全文获取类型
收费全文 | 2529篇 |
免费 | 525篇 |
国内免费 | 1034篇 |
专业分类
化学 | 2038篇 |
晶体学 | 108篇 |
力学 | 177篇 |
综合类 | 66篇 |
数学 | 372篇 |
物理学 | 1327篇 |
出版年
2024年 | 8篇 |
2023年 | 23篇 |
2022年 | 89篇 |
2021年 | 86篇 |
2020年 | 82篇 |
2019年 | 83篇 |
2018年 | 67篇 |
2017年 | 115篇 |
2016年 | 89篇 |
2015年 | 134篇 |
2014年 | 159篇 |
2013年 | 233篇 |
2012年 | 200篇 |
2011年 | 180篇 |
2010年 | 241篇 |
2009年 | 223篇 |
2008年 | 269篇 |
2007年 | 271篇 |
2006年 | 219篇 |
2005年 | 170篇 |
2004年 | 156篇 |
2003年 | 111篇 |
2002年 | 119篇 |
2001年 | 102篇 |
2000年 | 112篇 |
1999年 | 79篇 |
1998年 | 53篇 |
1997年 | 30篇 |
1996年 | 40篇 |
1995年 | 30篇 |
1994年 | 33篇 |
1993年 | 41篇 |
1992年 | 27篇 |
1991年 | 17篇 |
1990年 | 16篇 |
1989年 | 24篇 |
1988年 | 8篇 |
1987年 | 17篇 |
1986年 | 13篇 |
1985年 | 12篇 |
1984年 | 18篇 |
1983年 | 8篇 |
1982年 | 13篇 |
1981年 | 5篇 |
1980年 | 9篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1967年 | 3篇 |
1966年 | 3篇 |
排序方式: 共有4088条查询结果,搜索用时 0 毫秒
1.
Highly Selective Copper‐Catalyzed Asymmetric [3+2] Cycloaddition of Azomethine Ylides with Acyclic 1,3‐Dienes 下载免费PDF全文
María González‐Esguevillas Ana Pascual‐Escudero Dr. Javier Adrio Prof. Dr. Juan C. Carretero 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(12):4561-4565
The first examples of the catalytic asymmetric 1,3‐dipolar cycloaddition of azomethine ylides with acyclic activated 1,3‐dienes (and 1,3‐enynes) are described. Under copper catalysis, a selective cycloaddition at the terminal γ,δ‐C?C bond is observed. In addition, depending on the ligand used, either the exo or the endo adduct can be obtained with high selectivity. Under appropriate reaction conditions, the acyclic 1,6‐addition product is detected, suggesting a stepwise mechanism. The resulting C4‐alkenyl‐substituted pyrrolidines are suitable substrates for further access to polycyclic systems, as highlighted by the preparation of hexahydrochromeno[4,3‐b]pyrrole and the tetracyclic core of the alkaloid gracilamine. 相似文献
2.
ZHAO Shan-Rong CHEN Kai-Xian JIANG Hua-Liang WANG Qin-Mi JI Ru-YunShanghai Institute of Materia Medica Chinese Academy of Sciences Shanghai China 《中国化学》1997,15(1):84-89
We have used quantum mechanical method to study the transition states(TSs) of uridine phosphorolysis reaction. Comparing the four different reaction pathways and the five transition states obtained, we conclude that enzymatic uridine phosphorolysis takes place mainly according to acid-catalyzed SN2 mechanism. The proposed reaction pathway is consistent with many experimental results. 相似文献
3.
Alfons Pascual 《Helvetica chimica acta》1989,72(3):556-569
Preparation of Substituted 2-Aminooxazole-4-carbonitriles During our synthetic programme to convert 4-isoxazolylthioureas 3 into the corresponding carbodiimides 5 , a side reaction leading to the hitherto unknown 2-aminooxazole-4-carbonitriles 6 was observed. By selecting appropriate reaction conditions, it was possible to improve the yields of the carbodiimides 5 as well as of the novel oxazole-carbonitriles 6 at will, thus allowing the synthesis of 6 to be conducted in very good yields. To overcome the difficulty of isolating unstable carbodiimides, the synthesis of 6 is best carried out in a one-pot procedure. A limited mechanistic study showed that the formation of 6 proceeds via 5 as the only intermediate. The stability of the N?C?N bonds against base attack (depending strongly on both sterical hindrance and electronic-density factors) forms the only limitation of this new synthetic pathway to oxazole-4-carbonitriles. 相似文献
4.
Using the path integral method we derive quantum wave function and quantum fluctuations of charge andcurrent in the mesoscopic RLC circuit. We find that the quantum fluctuation of charge decreases with time, oppositely,the quantum fluctuation of current increases with time monotonously. Therefore there is a squeezing effect in the circuit.If some more charge devices are used in the mesoscopic-damped circuit, the quantum noise can be reduced. We also findthat uncertainty relation of charge and current periodically varies with the period π/2 in the under-damped case. 相似文献
5.
6.
测量金属表面生成氧化膜在电解质中的电位时,金属/氧化膜/电解质/参比电极构成了多电极体系,其中包括3个电池,由3个电池之间的关系、电池过程中所有的电化学反应、电荷传输步骤和化学反应步骤,导出电位的普遍适用的公式: E=Ea0+sum from s=1 to 3|ηt,s|-1/(ne)v7ΔG7, E=Ec0-sum from s=4 to 6|ηt,s|+1/(ne)v8ΔG8.当金属/氧化膜/电解质电池受到不同的步骤控制时,可以得到不同的简化公式. 相似文献
7.
8.
本文讨论AFC^*一代数中的一些映射的线性性质和它们的局部性质之间的关系.研究AFC^*一代数A上的保持乘法的局部自同构Ф在A中矩阵单位系上的作用。证明了Ф是A上的自同构。对于UHF代数上满等距的结构,还证明了UHF代数上的2-局部(满线性)等距是线性的. 相似文献
9.
A. Cerdán Vidal A. R. Maurí Aucejo M. Llobat Estellés C. Pascual Martí J. Simeón Martí 《Fresenius' Journal of Analytical Chemistry》1994,350(12):706-711
The possibility of identification of substances which have similar spectral behaviour by means of Apparent Content Curves has been studied. This study is carried out with absorption, excitation and emission spectra of several amphetamines of widespread pharmaceutical use. Results obtained show that amphetamine, phenylpropanolamine, pseudoephedrine, phenylephrine, epinephrine, dopamine and methoxyamphetamine can be identified with a probability of >95%. 相似文献
10.
SbPO4, a phosphate with a layered structure, was tested as an electrode material for lithium cells spanning the 3.0-0.0 V range. Two main electrochemical processes were detected as extensive plateaus at ca. 1.6 and 0.7 V in galvanostatic measurements. The first process was found to be irreversible, thus excluding a potential intercalation-like mechanism for the reaction and being better interpreted as a decomposition reaction leading to the formation of elemental Sb. This precludes the use of this compound as a cathodic material for lithium cells. By contrast, the process at 0.7 V is reversible and can be ascribed to the formation of lithium-antimony alloys. The best electrochemical response was obtained by cycling the cell at a C/20 discharge rate over the voltage range 1.25-0.25 V. Under these conditions, the cell delivers an average capacity of 165 Ah/kg—a value greater than those reported for other phosphates—upon successive cycling. 相似文献