全文获取类型
收费全文 | 818篇 |
免费 | 20篇 |
国内免费 | 11篇 |
专业分类
化学 | 582篇 |
力学 | 26篇 |
数学 | 78篇 |
物理学 | 163篇 |
出版年
2021年 | 12篇 |
2020年 | 10篇 |
2019年 | 18篇 |
2018年 | 7篇 |
2016年 | 17篇 |
2015年 | 14篇 |
2014年 | 14篇 |
2013年 | 29篇 |
2012年 | 31篇 |
2011年 | 45篇 |
2010年 | 21篇 |
2009年 | 21篇 |
2008年 | 28篇 |
2007年 | 27篇 |
2006年 | 27篇 |
2005年 | 25篇 |
2004年 | 20篇 |
2003年 | 17篇 |
2002年 | 25篇 |
2001年 | 15篇 |
2000年 | 13篇 |
1999年 | 12篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1996年 | 19篇 |
1995年 | 11篇 |
1994年 | 20篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 17篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 12篇 |
1985年 | 11篇 |
1984年 | 19篇 |
1983年 | 14篇 |
1982年 | 14篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 19篇 |
1978年 | 24篇 |
1977年 | 20篇 |
1976年 | 15篇 |
1975年 | 18篇 |
1974年 | 9篇 |
1973年 | 17篇 |
1971年 | 4篇 |
1968年 | 4篇 |
1943年 | 4篇 |
排序方式: 共有849条查询结果,搜索用时 187 毫秒
111.
The bridge-substituted calix[4]arene carboxylic acid, 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetramethoxy-calix[4]arene-2-carboxylic acid (1), can be readily converted to various esters 2-4 and reduced to the alcohol 5, which reacts with methyl iodide to give the ether 6. The alcohol can be dansylated to give 7, the fluorescence of which is selectively quenched by Cu(II) in acetonitrile. An attempt to convert the acid 1 to an amide resulted unexpectedly in the formation of a lactone 8. The conformational characteristics of 1-8 have been studied in solution and, in the cases of 2 and 4, in the solid state by determination of their single-crystal X-ray structures. With the exception of 8, in all these compounds the bridge substituent adopts an equatorial (lateral) orientation. 相似文献
112.
A series of allylpalladium dimers having metals connected by binucleating dialkyldithiooxamidate [N(R)SC-CS(R)N](2-) [R = methyl, ethyl, isopropyl, benzyl, isoamyl, (S)-1-(1-phenyl)ethyl, meso-(1-phenyl)ethyl, and rac-(1-phenyl)ethyl] were prepared by reacting the monochelate [(η(3)-allyl)Pd(N(R)SC-CS(R)NH κ-S,S Pd)] with [(η(3)-allyl)PdCl](2) in chloroform. At low temperature (20 °C), the bimetallic complexes [(η(3)-allyl)Pd](2)(μ-dialkyldithiooxamidate κ-N,N' Pd, κ-S,S' Pd') (kinetic compounds) are formed in a short reaction time (10 min). At a higher temperature (50 °C) and a longer reaction time (24 h), the corresponding bimetallic isomers [(η(3)-allyl)Pd](2)(μ-dialkyldithiooxamidate κ-N,S Pd, κ-N',S' Pd') (thermodynamic compounds) are obtained. Both kinetic and thermodynamic compounds can exist as endo or exo isomers, depending on the reciprocal orientation of the allyl cuspids. Both endo and exo isomers are only detectable in solution when the alkyl substituents are chiral alkyl groups. Moreover, diffractometric modeling agrees with the presence of both isomers in the solid state even when the alkyl substituent is an achiral alkyl group. In a chloroform solution, endo and exo isomers undergo isomeric conversion owing to the apparent allyl rotation that follows the Pd-N bond rupture in the (η(3)-allyl)Pd(N^N) frame of kinetic compounds or in the (η(3)-allyl)Pd(N^S) frame of thermodynamic compounds. The dithiooxamidate [N(R)SC-CS(R)N](2-), when engaged in a κ-N,S Pd, κ-N',S' Pd' coordination mode, behaves as a hybrid hemilabile binucleating ligand. At room temperature and in a chloroform solution, the kinetic compounds rearrange into the thermodynamically more stable isomers in about 3 or 4 days. The higher stability of the thermodynamic species was evaluated by means of computational studies in accordance with the maximum hardness principle. Finally, the crystal structures of [(η(3)-allyl)Pd](2)(μ-diethyldithiooxamidate κ-N,S Pd, κ-N',S' Pd'), [(η(3)-allyl)Pd](2)(μ-meso-(1-phenyl)ethyldithiooxamidate κ-N,S Pd, κ-N',S' Pd'), and [(η(3)-allyl)Pd](2)(μ-rac-(1-phenyl)ethyldithiooxamidate κ-N,N' Pd, κ-S,S' Pd') are reported. 相似文献
113.
114.
115.
Pier Domenico Lamberti Massimo Lanza de Cristoforis 《Mathematical Physics, Analysis and Geometry》2006,9(1):65-94
We consider the Dirichlet eigenvalue problem for the Laplace operator on a variable nonsmooth domain. We extend a result of Lupo and Micheletti concerning the structure of the set of domain perturbations which leave the multiplicity of an eigenvalue unchanged, and we study the set of perturbations which leave a certain eigenvalue unchanged. 相似文献
116.
117.
The sample is decomposed with 50% (v/v) aqua regia and the diluted solution is injected into the graphite furnace. The temperature program developed minimizes non-specific background signals, so that correction is not required. For a 100-mg sample, the 3σ detection limit is 70 ng Cr g?1. The relative standard deviation of the overall procedure is 5–7%. 相似文献
118.
Rosafluine, a Novel Diapocarotenediol from Rose Petals A strongly fluorescent trace compound from rose petals was identified as the as yet unknown 10,10′-diapocarotene-10,10′-diol ( = 4,9-dimethyldodeca-2,4,6,8,10-pentaene-1,12-diol; 2 ). 相似文献
119.
120.