全文获取类型
收费全文 | 1161篇 |
免费 | 53篇 |
国内免费 | 6篇 |
专业分类
化学 | 759篇 |
晶体学 | 7篇 |
力学 | 32篇 |
数学 | 190篇 |
物理学 | 232篇 |
出版年
2023年 | 9篇 |
2022年 | 20篇 |
2021年 | 26篇 |
2020年 | 33篇 |
2019年 | 36篇 |
2018年 | 25篇 |
2017年 | 14篇 |
2016年 | 37篇 |
2015年 | 46篇 |
2014年 | 45篇 |
2013年 | 65篇 |
2012年 | 94篇 |
2011年 | 111篇 |
2010年 | 57篇 |
2009年 | 51篇 |
2008年 | 88篇 |
2007年 | 72篇 |
2006年 | 57篇 |
2005年 | 48篇 |
2004年 | 40篇 |
2003年 | 28篇 |
2002年 | 22篇 |
2001年 | 10篇 |
2000年 | 13篇 |
1999年 | 7篇 |
1998年 | 12篇 |
1997年 | 6篇 |
1995年 | 8篇 |
1994年 | 5篇 |
1993年 | 9篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 9篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1981年 | 5篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 3篇 |
1971年 | 4篇 |
1969年 | 5篇 |
排序方式: 共有1220条查询结果,搜索用时 0 毫秒
21.
[reaction: see text] The temperature dependence of the dissociation of dimers formed from highly stabilized carbon-centered radicals has been examined. Analysis of the data yields the bond dissociation energy (BDE) for the central head-to-head C-C bond in these compounds. For example, for the dimer derived from 3-phenyl-2-coumaranone, BDE is 23.6 kcal/mol and the C-C bond length 1.596 A, a rather long value for a sigma bond. 相似文献
22.
A
cha Ali-Bey Hasna Fatiha Loukil Gilles Gosselin Andr Mathieu Abderrezzak Mesli 《Journal of heterocyclic chemistry》1982,19(3):617-624
Hydrolysis of the imine function of a series of Schiff bases derived from pyrrole-2-carboxaldehyde and substituted anilines (X = H, p-OCH3, p-OC2H5, p-CH3, p-Cl, p-Br, m-CN, m-NO2, p-NO2) was studied in all of the pH ranges. The hydrolysis curves log kabs (mn?1) = f(pH) were established in buffered aqueous methanol by polarography or amperometry. The shapes of the curves obtained for pH > 5 indicates that N-pyrrolylmethylene-2 anilines hydrolyse according to the same mechanism as N-benzylidene anilines. The particular stability of these products for pH < 5 permits one to obtain complete hydrolysis curves in acid media. A very good Hammett correlation (kobs = + 1,73) has been established from the maxima which appear for strong acidity. This stability is interpreted as due to the specific electron-donating effect of the pyrrole nucleus. The influence of the structural parameter X on the morphology of the curves log kabs = f(pH) and on the hydrolysis mechanism of the imine function is discussed in all of the pH ranges. 相似文献
23.
24.
Iron-catalyzed homo-coupling of simple and functionalized arylmagnesium reagents is described. The reaction is highly chemoselective (CN, COOEt and NO(2) groups are tolerated). The procedure was used to perform intramolecular couplings. This cyclization reaction is the key step of the total synthesis of the N-methylcrinasiadine. 相似文献
25.
26.
27.
Trombotto S Danel M Fitremann J Bouchu A Queneau Y 《The Journal of organic chemistry》2003,68(17):6672-6678
The base-catalyzed reaction of carboxymethyl 3,4,6-tri-O-acetyl-alpha-D-glucopyranoside 2-O-lactone (prepared from isomaltulose) with amino acids and fatty amines under basic catalysis gave a series of new pseudoglucopeptides, nonionic amphiphiles, and polymerizable derivatives. The same reaction applied to alcohols provided the corresponding 2-(alpha-D-glucopyranosyloxy)acetyl esters with either basic or acidic catalysts. 相似文献
28.
A new series of π-conjugated oligomers based on various combinations of thiophene and EDOT units and double bonds has been synthesized by Wittig-Horner reactions from phosphonate anions carrying EDOT or bis-EDOT units. Optical and electrochemical results evidence the crucial role of the EDOT moiety for modulating the electronic properties of the oligomers. The insertion of bis-EDOT unit in the middle of the molecule leads to a self-rigidification of the conjugated system due to non covalent S?O intramolecular interactions. The strong electron donor effect of the EDOT units explains the determining role of the relative position of the EDOT units on the localization and stabilization of the positive charges in the radical cation or dication states. 相似文献
29.
30.
We report on an innovative configuration of a supercontinuum laser based on an Yb-doped nonlinear microstructured fiber. Due to this particular fiber design combining amplification and nonlinear properties, a wideband output spectrum is generated in the fiber under the Q-switched regime. By exploiting stimulated Brillouin scattering, high peak power pulses are achieved and contribute to a spectral broadening due to a nonlinear wavelength conversion process. Both infrared (1000-1200 nm) and visible (650-750 nm) spectra are generated with an average power of 200 mW. 相似文献