全文获取类型
收费全文 | 7981篇 |
免费 | 324篇 |
国内免费 | 3篇 |
专业分类
化学 | 5690篇 |
晶体学 | 68篇 |
力学 | 169篇 |
数学 | 956篇 |
物理学 | 1425篇 |
出版年
2023年 | 68篇 |
2022年 | 44篇 |
2021年 | 91篇 |
2020年 | 144篇 |
2019年 | 132篇 |
2018年 | 163篇 |
2017年 | 137篇 |
2016年 | 322篇 |
2015年 | 258篇 |
2014年 | 263篇 |
2013年 | 566篇 |
2012年 | 533篇 |
2011年 | 597篇 |
2010年 | 273篇 |
2009年 | 245篇 |
2008年 | 543篇 |
2007年 | 529篇 |
2006年 | 434篇 |
2005年 | 416篇 |
2004年 | 305篇 |
2003年 | 302篇 |
2002年 | 284篇 |
2001年 | 189篇 |
2000年 | 185篇 |
1999年 | 119篇 |
1998年 | 82篇 |
1997年 | 81篇 |
1996年 | 104篇 |
1995年 | 68篇 |
1994年 | 75篇 |
1993年 | 92篇 |
1992年 | 76篇 |
1991年 | 44篇 |
1990年 | 39篇 |
1989年 | 33篇 |
1988年 | 29篇 |
1987年 | 28篇 |
1986年 | 24篇 |
1985年 | 63篇 |
1984年 | 56篇 |
1983年 | 38篇 |
1982年 | 35篇 |
1981年 | 31篇 |
1980年 | 25篇 |
1979年 | 12篇 |
1978年 | 23篇 |
1977年 | 17篇 |
1976年 | 11篇 |
1975年 | 17篇 |
1974年 | 15篇 |
排序方式: 共有8308条查询结果,搜索用时 0 毫秒
61.
[reaction: see text] Switching from a sulfinyl to a sulfonimidoyl group allows the reversal of the sense of asymmetric induction in thermal [5C + 2C] intramolecular pyrone-alkene cycloadditions. Removal of the sulfoximine unit from the resulting cycloadducts yields optically active oxabicyclic systems that are enantiomeric to those obtained using the sulfinyl chiral auxiliary. 相似文献
62.
Ramírez-Muñoz J 《Talanta》1966,13(1):87-101
Concepts of sensitivity in flame photometry are discussed. A distinction is made between sensitivity itself, concentration limits and dilution limits applied to qualitative and quantitative analysis by flame photometry. Sensitivity values, as well as the concentration limits, are considered from two different aspects: as a function of the slope of calibration curves-percentual values-and as a function of fluctutions-fluctuational values. The concepts are applied to the two main branches of flame photometry, emission and absorption. 相似文献
63.
Phosphoric acid at levels above 10-3 M depresses the S2 emission from sulphate, but concentrations up to 0.1 M may be necessary to eliminate cationic interferences. 相似文献
64.
García-Raso A Fiol JJ Tasada A Albertí FM Molins E Basallote MG Máñez MA Fernández-Trujillo MJ Sánchez D 《Dalton transactions (Cambridge, England : 2003)》2005,(23):3763-3772
Alkylidene-bis(2-aminopyrimidines) (pyr2Cx, x = 2-5) are useful ligands to interact with Ag(I) yielding discrete metallocycles. Crystal structures of the [(pyr2C2)Ag(NO3)]2 and [(H-pyr2C4)Ag(NO3)2]2 have been isolated where each macrocyclic moiety interacts with their surroundings through weak interactions, yielding 3D discrete structures, On the other hand, the solution study shows that the equilibrium constants for the formation of Ag(pyr2Cx)+ complexes are higher than the literature values for Ag(I) complexes with single pyrimidines, although the differences could be explained by invoking the solid-state structures of the Ag(I)-pyr2Cx complexes. 相似文献
65.
