全文获取类型
收费全文 | 1384篇 |
免费 | 21篇 |
国内免费 | 126篇 |
专业分类
化学 | 1349篇 |
晶体学 | 2篇 |
综合类 | 4篇 |
数学 | 63篇 |
物理学 | 113篇 |
出版年
2023年 | 108篇 |
2022年 | 16篇 |
2021年 | 49篇 |
2020年 | 39篇 |
2019年 | 63篇 |
2018年 | 60篇 |
2017年 | 34篇 |
2016年 | 34篇 |
2015年 | 31篇 |
2014年 | 47篇 |
2013年 | 111篇 |
2012年 | 44篇 |
2011年 | 65篇 |
2010年 | 49篇 |
2009年 | 83篇 |
2008年 | 71篇 |
2007年 | 89篇 |
2006年 | 96篇 |
2005年 | 84篇 |
2004年 | 79篇 |
2003年 | 32篇 |
2002年 | 29篇 |
2001年 | 14篇 |
2000年 | 19篇 |
1999年 | 22篇 |
1998年 | 35篇 |
1997年 | 11篇 |
1996年 | 18篇 |
1995年 | 10篇 |
1994年 | 12篇 |
1993年 | 15篇 |
1992年 | 7篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 9篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1968年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有1531条查询结果,搜索用时 15 毫秒
41.
Akiko Saito Mana Emoto Akira Tanaka Yuki Doi Kazuaki Shoji Yoshiyuki Mizushina Hiroshi Ikawa Hiromi Yoshida Nobuyasu Matsuura Noriyuki Nakajima 《Tetrahedron》2004,60(52):12043-12049
A simple method for the synthesis of procyanidin B3 substituted with a galloyl group at the 3 and 3″ position is described. Condensation of a benzylated catechin-3-O-gallate electrophile with a nucleophile, catechin and catechin-3-O-gallate, proceeded smoothly and stereoselectively to afford the corresponding dimer gallates, procyanidin B3-3-O-gallate and procyanidin B3-3,3″-di-O-gallate, in good yields. Further, their antioxidant activities on UV-induced lipid peroxide formation, DPPH radical scavenging activity and inhibitory activity of DNA polymerase were also investigated. Among three procyanidin B3 congeners (procyanidin B3, 3-O-gallate and 3,3″-di-O-gallate), the 3,3″-di-O-gallate derivative showed the strongest antioxidant and radical scavenging activity. Interestingly, the 3-O-gallate derivative was the strongest inhibitor of mammalian DNA polymerase with IC50 value of 0.26 μM, although it showed the weakest antioxidant and radical scavenging activity. It became apparent that the presence of a galloyl group at the C-3 position in the proanthocyanidin oligomer was very important for biological activity, however, the antioxidant activity of these compounds was not parallel to the DNA polymerase inhibitory activity. 相似文献
42.
Stéphanie Etienne C. Becker D. Ruch B. Grignard G. Cartigny C. Detrembleur C. Calberg R. Jerome 《Journal of Thermal Analysis and Calorimetry》2007,87(1):101-104
Silica nanoparticles of various sizes have
been incorporated by melt compounding in a poly(methyl methacrylate) (PMMA)
matrix to enhance its thermal and mechanical properties. In order to improve
nanoparticles dispersion, PMMA grafted particles have been prepared by atom
transfer radical polymerization (ATRP) from well-defined silica nanoparticles.
This strategy was expected to ensure compatibility between both components
of the PMMA nanocomposites. TEM analysis have been performed to evaluate the
nanosilica dispersion whereas modified and non-modified silica/PMMA nanocomposites
thermal stability and mechanical properties have been investigated by both
thermogravimetric and dynamical mechanical analysis. 相似文献
43.
The electronic absorption spectrum for the dibiphenylene-ethene radical anion (
), generated electrochemically in dimethylsulfoxide solutions, is reported. Based on calculations by the LCAO-MO method in theHückel approximation and on the analysis of the band positions it is suggested that in the
molecule two planar fluorenylidene fragments are twisted (60°) about the central C=C bond.
