全文获取类型
收费全文 | 2034篇 |
免费 | 194篇 |
国内免费 | 73篇 |
专业分类
化学 | 2240篇 |
晶体学 | 7篇 |
综合类 | 2篇 |
物理学 | 52篇 |
出版年
2023年 | 22篇 |
2022年 | 28篇 |
2021年 | 38篇 |
2020年 | 76篇 |
2019年 | 48篇 |
2018年 | 64篇 |
2017年 | 61篇 |
2016年 | 72篇 |
2015年 | 63篇 |
2014年 | 110篇 |
2013年 | 225篇 |
2012年 | 126篇 |
2011年 | 125篇 |
2010年 | 88篇 |
2009年 | 104篇 |
2008年 | 115篇 |
2007年 | 92篇 |
2006年 | 102篇 |
2005年 | 102篇 |
2004年 | 93篇 |
2003年 | 78篇 |
2002年 | 114篇 |
2001年 | 47篇 |
2000年 | 37篇 |
1999年 | 26篇 |
1998年 | 28篇 |
1997年 | 20篇 |
1996年 | 29篇 |
1995年 | 18篇 |
1994年 | 15篇 |
1993年 | 17篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 4篇 |
1988年 | 11篇 |
1987年 | 2篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 13篇 |
1979年 | 5篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1973年 | 1篇 |
1971年 | 2篇 |
1967年 | 1篇 |
排序方式: 共有2301条查询结果,搜索用时 15 毫秒
31.
选用HP-NINOWax毛细管色谱柱及氢火焰离子化检测器(FID),采用程序升温,建立了4种脂肪烷基二甲基叔胺的分析方法。结果表明,4种脂肪烷基二甲基叔胺质量浓度在0.005~1.0 g/L范围内,其峰面积与质量浓度有良好的线性关系,相关系数(R2)在0.9996以上。检出限(LODs,信噪比为3)在0.001~0.002 g/L之间,定量限(LOQs,信噪比为10)在0.003~0.005 g/L之间,回收率在90%~130%之间,相对标准偏差为1.3%~6.9%(n=6)。方法的线性范围宽、回收率高、选择性好,可用于叔胺的产品质量分析及生产过程控制分析。利用该方法对阳离子表面活性剂合成反应中十六烷基二甲基叔胺进行监测,结果很好地符合双分子亲核取代反应。该方法比滴定分析法更快速、精确,与液相色谱法相比,不需要进行柱前衍生或者使用色谱-质谱联用仪器。 相似文献
32.
Summary A derivatization procedure for the gas chromatographic analysis of bifunctional amines likeβ-adrenergic blocking drugs is described. The method consists of a two-step reaction with methyldichlorophosphine and sulfur
in presence of triethylamine to form cyclic methylphosphonothioic derivatives. The properties of these compounds are discussed
and the application of the method to the quantitation of ephedrine in human body fluids is presented.
Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984 相似文献
33.
The title compounds were synthesised and characterised as part of a study into new aromatic amines for charge transporting materials. Compounds 12-16, 20-24 and 27-34 are alkene linked triarylamines, compounds 35-36 are hydrazone derivatives and compounds 38-41 are pyrrole substituted triarylamines. Each compound was characterised by cyclic voltammetry. 相似文献
34.
Andrea Basso 《Tetrahedron letters》2004,45(3):587-590
Intramolecular Ugi reactions with bicyclic β-amino acids have been performed and the effects of the configuration and N-alkylation have been studied. We have proven that preferential ring contraction or nucleophilic attack by the solvent depend not only on the presence of N-alkylation but also on the relative disposition of the carboxyl group and the amine. Excellent results in terms of stereoselectivity have been obtained in the case of N-alkyl-3-exo-amino-7-oxabicyclo[2.2.1]-2-endo-carboxylic acids. 相似文献
35.
K. A. Frolov V. V. Dotsenko S. G. Krivokolysko V. P. Litvinov 《Russian Chemical Bulletin》2005,54(9):2226-2228
Reactions of (Z)-5-benzylidene-2-thiohydantoin with formaldehyde and primary aromatic amines afforded novel derivatives of imidazo[2,1-b][1,3,5]thiadiazine in 50–69% yields.
