全文获取类型
收费全文 | 3804篇 |
免费 | 142篇 |
国内免费 | 13篇 |
专业分类
化学 | 2819篇 |
晶体学 | 44篇 |
力学 | 45篇 |
数学 | 193篇 |
物理学 | 570篇 |
无线电 | 288篇 |
出版年
2023年 | 33篇 |
2022年 | 35篇 |
2021年 | 44篇 |
2020年 | 55篇 |
2019年 | 78篇 |
2018年 | 55篇 |
2017年 | 45篇 |
2016年 | 105篇 |
2015年 | 68篇 |
2014年 | 107篇 |
2013年 | 187篇 |
2012年 | 237篇 |
2011年 | 279篇 |
2010年 | 132篇 |
2009年 | 145篇 |
2008年 | 222篇 |
2007年 | 198篇 |
2006年 | 207篇 |
2005年 | 172篇 |
2004年 | 131篇 |
2003年 | 132篇 |
2002年 | 127篇 |
2001年 | 69篇 |
2000年 | 55篇 |
1999年 | 44篇 |
1998年 | 44篇 |
1997年 | 32篇 |
1996年 | 32篇 |
1995年 | 31篇 |
1994年 | 25篇 |
1993年 | 41篇 |
1992年 | 42篇 |
1991年 | 39篇 |
1990年 | 47篇 |
1989年 | 34篇 |
1988年 | 32篇 |
1987年 | 29篇 |
1986年 | 37篇 |
1985年 | 41篇 |
1984年 | 48篇 |
1983年 | 33篇 |
1982年 | 38篇 |
1981年 | 33篇 |
1980年 | 36篇 |
1979年 | 29篇 |
1978年 | 42篇 |
1977年 | 33篇 |
1976年 | 23篇 |
1974年 | 24篇 |
1973年 | 23篇 |
排序方式: 共有3959条查询结果,搜索用时 11 毫秒
61.
Construction of carbocyclic ring of indoles using ruthenium-catalyzed ring-closing olefin metathesis
The selective synthesis of substituted indoles was achieved by the ring-closing olefin metathesis (RCM)/elimination sequence or the RCM/tautomerization sequence of functionalized pyrrole precursors. The RCM/elimination sequence was also applied to double ring closure to yield a substituted carbazole. 相似文献
62.
Ailong Li Hideshi Ooka Nadge Bonnet Toru Hayashi Yimeng Sun Qike Jiang Can Li Hongxian Han Ryuhei Nakamura 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(15):5108-5112
Efficient, earth‐abundant, and acid‐stable catalysts for the oxygen evolution reaction (OER) are missing pieces for the production of hydrogen via water electrolysis. Here, we report how the limitations on the stability of 3d‐metal materials can be overcome by the spectroscopic identification of stable potential windows in which the OER can be catalyzed efficiently while simultaneously suppressing deactivation pathways. We demonstrate the benefits of this approach using gamma manganese oxide (γ‐MnO2), which shows no signs of deactivation even after 8000 h of electrolysis at a pH of 2. This stability is vastly superior to existing acid‐stable 3d‐metal OER catalysts, but cannot be realized if there is a deviation as small as 50‐mV from the stable potential window. A stable voltage efficiency of over 70 % in a polymer–electrolyte membrane (PEM) electrolyzer further verifies the availability of this approach and showcases how materials previously perceived to be unstable may have potential application for water electrolysis in an acidic environment. 相似文献
63.
Ehsan Jazaeri Liyuan Zhang Xungai Wang Takuya Tsuzuki 《Cellulose (London, England)》2011,18(6):1481-1485
The possibility of fabricating carbon nanofibers from cellulose nanofibers was investigated. Cellulose nanofiber of ~50 nm
in diameter was produced using ball milling in an eco-friendly manner. The effect of the drying techniques of cellulose nanofibers
on the morphology of carbon residue was studied. After pyrolysis of freeze-dried cellulose nanofibers below 600 °C, amorphous
carbon fibers of ~20 nm in diameter were obtained. The pyrolysis of oven-dried precursors resulted in the loss of original
fibrous structures. The different results arising from the two drying techniques are attributed to the difference in the spatial
distance between cellulose nanofiber precursors. 相似文献
64.
