全文获取类型
收费全文 | 2088篇 |
免费 | 73篇 |
国内免费 | 8篇 |
专业分类
化学 | 1345篇 |
晶体学 | 9篇 |
力学 | 46篇 |
数学 | 369篇 |
物理学 | 400篇 |
出版年
2023年 | 16篇 |
2022年 | 11篇 |
2021年 | 9篇 |
2020年 | 22篇 |
2019年 | 26篇 |
2018年 | 29篇 |
2017年 | 31篇 |
2016年 | 59篇 |
2015年 | 53篇 |
2014年 | 50篇 |
2013年 | 91篇 |
2012年 | 109篇 |
2011年 | 138篇 |
2010年 | 75篇 |
2009年 | 64篇 |
2008年 | 132篇 |
2007年 | 135篇 |
2006年 | 142篇 |
2005年 | 120篇 |
2004年 | 119篇 |
2003年 | 71篇 |
2002年 | 65篇 |
2001年 | 28篇 |
2000年 | 24篇 |
1999年 | 11篇 |
1998年 | 17篇 |
1997年 | 25篇 |
1996年 | 22篇 |
1995年 | 24篇 |
1994年 | 20篇 |
1993年 | 14篇 |
1992年 | 23篇 |
1991年 | 21篇 |
1990年 | 16篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1985年 | 20篇 |
1984年 | 16篇 |
1983年 | 13篇 |
1982年 | 25篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 16篇 |
1978年 | 13篇 |
1977年 | 17篇 |
1975年 | 11篇 |
1974年 | 15篇 |
1973年 | 23篇 |
1972年 | 11篇 |
1971年 | 10篇 |
排序方式: 共有2169条查询结果,搜索用时 31 毫秒
101.
Jan Scullman Gunnar Widmark 《International journal of environmental analytical chemistry》2013,93(2):29-36
A method has been developed for the determination of low concentrations of mercury in air (nanograms/m3), i.e. in the range of the believed natural levels of mercury in the atmosphere (20 ng/m3). Mercury vapour has been collected from up to 200 1 of air in glass tubes containing thin films of gold on sieved ceramic powder. In the laboratory the absorbed mercury was then released into a quartz-window cell by heating the tube in an oven at 500°C. In this paper it is demonstrated that, by using extremely thin films of precipitated gold, quantitative recovery is obtained and memory effects, which result from the use of thicker films, are avoided. 相似文献
102.
Rikard Westbom Sarah Josefsson Lennart Mathiasson Erland Björklund 《International journal of environmental analytical chemistry》2013,93(4):259-274
A method for studying PCB desorption behaviour from sediments using supercritical fluid extraction (SFE) and pressurized liquid extraction (PLE) is presented. Four sediments were investigated and extracted with supercritical carbon dioxide employing increasingly harsher extraction conditions ranging from 40°C and 12?MPa to 100°C and 36.5?MPa. To ensure quantitative extractions, the remaining SFE residues were also extracted with PLE. Resulting profiles identified at least three different PCB fractions within the four sediments. Furthermore, a distinct fraction was obtained with extraction for 2?h at 40°C and 36.5?MPa. This fraction has previously been found to correlate well with bioavailable fractions. The dependency of this fraction on sediment total organic carbon (TOC) was investigated, both for sediments examined in this study as well as for previously reported values using the same extraction conditions. It was found that TOC does not correlate to selective SFE, which indicates that the use of TOC to predict bioavailability is questionable. 相似文献
103.
Gunilla Niklasson Ingemar Kvarnström Björn Classon Bertil Samuelsson† Ulrika Nillroth Helena Danielson 《Journal of carbohydrate chemistry》2013,32(5):555-569
ABSTRACT D-Mannitol was used as precursor for the synthesis of acyclic C 2 symmetric potential HIV-1 protease inhibitors. The 1- and 6-hydroxy groups of D-mannitol were substituted by -NHBoc, -NHValZ, -SAr, -SOAr and -SO2Ar and the 2-and 5-hydroxy groups were benzylated. In some products one of the central hydroxyl groups was either inverted or deoxygenated. Despite a close structural similarity to previously published inhibitors none of the products showed significant inhibitory activity against HIV-1 protease. 相似文献
104.
Abstract FMOC-serine phenacylester was glycosylated with a derivative of the disaccharide α-D-Xylp-(1→3)-β-D-Glcp, and the product was treated with zinc to remove the phenacyl group. The title derivative is useful for synthesis of blood clotting factor IX glycopeptide fragments by the solid-phase approach. 相似文献
105.
