全文获取类型
收费全文 | 6155篇 |
免费 | 128篇 |
国内免费 | 17篇 |
专业分类
化学 | 4503篇 |
晶体学 | 64篇 |
力学 | 72篇 |
综合类 | 1篇 |
数学 | 253篇 |
物理学 | 1407篇 |
出版年
2022年 | 45篇 |
2021年 | 69篇 |
2020年 | 79篇 |
2019年 | 79篇 |
2018年 | 56篇 |
2017年 | 48篇 |
2016年 | 92篇 |
2015年 | 101篇 |
2014年 | 103篇 |
2013年 | 255篇 |
2012年 | 251篇 |
2011年 | 317篇 |
2010年 | 149篇 |
2009年 | 164篇 |
2008年 | 300篇 |
2007年 | 306篇 |
2006年 | 317篇 |
2005年 | 336篇 |
2004年 | 316篇 |
2003年 | 217篇 |
2002年 | 220篇 |
2001年 | 191篇 |
2000年 | 175篇 |
1999年 | 97篇 |
1998年 | 49篇 |
1997年 | 67篇 |
1996年 | 87篇 |
1995年 | 68篇 |
1994年 | 75篇 |
1993年 | 97篇 |
1992年 | 105篇 |
1991年 | 100篇 |
1990年 | 76篇 |
1989年 | 93篇 |
1988年 | 84篇 |
1987年 | 60篇 |
1986年 | 61篇 |
1985年 | 100篇 |
1984年 | 109篇 |
1983年 | 59篇 |
1982年 | 64篇 |
1981年 | 54篇 |
1980年 | 60篇 |
1979年 | 68篇 |
1978年 | 65篇 |
1977年 | 78篇 |
1976年 | 51篇 |
1975年 | 45篇 |
1974年 | 46篇 |
1973年 | 51篇 |
排序方式: 共有6300条查询结果,搜索用时 15 毫秒
991.
Thamina Akther Md.Monarul Islam Taisuke Matsumoto Junji Tanaka Pierre Thuéry Carl Redshaw Takehiko Yamato 《Tetrahedron》2018,74(2):329-335
The reductive coupling reaction of 1,4-bis(3-acetyl-5-tert-butyl-2-methoxyphenyl)butane 3 was carried out using TiCl4-Zn in pyridine followed by a McMurry coupling reaction to afford the compounds anti and syn 1,2-dimethyl[2.4]MCP-1-ene 4. Bromination of 4 with BTMA-Br3 in dry CH2Cl2 afforded the interesting compound 1,2-bis-(bromomethyl)-5,15-di-tert-butyl-8,18-dimethoxy[2.4]MCP-1-ene 6 and consecutive debromination with Zn and AcOH in CH2Cl2 solution afforded the stable solid 5,15-di-tert-butyl-8,18-dimethoxy-1,2-dimethylene[2.4]MCP 7 in 89% yield. Compound 7 was conveniently employed in a Diels–Alder reaction with dimethyl acetylenedicarboxylate (DMAD) to provide 2-(3′,6′-dihydrobenzo)-5,15-di-tert-butyl-8,18-dimethoxy[2.4]MCP-4′,5′-dimethylcarboxylate 8 in good yield. Diels–Alder adduct 8 was converted into a novel and inherently chiral areno-bridged compound [2.4]MCP 9 by aromatization. The chirality of the two conformers was characterized by circular dichroism (CD) spectra of the separated enantiomer which are perfect mirror images of each other. 相似文献
992.
M. Tanaka N. Tomita S. Shimomura S. Hiyamizu T. Higuchi N. Sano A. Adachi 《Journal of Crystal Growth》1995,150(1-4):388-393
GaAs/Al0.3Ga0.7As multi-layer structures were grown on GaAs (100) reverse-mesa etched substrates by glancing angle molecular beam epitaxy (GA-MBE). A(111)B facet was formed as a side-facet. Surface migration of Ga and Al atoms from the (100) flat region to the (111)B side-facet region has been investigated to fabricate T-shaped GaAs/AlGaAs quantum wells (QWs) under the condition that Ga and Al atoms impinge only an the (100) flat region and do not impinge on the (111)B side-facet. Observation of T-shaped GaAs/AlGaAs quantum wires (QWRs) by cross-sectional transmission electron microscopy (TEM) revealed that there is no migration of Al atoms from the (100) to the (111)B facet region at a substrate temperature (Ts) as high as 630°C, under a V/III ratio of 28 (in pressure ratio). On the other hand, very thin GaAs epitaxial layers grown on the (111)B side-facet region owing to the Ga migration were observed for substrate temperatures of 600 and 630°C. It was found that the mass flow of Ga atoms from the (100) region to the (111)B side-facet region increases, with the thermal activation energy of 2.0 eV, as the substrate temperature increases from 570 to 630°C. The GA-MBE growth on a reverse-mesa etched GaAs substrate at a low temperature 570°C or lower is desirable to fabricate a nm-scale GaAs/AlGaAs QWR structure with nm-scale precision. 相似文献
993.
Ohno H Okano A Kosaka S Tsukamoto K Ohata M Ishihara K Maeda H Tanaka T Fujii N 《Organic letters》2008,10(6):1171-1174
The palladium-catalyzed domino cyclization of propargyl bromides having two nucleophilic functional groups is described. Treatment of 1,7-diamino-5-bromohept-3-yne derivatives with catalytic Pd(PPh3)4 in the presence of NaH in MeOH gives the 2,7-diazabicyclo[4.3.0]non-5-enes in good yields. Interestingly, the regioselectivity of the reaction is completely controlled by the relative reactivity of the amine functional groups, irrespective of the position of the nucleophiles. The malonate derivative also undergoes domino cyclization to produce a hexahydroindole derivative. 相似文献
994.
Kouzou Matsumoto Dr. Toru Tanaka Sota Kugo Takuya Inagaki Yoshikazu Hirao Dr. Hiroyuki Kurata Dr. Takeshi Kawase Dr. Takashi Kubo Dr. 《化学:亚洲杂志》2008,3(12):2024-2032
Tetrakis(bithienyl)methane and tetrakis(terthienyl)methane have been synthesized from tetrakis(2‐thienyl)methane by use of Suzuki–Miyaura coupling as a key reaction. Their trimethylsilyl (TMS) derivatives are also synthesized. X‐ray analysis reveals that each oligothiophene moiety tends to adopt anti‐conformations and show relatively small torsion angles between the adjacent thiophene rings. While the longest absorption maxima of these tetrakis(oligothienyl)methanes exhibit only a slight bathochromic shift compared to the corresponding linear oligothiophene derivative, tetrakis(bithienyl)methane and its TMS derivative exhibit an appreciable red‐shift in their fluorescence spectra. The intramolecular interaction between thienyl groups of tetrakis(2‐thienyl)methane is supported by DFT calculation. 相似文献
995.
Okano A Mizutani T Oishi S Tanaka T Ohno H Fujii N 《Chemical communications (Cambridge, England)》2008,(30):3534-3536
Treatment of allenic bromoalkenes bearing a nucleophilic moiety with a catalytic amount of palladium(0) in the presence of TBAF or Cs(2)CO(3) in MeCN affords bicyclic heterocycles in good to high yields, through zipper-mode cascade cyclisation. 相似文献
996.
997.
998.
999.
1000.