全文获取类型
收费全文 | 24331篇 |
免费 | 2467篇 |
国内免费 | 5105篇 |
专业分类
化学 | 28014篇 |
晶体学 | 170篇 |
力学 | 269篇 |
综合类 | 130篇 |
数学 | 565篇 |
物理学 | 2755篇 |
出版年
2024年 | 51篇 |
2023年 | 364篇 |
2022年 | 556篇 |
2021年 | 845篇 |
2020年 | 1242篇 |
2019年 | 1051篇 |
2018年 | 1075篇 |
2017年 | 995篇 |
2016年 | 1088篇 |
2015年 | 1016篇 |
2014年 | 1527篇 |
2013年 | 2396篇 |
2012年 | 1341篇 |
2011年 | 1681篇 |
2010年 | 1296篇 |
2009年 | 1411篇 |
2008年 | 1505篇 |
2007年 | 1621篇 |
2006年 | 1514篇 |
2005年 | 1420篇 |
2004年 | 1421篇 |
2003年 | 1100篇 |
2002年 | 709篇 |
2001年 | 525篇 |
2000年 | 543篇 |
1999年 | 450篇 |
1998年 | 403篇 |
1997年 | 364篇 |
1996年 | 339篇 |
1995年 | 318篇 |
1994年 | 273篇 |
1993年 | 212篇 |
1992年 | 205篇 |
1991年 | 147篇 |
1990年 | 121篇 |
1989年 | 113篇 |
1988年 | 92篇 |
1987年 | 57篇 |
1986年 | 55篇 |
1985年 | 56篇 |
1984年 | 48篇 |
1983年 | 26篇 |
1982年 | 44篇 |
1981年 | 36篇 |
1980年 | 28篇 |
1979年 | 25篇 |
1978年 | 27篇 |
1977年 | 30篇 |
1976年 | 36篇 |
1974年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
Microwave-induced organic reaction enhancement (‘MORE’) chemistry technique (open vessel; controlled microwave energy to stay below the boiling point of the reaction mixture) was used for the N-formylation of aliphatic and aromatic amines and amino heterocycles with aq formic acid (80%) on a multiple gram scale in a few minutes. 相似文献
42.
P.A. Ramachandran 《Numerical Methods for Partial Differential Equations》2006,22(4):831-846
Time‐dependent differential equations can be solved using the concept of method of lines (MOL) together with the boundary element (BE) representation for the spatial linear part of the equation. The BE method alleviates the need for spatial discretization and casts the problem in an integral format. Hence errors associated with the numerical approximation of the spatial derivatives are totally eliminated. An element level local cubic approximation is used for the variable at each time step to facilitate the time marching and the nonlinear terms are represented in a semi‐implicit manner by a local linearization at each time step. The accuracy of the method has been illustrated on a number of test problems of engineering significance. © 2005 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2006 相似文献
43.
Debendra K. Mohapatra Dhananjoy Mondal Mukund S. Chorghade Mukund K. Gurjar 《Tetrahedron letters》2006,47(52):9215-9219
The non-proteinogenic amino acid 3-hydroxy-4-methylproline (HMP) is an active constituent of some potent antimicrobials including echinocandins, nostopeptins, pneumocandins, sporiofungin and mulundocandins. A synthesis has been achieved in 10 steps with 29% overall yield; the Evans’ aldol reaction using Crimmins’ modified method was pivotal to the success of the strategy. 相似文献
44.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
45.
A practical access to alkyl- and aryl-substituted (E)-2-(azidomethyl)alkenoates and related azido compounds from the corresponding allylic bromides in aqueous acetone is described. An alternative method to obtain the starting bromides based on heterogeneous catalysis under mild conditions was also investigated. 相似文献
46.
Cosmic dust and our origins 总被引:1,自引:0,他引:1
J Mayo Greenberg 《Surface science》2002,500(1-3):793-822
The small solid particles in the space between the stars provide the surfaces for the production of many simple and complex molecules. Processes involving the effects of ultraviolet irradiation of the thin (hundredth micron) mantles are shown to produce a wide range of molecules and ions also seen in comets. Some of the more complex ones inferred from laboratory experiments are expected to play an important role in the origin of life. An outline of the chemical evolution of interstellar dust as observed and as studied in the laboratory is presented. Observations of comets are shown to provide substantial evidence for their being fluffy aggregates of interstellar dust as it was in the protosolar nebula, i.e. the interstellar cloud which collapsed to form the solar system. The theory that comets may have brought the progenitors of life to the earth is summarized. 相似文献
47.
Chemical bonding in isolated molecules and crystals of zwitterionic pentacoordinate silicon chelates
The electronic structures of a number of zwitterionic pentacoordinate silicon chelates were investigated using the results of X-ray diffraction studies and quantum-chemical calculatoins by the MPW1PW91/6-311G(d) method. The topological analysis of the electron density distribution function and the study in the framework of the natural bond orbital partitioning scheme showed that the character of chemical bonding in the axial fragments of the molecules under consideration changes from dative to three-center, four-electron as the silicon atom assumes a trigonal-bipyramidal coordination. 相似文献
48.
49.
1 INTRODUCTION Carbon nanotubes have many potential applica- tions due to their unique structures and properties[1~4]. Physicists have been studying their electrical, me- chanical and other properties since their discove- ries[5, 6]. Recently, chemists are interested in carbon nanotubes because their properties can be altered by chemical functionalization[7~14], and these functiona- lized nanotubes can undergo further chemical trea- tment. So the potential application range of such na- … 相似文献
50.
G.P. Romanelli D. Bennardi D.M. Ruiz G. Baronetti H.J. Thomas 《Tetrahedron letters》2004,45(48):8935-8939
Substituted coumarins are synthesized from phenols and β-ketoesters by the Pechmann reaction, using a Wells-Dawson heteropolyacid (H6P2W18O62·24H2O) as catalyst by a solvent-free procedure. This one requires low reaction times, 130 °C temperature and as little as 1 mol % of Wells-Dawson acid, obtaining good to excellent yields of coumarins. The catalyst showed to be reusable with no differences in the yields. The results are compared with those of the reactions performed in toluene solution. The presented synthetic procedure is a convenient, clean and fast alternative for synthesizing 4-substituted coumarins (17 examples). 相似文献