全文获取类型
收费全文 | 2013篇 |
免费 | 45篇 |
国内免费 | 4篇 |
专业分类
化学 | 1084篇 |
晶体学 | 5篇 |
力学 | 20篇 |
数学 | 565篇 |
物理学 | 388篇 |
出版年
2021年 | 20篇 |
2020年 | 23篇 |
2019年 | 24篇 |
2017年 | 18篇 |
2016年 | 30篇 |
2015年 | 30篇 |
2014年 | 41篇 |
2013年 | 68篇 |
2012年 | 55篇 |
2011年 | 81篇 |
2010年 | 52篇 |
2009年 | 38篇 |
2008年 | 63篇 |
2007年 | 71篇 |
2006年 | 67篇 |
2005年 | 51篇 |
2004年 | 37篇 |
2003年 | 40篇 |
2002年 | 41篇 |
2001年 | 36篇 |
2000年 | 27篇 |
1998年 | 24篇 |
1997年 | 25篇 |
1996年 | 37篇 |
1995年 | 39篇 |
1994年 | 35篇 |
1993年 | 28篇 |
1992年 | 35篇 |
1991年 | 25篇 |
1990年 | 22篇 |
1989年 | 31篇 |
1988年 | 30篇 |
1987年 | 23篇 |
1986年 | 22篇 |
1985年 | 34篇 |
1984年 | 15篇 |
1983年 | 16篇 |
1982年 | 14篇 |
1981年 | 15篇 |
1979年 | 19篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1975年 | 13篇 |
1974年 | 17篇 |
1973年 | 22篇 |
1970年 | 18篇 |
1969年 | 17篇 |
1968年 | 15篇 |
1966年 | 20篇 |
1923年 | 15篇 |
排序方式: 共有2062条查询结果,搜索用时 0 毫秒
1.
We prove that for every fixed k and ? ≥ 5 and for sufficiently large n, every edge coloring of the hypercube Qn with k colors contains a monochromatic cycle of length 2 ?. This answers an open question of Chung. Our techniques provide also a characterization of all subgraphs H of the hypercube which are Ramsey, that is, have the property that for every k, any k‐edge coloring of a sufficiently large Qn contains a monochromatic copy of H. © 2006 Wiley Periodicals, Inc. J Graph Theory 53: 196–208, 2006 相似文献
2.
The hydrogen-deuterium exchange of H-2 in 4-pyrimidone, a number of 1- and 3-alkyl-4-pyrimidones and several of their thione analogs in deuterium oxide at moderate temperatures is reported. This reaction is apparently not susceptible to acid or base catalysis. Cations of the 4-pyrimidones were found not to exchange H-2 at all in acid media. Catalytic amounts of bases do not alter the exchange rates whereas more concentrated alkali decompose the pyrimidone. However, 1,4(3,4)-dihydro-1,3-dimethyl-4-oxopyrimidinium iodide (and its 6-methyl analog) exchange H-2 quite rapidly in neutral deuterium oxide, almost instantly when triethylamine is added as a catalyst, and very slowly in dilute solutions of deuterium chloride in deuterium oxide. Mechanisms are proposed to account for these phenomena. 相似文献
3.
Bernard A. Mikrut Krishan K. Khullar Pamela Y. P. Chan John M. Kokosa Ludwig Bauer Richard S. Egan 《Journal of heterocyclic chemistry》1974,11(5):713-718
The reaction of pyridine 1-oxide with 1-adamantanethiol in acetic anhydride produced a mixture of 2- and 3-(1-adamantanethio)pyridines, 1-aeetyl-2-(1-adamantanethio)-3-hydroxy-4-acetoxy-1,2,3,4-telrahydropyridine and the corresponding 3-acetoxyderivative. Pure substances were separated by means of column chromatography on alumina. The tetrahydropyridines were identified by means of their proton magnetic and mass spectra. 4-(1-Adamantanethio)pyridine was synthesized from 4-chloropyridinc and 1-adamantanethiol. The three isomeric (1-adamantanethio)-pyridines were, each, cleaved by concentrated hydrochloric acid to give 1-chloroadamantane and the corresponding pyridinethiol. 相似文献
4.
5.
Nanosecond fluorescence spectroscopy has been used to study the interaction of 2-anilinonaphthalene with polar solvent molecules which is shown to result in stoichiometric complex formation at low polar solvent concentrations. This is followed by reorientation of the solvent cage when the concentration of polar solvent is high. 相似文献
6.
7.
8.
Yang Nan Subhash P. Upadhyaya Wei Xu Kathrine E. Hughes William J. Dunn Ludwig Bauer Hemendra N. Bhargava George A. Doss 《Journal of heterocyclic chemistry》1996,33(2):399-407
A number of (E)-7-arylidenenaltrexones were synthesized by azeotropic distillation of water from a benzene solution of naltrexone and an aromatic aldehyde (benzaldehyde, 4-chloro- and 4-fluorobenzaldehyde, 3-and 4-pyridinecarboxaldehyde and 1-methyl-2-imidazolecarboxaldehyde) using piperidine as a catalyst. In addition, (E)-7-benzylidenenaloxone was prepared by the previously published Claisen-Schmidt condensation using sodium hydroxide in methanol. The stereochemistry of these arylidene derivatives 3–9 was determined to be (E) by means of nuclear Overhauser enhancement experiments. The 13C nmr spectra of (E)- 3–9 are recorded in deuteriochloroform and those of the hydrochlorides in deuteriodimethyl sulfoxide. 相似文献
9.
Ludwig Maier 《Helvetica chimica acta》1968,51(7):1608-1616
Further examples of the α-aminoalkylation of white phosphorus and of a biphosphine are reported. The reaction is described using C-methyl piperidines, methylamine, ethylamine or propylamine as the amine component, and formaldehyde or benzaldehyde as the carbonyl component. The reaction mechanism has been clarified. Furthermore the direct synthesis of the diethylamine salt of phosphorous acid is reported. In addition, some physical and chemical properties of tris (dialkylamino-methyl)-phosphine oxides are given and their coordination compounds with nickel halides are described. 相似文献
10.
2-Vinylindole ( 1a ) and its donor- and acceptor-substituted (E)-derivatives 1b – e react highly locoselectively with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate ( 3 ) to form the novel (indol-2-yl)-1,4-dihydropyridazines 4a and 7 as well as the heterocyclic annellated pyridazines 4b , 5 , and 6 . The reactions of the structurally related 3-vinylindoles 2a – e with 3 also gave rise to new indol-3-ylpyridazines 8 , 9 , and 10 . The locoselectivities of these Diels-Alder reactions were controlled mainly by steric effects. 相似文献