Das Elektronenabsorptionsspektrum des Dibiphenylen-Radikalanions (Kurze Mitteilung)
Zusammenfassung Das Dibiphenylenethen-Radikalanion ( ) wurde elektrochemisch hergestellt und das Elektronenabsorptionsspektrum aufgenommen. Auf Grund von LCAO-MO Berechnungen in derHückel-Annäherung und der Analyse der Bandenlagen wird nahegelegt, daß im -Molekül zwei planare Fluorenyliden-Fragmente ca. 60° um die zentrale C=C-Bindung verdrillt angeordnet sind.相似文献
44.
自由基活性聚合及其最新进展 总被引:5,自引:2,他引:5
本文介绍了实现自由基活性聚合的动力学、热力学条件及可能的途径。在这些条件下自由基保持稳定的低浓度,增长链自由基与休眠种处于动力学平衡动态,可有效地控制聚合反应,使聚合反应具有聚合物分子量随反应时间、单体转化率成线性增长关系及所得聚合物分子量分布较窄的特征,并且在加入第二单体时可继续生成嵌段共聚物。 相似文献
45.
Vitaliy Petrik 《Tetrahedron letters》2007,48(19):3327-3330
The easy radical trifluoromethylation of a series of 1,3-dicarbonyl compounds with CF3I is described. The reaction occurs in the presence of a nitrogen base and sodium dithionite in CH3CN-H2O solution. Additionally, we report a new access to ammonium triflinates. 相似文献
46.
Thiam Seong Chong 《Journal of organometallic chemistry》2006,691(4):687-692
The rate constant for the methyl abstraction reaction of CpFe(CO)2Me has been measured with the benzyl radical clock as (1.1 ± 0.2) × 105 M−1 s−1 at room temperature. Time-resolved Fourier-transform Infrared (FTIR) absorption spectroscopy pointed towards the formation of the CpFe(CO)2 radical upon benzyl abstraction. The main stable product has been established by a linear scan of the reaction mixture as Cp2Fe2(CO)4 produced by the dimerization of the CpFe(CO)2 radicals. The transition state structure for the abstraction process was also found at UB3LYP/6-311+G* level of theory to contain a planar CH3 group. 相似文献
47.
洋葱色素的提取及抗氧化活性的测定 总被引:6,自引:0,他引:6
分别用芬顿(Fenton)反应法和1,1-二苯-2-苦肼基(DPPH)法测定了洋葱色素的抗氧化作用。结果表明,洋葱色素对羟自由基和DPPH自由基均有一定的清除作用,其清除率大小依次为42.4%,58.2%。洋葱色素是十分具有开发利用价值的天然色素。 相似文献
48.
W. Russell Bowman Anthony J. Fletcher Peter J. Lovell Elena Hernández López Graeme B.S. Potts 《Tetrahedron》2007,63(1):191-203
Amides have been successfully used as precursors of imidoyl radicals for radical cyclisation. The amides have been converted to imidoyl selanides via reaction with phosgene to yield imidoyl chlorides followed by reaction with potassium phenylselanide. Imidoyl selanides were reacted with tributyltin hydride (Bu3SnH) as the radical mediator with triethylborane or AIBN as initiators to yield imidoyl radicals for cyclisation reactions. Imidoyl radicals have been cyclised onto alkenes to yield 2,3-substituted-indoles and -quinolines and also onto pyrroles and indoles to give bi- and tricyclic heteroarenes. 相似文献
49.
Sequential ring-closing metathesis (RCM)-Kharasch cyclizations are promoted by the Grubbs metathesis catalysts and provide rapid access to bicyclic lactones and lactams. 相似文献
50.
N-芳基丙烯酰胺是-类功能性单体,它能与丙烯酸酯类单体进行自由基共聚合。本文以4,4'-二氨基二苯基甲烷为起始原料,合成了新的带有较长侧基的甲基丙烯酰胺类单体,即4-乙酰氨基-4'-甲基丙烯酰氨基-二苯基甲烷(AMDPM)。它能进行自由基聚合或与丙烯酸甲酯(MA)共聚合。 相似文献