A rhodium-catalyzed 1,4-addition of sodium tetraarylborates to β,β-disubstituted α,β-unsaturated esters has been developed. Highly efficient asymmetric catalysis has also been described to create quaternary carbon stereocenters at the β-position of esters by tuning the ester group of substrates and employing a readily available chiral diene ligand. 相似文献
65.
Shotaro Hayashi Minami Sakamoto Fumitaka Ishiwari Takanori Fukushima Shin-ichi Yamamoto Toshio Koizumi 《Tetrahedron》2019,75(8):1079-1084
Here we report the facile derivatization of a cyano-substituted stilbene into higher π-extended analogues. The cyano-substituted stilbene, which serves as a synthetic scaffold, has a bromo group and a formyl group on its 4- and 4′-position of the phenylene rings and thus readily undergoes selective transformation into other functional groups using various simple organic reactions. The resultant π-conjugated molecules that contain a cyano-substituted stilbene skeleton exhibit fluorescence in solution and in the solid state. 相似文献
66.
67.
Masahiro Irie Setsuko Tomimoto Koichiro Hayashi 《Journal of polymer science. Part A, Polymer chemistry》1972,10(11):3235-3242
The dependence of the rate of polymerization on light intensity and the stereoregularity of the polymer was studied to elucidate the propagation and termination mechanisms of the photoinduced cationic polymerization of α-methylstyrene in the presence of tetracyanobenzene in methylene chloride. The rate of polymerization was proportional to the light intensity. The polymer is highly syndiotactic, and the stereoregularity is similar to that of polymers obtained by radiation-induced cationic polymerization. The initiation mechanism was also studied by electron spin resonance, by which the anion radical of tetracyanobenzene formed from a photoexcited complex between α-methylstyrene and tetracyanobenzene was observed. The cation radical of α-methylstyrene, counterpart of the anion radical, is believed to initiate the polymerization. 相似文献
68.
Hayashi K Kujime E Katayama H Sano S Nagao Y 《Chemical & pharmaceutical bulletin》2007,55(12):1773-1775
In the imine aldol reactions of 1 with aromatic aldehydes using magnesium salts in the presence of amines, the threo/erythro ratios of products increased in the order Mg(ClO4)2>MgI2>MgBr2>MgCl2>Mg(OTf)2 and N,N,N',N'-tetramethylethylenediamine (TMEDA)>Et3N. This increase in the threo/erythro ratios of products was estimated to be caused by a retro-imine aldol reaction under thermodynamic control. 相似文献
69.
Dr. Jun Nakamura Hidenori Yamashiro Sayaka Hayashi Mami Yamamoto Kenji Miura Dr. Shu Xu Prof. Dr. Takayuki Doi Dr. Hideki Maki Osamu Yoshida Prof. Dr. Hirokazu Arimoto 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(40):12681-12689
Covalently linked vancomycin dimers have attracted a great deal of attention among researchers because of their enhanced antibacterial activity against vancomycin‐resistant strains. However, the lack of a clear insight into the mechanisms of action of these dimers hampers rational optimization of their antibacterial potency. Here, we describe the synthesis and antibacterial activity of novel vancomycin dimers with a constrained molecular conformation achieved by two tethers between vancomycin units. Conformational restriction is a useful strategy for studying the relationship between the molecular topology and biological activity of compounds. In this study, two vancomycin units were linked at three distinct positions of the glycopeptide (vancosamine residue (V), C terminus (C), and N terminus (N)) to form two types of novel vancomycin cyclic dimers. Active NC‐VV‐linked dimers with a stable conformation as indicated by molecular mechanics calculations selectively suppressed the peptidoglycan polymerization reaction of vancomycin‐resistant Staphylococcus aureus in vitro. In addition, double‐disk diffusion tests indicated that the antibacterial activity of these dimers against vancomycin‐resistant enterococci might arise from the inhibition of enzymes responsible for peptidoglycan polymerization. These findings provide a new insight into the biological targets of vancomycin dimers and the conformational requirements for efficient antibacterial activity against vancomycin‐resistant strains. 相似文献
70.
Cellulose - Cellulose I crystals swell on exposure to ethylenediamine (EDA) molecules to form a cellulose I–EDA complex, and successive extraction of EDA molecules converts the complex... 相似文献