Zhengchun Liu Björn Classon Per J. Garegg Bertil Samuelsson 《Journal of carbohydrate chemistry》2013,32(1):65-76
ABSTRACT Ammonium 2,3,6-trideoxy-2,6-epithio-D-manno-2-octenoate (8), ammonium 2,3,6-trideoxy-2,6-epithio-D-glycero-D-talo-octanoate (10a), ammonium 2,3,6-trideoxy-2,6-epithio-D-glycero-D-galacto-octanoate (10b) and ammonium 2,3,6-trideoxy-2,6-epithio-oxa-D-glycero-D-galacto-octanoate (13) have been synthesised as potential inhibitors of the enzyme CMP-KDO synthetase. The key step in the synthesis of 8 was the elimination of water from methyl 3,6-dideoxy-4,5:7,8-di-O-isopropylidene-6-thio-D-manno-2-octulosonate (4) using chlorodiphenylphosphine, imidazole and bromine to give the unsaturated methyl 2,3,6-trideoxy-2,6-epithio-4,5:7,8-di-O-isopropylidene-D-manno-2-octenoate (5). For the synthesis of 10a and 10b, zinc reduction of methyl 3,6-dideoxy-4,5:7,8-di-O-isopropylidene-6-S-(4-methoxybenzyl)-6-thio-2-O-(trichloro-tert-butoxycarbonyl)-D-manno-2-octenoate (2) gave an epimeric mixture of an α-hydroxyester 6 which was ring closed by in situ activation of the hydroxyl group using triphenylphosphine and tri-iodoimidazole followed by cleavage of the p-methoxybenzyl group to give 7a and 7b, which then were deprotected to give 10a and 10b. 相似文献
106.
Giovanna Luongo Gunnar Thorsén Conny Östman 《Analytical and bioanalytical chemistry》2014,406(12):2747-2756
A production process in which the use of various types of chemicals seems to be ubiquitous makes the textile industry a growing problem regarding both public health as well as the environment. Among several substances used at each stage, the present study focuses on the quinolines, a class of compounds involved in the manufacture of dyes, some of which are skin irritants and/or classified as probable human carcinogens. A method was developed for the determination of quinoline derivatives in textile materials comprising ultrasound-assisted solvent extraction, solid phase extraction cleanup, and final analysis by gas chromatography/mass spectrometry. Quinoline and ten quinoline derivatives were determined in 31 textile samples. The clothing samples, diverse in color, material, brand, country of manufacture, and price, and intended for a broad market, were purchased from different shops in Stockholm, Sweden. Quinoline, a possible human carcinogen, was found to be the most abundant compound present in almost all of the samples investigated, reaching a level of 1.9 mg in a single garment, and it was found that quinoline and its derivatives were mainly correlated to polyester material. This study points out the importance of screening textiles with nontarget analysis to investigate the presence of chemicals in an unbiased manner. Focus should be primarily on clothing worn close to the body. 相似文献
107.
Matthias Ermer Julian Mehler Björn Rosenberger Dr. Marcus Fischer Dr. Peter S. Schulz Prof. Martin Hartmann 《ChemistryOpen》2021,10(2):233-242
Using ionic liquids (ILs) as linker precursors, the well-known metal-organic framework (MOF) UiO-66 (Universitetet i Oslo) and the recently reported MOF hcp UiO-66 (hexagonal closed packed) have been successfully synthesized and characterized. The advantage of the applied novel synthesis approach is an economically and environmentally benign work-up procedure, due to the better solubility of the IL. Additionally, the reactivity of the terephthalate anions is increased compared to terephthalic acid, resulting in faster MOF formation with an increased amount of defects in the MOF structure. In order to explore to the influence of defects on the catalytic performance, the cyclisation of citronellal to isopulegol was employed as test reaction. The activity of hcp UiO-66 and fcc UiO-66 (face centered cubic) is improved compared to other MOF or zeolite based catalysts, while the selectivity is similar. 相似文献
108.
109.
Christian Schneider Dr. Serhiy Demeshko Prof. Dr. Franc Meyer Dr. C. Gunnar Werncke 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(20):6348-6353
A [Fe-S-Fe] subunit with a single sulfide bridging two low-coordinate iron ions is the supposed active site of the iron-molybdenum co-factor (FeMoco) of nitrogenase. Here we report a dinuclear monosulfido bridged diiron(II) complex with a similar complex geometry that can be oxidized stepwise to diiron(II/III) and diiron(III/III) complexes while retaining the [Fe-S-Fe] core. The series of complexes has been characterized crystallographically, and electronic structures have been studied using, inter alia, 57Fe Mössbauer spectroscopy and SQUID magnetometry. Further, cleavage of the [Fe-S-Fe] unit by CS2 is presented. 相似文献
110.
Grégoire Sieg Quentin Pessemesse Sascha Reith Stefan Yelin Prof. Dr. Christian Limberg Prof. Dr. Dominik Munz Dr. C. Gunnar Werncke 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(67):16760-16767
Carbonyl and iminyl based radical anions are reactive intermediates in a variety of transformations in organic synthesis. Herein, the isolation of ketyl, and more importantly unprecedented ketiminyl and aldiminyl radical anions coordinated to cobalt and iron complexes is presented. Insights into the electronic structure of these unusual metal bound radical anions is provided by X-Ray diffraction analysis, NMR, IR, UV/Vis and Mössbauer spectroscopy, solid and solution state magnetometry, as well as a by a detailed computational analysis. The metal bound radical anions are very reactive and facilitate the activation of intra- and intermolecular C−H bonds. 相似文献