Dedicated to Academician N. S. Zefirov on the occasion of his 70th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2158–2160, September, 2005. 相似文献
36.
Summary Tokaj wines (Szamorodni and Aszu wines) of Hungarian origin were investigated on the basis of free amino acids and biogenic
amines. The separation and determination of these compounds was performed by an amino acid analyser equipped with an ion-exchange
resin column. The total amount of free amino acids and biogenic amines was higher in Aszu wines than in Szamorodni wines.
The main amino acids were proline and arginine, while the major biogenic amines were tyramine and putrescine. The free amino
acid and biogenic amine content of Aszu wines depended on the vineyards the wines originated from.
Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001 相似文献
37.
Sophie Guillard 《Tetrahedron》2006,62(24):5608-5616
This work describes advances in the study of the internal condensation of ammonium salts of N-acylsulfonamides. N-Acyltrifluoromethanesulfonamides show considerable advantages over the non fluorinated analogues by virtue of their higher reactivity and acidity. The reaction chemoselectivity has been investigated using a wide range of amines. The sensitivity of the reaction to steric and electronic effects confirms the potential application of these reagents in chemoselective acylation of polyamines. 相似文献
38.
A convenient general method of preparing 4-acylpyridazines (4) is reported. It involves homolytic acylation of ethyl-4-pyridazinecarboxylate yielding ethyl-5-acyl-4-pyridazinecarboxylates (2) which easily can be converted to4 by alkaline hydrolysis followed by decarboxylation. The hitherto unknown pyridazino[4,5-d]pyridazine-1 (2H) ones bearing an alkyl- or arylsubstituent on C-4 (5a-g) were prepared in quantitative yields by reaction of2 with hydrazine.
10. Mitt.:G. Heinisch, A. Jentzsch undI. Kirchner, Tetrahedron Lett.1978, 619. 相似文献
39.
A microchip CE-amperometric detection (AD) system has been fabricated by integrating a two-dimensionally adjustable CE microchip and an AD cell containing a one-dimensionally adjustable disk detection electrode in a Plexiglas holder. It facilitates the precise 3-D alignment between the channel outlet and the detection electrode without a complicated 3-D manipulator. The performance of this unique system was demonstrated by separating five aromatic amines (1,4-phenyldiamine, aniline, 2-methylaniline, 4-chloroaniline, and 1-naphthylamine) of environmental concern. Factors influencing their separation and detection processes were examined and optimized. The five analytes have been well separated within 140 s in a 74 cm long separation channel at a separation voltage of +2500 V using a 10 mM phosphate buffer (pH 3.5). Highly linear response is obtained for the five analytes over the range 20-200 microM with the detection limits ranging from 0.46 to 1.44 microM, respectively. The present system demonstrated long-term stability and reproducibility with RSDs of less than 5% for the peak current (n = 9). The new approach for the microchannel-electrode alignment should find a wide range of applications in CE, flowing injection analysis, and other microfluidic analysis systems. 相似文献
40.
5,6,7,8-Tetrafluoro-4-hydroxycoumarin reacted with benzylamine under mild conditions to give a stable salt, while its refluxing
with aniline or benzylamine in xylene afforded 5,6,7,8-tetrafluoro-4-phenyl(benzyl)aminocoumarins. Reactions of 3-acetyl(acetimidoyl)-5,6,7,8-tetrafluoro-4-hydroxycoumarins
with benzylamine followed different pathways, depending on the solvent. Condensation at the acyl substituent can be accompanied
by replacement of the F atom in position 7. 3-Acetylcoumarin formed a salt, while 3-acetimidoylcoumarin yielded a 7-monosubstituted
product. 3-Acetyl(acetimidoyl)-5,6,7,8-tetrafluoro-4-hydroxycoumarins reacted with aniline to give only 5,6,7,8-tetrafluoro-4-hydroxy-3-(N-phenylacetimidoyl)coumarin.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1170–1174, July, 2006. 相似文献