全文获取类型
收费全文 | 35470篇 |
免费 | 250篇 |
国内免费 | 284篇 |
专业分类
化学 | 14121篇 |
晶体学 | 388篇 |
力学 | 1544篇 |
综合类 | 12篇 |
数学 | 10111篇 |
物理学 | 9828篇 |
出版年
2021年 | 108篇 |
2020年 | 120篇 |
2019年 | 139篇 |
2018年 | 1334篇 |
2017年 | 1584篇 |
2016年 | 956篇 |
2015年 | 735篇 |
2014年 | 739篇 |
2013年 | 1272篇 |
2012年 | 3423篇 |
2011年 | 2639篇 |
2010年 | 2073篇 |
2009年 | 1776篇 |
2008年 | 870篇 |
2007年 | 898篇 |
2006年 | 848篇 |
2005年 | 4619篇 |
2004年 | 4093篇 |
2003年 | 2468篇 |
2002年 | 649篇 |
2001年 | 417篇 |
2000年 | 211篇 |
1999年 | 264篇 |
1998年 | 207篇 |
1997年 | 136篇 |
1996年 | 123篇 |
1995年 | 134篇 |
1994年 | 96篇 |
1993年 | 62篇 |
1992年 | 232篇 |
1991年 | 197篇 |
1990年 | 192篇 |
1989年 | 161篇 |
1988年 | 162篇 |
1987年 | 105篇 |
1986年 | 95篇 |
1985年 | 128篇 |
1984年 | 130篇 |
1983年 | 85篇 |
1982年 | 77篇 |
1981年 | 85篇 |
1980年 | 91篇 |
1979年 | 83篇 |
1978年 | 87篇 |
1977年 | 82篇 |
1976年 | 141篇 |
1975年 | 87篇 |
1974年 | 85篇 |
1973年 | 88篇 |
1972年 | 60篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Adducts of the conjugate addition of benzylamine to enantiopure amides of aroylacrylic acid possess high enantiomeric and diastereomeric purity. A high degree of stereoselectivity has been achieved by means of crystallization-induced asymmetric transformation. A practical synthesis leading to dipeptides containing homophenylalanine is depicted. 相似文献
102.
Janusz Mierczyński Wenxian Shen Xiao-Qiang Zhao 《Journal of Differential Equations》2004,204(2):471-510
The purpose of this paper is to investigate uniform persistence for nonautonomous and random parabolic Kolmogorov systems via the skew-product semiflows approach. It is first shown that the uniform persistence of the skew-product semiflow associated with a nonautonomous (random) parabolic Kolmogorov system implies that of the system. Various sufficient conditions in terms of the so-called unsaturatedness and/or Lyapunov exponents for uniform persistence of the skew-product semiflows are then provided. Among others, it is shown that if the associated skew-product semiflow has a global attractor and its restriction to the boundary of the state space has a Morse decomposition which is unsaturated or whose external Lyapunov exponents are positive, then it is uniformly persistent. More specific conditions are discussed for uniform persistence in n-species, particularly 3-species, random competitive systems. 相似文献
103.
JPC – Journal of Planar Chromatography – Modern TLC - Emodin and twelve phenolic acids (ellagic, gallic, protocatechuic, homoprotocatechuic, caffeic, p-hydroxybenzoic, p-coumaric,... 相似文献
104.
Efficient synthesis of ferrocenylenones using a Friedel-Crafts acylation reaction is described. Acryloyl, methacryloyl, crotonoyl, cinnamoyl, and β-methylcrotonoyl chlorides react with ferrocene in the presence of a Lewis acid (EtAlCl2 or EtAlCl2-Me3Al) to give the corresponding ferrocenylenones (acryloyl, methacryloyl, crotonoyl, cinnamoyl, and methylcrotonoylferrocenes) in good isolated yields. Besides ferrocenylenones, chloroactylferrocene is also synthesised by this method. 相似文献
105.
106.
Titanium tetrachloride promoted reaction of silyl ketene acetals with epoxides, followed by acidic work-up, affords butanolides in moderate/good yields. With epihalohydrins the reaction is regioselective and occurs at the less substituted end of the epoxide; the γ-haloalkyl-γ-butanolides thus obtained can be further transformed into various products. 相似文献
107.
The problem of homoaromaticity in mono-, di- and polyfluorinated- homotropylium cations is addressed by the B3LYP/6-311++G** DFT method. The energetic, structural and magnetic criteria are used for this purpose. They convincingly show that the ground state equilibrium species are aromatic, or in other words that the homoaromaticity is preserved by the (poly)fluorination. In contrast, a considerable decrease in the aromatic stabilization is observed in the transition structures (TS). According to the NICS(0) index, they vary form strongly antiaromatic, via weakly and non-aromatic to slightly aromatic transition states. However, the hierarchy of the aromaticity in fluorinated homotropylium ions predicted by NICS(0) is completely unrelated to that obtained by using the energy criterion assuming a kinetic definition of aromaticity. On the other hand the latter is closely related to geometric parameters of the equilibrium and transition structures. 相似文献
108.
109.
Inés CarrancoJosé Luis D?&#x;az Oscar JiménezRodolfo Lavilla 《Tetrahedron letters》2003,44(46):8449-8452
The Lewis acid-catalyzed interaction of dihydropyridines with ethyl glyoxalate and primary aliphatic amines yields, depending on the reaction conditions, bicyclic aminals or rearranged tetrahydropyridines in one-pot multicomponent reactions. 相似文献
110.
Gravimetric measurements of thermodesorption of n-hexane and n-heptane were performed under quasi-equilibrium conditions. Differential thermodesorption profiles for ZSM-5 and ZSM-11 showed two peaks, but for Y zeolites, only one thermodesorption peak was observed. A model function, derived from the Langmiur adsorption model, was fitted to the experimental data, and the model parameters (the adsorption entropy and enthalpy) were estimated. The two-step desorption profiles observed for ZSM-5 and ZSM-11 were attributed to the commensurate freezing effect, i.e. a transition in the adsorbed phase resulting in ordering of the adsorbed molecules in the zeolite channels. The results observed for ZSM-11 indicate that the zigzag channels typical for ZSM-5 micropore system are not necessary for this transition to occur. 相似文献