全文获取类型
收费全文 | 1986篇 |
免费 | 69篇 |
国内免费 | 17篇 |
专业分类
化学 | 1421篇 |
晶体学 | 4篇 |
力学 | 17篇 |
数学 | 296篇 |
物理学 | 240篇 |
综合类 | 94篇 |
出版年
2023年 | 12篇 |
2022年 | 22篇 |
2021年 | 26篇 |
2020年 | 27篇 |
2019年 | 31篇 |
2018年 | 29篇 |
2017年 | 23篇 |
2016年 | 42篇 |
2015年 | 43篇 |
2014年 | 46篇 |
2013年 | 101篇 |
2012年 | 158篇 |
2011年 | 201篇 |
2010年 | 78篇 |
2009年 | 58篇 |
2008年 | 151篇 |
2007年 | 127篇 |
2006年 | 148篇 |
2005年 | 131篇 |
2004年 | 127篇 |
2003年 | 112篇 |
2002年 | 98篇 |
2001年 | 15篇 |
2000年 | 21篇 |
1999年 | 20篇 |
1998年 | 19篇 |
1997年 | 14篇 |
1996年 | 18篇 |
1995年 | 20篇 |
1994年 | 10篇 |
1993年 | 10篇 |
1992年 | 8篇 |
1991年 | 11篇 |
1990年 | 9篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 13篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1981年 | 3篇 |
1980年 | 12篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 3篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 4篇 |
1971年 | 2篇 |
排序方式: 共有2072条查询结果,搜索用时 31 毫秒
31.
Jeremy Haefner 《Proceedings of the American Mathematical Society》1996,124(4):1013-1021
Let be a ring graded by a group . We are concerned with describing those -graded rings that are graded equivalent to -crossed products. We give necessary and sufficient conditions for when a strongly graded ring is graded equivalent to a crossed product, provided that the 1-component is either Azumaya or semiperfect. Our result uses the torsion product theorem of Bass and Guralnick. We also construct various examples of such rings.
32.
The method of moments is used to show how the structures of molecules and solids is sensitive to the number of valence electrons. 相似文献
33.
Jagoda M Warzeska S Pritzkow H Wadepohl H Imhof P Smith JC Krämer R 《Journal of the American Chemical Society》2005,127(43):15061-15070
For a number of phosphoryltransfer enzymes, including the exonuclease subunit of DNA polymerase I, a mechanism involving two-metal ions and double Lewis-acid activation of the substrate, combined with leaving group stabilization, has been proposed. Inspired by the active site structure of this enzyme, we have designed as a synthetic phosphoryl transfer catalyst the dicopper(II) macrocyclic complex LCu(2). Crystal structures of complexes [(L)Cu(2)(mu-NO(3))(NO(3))](NO(3))(2) (1), [(L)Cu(2)(mu-CO(3))(CH(3)OH)](BF(4))(2) (2), and [(L)Cu(2)(mu-O(2)P(OCH(3))(2))(NO(3))](NO(3))(2) (3) illustrate various possibilities for the interaction of oxoanions with the dicopper(II) site. 1 efficiently promotes the transesterification of dimethyl phosphate (DMP) in CD(3)OD, k(cat) = 2 x 10(-)(4) s(-)(1) at 55 degrees C. 1 is the only available catalyst for the smooth transesterification of highly inert simple dialkyl phosphates. From photometric titrations and the pH dependence of reactivity, we conclude that a complex [(L)Cu(2)(DMP)(OCH(3))](2+) is the reactive species. Steric bulk at the -OR substituents of phosphodiester substrates O(2)P(OR)(2)(-) drastically reduces the reactivity of 1. This is explained with -OR leaving group stabilization by Cu coordination, an interaction which is sensitive to steric crowding at the alpha-C-atom of substituent R. A proposed reaction mechanism related to that of the exonuclease unit of DNA polymerase I is supported by DFT calculations on reaction intermediates. The complex [(L)Cu(3)(mu(3)-OH)(mu-CH(3)O)(2)(CH(3)CN)(2)](ClO(4))(3) (4) incorporates a [Cu(OH)(OCH(3))(2)(CH(3)CN)(2)](-) complex anion, which might be considered as an analogue of the [PO(2)(OCH(3))(2)(OCD(3))](2)(-) transition state (or intermediate) of DMP transesterification catalyzed by LCu(2). 相似文献
34.
A bulky tripodal tris(carbene)borate ligand, prepared from 1-tert-butylimidazole, is cleanly transferred to iron(II) by a magnesium reagent. 相似文献
35.
