全文获取类型
收费全文 | 6021篇 |
免费 | 211篇 |
国内免费 | 22篇 |
专业分类
化学 | 4104篇 |
晶体学 | 37篇 |
力学 | 91篇 |
数学 | 731篇 |
物理学 | 1291篇 |
出版年
2022年 | 54篇 |
2021年 | 60篇 |
2020年 | 101篇 |
2019年 | 65篇 |
2018年 | 47篇 |
2017年 | 57篇 |
2016年 | 160篇 |
2015年 | 149篇 |
2014年 | 152篇 |
2013年 | 257篇 |
2012年 | 324篇 |
2011年 | 370篇 |
2010年 | 211篇 |
2009年 | 165篇 |
2008年 | 277篇 |
2007年 | 284篇 |
2006年 | 252篇 |
2005年 | 267篇 |
2004年 | 247篇 |
2003年 | 202篇 |
2002年 | 187篇 |
2001年 | 124篇 |
2000年 | 104篇 |
1999年 | 97篇 |
1998年 | 65篇 |
1997年 | 65篇 |
1996年 | 77篇 |
1995年 | 82篇 |
1994年 | 76篇 |
1993年 | 83篇 |
1992年 | 98篇 |
1991年 | 73篇 |
1990年 | 62篇 |
1989年 | 53篇 |
1988年 | 60篇 |
1987年 | 61篇 |
1986年 | 47篇 |
1985年 | 77篇 |
1984年 | 69篇 |
1983年 | 63篇 |
1982年 | 61篇 |
1981年 | 61篇 |
1980年 | 56篇 |
1979年 | 57篇 |
1978年 | 57篇 |
1977年 | 53篇 |
1976年 | 44篇 |
1975年 | 35篇 |
1974年 | 48篇 |
1973年 | 53篇 |
排序方式: 共有6254条查询结果,搜索用时 15 毫秒
131.
132.
Multivalent cations can cause DNA to condense from its extended state in solution into high-density toroid-shaped particles. Developing methods to control the size and size distribution of DNA toroids is an important goal for the development of artificial gene delivery systems. Here we demonstrate that changes in salt conditions, prior to condensation by multivalent cations, can significantly affect DNA condensation. Specifically, millimolar concentrations of MgCl2 are shown to cause the formation of toroid clusters, whereas NaCl at the same ionic strength does not. 相似文献
133.
Fritz Vögtle Edwin Weber 《Journal of inclusion phenomena and macrocyclic chemistry》1992,12(1-4):75-119
We thank the editors of this issue for the opportunity to present the historic development of crown chemistry at the Universities of Wurzburg and Bonn in memory of C. J. Pedersen, the originator of the crown ethers. His legacy of science has tremendously stimulated research at both universities.This paper is dedicated to the memory of the late Dr C. J. Pedersen. 相似文献
134.
135.
The structures of the inclusion compounds formed by the host 9,9’-(ethyne-1,2-diyl)bis(fluoren-9-01) with pyridine and picolines are similar and display tubular topology. The host discriminates poorly between
these guests. The kinetics of desorption of the pyridine compound is governed by the Avrami-Erofe’ev equation A2, with an
activation energy of 111(7) kJ mol−1. 相似文献
136.
Ingeborg Csöregh Mátyás Czugler Anne Ertan Edwin Weber Jochen Ahrendt 《Journal of inclusion phenomena and macrocyclic chemistry》1990,8(3):275-287
The crystal structures of four dimethyl sulphoxide (DMSO) inclusion compounds with different carboxylic acid hosts,1–4, have been studied by single crystal X-ray analysis. Crystals of thetrans-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid inclusion compound (1a), [1 · DMSO (1: 1)] show monoclinic (P21/n) symmetry with the unit cell dimensionsa = 11.522(4),b = 18.658(2),c = 8.709(1) Å and = 98.92(2)°. The clathrate of the 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid (2a), [2 · DMSO (1: 2)] is triclinic (P) with the cell dimensionsa = 15.043(7),b =9.657(4),c = 8.118(7) Å, = 101.81(5), = 96.05(4) and = 100.04(4)°. Triclinic (P) symmetry is shown also by the inclusion compound of 9,10-dihydro-9,10-ethanoanthracene-11-monocarboxylic acid (3a) [3 · DMSO (1:1)] with the cell dimensionsa=6.3132(1),b=7.9846(2),c=17.5314(4) Å, = 96.46(2), = 87.08(2) and = 106.02(2)°. The 9,9-bianthryl-2-monocarboxylic acid clathrate (4a) [4 · DMSO (1:1)] is monoclinic (P21/n) and the cell dimensions area = 19.625(18),b = 8.817(1),c = 14.076(8) Å and = 97.92(6)°. In all these structures, the hosts show the same basic recognition pattern for the DMSO guest, involving a strong O-H ... O bond from the COON to the S=O group, and a possible C-H ... O type interaction between the carbonyl O atom of the host and a CH3 group of the guest. The crystals consist of discrete host-guest aggregates which are mainly held together by weak intermolecular interactions of the Van der Waals' type. The stoichiometries of the aggregates are, however, different. 相似文献
137.
H. P. Weber 《Helvetica chimica acta》1974,57(3):623-630
The crystal structure of a complex prepared from sulfuric acid, cyanoguanidin and lupetidin has been determined by X-ray analysis and refined to R = 0.053. It consists of a sulfate ion, multiply hydrogen bonded to two protonated molecules of lupetidin and a neutral molecule of cyanoguanidin. 相似文献
138.
139.
The effect of an electron attracting substituent in the Lewis acid catalyzed oligomerization of flavanols was investigated. The results showed that the presence of a COCF3, at the 8 position of a (+)-catechin unit strongly influenced the attack of the 6 free nucleophile flavanol position by the electrophile generated from a 4-O-alkyloxy protected catechin unit. This was observed either with TiCl4 or TMSOTf as Lewis acids in which the carbon-carbon bond formation was inhibited and the corresponding dimer was detected in small amount. On the contrary, the formation of a carbon-oxygen bond was observed and the corresponding C-4→O→C-3 ether linked procyanidin dimer was isolated in a good yield. In order to avoid the participation of the C-3 hydroxyl group in the dimerization reaction, the two flavanol units were forced into C-4→C-8 coupling by use of an internal link. The structural elucidation of the isolated compounds was achieved through MS and NMR spectroscopy. 相似文献
140.
Edwin Weber Orm Hager Concepcion Foces-Foces Antonio L. Llamas-Saiz 《Journal of inclusion phenomena and macrocyclic chemistry》1999,34(2):199-220
New chiral host compounds based on mandelic acid derivatives having methyl (6a, b and 8a, b) or bromo substituents (7a, b) attached to the phenyl ring of mandelic acid and involving additional aromatic groups were synthesized. The inclusion properties of both the racemic and the optically resolved host species are reported, including solvent co-crystallization as well as chiroselective and vapour sorptive inclusion. The structures of the free racemic host compound 6b and of the DMSO inclusion compounds of optically resolved and racemic 8 (8a and 8b, respectively) have been determined by X-ray analysis. Enantiomeric pairs of molecules in 6b form centro-symmetric dimers by mutual hydrogen bonding of one hydroxyl group while the other is involved in O-–H ... interactions. The guest molecules in the DMSO complexes of 8a and 8b are bound via hydrogen bonds to two host molecules related by translation along crystallographic axes. Parallels to previous hosts of this type are drawn. 相似文献