篇 |
1974年 | 28篇 |
1972年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
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
82.
Hong SJ Ryu JY Lee JY Kim C Kim SJ Kim Y 《Dalton transactions (Cambridge, England : 2003)》2004,(17):2697-2701
The structures of new polymeric compounds containing Cu(II) ions and btp (2,6-bis(N'-1,2,4-triazolyl)pyridine) ligands have been determined. The btp ligands bridge Cu(II) ions to form double zigzag chains, [Cu(ClO4)2(btp)2] 3 with perchlorate anions, and form single zigzag chains, [Cu(btp)(H2O)4](SO4).2H2O 4 with sulfate anions. The polymeric compound 3 was found to effectively catalyze the epoxide ring-opening reaction with methanol, while polymeric compound 4 was almost inactive with epoxides under the same conditions. The polymeric compound 3 showed an efficient catalytic activity and regioselective reactivity in the ring opening of epoxides and allowed reuse without a significant loss of activity through three runs with epoxides. 相似文献
83.
The action of a series of lanthanide chelate complexes LnL3, where Lu=La, Pr, Nd, Sm, Eu, Dy, Ho, Er and Yb; L= dbm (dibenzoylmethanato), dpm (dipivaloylmethanato), and acac acetylactonato), on nmr spectra of 2-butanol has been studied. The origin of shifting abilities and shifting sign of various lanthanide ions are explained by means of Bleaney's theory. 相似文献
84.
Kang SO VanderVelde D Powell D Bowman-James K 《Journal of the American Chemical Society》2004,126(39):12272-12273
Multiple deuterium exchange between DMSO-d6 and amide hydrogens in two hexaamido cryptand fluoride receptors has been verified by 19F and 2H NMR and FAB mass spectral studies. Structural results for one of the complexes indicate a tricapped trigonal prism hydrogen bond coordination geometry around an encapsulated fluoride, with hydrogen bonds from fluoride to six amide and three phenyl hydrogens. 相似文献
85.
Summary A set of algorithms designed to enhance the display of protein binding cavities is presented. These algorithms, collectively entitled CAVITY SEARCH, allow the user to isolate and fully define the extent of a particular cavity. Solid modeling techniques are employed to produce a detailed cast of the active site region, which can then be color-coded to show electrostatic and steric interactions between the protein cavity and a bound ligand. 相似文献
86.
An improved method for constructing a full-length enriched cDNA library using small amounts of total RNA as a starting material 总被引:1,自引:0,他引:1
We have developed an improved method for constructing a full-length cDNA library using small quantity of material by modifying the original oligo-capping method. In our devised method, total RNAs are used in sequential oligo-capping steps directly without preliminary mRNA purification. Using this method, we constructed full- length cDNA libraries from 100 mg of total RNA. These libraries contained 8x10(5) to 8x10(6) independent clones with average insert sizes of 2.0 kb. Moreover, the number of full-length cDNAs containing the translation initiation codon ATG in the constructed libraries was estimated to 60-70%. In addition, 54% of the known cDNAs had a longer 5' end than the corresponding genes in the public database. Our results show that the method can be effectively used to construct full-length enriched cDNA libraries, especially, if starting material is limited. 相似文献
87.
D. S. Hwang W. M. Choung Y. K. Kim J. H. Park S. J. Park 《Journal of Radioanalytical and Nuclear Chemistry》2002,254(2):255-262
The formation property of Mo precipitate was investigated and improved the existing process was using H2O2 that acts as an interfering compound in a subsequent alumina adsorption process. The property of the Mo precipitate was investigated by using SEM, FTIR, TG-DTA, and XRD. The simulated solution consisted of 1M nitric acid containing seven elements (Mo, I, Ru, Zr, Ce, Nd, Sr) and their radioactive tracers. As a result, the precipitate was composed of the Mo precipitate and re-precipitated a-benzoinoxime which was added excessively for increasing the precipitation efficiency. It was confirmed that the Mo precipitate was formed by the reaction of two a-benzoinoxime molecules and one MoO2
2+. Molybdenum precipitate was dissolved in 0.4M NaOH solution within 5 minutes without H2O2. Hydrogen peroxide induced only the rapid dissolution of the a-benzoinoxime re-precipitate. Also, the dissolution method without H2O2 was favorable in the purification aspect because Zr and Ru were contained as a small fraction of 1.3% and 7.7%, respectively, in the dissolving solution. 相似文献
88.
Poly(dimethylsiloxane) microchip for precolumn reaction and micellar electrokinetic chromatography of biogenic amines 总被引:2,自引:0,他引:2
We have demonstrated that precolumn derivatization and capillary electrophoresis separation on a poly(dimethylsiloxane) (PDMS) microchip can be realized as efficient as those on glass microchips. In an optimized condition of micellar electrokinetic chromatography (MEKC), using 25 mM sodium borate buffer (pH 10.0) with 25 mM sodium dodecyl sulfate (SDS) and 5% v/v methanol, the electroosmotic flow in an oxidized PDMS microchip is stabilized within 3% for days. By employing a fluorometric derivatization with o-phthaldialdehyde (OPA) in an optimally designed reaction chamber, four most important biogenic amines occurring in foods, histamine, tyramine, putrescine, and tryptamine, are quantitatively determined in less than 1 min at the levels applicable to real samples. The migration behaviors of anionic OPA-derivatized biogenic amines under the MEKC conditions are analyzed, and it has been found that under our separation conditions, the electrophoretic mobility of the SDS micelles is significantly greater than those of the anions in the aqueous phase. The channel manifold in a PDMS substrate is fabricated using replica molding against a thick photoresist, SU-8, pattern generated by photolithography. The plate with the microchannel pattern is strongly, irreversibly bonded to another PDMS plate by using a new bonding technique, which employs surface oxidation by corona discharge generated from a cheap, handy source, Tesla coil. 相似文献
89.
Simultaneous anodic oxidation and cathodic reduction of aldoses in a divided flow cell were studied. The stream of the anolyte was an aqueous solution containing D-glucose, sodium bromide, and sodium bicarbonate. The stream of the catholyte was also an aqueous solution containing xylose and sodium sulfite. The factors which affected both the anodic and cathodic reactions were studied. The results indicate that the flow rates and temperatures of the anolyte and the catholyte, concentrations of the aldoses, pH values and the material of electrodes significantly affect both anodic and cathodic yields. The selectivities of gluconic acid in the anode and xylitol in the cathode were very high. The power consumption of paired electrolysis in the flow system was less than paired electrolysis in a batch system. 相似文献
90.
Dal Ho Huh 《Tetrahedron》2004,60(44):9857-9862
Various 2-arylmethylimidazolines were prepared by treating readily available 2-aryl-1,1-dibromoethenes with ethylenediamine under mild conditions and further converted into the corresponding imidazoles smoothly with Swern oxidation. 相似文献