全文获取类型
收费全文 | 2604篇 |
免费 | 89篇 |
国内免费 | 9篇 |
专业分类
化学 | 1921篇 |
晶体学 | 7篇 |
力学 | 28篇 |
数学 | 405篇 |
物理学 | 341篇 |
出版年
2021年 | 20篇 |
2020年 | 41篇 |
2019年 | 44篇 |
2018年 | 21篇 |
2016年 | 62篇 |
2015年 | 63篇 |
2014年 | 75篇 |
2013年 | 89篇 |
2012年 | 122篇 |
2011年 | 136篇 |
2010年 | 89篇 |
2009年 | 61篇 |
2008年 | 100篇 |
2007年 | 127篇 |
2006年 | 133篇 |
2005年 | 119篇 |
2004年 | 82篇 |
2003年 | 75篇 |
2002年 | 84篇 |
2001年 | 55篇 |
2000年 | 37篇 |
1999年 | 40篇 |
1998年 | 38篇 |
1997年 | 28篇 |
1996年 | 24篇 |
1995年 | 32篇 |
1994年 | 34篇 |
1993年 | 29篇 |
1992年 | 45篇 |
1991年 | 29篇 |
1990年 | 28篇 |
1989年 | 31篇 |
1988年 | 36篇 |
1987年 | 39篇 |
1986年 | 31篇 |
1985年 | 43篇 |
1984年 | 31篇 |
1983年 | 23篇 |
1982年 | 21篇 |
1981年 | 29篇 |
1980年 | 33篇 |
1979年 | 22篇 |
1978年 | 30篇 |
1977年 | 23篇 |
1976年 | 28篇 |
1975年 | 32篇 |
1974年 | 34篇 |
1973年 | 32篇 |
1971年 | 19篇 |
1970年 | 16篇 |
排序方式: 共有2702条查询结果,搜索用时 46 毫秒
71.
Harald Greger Otmar Hofer Andreas Werner 《Monatshefte für Chemie / Chemical Monthly》1985,116(2):273-277
In addition to the well known affinin [=spilanthol, (2E,6Z,8E)-deca-2,6,8-trienoic acid isobutylamide (1)], the corresponding 2-methyl-butylamide (2), and two new acetylenic alkamides were isolated fromSpilanthes oleracea L. by reversed phase medium pressure chromatography: (Z)-non-2-en-6,8-diynoic acid isobutylamide (3) and (Z)-dec-2-en-6,8-diynoic acid isobutylamide (4). The structures and their stereochemistries were elucidated by1H-NMR,13C-NMR (2 and3), MS, UV, IR, and CD (2). The chemotaxonomic significance of the distribution of alkamides within theCompositae tribeHeliantheae is briefly discussed.
Neue Amide ausSpilanthes oleracea (Kurze Mitteilung)
Zusammenfassung AusSpilanthes oleracea wurden neben dem seit langem bekannten Affinin [=Spilanthol, (2E,6Z,8E)-Deca-2,6,8-triensäureisobutylamid (1)] durch Umkehrphasen-Mitteldruckchromatographie das entsprechende 2-Methylbutylamid (2) und zwei neue acetylenische Alkamide isoliert: (Z)-Non-2-en-6,8-diinsäure-isobutylamid (3) und (Z)-Dec-2-en-6,8-diinsäure-isobutylamid (4). Die Strukturen und deren Stereochemie wurden mittels1H-NMR,13C-NMR (2 und3), MS, UV, IR und CD (2) aufgeklärt. Die chemotaxonomische Bedeutung der Verbreitung von Alkamiden innerhalb der Compositen-TribusHeliantheae wird kurz diskutiert.相似文献
72.
Alkynylxenon(II) fluorides, RCC(triple bond)XeF, have been prepared from the reactions of the corresponding trimethyl(alkynyl)silanes, Me3(-)SiC(triple bond)CR, and XeF2 in the presence of [NMe4F in common organic solvents at low temperature. The existence of the linear unit C(triple bond)C-Xe-F was proved for PhC(triple bond)CXeF by the 19F-13C NMR correlation method using the HMBC pulse sequence. 相似文献
73.