C. Sánchez-Pedreño M.Hernández Córdoba P.Viñas López-Pelegrín 《Microchemical Journal》1985,32(2):242-248
A kinetic method for the silver determination based on the catalytic effect of Ag(I) on the oxidation of o-dianisidine by persulfate at pH = 4.5 in the presence of 1,10-phenanthroline is proposed. Ag(I) can be determined by measuring the absorbance of the reaction product at 450 nm, using the fixed time method. Co(II) does not have a catalytic effect in the reaction, but in the simultaneous presence of silver and cobalt the reaction shows an induction period the length of which is related to the Ag/Co molar ratio. As a consequence, two new methods for the determination of cobalt and silver using the induction period method are proposed. The two methods for silver are applied to the determination of silver in lead. 相似文献
66.
Simple and rapid determination of the drug naproxen in pharmaceutical preparations by heavy atom-induced room temperature phosphorescence 总被引:1,自引:0,他引:1
Segura Carretero A Cruces-Blanco C Ramírez García MI Cañabate Díaz B Fernández Gutiérrez A 《Talanta》1999,50(2):401-407
A simple, selective and sensitive heavy atom-induced room temperature phosphorimetric method (HAI-RTP) is described for the determination of naproxen (NAP) in pharmaceutical preparations. The phosphorescence signals are a consequence of intermolecular protection when analytes are, exclusively, in presence of a heavy atom salt and sodium sulfite as an oxygen scavenger to minimize RTP quenching. These variables selection constitute the basis of a HAI-RTP method for the determination of naproxen (detection limit 17.6 ng ml(-1); 1.71% relative standard deviation at 250 ng ml(-1)). The method has been applied satisfactorily to the analysis of pharmaceutical preparations. 相似文献
67.
Peña D López F Harutyunyan SR Minnaard AJ Feringa BL 《Chemical communications (Cambridge, England)》2004,(16):1836-1837
Highly enantioselective Cu-catalysed 1,4-addition of diphenylzinc to cyclohexenone has been achieved for the first time using a monodentate phosphoramidite ligand. 相似文献
68.
Miguel A. Bañares Amador Angoso Juan L. Manzano Emilio Rodríguez 《Transition Metal Chemistry》1987,12(2):149-152
Summary Cobalt(II) chlorocomplexes with empirical formulae (bnH2)2CoCl6·2H2O (1) and (bnH2)2CoCl6 (2), where bnH
2
2+
represents the diprotonated species of putrescine (1,4-butanediamine), were prepared.The visible spectra in the solid state and measurement of the magnetic moments permit the assignation of an octahedral geometry for cobalt in (1) and a tetrahedral geometry for (2). 相似文献
69.
Rapid and direct procedures for the determination of molybdenum, chromium and aluminium in human urine samples are developed. Fast-programme methodology is used to simplify the heating cycles. Hydrogen peroxide, nitric acid and Triton X-100 are added to the urine samples which are directly introduced into the furnace. For molybdenum, two successive injection steps are required due to the low level of this element in the samples analyzed. Calibration is carried out using aqueous standards for aluminium and the standard additions method for both molybdenum and chromium. The reliability of the procedures is checked by analyzing two certified reference materials. 相似文献
70.
Peña-Alvarez A Díaz L Medina A Labastida C Capella S Vera LE 《Journal of chromatography. A》2004,1027(1-2):131-136
Steam distillation (SD) extraction-solid-phase microextraction coupled to GC-MS was developed for the determination of terpenes and Bligh-Dyer extraction-derivatization coupled with GC for the determination of fatty acids such as ethyl esters were used. It was found that the three different Agave species have the same profile of fatty acids; the quantity of these compounds is different in each Agave variety. On the other hand, different terpenes were identified in the three Agave plants studied: nine in A. salmiana, eight in A. angustifolia and 32 in A. tequilana Weber var. azul. 相似文献