全文获取类型
收费全文 | 17799篇 |
免费 | 1521篇 |
国内免费 | 1043篇 |
专业分类
化学 | 18151篇 |
晶体学 | 334篇 |
力学 | 21篇 |
综合类 | 25篇 |
数学 | 4篇 |
物理学 | 1828篇 |
出版年
2024年 | 13篇 |
2023年 | 172篇 |
2022年 | 307篇 |
2021年 | 549篇 |
2020年 | 606篇 |
2019年 | 464篇 |
2018年 | 384篇 |
2017年 | 377篇 |
2016年 | 648篇 |
2015年 | 658篇 |
2014年 | 725篇 |
2013年 | 1399篇 |
2012年 | 873篇 |
2011年 | 794篇 |
2010年 | 864篇 |
2009年 | 895篇 |
2008年 | 910篇 |
2007年 | 912篇 |
2006年 | 880篇 |
2005年 | 831篇 |
2004年 | 881篇 |
2003年 | 783篇 |
2002年 | 1473篇 |
2001年 | 431篇 |
2000年 | 370篇 |
1999年 | 380篇 |
1998年 | 355篇 |
1997年 | 255篇 |
1996年 | 476篇 |
1995年 | 440篇 |
1994年 | 186篇 |
1993年 | 163篇 |
1992年 | 169篇 |
1991年 | 109篇 |
1990年 | 72篇 |
1989年 | 84篇 |
1988年 | 77篇 |
1987年 | 44篇 |
1986年 | 54篇 |
1985年 | 66篇 |
1984年 | 50篇 |
1983年 | 30篇 |
1982年 | 30篇 |
1981年 | 24篇 |
1980年 | 19篇 |
1979年 | 14篇 |
1978年 | 15篇 |
1975年 | 6篇 |
1973年 | 6篇 |
1972年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Highly stereocontrolled additions to achiral acyclic (E) olefins are achieved via incorporation into an ansa macrolide with a non-racemic stilbene diol (molecular workbench approach). 相似文献
92.
93.
人工神经网络方法预测气相色谱保留值 总被引:8,自引:2,他引:8
本文运用一典型的人工神经网络模型-“反向传播“模型的改进形式,研究了诱导效应指数I,摩尔折射度Ro,疏水亲脂参数IgP,以及分子联通性指数与气象色谱保留行为的关系,实现了对色谱保留植的预测。神经网络预测模型的最大相对误差不超过8.7%。结果表明,该方法性能良好,可望成为色谱保留值预测的有效手段。 相似文献
94.
本文讨论了芳烃金属配合物中的芳香部份在与金属配合后性质上的变化,以及这些变化后的性质在有机合成中的应用。 相似文献
95.
96.
以2-(3,4-环己基类二氧-5-异丙基)苯基乙醇(2)和3-异丙氧基-5,5-二甲基-1,3-环己烯酮(4)为原料,利用羰基α-位的烷基化反应和分子内的Friedel-Carfts反应为关键步骤,经9步反应,合成了多氧芳香型三环二萜类天然产物Salvinolone。 相似文献
97.
Catalytic nitrobenzene oxidation of lignins 总被引:1,自引:0,他引:1
A. F. Gogotov N. A. Rybal'chenko T. I. Makovskaya V. A. Babkin 《Russian Chemical Bulletin》1996,45(12):2854-2857
Alkaline nitrobenzene oxidation of hardwood and softwood lignins in the presence of redox and phase-transfer catalysts was studied. The selectivity of oxidation of lignins increased by 1.7 to 1.9 times. A possible mechanism of catalysis is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 3004–3007, December, 1996. 相似文献
98.
Summary Both the separation of polycyclic aromatic hydrocarbons (PAH's) on tailor made HPLC phases, and their chromatographic enrichment
and quantitative determination are of great interest. Because of the generally low concentrations of PAH's in the environment,
and the low limiting values in German legislation for drinking water, methods for selective enrichment and sensitive detection
are indispensable. On account of their relatively high amounts in soil, the determination of PAH's is valuable for the assessment
of the potential danger to ground water by waste materials. These requirements can be fulfilled by used of solid phase extraction
on enrichment columns, and fluorescence or UV/VIS diode array detection.
For the detection of PAH's in the picogram range, the wavelengths for excitation and emission were time programmed over the
chromatogram. With this feature, it is possible to detect all the individual compounds at the highest sensitivity, over the
entire analysis. 相似文献
99.
Jörg Teichgräber 《Journal of organometallic chemistry》2005,690(23):5255-5263
The synthesis, characterization, and thermal decomposition behavior of dicopper(I) oxalato complexes L4Cu2(C2O4) (L = CNtBu (2a), CNCMe2CH2tBu (2b), CNC6H3Me2-2,6 (2c)) is reported. 2c can be prepared in a straightforward manner by the reaction of stoichiometric amounts of Cu2O and oxalic acid with four equivalents of CNC6H3Me2-2,6, while those complexes with aliphatic isocyanides are better prepared from a copper(I) oxalato complex with alkine capping ligands (Me3SiCCSiMe3)2Cu2(C2O4) (1) via ligand exchange. Crystallographic and spectroscopic evidence for 2a-c confirms the anticipated dinuclear structure with the oxalate in a μ-1,2,3,4 bridging mode and an essentially σ-character of the terminal isocyanides. In solid form the complexes are stable at room temperature and can be handled in air for some time. Their decomposition was studied by thermal gravimetric analysis coupled with mass spectrometry, and the degradation pathway was shown to depend on the type of isocyanide capping ligand. Decomposition of 2a,b takes place between 150 and 200 °C to give CuCN in a clean process that involves isobutene elimination from the terminal ligands, with elimination of (CN)2 and conversion to elemental copper at higher temperatures. Heating of 2c leads to CuO (and then to Cu2O) via release of the intact isocyanide, CO2, and CO in a well-behaved thermal process around 200-280 °C. 相似文献
100.
Indoles undergo smooth alkylation with cyclopropyl ketones in the presence of the CeCl3·7H2O/LiI reagent system in refluxing acetonitrile under neutral conditions to produce the corresponding C-3 substituted indole derivatives in good to high yields and with high selectivity. The use of CeCl3·7H2O/LiI makes this method simple, convenient, and cost-effective. 相似文献