全文获取类型
收费全文 | 13425篇 |
免费 | 1008篇 |
国内免费 | 3852篇 |
专业分类
化学 | 10727篇 |
晶体学 | 45篇 |
力学 | 171篇 |
综合类 | 131篇 |
数学 | 645篇 |
物理学 | 730篇 |
综合类 | 5836篇 |
出版年
2024年 | 53篇 |
2023年 | 173篇 |
2022年 | 353篇 |
2021年 | 398篇 |
2020年 | 472篇 |
2019年 | 490篇 |
2018年 | 439篇 |
2017年 | 445篇 |
2016年 | 485篇 |
2015年 | 501篇 |
2014年 | 809篇 |
2013年 | 907篇 |
2012年 | 836篇 |
2011年 | 947篇 |
2010年 | 702篇 |
2009年 | 761篇 |
2008年 | 845篇 |
2007年 | 957篇 |
2006年 | 865篇 |
2005年 | 863篇 |
2004年 | 779篇 |
2003年 | 697篇 |
2002年 | 540篇 |
2001年 | 501篇 |
2000年 | 512篇 |
1999年 | 402篇 |
1998年 | 361篇 |
1997年 | 364篇 |
1996年 | 305篇 |
1995年 | 300篇 |
1994年 | 241篇 |
1993年 | 271篇 |
1992年 | 170篇 |
1991年 | 120篇 |
1990年 | 127篇 |
1989年 | 103篇 |
1988年 | 72篇 |
1987年 | 38篇 |
1986年 | 19篇 |
1985年 | 20篇 |
1984年 | 9篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1979年 | 7篇 |
1978年 | 1篇 |
1959年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
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
22.
Rhodium catalysts have been prepared on palygorskite and montmorillonite (clay) supports by reduction with hydrogen (1 atmosphere) at room temperature of a cationic organometallic rhodium compound anchored to the support. The activity of these catalysts for the hydrogenation of liquid-phase 1-hexene remains constant with increase of prehydrogenation time and with re-use for several runs. No rhodium leaching is observed. 相似文献
23.
24.
高的氢压和反应温度有利于片硼酸钠与镁和氢反应生成硼氢化钠,但温度接近镁的熔点时不利于硼氢化钠的生成。在反应物中添加铁、镍和钴将加速硼氢化钠的生成,而铜的加入却阻碍硼氢化钠的生成。 相似文献
25.
Grindstone Chemistry--a greatly evolved version of Toda’s method of grinding solids together for solvent-free chemical reactions--has been described and its usefulness illustrated by the successful application of this technique to a simplified process for conducting the multi-component Biginelli reaction for the synthesis of physiologically active tetrahydropyrimidinones. 相似文献
26.
Shuichi Oi Mitsutoshi MoroHiroe Fukuhara Takanori KawanishiYoshio Inoue 《Tetrahedron》2003,59(24):4351-4361
The addition of arylstannanes to the carbon-heteroatom double bond in the presence of a catalytic amount of a cationic rhodium complex ([Rh(cod)(MeCN)2]BF4) was examined. The reactions of aldehydes, α-dicarbonyl compounds, and N-substituted aldimines with the arylstannanes gave corresponding alcohols, α-hydroxy carbonyl compounds, and amines, respectively. An arylrhodium complex generated by the transmetalation with the arylstannane was probably the active catalytic species. 相似文献
27.
Using a double Suzuki cross-coupling reaction ortho- or para-bromobenzyl bromides are easily transformed into unsymmetrical diaryl (or heteroaryl) methanes 相似文献
28.
A variety of aryl nitriles were prepared in excellent yields from the palladium acetate catalyzed coupling of aryl halides with Zn(CN)2 using polymer-supported triphenyl phosphine as the ligand and dimethylformamide as solvent under microwave irradiation conditions. 相似文献
29.
Qi Wang Huaxiang Yang Zhiqiang Fan Hong Xu 《Journal of polymer science. Part A, Polymer chemistry》2004,42(5):1093-1099
Ethylisobutylaluminoxane (EBAO) and its analogues were synthesized by a reaction between an triethylaluminum (Et3Al)/triisobutylaluminum (i‐Bu3Al) mixture and 4‐fluorobenzeneboronic acid, phenylboronic acid, or n‐butaneboronic acid and subsequent hydrolysis with water. They were used as cocatalysts in ethylene polymerization catalyzed by an iron complex {[(ArN?C(Me))2C5H3N]FeCl2, where Ar is 2,6‐diisopropylphenyl}. Polyethylene with a high molecular weight and a narrow molecular weight distribution was prepared with modified EBAOs, and the performance of the iron complex at high polymerization temperatures was greatly improved. The activators for the iron complex also affected the polymerization activity and the molecular weight of the resultant polyethylene. It was suggested that the stereo and electronic effects of the substitute groups of aluminoxane contributed to the improved performance of the new activators. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1093–1099, 2004 相似文献
30.
Novel Chiral Ferrocenyl Aziridino Alcohol Catalysts Promoting Asymmetric Addition of Diethylzinc to Aldehydes 总被引:3,自引:0,他引:3
Optically active aziridino alcohols containing ferrocenyl groups were prepared from commercially available L-threonine in excellent yields and used as catalysts to promote the asymmetric addition of diethylzinc to aryaldehydes to afford 1-arylpropanol in up to 84% enantiomeric excesses with moderate to good yields. 相似文献