全文获取类型
收费全文 | 28132篇 |
免费 | 1052篇 |
国内免费 | 154篇 |
专业分类
化学 | 20141篇 |
晶体学 | 247篇 |
力学 | 513篇 |
数学 | 3701篇 |
物理学 | 4736篇 |
出版年
2023年 | 186篇 |
2022年 | 275篇 |
2021年 | 370篇 |
2020年 | 498篇 |
2019年 | 493篇 |
2018年 | 329篇 |
2017年 | 291篇 |
2016年 | 761篇 |
2015年 | 651篇 |
2014年 | 776篇 |
2013年 | 1333篇 |
2012年 | 1683篇 |
2011年 | 1878篇 |
2010年 | 1075篇 |
2009年 | 919篇 |
2008年 | 1596篇 |
2007年 | 1495篇 |
2006年 | 1475篇 |
2005年 | 1417篇 |
2004年 | 1208篇 |
2003年 | 939篇 |
2002年 | 976篇 |
2001年 | 480篇 |
2000年 | 408篇 |
1999年 | 396篇 |
1998年 | 398篇 |
1997年 | 370篇 |
1996年 | 400篇 |
1995年 | 332篇 |
1994年 | 371篇 |
1993年 | 343篇 |
1992年 | 296篇 |
1991年 | 220篇 |
1990年 | 252篇 |
1989年 | 215篇 |
1988年 | 228篇 |
1987年 | 201篇 |
1986年 | 185篇 |
1985年 | 299篇 |
1984年 | 291篇 |
1983年 | 202篇 |
1982年 | 223篇 |
1981年 | 225篇 |
1980年 | 217篇 |
1979年 | 195篇 |
1978年 | 218篇 |
1977年 | 207篇 |
1976年 | 159篇 |
1975年 | 156篇 |
1974年 | 181篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
A diastereoselective [4+2] annulation of vinyl carbodiimides with chiral N-alkyl imines has been developed to access the stereochemically rich tricyclic core of the batzelladine alkaloids. Its application to the asymmetric synthesis of batzelladine D permitted the use of long-range, directed hydrogenation and stereoselective intramolecular iodoamination as additional key steps to establish the remaining stereocenters within the natural product with excellent stereocontrol. 相似文献
142.
143.
144.
145.
146.
Thomas C. Amu 《Journal of solution chemistry》1981,10(1):69-78
Interdiffusion coefficient have been measured for 1,1,2,2-tetrabromoethane in 21 organic solvents in order to ascertain the influence of shape, size, viscosity, nature and degree of branching, of the solvent molecules on transport properties. None of the existing correlations for diffusion in binary liquid systems appear to fit all the experimental data. Steric effects due to the solvent molecules are shown to be very important in the interpretation of the diffusion process. 相似文献
147.
148.
Kohji Chihara Takeshi Takemura Tsuguo Yamaoka N. Yamamoto Arnold Schaffer Ralph S. Becker 《Photochemistry and photobiology》1979,29(5):1001-1008
Abstract— The absorption and emission spectra, lifetimes and quantum yields of all-trans retinol have been examined as a function of solvent and temperature. In addition, the spectroscopy of retinyl ether and 2 other polyene alcohols have been determined. Based on the results obtained, we conclude that the singlet excited state of retinol and retinyl cther is of a forbidden character and of the type loosely called 1 A- g . Retinol forms a dimer in an alkane solvent when cooling from 298 to 77 K. A general structure for the dimer is proposed. 相似文献
149.
150.
PHOTOCYTOTOXICITY OF CURCUMIN 总被引:5,自引:0,他引:5
Thomas A. Dahll Piotr Bilski Krzysztof J. Reszka Colin F. Chignell 《Photochemistry and photobiology》1994,59(3):290-294
Curcumin, bis (4-hydroxy-3-methoxyphenyl)-l,6-diene-3,5-dione, is a yellow-orange dye derived from the rhizome of the plant Curcuma longa. Curcumin has demonstrated phototoxicity to several species of bacteria under aerobic conditions (Dahl, T. A., et al. , 1989, Arch. Microbiol. 151 183), denoting photodynamic inactivation. We have now found that curcumin is also phototoxic to mammalian cells, using a rat basophilic leukemia cell model, and that this phototoxicity again requires the presence of oxygen. The spectral and photochemical properties of curcumin vary with environment, resulting in the potential for multiple or alternate pathways for the exertion of photodynamic effects. For example, curcumin photogenerates singlet oxygen and reduced forms of molecular oxygen under several conditions relevant to cellular environments. In addition, we detected carbon-centered radicals, which may lead to oxidation products (see accompanying paper). Such products may be important reactants in curcumin's phototoxicity since singlet oxygen and reduced oxygen species alone could not explain the biological results, such as the relatively long lifetime (t12 = 27 s) of the toxicant responsible for decreased cell viability. 相似文献