Alvarez-Bercedo P Bond AD Haigh R Hopkins AD Lawson GT McPartlin M Moncrieff D Mosquera ME Rawson JM Woods AD Wright DS 《Chemical communications (Cambridge, England)》2003,(11):1288-1289
The reaction of CyPHNa with Sn(NMe2)2 in the presence of PMDETA (= (Me2NCH2CH2)2NMe) gives the title compound [(Sn(mu-PCy))3(Na x PMDETA)2] (1), containing an electron-deficient [(Sn(mu-PCy))]3(2-) dianion with a novel two-electron, three centre (2e-3c) bonding arrangement. 相似文献
36.
Jeremy I. Musher 《Tetrahedron》1974,30(13):1747-1751
The theory of bonding in ylides and their oxides, acetylacetonates and π-cyclopentadienyl complexes has been discussed from the viewpoint of hypervalent bonding. The existing ylides and acetylacetonates are discussed and new ones are proposed. The π-cyclopentadienyl complexes are only speculative. 相似文献
37.
Orbital correlation diagrams for the black phosphorus to A7 (arsenic) phase transformation for various points in the Brillouin zone are presented and analyzed in symmetry terms. The level doublings observed at the black phosphorus geometry for several points are directly attributable to the nonsymmorphic nature of the space group (Herring's theorem). Doublings observed at the A7 structure derive from the choice of a four-atom, rather than two-atom, unit cell to view the geometrical transformation. The space group requirements at the beginning and end points of the transformation process simply control the form of orbital correlation diagrams. In general, crossings are found at where bonds are made and broken along the x direction, a result with simple analogies to more easily visualized one-dimensional problems. 相似文献
38.
Youngtai Yoo Larry K. Johnson Stephen N. Falling Jeremy R. Lizotte Timothy E. Long 《Journal of polymer science. Part A, Polymer chemistry》2002,40(16):2789-2798
Accelerated crosslinking of novel poly(3,4‐epoxy‐1‐butene) (3,4‐PEPB) oligomers in the presence of a cobalt‐based redox catalyst was investigated. Previous studies using model compounds, 3,4‐dimethoxy‐1‐butene and 1,4‐dimethoxy‐2‐butene, suggested that maleation of hydroxyl‐terminated 3,4‐PEPB oligomers would result in more rapid crosslinking in thin films. Novel maleated oligomers offered a unique combination of both electron‐rich and electron‐poor olefinic sites, and quantitative maleation significantly increased the crosslinking rate of 3,4‐PEPB. Efficient copolymerization between terminal maleate groups and olefinic groups in the repeating unit was proposed to account for accelerated crosslinking rates. Furthermore, the addition of novel reactive diluents, such as maleic acid mono‐ethyl ester, also effectively improved the 3,4‐PEPB crosslinking rate. Sol fraction measurements as a function of coating thickness revealed that the crosslinking rate versus oxygen diffusion was less significant for the maleated oligomers because of the presence of reactive electron‐poor olefins. Sol fractions were constant for catalyst concentrations greater than 0.25–0.50 wt % (as compared with oligomer feed). This observation suggested that a redox process was not a dominant factor in determining crosslinking rates at various experimental conditions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2789–2798, 2002 相似文献
39.
A peptidomimetic of the turn in the helix-turn-helix (HTH) motif of DNA-binding proteins was designed and synthesized. Conformational constraint was achieved by an unusual linking of two amino acids with a side chain carbon-carbon bond. A phenyl ring provides the potential for new hydrophobic contacts with the hydrophobic core of the HTH motif. In the mimic, the peptide backbone and the central residue were retained in native form within a 12-membered cyclic tripeptide. The target compound 1b was synthesized by two sequential Horner-Wittig couplings followed by enantioselective hydrogenation with Rh(MeDuPHOS) in eight steps and 35% overall yield. The stereochemical outcome of the key hydrogenation was determined by aromatic ring oxidation with RuO(2)/NaIO(4) to give 2 equiv of Boc-Asp-OMe. 相似文献
40.
Masterson DS Yin H Chacon A Hachey DL Norris JL Porter NA 《Journal of the American Chemical Society》2004,126(3):720-721
Strategies are reported that combine in one step a predictable chemical-based protein digestion with mass spectrometry. Lysine residue amino groups in peptides and proteins are modified by reaction with a peroxycarbonate derived from p-nitrophenol, and tert-butyl hydroperoxide. The peroxycarbonate reacts with lysine residues in peptides and proteins, and the resulting lysine peroxycarbamates undergo homolytic fragmentation under conditions of low-energy collision-induced dissociation (CID). Observed fragmentation of the peptides involves apparent free radical processes including Hofmann-L?ffler-type rearrangements that lead to peptide chain fragmentation. Strategies for directed cleavage of peptides by free radical promoted processes are feasible, and such strategies may well simplify schemes for protein analysis. 相似文献