全文获取类型
收费全文 | 1318篇 |
免费 | 42篇 |
国内免费 | 4篇 |
专业分类
化学 | 983篇 |
晶体学 | 11篇 |
力学 | 11篇 |
数学 | 110篇 |
物理学 | 249篇 |
出版年
2021年 | 11篇 |
2020年 | 15篇 |
2019年 | 17篇 |
2018年 | 13篇 |
2017年 | 12篇 |
2016年 | 29篇 |
2015年 | 30篇 |
2014年 | 27篇 |
2013年 | 62篇 |
2012年 | 55篇 |
2011年 | 80篇 |
2010年 | 45篇 |
2009年 | 50篇 |
2008年 | 79篇 |
2007年 | 82篇 |
2006年 | 80篇 |
2005年 | 72篇 |
2004年 | 80篇 |
2003年 | 48篇 |
2002年 | 49篇 |
2001年 | 37篇 |
2000年 | 23篇 |
1999年 | 10篇 |
1998年 | 19篇 |
1997年 | 10篇 |
1996年 | 17篇 |
1995年 | 10篇 |
1994年 | 12篇 |
1993年 | 11篇 |
1992年 | 18篇 |
1991年 | 10篇 |
1990年 | 19篇 |
1989年 | 19篇 |
1988年 | 13篇 |
1987年 | 13篇 |
1985年 | 15篇 |
1984年 | 23篇 |
1983年 | 12篇 |
1982年 | 15篇 |
1981年 | 17篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1978年 | 13篇 |
1977年 | 10篇 |
1976年 | 7篇 |
1975年 | 10篇 |
1974年 | 8篇 |
1973年 | 6篇 |
1967年 | 4篇 |
1966年 | 4篇 |
排序方式: 共有1364条查询结果,搜索用时 15 毫秒
41.
42.
43.
Harada S Kumagai N Kinoshita T Matsunaga S Shibasaki M 《Journal of the American Chemical Society》2003,125(9):2582-2590
Full details of our direct Michael addition of unmodified ketones using new asymmetric zinc catalysis are described. Et(2)Zn/(S,S)-linked-BINOL complexes were successfully applied to direct 1,4-addition reactions of hydroxyketones. The first generation Et(2)Zn/(S,S)-linked-BINOL 1 = 2/1 system was effective for 1,4-addition of 2-hydroxy-2'-methoxyacetophenone (3). Using 1 mol % of (S,S)-linked-BINOL 1 and 2 mol % of Et(2)Zn, we found that a 1,4-addition reaction of beta-unsubstituted enone proceeded smoothly at 4 degrees C to afford products in high yield (up to 90%) and enantiomeric excess (up to 95%). In the case of beta-substituted enones, however, the first generation Et(2)Zn/(S,S)-linked-BINOL 1 = 2/1 system was not at all effective. The second generation Et(2)Zn/(S,S)-linked-BINOL 1 = 4/1 with MS 3A system was developed and was effective for various beta-substituted enones to afford products in good dr, yield (up to 99%), and high enantiomeric excess (up to 99% ee). With the Et(2)Zn/1 = 4/1 systems, catalyst loading for beta-unsubstituted enone was reduced to as little as 0.01 mol % (substrate/chiral ligand = 10 000). The new system was also effective for 1,4-addition reactions of 2-hydroxy-2'-methoxypropiophenone (9) to afford chiral tert-alcohol in high enantiomeric excess (up to 96% ee). Mechanistic investigations as well as transformations of the Michael adducts into synthetically versatile intermediates are also described. 相似文献
44.
Some molecules with a molecular skeleton similar to that of stilbenes and azobenzenes show orientational disorder in the crystals. If the disorder is dynamic, a conformational interconversion takes place through a pedal motion. In this study X-ray diffraction analyses of (E)-stilbene (1) and azobenzene (2) were carried out at various temperatures. We succeeded in observing thermodynamic nonequilibrium states that were generated by fast freezing of the conformational interconversion at low temperatures. The populations of the two conformers in crystals of 1 at 90 K varied with the cooling rate. Flash cooling of a crystal of 2 from room temperature to 90 K made it possible to observe the minor unstable conformer that does not exist in the equilibrium state at the same temperature. 相似文献
45.
The sequential Sonogashira reaction and the cyclization reaction of various 2-iodoanilines and terminal alkynes in the presence of a palladium catalyst and tetrabutylammonium fluoride (TBAF) gave the corresponding 2-substituted indoles in good yields. 相似文献
46.
Drying dissipative structural patterns formed in the course of drying colloidal crystals of silica spheres (110 nm in diameter)
in water, methyl alcohol, ethyl alcohol, 1-propyl alcohol, diethyl ether, and in the mixtures of ethyl alcohol with the other
solvents above have been studied on a cover glass. The macroscopic broad rings were formed in the outside edges of the dried
film for all the solvents examined. Furthermore, much distinct broad rings appeared in the inner area when the solvents were
ethyl alcohol, methyl alcohol, and their mixtures. Profiles of the thickness of the dried films were sensitive to the organic
solvents and explained well with changes in the surface tensions, boiling points, and viscosities of the solvents. The macroscopic
and microscopic spoke-like crack patterns formed. The drying area (or the drying time) increased (or decreased) as the surface
tension of the solvent decreased. However, the absolute values of these drying parameters are determined also by the boiling
points of the solvents. Importance of the fundamental properties of the solvents is supported in addition to the characteristics
of colloidal particles in the drying dissipative pattern formation. 相似文献
47.
A porphyrin nanotube: size-selective inclusion of tetranuclear molybdenum-oxo clusters 总被引:2,自引:0,他引:2
Harada R Matsuda Y Okawa H Kojima T 《Angewandte Chemie (International ed. in English)》2004,43(14):1825-1828
48.
Tokuda N Sasaki N Watanabe H Miki K Yamasaki S Hasunuma R Yamabe K 《The journal of physical chemistry. B》2005,109(26):12655-12657
Electroless deposition of Ag on atomically flat H-terminated Si(111) surfaces in aqueous alkaline solutions containing Ag ions produced two different sizes of Ag nanowires along atomic step edges: (1) a narrow nanowire of 10 nm in width and 0.5 nm in height and (2) a wide nanowire of 35 nm in width and 11 nm in height. The narrow and wide nanowires were formed by immersion in the solutions containing less than 1 ppb and 8 ppm dissolved-oxygen concentrations, respectively. This result indicates that the dissolved oxygen initiates the formation of Ag nucleation sites and that the fabrication method has a possibility of controlling the size of Ag nanowires. 相似文献
49.
It has been shown by a series of experiments on 5-alkyl, 5-halo-, and other 5-substituted 1-naphthol derivatives that the product ratio of the ortho- and para-naphthoquinones formed on oxidation with Fremy's salt is controlled by the bulkiness of the C5-substituent. 相似文献
50.
Asymmetric catalytic hydrogenations of three kinds of chiral pyruvamides were carried out using palladium on charcoal as a catalyst to give lactamides with diastereomeric ratios ranging from 76:24 to 98:2. 相似文献