全文获取类型
收费全文 | 1945篇 |
免费 | 49篇 |
国内免费 | 6篇 |
专业分类
化学 | 1362篇 |
晶体学 | 21篇 |
力学 | 29篇 |
数学 | 112篇 |
物理学 | 476篇 |
出版年
2021年 | 17篇 |
2020年 | 17篇 |
2019年 | 26篇 |
2018年 | 22篇 |
2016年 | 31篇 |
2015年 | 29篇 |
2014年 | 36篇 |
2013年 | 86篇 |
2012年 | 78篇 |
2011年 | 102篇 |
2010年 | 51篇 |
2009年 | 58篇 |
2008年 | 89篇 |
2007年 | 102篇 |
2006年 | 104篇 |
2005年 | 109篇 |
2004年 | 93篇 |
2003年 | 80篇 |
2002年 | 97篇 |
2001年 | 30篇 |
2000年 | 28篇 |
1999年 | 34篇 |
1998年 | 26篇 |
1997年 | 19篇 |
1996年 | 24篇 |
1995年 | 26篇 |
1994年 | 26篇 |
1993年 | 20篇 |
1992年 | 27篇 |
1991年 | 18篇 |
1990年 | 11篇 |
1989年 | 17篇 |
1988年 | 22篇 |
1987年 | 17篇 |
1986年 | 23篇 |
1985年 | 21篇 |
1984年 | 34篇 |
1983年 | 20篇 |
1982年 | 20篇 |
1981年 | 27篇 |
1980年 | 21篇 |
1979年 | 30篇 |
1978年 | 13篇 |
1977年 | 20篇 |
1976年 | 18篇 |
1975年 | 27篇 |
1974年 | 27篇 |
1973年 | 19篇 |
1972年 | 19篇 |
1965年 | 12篇 |
排序方式: 共有2000条查询结果,搜索用时 31 毫秒
21.
Infrared and Raman spectra of 1,2-dibromoethane CH2BrCH2Br and CD2BrCD2Br were observed in the liquid state, and the fundamental frequencies were determined by comparison with those of related molecules. Infrared absorption intensities of fundament bands were measured in the liquid state, and the intensity data were interpreted on the basis of the valence-optical theory. From the converged value of a population ratio, the energy difference between the trans and gauche isomers was determined, which was in good agreement with the value obtained from the temperature effect of the IR spectrum. 相似文献
22.
Aldehyde-2,4-dinitrophenylhydrazones exist as (E)- and (Z)-geometrical isomers, and adventitious isomerization during sample preparation can cause analytical errors. Purified alkenal-2,4-dinitrophenylhydrazone derivatives comprise only the (E)-isomer. However, partial isomerization to the (Z)-isomer occurs upon addition of acid to attain an equilibrium isomer ratio. The UV-visible spectral properties of the isomers differ; the (Z)-isomer exhibiting a 6-10 nm lower absorption maximum compared to the (E)-isomer. Alkenal-2,4-dinitrophenylhydrazones having a CC double bond at the 2- or 3-position of the alkenal exhibited similar absorption maxima with an equilibrium isomer ratio (0.035) that was much lower than those of other alkenals. The CC double bond at the 3-position migrates to a position of conjugation with the CN double bond during hydrazone synthesis to form a stabilized molecular structure. Alkenal-2,4-dinitrophenylhydrazones having a double bond at the 4-position or greater exhibited a similar absorption maxima equilibrium isomer ratio (0.14) to alkanal-2,4-dinitrophenylhydrazones. The quantitative analysis of carbonyl compounds in air or water using DNPH is usually conducted in the presence of an acid catalyst. Consequently, the solution of the direct extract prepared for HPLC or GC analysis contains both (E)- and (Z)-isomers. 相似文献
23.
A simple synthesis of 3,3,3-trifluoropropenyl compounds by means of the TBAF-mediated Horner reaction is described. The reagent, 2,2,2-trifluoroethyldiphenylphosphine oxide, was readily prepared either by Arbuzov reaction of ethyl diphenylphosphinite with 2,2,2-trifluoroethyl iodide or by treating chlorodiphenylphosphine with trifluoroacetic acid and water. Treatment of the phosphine oxide with aromatic aldehydes in the presence of TBAF at room temperature afforded the corresponding 3,3,3-trifluoropropenyl compounds in good yields. The present method is very convenient for preparing 3,3,3-trifluoropropenyl compounds from aromatic aldehydes in terms of availability of the reagent, operational simplicity, and good yields of the products. 相似文献
24.
Cyclopropyltriphenylphosphonium bromide (2a) was conveniently prepared from 3-bromopropyltriphenylphosphonium bromide (1a). The Wittig reaction of cyclopropyltriphenylphosphonium bromide (2a) with carbonyl compounds gave alkylidenecyclopropanes (4, 6 and 7). Successive treatment of 1a with two equivalents of base and carbonyl compounds gave alkylidenecyclopropanes (4 and 5) without isolation of intermediary 2a. 2-Methylcyclopropyltriphenylphosphonium bromide (2b) was prepared and allowed to react with carbonyl compounds. 相似文献
25.
Sadao Arai Masanori Ishikura Kiyoshi Sato Takamichi Yamagishi 《Journal of heterocyclic chemistry》1995,32(3):1081-1083
The 16c-azonia[6]helicene salts, in which a carbon atom at the inner helix skeleton of [6]helicene is replaced by a quaternary nitrogen atom, have been synthesized by photo-induced intramolecular quater-nization of 2-[2-(1-chloronaphthyl)vinyl]benzo[h]quinolines in acetonitrile. 相似文献
26.
Bile acids, such as cholic, chenodeoxycholic, deoxycholic, lithocholic and ursodeoxycholic acids, were allowed to react with hexafluoroisopropanol and tri-fluoracetic anhydride at 37 for 30 min. The resulting derivatives were gas chromatographed on QF-1, with flame ionization detection, and were identified by gas chromatography-mass spectrometry. Separation was good. By using this method, these acids were detected in samples of human duodenal fluid; the ratios of each were 24.4, 41.5, 24.9, 2.3 and 6.9%, respectively. 相似文献
27.
The nitro group in tertiary or secondary aliphatic nitro compounds is replaced by hydrogen or deuterium on treatment with tributyltin hydride or tributyltin deuteride, respectively. 相似文献
28.
Masatoshi Sakae Masanori Katsurada Mayumi Watanabe Fumio Yoneda Sadao Nishigaki Toshimasa Ishida 《Journal of heterocyclic chemistry》1997,34(1):157-160
The reaction of 2-bromoethylamine 1 with methylisothiocyanate 2 under mild condition gave 2-methyl-amino-2-thiazoline 3 as the major product together with two kinds of byproducts, 3-(N-methylthiocar-bamoyl)-2-methyliminothiazolidine 4 and N,N′-dimethyl-N-(2-thiazolin-2-yl)thiourea 5. Thermal isomer-ization of 5 to 4 was observed. The structures of the byproducts were confirmed by X-ray crystallography. 相似文献
29.
Novel synthetic procedure of heterocycles was developed using nickel-mediated alkylative carboxylation, and the total synthesis of erythrocarine was achieved using this method and RCM of dienyne as the key steps. [reaction: see text] 相似文献
30.
This review covers newly isolated simple indole alkaloids, structure determinations, total syntheses and biological activities reported in the literature in 2003. 相似文献