全文获取类型
收费全文 | 6477篇 |
免费 | 256篇 |
国内免费 | 46篇 |
专业分类
化学 | 4785篇 |
晶体学 | 24篇 |
力学 | 208篇 |
数学 | 883篇 |
物理学 | 879篇 |
出版年
2023年 | 67篇 |
2022年 | 87篇 |
2021年 | 126篇 |
2020年 | 118篇 |
2019年 | 148篇 |
2018年 | 91篇 |
2017年 | 82篇 |
2016年 | 197篇 |
2015年 | 166篇 |
2014年 | 205篇 |
2013年 | 369篇 |
2012年 | 465篇 |
2011年 | 503篇 |
2010年 | 271篇 |
2009年 | 239篇 |
2008年 | 424篇 |
2007年 | 435篇 |
2006年 | 391篇 |
2005年 | 361篇 |
2004年 | 338篇 |
2003年 | 299篇 |
2002年 | 249篇 |
2001年 | 84篇 |
2000年 | 80篇 |
1999年 | 65篇 |
1998年 | 57篇 |
1997年 | 60篇 |
1996年 | 74篇 |
1995年 | 49篇 |
1994年 | 36篇 |
1993年 | 27篇 |
1992年 | 32篇 |
1991年 | 33篇 |
1990年 | 35篇 |
1989年 | 27篇 |
1988年 | 18篇 |
1986年 | 21篇 |
1985年 | 26篇 |
1984年 | 39篇 |
1983年 | 28篇 |
1982年 | 38篇 |
1981年 | 25篇 |
1980年 | 37篇 |
1979年 | 24篇 |
1978年 | 29篇 |
1977年 | 37篇 |
1976年 | 27篇 |
1975年 | 28篇 |
1974年 | 16篇 |
1973年 | 18篇 |
排序方式: 共有6779条查询结果,搜索用时 15 毫秒
101.
102.
The Mitsunobu reaction is a widely used and versatile method for the dehydrative oxidation–reduction condensation of an acid/pronucleophile usually with a primary or secondary alcohol that requires the combination of a reducing phosphine reagent together with an oxidizing azo reagent. The utility of this reaction stems from the fact that it is generally highly stereoselective and occurs with inversion of the stereochemical configuration of the alcohol starting material. Furthermore, as carboxylic acids, phenols, imides, sulfonamides, and other compounds can be used as the acid/pronucleophile, this reaction is useful for the preparation of a wide variety of functional groups. This Focus Review of the Mitsunobu reaction summarizes its origins, the current understanding of its mechanism, and recent improvements and applications. 相似文献
103.
Patrick Koch Carl Djerassi Vijai Lakshmi Francis J. Schmitz 《Helvetica chimica acta》1983,66(8):2431-2436
Thirty-two sterols are identified in the sponge Hyrtios sp. In addition to fourteen monohydroxylated compounds, the sterol fraction contains eight functionalized sidechain sterols, three of them new: 3 β-hydroxy-24-norchol-5-en-23-al (19) , (22-trans)-3 β-hydroxycholest-5, 22-dien-24-one (20) and (22R, 23R, 24S) or (22S, 23S, 24S)- 22,23-epoxy-24-methylchoest-5-en-3 β-ol (24). The probable biological origin, rather than artifact production, of these undescribed components is discussed. 相似文献
104.
Stable radical 2-(6-uradinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl shows antiferromagnetic spin pairing with 2J/k= -14 K, attributable to a close contact between unpaired spin density on the imidazole-type nitrogen atoms; hydrogen bonds aid dimer formation, but do not appear to play an eletronic role in the magnetic behaviour. 相似文献
105.
Aromí G Ribas J Gamez P Roubeau O Kooijman H Spek AL Teat S MacLean E Stoeckli-Evans H Reedijk J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(24):6476-6488
Coordination complexes of the ligand H3L [1,3-bis(3-oxo-3-phenylpropionyl)-2-hydroxy-5-methylbenzene] with Cu(II) are reported. Clusters showing various nuclearities or modes of supramolecular organization have been prepared by slightly changing the reaction conditions and have been crystallographically characterized. The reaction of H3L with one equivalent of Cu(OAc)2 in DMF yields the dinuclear complex [Cu2(HL)2(dmf)2] (1). Reaction in MeOH of H3L with an increased amount of metal, in the form of Cu(NO3)2, and excess strong base (nBu4NOH) affords the cluster [Cu8(L)2(OMe)8(NO3)2] (2). Complex 2 is a dimer of two linear [Cu4] arrays bridged by methoxide ligands, where the polynucleating ligand is fully deprotonated. The [Cu4]2 clusters are linked to each other by NO3- bridges to form one-dimensional coordination polymers. The link between [Cu8] units and their relative spatial positioning can be modified by changing the anion of the Cu(II) salt, as demonstrated by the synthesis of the cluster polymers [Cu8(L)2(OMe)8Cl2] (3) and [Cu8(L)(OMe)7.86Br2.14] (4), where only NO3- has been replaced by Cl- or Br-, respectively. Similarly, when ClO4- is used, compound [Cu8(L)2(OMe)8(ClO4)2(MeOH)4] (5) can be isolated. It contains independent [Cu8] units. A slight change in the stoichiometry of the reaction leading to 2 affords the related complex catena-[Cu4(L)(OMe)3(NO3)2(H2O)0.36] (6). This polymer contains essentially the same [Cu4] moiety as 2, albeit organized in a completely different arrangement. Each [Cu4] unit in 6 is linked by OMe- ligands to two such equivalent groups to form an infinite chain. Magnetic susceptibility measurements reveal weak antiferromagnetic exchange between Cu(II) centers in 1 (J = -0.73 cm(-1)) and strong antiferromagnetic coupling within [Cu4] chains in 2, 5, and 6 (most negative J values of -113.8 and -177.3 cm(-1) for 2 and 6, respectively). 相似文献
106.
