全文获取类型
收费全文 | 39905篇 |
免费 | 2281篇 |
国内免费 | 6354篇 |
专业分类
化学 | 39413篇 |
晶体学 | 764篇 |
力学 | 300篇 |
综合类 | 227篇 |
数学 | 1995篇 |
物理学 | 5841篇 |
出版年
2024年 | 42篇 |
2023年 | 375篇 |
2022年 | 628篇 |
2021年 | 840篇 |
2020年 | 962篇 |
2019年 | 1877篇 |
2018年 | 996篇 |
2017年 | 1886篇 |
2016年 | 1310篇 |
2015年 | 1165篇 |
2014年 | 1592篇 |
2013年 | 3509篇 |
2012年 | 2343篇 |
2011年 | 2568篇 |
2010年 | 1949篇 |
2009年 | 2273篇 |
2008年 | 2525篇 |
2007年 | 2622篇 |
2006年 | 2455篇 |
2005年 | 2258篇 |
2004年 | 2194篇 |
2003年 | 1757篇 |
2002年 | 1457篇 |
2001年 | 1168篇 |
2000年 | 1170篇 |
1999年 | 920篇 |
1998年 | 823篇 |
1997年 | 711篇 |
1996年 | 621篇 |
1995年 | 618篇 |
1994年 | 514篇 |
1993年 | 444篇 |
1992年 | 406篇 |
1991年 | 292篇 |
1990年 | 192篇 |
1989年 | 181篇 |
1988年 | 138篇 |
1987年 | 83篇 |
1986年 | 90篇 |
1985年 | 78篇 |
1984年 | 75篇 |
1983年 | 33篇 |
1982年 | 65篇 |
1981年 | 72篇 |
1980年 | 44篇 |
1979年 | 73篇 |
1978年 | 36篇 |
1977年 | 24篇 |
1976年 | 27篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
A practical access to alkyl- and aryl-substituted (E)-2-(azidomethyl)alkenoates and related azido compounds from the corresponding allylic bromides in aqueous acetone is described. An alternative method to obtain the starting bromides based on heterogeneous catalysis under mild conditions was also investigated. 相似文献
22.
Atsuo Nakazaki 《Tetrahedron》2006,62(26):6264-6271
The stereoselective total syntheses of (±)-α-vetispirene, (±)-hinesol, and (±)-β-vetivone were accomplished based on a Claisen rearrangement in an alkenyl bicyclic dihydropyran system. The most striking feature of this approach is that the Claisen rearrangement of bicyclic dihydropyran proceeds stereoselectively to provide a multi-functionalized spiro[4.5]decane, which is an efficient precursor for the synthesis of the vetivane sesquiterpenes. 相似文献
23.
A. P. Vink E. van der Kolk P. Dorenbos C. W. E. van Eijk 《Optics Communications》2002,210(3-6):277-284
The optical properties of Ce3+ in CaSO4, SrSO4 and BaSO4 are reported. The Ce3+ ion shows 4f05d1→2F5/2,2F7/2 luminescence in all three sulphates. Co-doping with Na+ does not change the local surrounding of the Ce3+ ion, but enhances the amount of Ce3+ ions built in. Under optical excitation, besides the typical Ce3+ doublet emission in the ultraviolet spectral region, band emission around 445 nm was observed. This band emission was not assigned to emission from a Ce3+ centre, but to emission from an impurity-trapped exciton. Under X-ray excitation, both Ce3+ emission and an emission band around 380 nm was observed. This band was assigned to emission from a self-trapped exciton. 相似文献
24.
25.
主要介绍了一个引理,这个引理奠定了K4-同胚图K4(α,1,1,δ,ε,η)色性研究的基础。 相似文献
26.
[Na(18-C-6)]2[Cu(i-mnt)2]的合成与结构分析 总被引:2,自引:2,他引:0
研究了18-冠-6与Na2[Cu(i-mnt)2][i-mnt=异丁二腈烯二硫醇阴离子,S2CC(CN)2-2]的反应,得到的配合物[Na(18-C-6)]2[Cu(i-mnt)2](1)通过元素分析、红外光谱、X射线单晶衍射进行了结构分析.配合物为单斜晶系,空间群P2(1)/c.晶体学结构数据a=1.2819(11),b=1.1793(10),c=1.4928(13)nm,β=99.121(16)°,V=2.228(3)nm3,Z=2,Dcaled.=1.369g/cm3,F(000)=958,R1=0.0521,wR2=0.1003.1中的[Cu(i-mnt)2]基团通过配体i-mnt的氮原子与两个[Na(18-C-6)]基团中的钠原子成键,形成稳定的中性配合物. 相似文献
27.
P. R. Latthe P. S. Shinge Bharati V. Badami P. B. Patil S. N. Holihosur 《Journal of Chemical Sciences》2006,118(3):249-256
3-[4-(Azidocarbonyl)]phenylsydnone (2) obtained from 3-(4-hydrazinocarbonyl) phenylsydnone (1) on Curtius rearrangement with alcohols, water and amines afforded the corresponding carbamates (3a-h), 4,4′-(sydnone-3-yl) diphenyl urea (4) and 4-(heterocyclyl)phenyl ureas (5a-l). Compounds (5a-l) on one-pot ring conversion yielded the 1,3,4-oxadiazolin-2-one derivatives (6a-l), which on reaction with N2H4 gave the 4-amino-1,2,4-triazolin-3-ones (7a-l). All these compounds exhibited moderate antimicrobial activity against the few microbes tested. The carbamates have been
found to be more toxic against fourth instar larvae ofAedes aegypti, in particular, then-butyl derivative (3e). 相似文献
28.
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
29.
30.