Zou B Wang M Qiu D Zhang X Chi L Fuchs H 《Chemical communications (Cambridge, England)》2002,(9):1008-1009
Stable surface nanostructures with different morphology have been successfully constructed by modifying the chemical structure of synthetic amphiphiles; by introducing mesogenic groups into bolaform amphiphiles, stable spaghetti-like or stripe-like nanostructures can be obtained; it is believed that such a kind of surface structure could be used for templating synthesis and assembly. 相似文献
74.
An easy to handle and stable racemization catalyst for secondary alcohols is obtained by an in situ mixture of readily available [Ru(cymene)Cl2]2 with chelating aliphatic diamines. Optimization of the reaction revealed that N,N,N′,N′-tetramethyl-1,3-propanediamine as ligand racemizes aromatic alcohols completely within 5 h. This easy to handle and stable catalytic system is combined with a lipase-catalyzed resolution to provide an efficient dynamic kinetic resolution of secondary alcohols. 相似文献
75.
The reaction of linear (Si(n)Cl(2)(n)(+2); n = 3-5) and cyclic (Si(5)Cl(10)) perchloropolysilanes with 1 or 2 equiv of LiN(SiMe(3))(2) results in the formation of the bis(trimethylsilyl)amino derivatives (Me(3)Si)(2)NSi(3)Cl(7) (1), (Me(3)Si)(2)NSi(4)Cl(9) (2), (Me(3)Si)(2)N(SiCl(2))(n)N(SiMe(3))(2) (n = 3, 4; n = 4, 5; n = 5, 6), cyclo-(Me(3)Si)(2)NSi(5)Cl(9) (7), and cyclo-[(Me(3)Si)(2)N](2)Si(5)Cl(8) (8). 1-8 easily can be hydrogenated with LiAlH(4) to give the corresponding amino and diamino polysilanyl hydrides. The monosubstituted and cyclic compounds 1, 2, 7, and 8 additionally afford Si-Si bond scission products, which cannot be separated in all cases. Chloro- and dichloro derivatives of Si(3)H(8), n-Si(4)H(10), and n-Si(5)H(12) are obtained from the corresponding aminosilanes and dry HCl. All compounds were characterized by standard spectroscopic techniques. For Si-H derivatives the coupled (29)Si NMR spectra were analyzed to obtain an unequivocal structural proof. 相似文献
76.
France S Shah MH Weatherwax A Wack H Roth JP Lectka T 《Journal of the American Chemical Society》2005,127(4):1206-1215
We report a mechanistically based study of bifunctional catalyst systems in which chiral nucleophiles work in conjunction with Lewis acids to produce beta-lactams in high chemical yield, diastereoselectivity, and enantioselectivity. Chiral cinchona alkaloid derivatives work best when paired with Lewis acids based on Al(III), Zn(II), Sc(III), and, most notably, In(III). Homogeneous bifunctional catalysts, in which the catalyst contains both Lewis acidic and Lewis basic sites, were also studied in detail. Mechanistic evidence allows us to conclude that the chiral nucleophiles form zwitterionic enolates that react with metal-coordinated imines. Alternative scenarios, which postulated metal-bound enolates, were disfavored on the basis of our observations. 相似文献
77.
78.
79.
80.
Harald Schfer 《无机化学与普通化学杂志》1982,489(1):154-160
Structure and Properties of Cesium Hydroxide Monohydrate, a Compound Characterized by Layered [H3O2?] Polyanions in its High Temperature Form Cesium hydroxide monohydrate, which was formed at the synthesis of cesium hydride as a by-product, was obtained in form of single crystals by recrystallization from ammonia in high pressure autoclaves. Temperature dependent X-ray structure investigations and measurements of the specific heat show the occurance of several modifications. At 293 K X-ray data prove that it is possible to distinguish between OH? ions and H2O molecules. This can also be confirmed by IR spectroscopy. At 355 K and 400 K the investigations on single crystals show layered [H3O2?] polyanions, which are separated by layers of cesium ions in a hexagonal unit cell. 相似文献