Storr T Obata M Fisher CL Bayly SR Green DE Brudziñska I Mikata Y Patrick BO Adam MJ Yano S Orvig C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,11(1):195-203
Seven discrete sugar-pendant diamines were complexed to the {M(CO)(3)}(+) ((99m)Tc/Re) core: 1,3-diamino-2-propyl beta-D-glucopyranoside (L(1)), 1,3-diamino-2-propyl beta-D-xylopyranoside (L(2)), 1,3-diamino-2-propyl alpha-D-mannopyranoside (L(3)), 1,3-diamino-2-propyl alpha-D-galactopyranoside (L(4)), 1,3-diamino-2-propyl beta-D-galactopyranoside (L(5)), 1,3-diamino-2-propyl beta-(alpha-D-glucopyranosyl-(1,4)-D-glucopyranoside) (L(6)), and bis(aminomethyl)bis[(beta-D-glucopyranosyloxy)methyl]methane (L(7)). The Re complexes [Re(L(1)-L(7))(Br)(CO)(3)] were characterized by (1)H and (13)C 1D/2D NMR spectroscopy which confirmed the pendant nature of the carbohydrate moieties in solution. Additional characterization was provided by IR spectroscopy, elemental analysis, and mass spectrometry. Two analogues, [Re(L(2))(CO)(3)Br] and [Re(L(3))(CO)(3)Br], were characterized in the solid state by X-ray crystallography and represent the first reported structures of Re organometallic carbohydrate compounds. Conductivity measurements in H(2)O established that the complexes exist as [Re(L(1)-L(7))(H(2)O)(CO)(3)]Br in aqueous conditions. Radiolabelling of L(1)-L(7) with [(99m)Tc(H(2)O)(3)(CO)(3)](+) afforded in high yield compounds of identical character to the Re analogues. The radiolabelled compounds were determined to exhibit high in vitro stability towards ligand exchange in the presence of an excess of either cysteine or histidine over a 24 h period. 相似文献
107.
This communication describes the catalytic asymmetric 1,2-addition of vinylzinc reagents to aromatic, alpha,beta-unsaturated, and dialkyl ketones with enantioselectivities between 79 and 97% and with yields ranging from 84 to 98%. The products of these reactions are tertiary allylic alcohols with chiral quaternary centers that are useful in organic synthesis. The reaction involves hydrozirconation of a terminal alkyne, transmetalation to zinc, and addition to a ketone in the presence of a chiral titanium-based Lewis acid catalyst. The reactions proceed smoothly at room temperature in under 24 h. 相似文献
108.
May JP Fournier P Pellicelli J Patrick BO Perrin DM 《The Journal of organic chemistry》2005,70(21):8424-8430
This work describes a rapid and high yielding oxidation of 14 tryptophanylated amino acid methyl esters to the corresponding 3a-hydroxypyrrolo[2,3-b]indoline (Hpi) amino acids with generally facile separation of syn-cis and anti-cis diastereomers. Structural X-ray diffraction data are presented for both diastereomers of Tr-Hpi-Gly-OMe, which allow for a putative assignment of the other 13 pairs of diastereomers reported herein, based on correlations with 1H NMR chemical shifts. Selective and high yielding deprotection at either the N or C terminus is described, allowing the Hpi motif to be introduced efficiently into potential targets with minimal protecting group manipulation. Two tripeptides containing Hpi and cysteine were prepared and treated with acid in the Savige-Fontana reaction to produce a cyclic tryptathionine linkage, characteristic of both amatoxins and phallotoxins. 相似文献
109.
Kempf JG Jung JY Sampson NS Loria JP 《Journal of the American Chemical Society》2003,125(40):12064-12065
Current solution NMR experiments for characterizing conformational exchange processes in large proteins are limited to exchange rates ca. 500-3000 s-1. A TROSY-based constant relaxation time (R1rho - R1) experiment is designed to extend this capability to measure motion with rates up to 105 s-1 in large macromolecules. The experiment combines off-resonance spin-lock rf fields, which provide access to the faster time-scale dynamics, with TROSY coherence selection, which extends the molecular-weight range available for study. When implemented on the 53-kDa dimeric enzyme triosephosphate isomerase, the experiment yielded substantial gains in signal-to-noise (up to 60%) over current experiments at modest static magnetic fields (14.1 T). The TROSY (R1rho - R1) experiment should therefore be of general utility for investigation of fast conformational exchange events in large proteins. 相似文献
110.
Liu SX Dolder S Franz P Neels A Stoeckli-Evans H Decurtins S 《Inorganic chemistry》2003,42(16):4801-4803
Three divalent transition metal complexes of 4,5-bis(2-pyridylmethylsulfanyl)-4',5'-ethylenedithiotetrathiafulvalene have been prepared and crystallographically characterized. The isostructural Co(II) and the Ni(II) complexes show octahedral geometries around the metal ions with the coordination sites occupied by the pyridyl nitrogen atoms and the thioether sulfur atoms of the ligand and cis coordination of the halide ions. Cyclic voltammetry reveals that the complexation leads to a small anodic shift in the first oxidation potential of the TTF system. 相似文献