全文获取类型
收费全文 | 3199篇 |
免费 | 120篇 |
国内免费 | 28篇 |
专业分类
化学 | 2144篇 |
晶体学 | 19篇 |
力学 | 74篇 |
数学 | 559篇 |
物理学 | 551篇 |
出版年
2023年 | 24篇 |
2022年 | 44篇 |
2021年 | 75篇 |
2020年 | 71篇 |
2019年 | 73篇 |
2018年 | 71篇 |
2017年 | 60篇 |
2016年 | 111篇 |
2015年 | 82篇 |
2014年 | 112篇 |
2013年 | 225篇 |
2012年 | 207篇 |
2011年 | 257篇 |
2010年 | 165篇 |
2009年 | 135篇 |
2008年 | 189篇 |
2007年 | 184篇 |
2006年 | 178篇 |
2005年 | 158篇 |
2004年 | 114篇 |
2003年 | 90篇 |
2002年 | 96篇 |
2001年 | 46篇 |
2000年 | 45篇 |
1999年 | 24篇 |
1998年 | 28篇 |
1997年 | 31篇 |
1996年 | 34篇 |
1995年 | 22篇 |
1994年 | 22篇 |
1993年 | 26篇 |
1992年 | 17篇 |
1991年 | 30篇 |
1990年 | 19篇 |
1989年 | 11篇 |
1988年 | 20篇 |
1987年 | 10篇 |
1986年 | 29篇 |
1985年 | 27篇 |
1984年 | 21篇 |
1983年 | 19篇 |
1982年 | 28篇 |
1981年 | 23篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1975年 | 8篇 |
1974年 | 7篇 |
排序方式: 共有3347条查询结果,搜索用时 15 毫秒
61.
P. Cloudy J. M. Létoffé P. Germain J. P. Bastide A. Bayol S. Blasquez R. C. Rao B. Gonzalez 《Journal of Thermal Analysis and Calorimetry》1991,37(11-12):2497-2506
Study and characterization of molecular complexes between cholesterol and beta cyclodextrin has been done using X-ray diffraction, thermogravimetric analysis (TG), differential scanning calorimetry (DSC) and nuclear magnetic resonance spectroscopy (13C NMR). Whatever the value of the molar ratio cholesterol/βCD used during the preparation, the same compound is always obtained. Corresponding to a molar ratio 1/3 (cholesterol/βCD), this compound is a stable hydrate which, contrary toβCD, contains at room temperature a large amount of molecules of water. It can be dehydrated under low pressure but the thermal degradation occurs at 200°C (250°C forβCD). This implies that cholesterol is strongly bounded toβCD. 相似文献
62.
Alkaloids such as cinchonidine, quinine and N-methylephedrine have been N-alkylated using polymeric benzyl halides or co-polymerized and then N-alkylated, thus affording a series of polymer-supported chiral ammonium salts which have been employed as phase-transfer catalysts in the asymmetric benzylation of an N-(diphenylmethylene)glycine ester. These new polymeric catalysts can be easily recovered by simple filtration after the reaction and reused. The best ee's were achieved when Merrifield resin-anchored cinchonidinium ammonium salts were employed. 相似文献
63.
Analysis of sterol oxidation products in foods 总被引:1,自引:0,他引:1
The main aspects related to the analysis of sterol oxidation products (SOP) in foods are comprehensively reviewed. Special emphasis is placed on the critical and controversial points of this analysis because these points affect crucial analytical parameters such as precision, accuracy, selectivity, and sensitivity. The effect of sample preparation and the conditions of quantification by gas chromatography and liquid chromatography on these parameters are also reviewed. The results show that, in order to choose an adequate method to analyze SOP in a certain food, the analyst must consider its SOP concentration and matrix complexity. The term SOP includes both cholesterol oxidation products (COP) and phytosterol oxidation products (POP). The state of the art of COP and POP analysis is quite different; many more studies have dealt with the analysis of COP than of POP. However, most of the results presented here about COP analysis may be extrapolated to POP analysis because both groups of compounds show similar structures and characteristics. 相似文献
64.
