全文获取类型
收费全文 | 663篇 |
免费 | 19篇 |
国内免费 | 3篇 |
专业分类
化学 | 467篇 |
晶体学 | 3篇 |
力学 | 26篇 |
数学 | 44篇 |
物理学 | 145篇 |
出版年
2023年 | 7篇 |
2022年 | 3篇 |
2021年 | 3篇 |
2020年 | 15篇 |
2019年 | 7篇 |
2018年 | 6篇 |
2016年 | 15篇 |
2015年 | 12篇 |
2014年 | 8篇 |
2013年 | 25篇 |
2012年 | 30篇 |
2011年 | 38篇 |
2010年 | 21篇 |
2009年 | 13篇 |
2008年 | 35篇 |
2007年 | 33篇 |
2006年 | 33篇 |
2005年 | 37篇 |
2004年 | 20篇 |
2003年 | 24篇 |
2002年 | 19篇 |
2001年 | 13篇 |
2000年 | 15篇 |
1999年 | 3篇 |
1997年 | 3篇 |
1996年 | 15篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 10篇 |
1992年 | 15篇 |
1991年 | 3篇 |
1990年 | 7篇 |
1988年 | 11篇 |
1987年 | 12篇 |
1986年 | 6篇 |
1985年 | 15篇 |
1984年 | 9篇 |
1983年 | 12篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 12篇 |
1975年 | 10篇 |
1974年 | 14篇 |
1973年 | 7篇 |
1969年 | 2篇 |
1930年 | 2篇 |
排序方式: 共有685条查询结果,搜索用时 500 毫秒
171.
Enolates derived from α-(ortho-haloaryl)-substituted ketones undergo palladium-catalysed C-O bond formation to deliver benzofuran products in good yield. A catalyst generated from Pd2(dba)3 and the ligand DPEphos effects the key bond formation to deliver a variety of substituted products from both cyclic and acyclic precursors. The analogous thio-ketones undergo C-S bond formation using identical reaction conditions and are converted to benzothiophene products. A cascade sequence that produces the required α-aryl ketones in situ has also been developed, although the substrate scope is more restricted. 相似文献
172.
A new method for the synthesis of the octahydropyrrolo[3,2,1-ij]quinoline ring system that possesses the characteristic skeleton of the aspidosperma family of alkaloids has been developed. The method utilizes an intramolecular Diels-Alder reaction of an amido-substituted furan across a tethered indole pi-bond. To apply this strategy to the synthesis of the indole alkaloid spegazzinidine, it was necessary to address the problem of assembling the final D-ring of the pentacyclic skeleton. Radical cyclization of a model N-allyl-7-bromo-3a-methylhexahydroindolinone system was found to preferentially lead to the 6-endo-trig cyclization product, with the best yield being obtained under high dilution conditions. The six-membered cyclized product is generated through two reaction pathways: (a) 6-endo-trig ring closure and (b) rearrangement of an intermediate methylene-cyclopentyl radical obtained by 5-exo-trig cyclization. A number of related 7-bromo-substituted hexahydroindolinones containing tethered olefinic groups were prepared and found to undergo efficient cyclization under both radical and palladium-mediated reaction conditions. Vinyl radical cyclization with several N-butenyl-substituted systems afforded a mixture of 6-exo and 7-endo cyclization products. A protocol to introduce an ethyl substituent into the C20-position of the aspidospermidine skeleton was also developed. 相似文献
173.
174.
Nikki E. Stainforth Gary A. Cutting Matthew P. John Michael C. Willis 《Tetrahedron: Asymmetry》2009,20(6-8):741-743
The combination of Mg(ClO4)2, 2,2′-bipyridine and N-methylmorpholine generates an effective catalyst system for the direct addition of α-carbonate-substituted ketones to aryl N-Ts imines. Methyl-carbonate-substituted ketones deliver acyclic α-hydroxy-β-aminoketone derivates, while ketones substituted with α-iso-propenyl-carbonates furnish cyclic carbamate adducts. In both cases the anti-configured Mannich products dominate. 相似文献
175.
Recent papers on chevron surface stabilized ferroelectric liquid crystal cells claim that the chevron layer structure can be reversibly uprighted by application of the low to moderate electric fields typically employed to produce director reorientation. In this paper we show, using optical microscopy and X-ray scattering, that there is no significant change in the smectic layer thickness or chevron layer structure of our chevron surface stabilized ferroelectric liquid crystal cells under typical director switching conditions. Furthermore, we present arguments, based on the known elastic properties of smectics, that there is not likely to be a significant elastic layer response to these levels of applied electric field in any surface stabilized ferroelectric liquid crystal cell with anchored layers. Both the switching and observed continuous optical response to applied field can be understood on the basis of electric field induced reorientation of a non-uniform molecular director distribution. We further show that the typically observed broad distribution of layer orientations about the mean chevron structure arises from localized layering defects. 相似文献
176.
We describe the asymptotic behavior of automorphisms of totally disconnected locally compact groups in terms of a set of `directions'
which comes equipped with a natural pseudo-metric. The structure at infinity obtained by completing the induced metric quotient
space of the set of directions recovers familiar objects such as: the set of ends of the tree for the group of inner automorphisms
of the group of isometries of a regular locally finite tree; and the spherical Bruhat-Tits building for the group of inner
automorphisms of the set of rational points of a semisimple group over a local field.
Research supported by A.R.C. Grant DP0208137. 相似文献
177.
178.
Le Sann C Munoz DM Saunders N Simpson TJ Smith DI Soulas F Watts P Willis CL 《Organic & biomolecular chemistry》2005,3(9):1719-1728
A versatile approach for the enantioselective synthesis of functionalised beta-hydroxy N-acetylcysteamine thiol esters has been developed which allows the facile incorporation of isotopic labels. It has been shown that a remarkable reversal of selectivity occurs in the titanium mediated aldol reaction of acyloxazolidinone using either (S)- or (R)-tert-butyldimethylsilyloxybutanal. The aldol products are valuable intermediates in the synthesis of 4-hydroxy-6-substituted delta-lactones. 相似文献
179.
Oligo(p-phenylene) rigid rod 10 is synthesized via a functional group-tolerant molecular doubling approach. Preparative chromatographic methods, protecting groups, boronic acid isolations, and Grignard or organolithium reagents are not used. The convenient synthesis of well-defined, polar-functionalized oligophenylene rigid rods could afford ready access to a variety of useful electronic organic materials. 相似文献
180.
Energy spectra for decaying 2D turbulence in a bounded domain 总被引:1,自引:0,他引:1
We use results derived in the framework of the replica approach to study the liquid-glass thermodynamic transition. The main results are derived without using replicas and applied to the study of the Lennard-Jones binary mixture introduced by Kob and Andersen. We find that there is a phase transition due to the entropy crisis. We compute both analytically and numerically the value of the phase transition point T(K) and the specific heat in the low temperature phase. 相似文献