全文获取类型
收费全文 | 1523篇 |
免费 | 67篇 |
国内免费 | 14篇 |
专业分类
化学 | 1111篇 |
晶体学 | 16篇 |
力学 | 11篇 |
数学 | 105篇 |
物理学 | 361篇 |
出版年
2023年 | 9篇 |
2022年 | 15篇 |
2021年 | 18篇 |
2020年 | 22篇 |
2019年 | 28篇 |
2018年 | 23篇 |
2017年 | 11篇 |
2016年 | 38篇 |
2015年 | 34篇 |
2014年 | 41篇 |
2013年 | 86篇 |
2012年 | 88篇 |
2011年 | 124篇 |
2010年 | 51篇 |
2009年 | 63篇 |
2008年 | 79篇 |
2007年 | 76篇 |
2006年 | 86篇 |
2005年 | 78篇 |
2004年 | 59篇 |
2003年 | 79篇 |
2002年 | 45篇 |
2001年 | 26篇 |
2000年 | 34篇 |
1999年 | 16篇 |
1998年 | 21篇 |
1997年 | 19篇 |
1996年 | 10篇 |
1995年 | 11篇 |
1994年 | 13篇 |
1992年 | 14篇 |
1991年 | 11篇 |
1990年 | 15篇 |
1989年 | 11篇 |
1988年 | 13篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 17篇 |
1984年 | 18篇 |
1983年 | 11篇 |
1982年 | 8篇 |
1981年 | 17篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1978年 | 13篇 |
1977年 | 17篇 |
1975年 | 11篇 |
1974年 | 10篇 |
1973年 | 12篇 |
1972年 | 11篇 |
排序方式: 共有1604条查询结果,搜索用时 273 毫秒
41.
42.
43.
44.
45.
[reaction: see text] Diphenylphosphine oxide and related P(O)H compounds react with propargyl alcohols at room temperature in the presence of a catalytic amount of Ni(0) complex and Ph(2)P(O)OH to produce high yields of phosphinoyl 1,3-dienes though an efficient in situ dehydration process. 相似文献
46.
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
47.
48.
49.
The oxygen ions of the β-VOPO4 catalyst were exchanged with an
tracer by a reduction–oxidation method and by a catalytic oxidation of but-1-ene using
2. The bands at 992 and 900 cm−1 were more shifted to lower frequencies than those at 1076 and 1002 cm−1. Applying the correlation between the Raman bands and stretching vibrations in the literature, the exchanged oxygen species were estimated. The results suggest that the P–O–V vacancies corresponding to 992 and 900 cm−1 were responsible for reoxidation and the V=O oxygen corresponding to the 1002 cm−1 band of β-VOPO4 was not. The (VO)2P2O7 was oxidized to β-VOPO4 by O2 above 823 K. The insertion position of oxygen was determined at the bands at 992 and 900 cm−1 of β-VOPO4 using
2, which is the same as the exchanged position. 相似文献
50.
Determination of chemical form of antimony in contaminated soil around a smelter using X-ray absorption fine structure. 总被引:1,自引:0,他引:1
Masaki Takaoka Satoshi Fukutani Takashi Yamamoto Masato Horiuchi Naoya Satta Nobuo Takeda Kazuyuki Oshita Minoru Yoneda Shinsuke Morisawa Tsunehiro Tanaka 《Analytical sciences》2005,21(7):769-773
Only limited information is available about the behavior of antimony (Sb) in contaminated soils. However, understanding the behavior of Sb in contaminated soils is important, because the toxicity or solubility of this element depends on its chemical state. In this study, we investigated the levels of Sb and the chemical forms of Sb in the soil around a smelter using X-ray absorption fine structure (XAFS) spectra. The highest Sb concentration in the contaminated soil was 2900 mg/kg dry soil. According to Sb-K edge X-ray absorption near edge (XANE) spectra, the Sb in the soil was in the form of Sb(V) compounds. The similarity of extended XAFS (EXAFS) spectra suggests that Sb speciation was independent of the sampling site, which indicates that Sb or Sb2O3 emitted from the smelter was converted into Sb(V) compounds in the soil. 相似文献