全文获取类型
收费全文 | 3202篇 |
免费 | 127篇 |
国内免费 | 10篇 |
专业分类
化学 | 2274篇 |
晶体学 | 7篇 |
力学 | 50篇 |
数学 | 470篇 |
物理学 | 538篇 |
出版年
2023年 | 26篇 |
2022年 | 24篇 |
2021年 | 54篇 |
2020年 | 60篇 |
2019年 | 69篇 |
2018年 | 37篇 |
2017年 | 32篇 |
2016年 | 91篇 |
2015年 | 115篇 |
2014年 | 109篇 |
2013年 | 184篇 |
2012年 | 236篇 |
2011年 | 231篇 |
2010年 | 132篇 |
2009年 | 127篇 |
2008年 | 158篇 |
2007年 | 176篇 |
2006年 | 179篇 |
2005年 | 156篇 |
2004年 | 179篇 |
2003年 | 124篇 |
2002年 | 110篇 |
2001年 | 62篇 |
2000年 | 42篇 |
1999年 | 38篇 |
1998年 | 48篇 |
1997年 | 44篇 |
1996年 | 30篇 |
1995年 | 45篇 |
1994年 | 42篇 |
1993年 | 25篇 |
1992年 | 26篇 |
1991年 | 25篇 |
1990年 | 18篇 |
1989年 | 23篇 |
1988年 | 13篇 |
1987年 | 12篇 |
1985年 | 11篇 |
1984年 | 13篇 |
1983年 | 11篇 |
1982年 | 17篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 26篇 |
1978年 | 17篇 |
1977年 | 16篇 |
1976年 | 11篇 |
1974年 | 9篇 |
1973年 | 15篇 |
1972年 | 9篇 |
排序方式: 共有3339条查询结果,搜索用时 0 毫秒
991.
992.
The Enthalpy of Formation for TeX4 The enthalpies of formation were derived for the tetrahalogenides TeCl4, TeBr4, and TeJ4 from the enthalpy of solution in KOH. The determination based on the enthalpy of formation for TeO2 and the KX and their enthalpy of solution in KOH. Results see ?Inhaltsübersicht”?. 相似文献
993.
994.
The first enantiocontrolled total synthesis of the marine natural product oscillarin is described. The proposed structure and absolute configuration of oscillarin is thus confirmed, and a previously assigned structure of a subunit was shown to be incorrect. The X-ray structure of an oscillarin-thrombin complex was resolved at 2.0 A resolution, which validated its potent inhibitory activity against the enzyme with an IC(50) = 28 nM. Methodology was developed for the synthesis of enantiopure octahydroindole-2-carboxylic acids with usable functionality at C-6. The method consists of the halocarbocyclization of N-acyloxyiminium ions containing an olefinic tether in the presence of tin tetrachloride or tin tetrabromide. This N-acyloxyiminium ion aza-Prins carbocyclization proved to be general for the construction of octahydroindole and perhydroquinoline 2-carboxylic acids. Mechanistic rationales are based on an antiperiplanar attack of the terminal alkene on the iminium ion, leading to an incipient secondary carbocation which is trapped by halide via an equatorial attack. X-ray crystal structures of products corroborate the expected stereochemistry. 相似文献
995.
A novel NMR method characterizes slow motions in proteins by multiple refocusing of double- and zero-quantum coherences of amide protons and nitrogen-15 nuclei. If both nuclei experience changes in their isotropic chemical shifts because of internal motions on slow time scales (mus - ms), this leads to a difference in the relaxation rates of double- and zero-quantum coherences. This is due to CSM/CSM (chemical shift modulation) cross-correlation effects that are related to the well-known chemical exchange contribution Rex to the decay rate R2 = 1/T2 of nitrogen-15 nuclei. The CSM/CSM contributions can be distinguished from other mechanisms through their dependence on the repetition rate of a Carr-Purcell-Meiboom-Gill (CPMG) multiple refocusing sequence. In ubiquitin, motional processes can be identified that could hitherto not be observed by conventional CPMG nitrogen-15 NMR. 相似文献
996.
Rossmeisl J Nørskov JK Jacobsen KW 《Journal of the American Chemical Society》2004,126(40):13140-13143
We present extensive density functional theory calculations of the bonding between strands in beta-sheets. We identify a significant cooperative effect whereby the interaction increases in strength with the number of strands. We show that the effect is related to a coupling between interstrand bonding and intrastrand elastic properties. It is found that a direct consequence of this coupling is that the pitch of beta-sheets should contract with increasing number of strands, and we show that the effect can be observed directly in experimental data from the Protein Data Bank. 相似文献
997.
Small amounts of simple methoxy poly(ethylene glycol)s (MPEGs) have a beneficial effect on catalyzed asymmetric aryl and alkyl transfer reactions onto aldehydes. The enantiomeric excesses of the products are improved, and this "MPEG effect" allows a reduction of the catalyst loading by a factor of 10. 相似文献
998.
Maire P Deblon S Breher F Geier J Böhler C Rüegger H Schönberg H Grützmacher H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(17):4198-4205
Iridium(I) complexes containing a (5H-dibenzo[a,d]cyclohepten-5-yl)-phosphane (tropp(R); R = phosphorus-bound substituent = Ph, Cyc) as a rigid, concave-shaped, mixed phosphane olefin ligand were prepared and tested as catalyst precursors in the hydrogenation of imines. With the complex [Ir(tropp(Cyc))(cod)]OTf, turnover frequencies (TOFs) of >6000 h(-1) were reached in the hydrogenation of N-phenyl-benzylidenamine, PhN==CHPh. Lower activities (TOF>80 h(-1)) are observed with N-phenyl-(1-phenylethylidene)amine, PhN==CMePh. Chiral tropp-type ligands were prepared in few simple steps. Monosubstitution of the olefinic unit in the dibenzo[a,d]cycloheptenyl moiety with (R)- or (S)-mentholate gave mixtures of diastereomers that could be separated and isolated in enantiomerically pure form. Iridium(I) complexes with these ligands are rare examples of side-on bonded enolether complexes. In catalytic imine hydrogenations, complete conversion (>98 %) was reached in all cases (conditions: p[H(2)] = 50 bar, T = 50 degrees C, t = 2 h, substrate/catalyst 100:1). The best enantiomeric excess (ee = 86 % S isomer) was reached with PhN==CMePh as substrate and the R,R form of the (10-menthyloxy-5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane ligand. The iridium(I) complex containing the same phosphane gave a 60 % ee (S isomer) with the enamide N-(1-phenylvinyl)acetamide as substrate (conditions: p[H(2)] = 4 bar, T = 50 degrees C, t = 18 h, substrate/catalyst = 50:1). These reactions constitute the first examples in which chiral olefins have been used as steering ligands in catalytic enantioselective hydrogenations. 相似文献
999.
1000.