Alvarez-Bercedo P Bond AD Haigh R Hopkins AD Lawson GT McPartlin M Moncrieff D Mosquera ME Rawson JM Woods AD Wright DS 《Chemical communications (Cambridge, England)》2003,(11):1288-1289
The reaction of CyPHNa with Sn(NMe2)2 in the presence of PMDETA (= (Me2NCH2CH2)2NMe) gives the title compound [(Sn(mu-PCy))3(Na x PMDETA)2] (1), containing an electron-deficient [(Sn(mu-PCy))]3(2-) dianion with a novel two-electron, three centre (2e-3c) bonding arrangement. 相似文献
65.
Energy controlled insertion of polar molecules in dense fluids 总被引:1,自引:0,他引:1
De Fabritiis G Delgado-Buscalioni R Coveney PV 《The Journal of chemical physics》2004,121(24):12139-12142
We present a method to search low energy configurations of polar molecules in the complex potential energy surfaces associated with dense fluids. The search is done in the configurational space of the translational and rotational degrees of freedom of the molecule, combining steepest-descent and Newton-Raphson steps which embed information on the average sizes of the potential energy wells obtained from prior inspection of the liquid structure. We perform a molecular dynamics simulation of a liquid water shell which demonstrates that the method enables fast and energy-controlled water molecule insertion in aqueous environments. The algorithm finds low energy configurations of incoming water molecules around three orders of magnitude faster than direct random insertion. This method represents an important step towards dynamic simulations of open systems and it may also prove useful for energy-biased ensemble average calculations of the chemical potential. 相似文献
66.
[reaction: see text] Condensation of N-substituted glycines with chiral 3-allyl-2-formyl perhydro-1,3-benzoxazines forms an azomethine ylide that cyclizes to give octahydropyrrolo[3,4-b]pyrrole derivatives. The [3 + 2] dipolar cycloadditions are stereoespecific leading to a single diastereoisomer. The chemical yields are dependent on the reaction temperature and the presence or absence of a base. 相似文献
67.
Y. BallesterosM.J. Gonzalez de la Huebra M.C. QuintanaP. Hernandez L. Hernandez 《Microchemical Journal》2003,74(2):193-202
The electrochemical behaviour of kinetin (6-furfurylaminopurine) on a carbon paste modified with OV-17 silicone electrode, is studied. The determination of kinetin is possible working in square wave voltammetric techniques, reaching limits of determination of 38.7 ng ml−1. The proposed method was successfully applied to determine the cytokinin in extracts of apples (previously spiked with kinetin) and the obtained results were in accordance with the results obtained with HPLC-UV. 相似文献
68.
Treatment of steroid sapogenins with diacetoxyiodobenzene (DIB) and boron trifluoride ethyl etherate in acetic acid led to the introduction of an axial acetoxyl group at position C-23 of the side chain. 相似文献
69.
Angel Alberola Celia Andrs Alfonso Gonzlez Ortega Rafael Pedrosa Martina Vicente 《Journal of heterocyclic chemistry》1987,24(3):709-713
Unsymmetrically substituted β-aminoenones react with malononitrile, cyanomethylphenylsulfone, benzoyl-acetonitrile and ethyl cyanoacetate, in very mild conditions, to yield regiospecifically 3-functionalized 2(1H)-pyridones in high yields. 相似文献
70.
Sandra Jiménez-Alonso Ana Estévez-Braun Ángel G. Ravelo Rafael Zárate Matías López 《Tetrahedron》2007,63(14):3066-3074
Several bis-pyrano-l,4-benzoquinones have been synthesized by a double domino Knoevenagel hetero Diels-Alder reaction. The synthetic approach is highly efficient allowing the construction of complex polycyclic scaffolds with six new σ-bonds. These reactions performed more efficiently and more rapidly using microwave irradiation. The resulting bis-pyrano-1,4-benzoquinones are the first examples of a double domino Knoevenagel hetero Diels-Alder reaction. Our approach represents a novel contribution to the chemistry of 2,5-dihydroxy-l,4-benzoquinones and the first general method for the synthesis of bis-pyranobenzoquinones. 相似文献