全文获取类型
收费全文 | 11728篇 |
免费 | 372篇 |
国内免费 | 34篇 |
专业分类
化学 | 7813篇 |
晶体学 | 38篇 |
力学 | 162篇 |
综合类 | 1篇 |
数学 | 2151篇 |
物理学 | 1514篇 |
无线电 | 455篇 |
出版年
2020年 | 91篇 |
2019年 | 98篇 |
2016年 | 205篇 |
2015年 | 200篇 |
2014年 | 250篇 |
2013年 | 492篇 |
2012年 | 324篇 |
2011年 | 313篇 |
2010年 | 270篇 |
2009年 | 240篇 |
2008年 | 331篇 |
2007年 | 327篇 |
2006年 | 356篇 |
2005年 | 334篇 |
2004年 | 296篇 |
2003年 | 298篇 |
2002年 | 307篇 |
2001年 | 228篇 |
2000年 | 198篇 |
1999年 | 143篇 |
1998年 | 130篇 |
1997年 | 162篇 |
1996年 | 167篇 |
1995年 | 142篇 |
1994年 | 158篇 |
1993年 | 180篇 |
1992年 | 172篇 |
1991年 | 136篇 |
1990年 | 154篇 |
1989年 | 158篇 |
1988年 | 154篇 |
1987年 | 158篇 |
1986年 | 141篇 |
1985年 | 257篇 |
1984年 | 220篇 |
1983年 | 139篇 |
1982年 | 180篇 |
1981年 | 172篇 |
1980年 | 221篇 |
1979年 | 217篇 |
1978年 | 217篇 |
1977年 | 243篇 |
1976年 | 214篇 |
1975年 | 184篇 |
1974年 | 146篇 |
1973年 | 201篇 |
1972年 | 107篇 |
1971年 | 99篇 |
1970年 | 79篇 |
1933年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Two Optically Active Luteochromes: Preparation, Spectra and Chiroptical and Chromatographic Properties Selective rearrangement of (5R,6S,5′ R,6′S)-5,6:5′, 6′ -diepoxy-5,6,5′, 6′ -tetrahydro- β,β -carotene with the aid ofan ethereal solution of malonic acid leads to the mixture of the C(8′)-epimeric luteochromes. HPLC separation gave the pure isomers. This is the first report on th preparation of optically active luteochromes. 相似文献
72.
73.
Mycotoxins are secondary metabolites, formed by the action of fungi on agricultural crops in the field or during storage. These metabolites are highly toxic to animals and humans and high levels have been measured in agricultural crops. In order to evaluate human risks due to ingestion of mycotoxin-contaminated food different methods have been developed for analysis of mycotoxins in cereals and maize. In this project the focus was on mycotoxins in agricultural soil and the fate of these toxins in the soil-water-plant system. Two different mycotoxins were selected in the study: zearalenone (ZON) produced by species of Fusariumor Aspergillusand ochratoxin A (OTA) produced by species of Penicillium. We developed a method for analysis of these toxins in soil. Soil samples were extracted with methanol-water (9:1) and purified by solid-phase extraction (SPE, C8-columns). The final extract was analysed using high-pressure liquid chromatography (HPLC) with fluorescence detection. A Phenyl Hexyl column was used to separate the toxins. The detection limits obtained were 0.1 and 1.0 microg kg(-1) dry weight (dw) for OTA and ZON, respectively. The developed method has been used for analysis of different soils in connection with growth chamber experiments. The soil types used in the growth chamber experiments were a sandy soil, a sandy clay soil, and a soil with high content of organic matter. The recovery was determined as 85.8 and 93.4% and the repeatability to 5.1 and 12.8% for OTA and ZON, respectively. The reproducibility obtained was 8.5 and 15.0% for soil samples, representing concentration levels from 0.2-30 microg kg(-1) dw (OTA) and from 1.0-100 microg kg(-1) dw (ZON). 相似文献
74.
The synthesis of a series of tetra‐ and pentadentate bispidine‐type ligands (bispidine=3,7‐diazabicyclo[3.3.1]nonane) – tetradentate ligands are donor‐substituted at C(2) and C(4), pentadentate ligands have an additional donor at N(3) or N(7), with pyridine, 2‐methylpyridine, or quinoline donor moieties – and of their CuII complexes are reported, together with single‐crystal structural analyses and solution studies (electrochemistry, electronic and EPR spectroscopy). Depending on the ligand geometry and on the co‐ligands (solvent or counter anion), there are various structural forms (pseudo‐Jahn–Teller elongation along all three molecular axes), and the structural data are correlated with the spectroscopic and electrochemical parameters. 相似文献
75.
76.
77.
Hans Zimmer R. Palmer-Sungail Douglas Ho Adel Amer 《Journal of heterocyclic chemistry》1993,30(1):161-167
The reaction pathway towards formation of 4-aroyl-3-hydroxy-2(5H)-furanones 1 from the base catalyzed reaction between an acetophenone, diethyl oxalate and formaldehyde was investigated. If methoxide was used as base, a transesterification was found to occur in the first step, while a side reaction, namely the retro Claisen reaction, was in competition with the desired lactone formation in the second step. The nature of the solvent and the acidic character of 1 as well as the basicity of the aminoarylene were found to have a profound influence on the course of the reaction of 1 with aminoarylenes. 相似文献
78.
79.
80.
The gas-phase rearrangement of (1Z, 2′E)-, (1Z, 2′Z)-, (1E, 2′E)-, and (1E, 2′Z)- propenyl but-2′-enyl ether (Z, E)-, (Z, Z)-, (E, E)-, and (E, Z)-1) into erythro- and threo-2, 3-dimethyl-pent-4-en-al (erythro- and threo-2) was investigated over a temperature range from 142,5° to 190,0° at 20–35 Torr (for kinetic data and activation parameters see table 2). All four stereoisomeric ethers 1 rearrange preferentially via a chair-like transition state C into the aldehydes 2 (ΔΔG≠ (160°) = 2,5–2,7 kcal/mol for B – C (B = boat-like transition state). The relative rates (krel) for (Z, Z)-1, (Z, E)-1, (E,Z)-1, and (E,E)-1 at 160° are 1,0, 2,9, 4,3 and 9,0 respectively (see table 5). Taking into account the relative enthalpies of the ethers 1 and the steric interaction in the C transition state of the ethers 1 (see table 6), krel values can be estimated. They are in good agreement with those observed (see table 5). 相似文献