全文获取类型
收费全文 | 669篇 |
免费 | 19篇 |
国内免费 | 16篇 |
专业分类
化学 | 617篇 |
晶体学 | 4篇 |
力学 | 2篇 |
综合类 | 2篇 |
数学 | 3篇 |
物理学 | 76篇 |
出版年
2024年 | 1篇 |
2023年 | 8篇 |
2022年 | 21篇 |
2021年 | 24篇 |
2020年 | 18篇 |
2019年 | 13篇 |
2018年 | 7篇 |
2017年 | 13篇 |
2016年 | 21篇 |
2015年 | 15篇 |
2014年 | 14篇 |
2013年 | 44篇 |
2012年 | 20篇 |
2011年 | 23篇 |
2010年 | 18篇 |
2009年 | 27篇 |
2008年 | 15篇 |
2007年 | 30篇 |
2006年 | 32篇 |
2005年 | 23篇 |
2004年 | 22篇 |
2003年 | 27篇 |
2002年 | 162篇 |
2001年 | 12篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1972年 | 1篇 |
1970年 | 2篇 |
1969年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有704条查询结果,搜索用时 15 毫秒
71.
It is shown for the first time that the Wurtz reaction can be realized by the action of an organic substance (triphenylphosphine) on an alkyl halide (perfluoroalkyl iodide) without the use of metals. It was found that when trying to prepare the bis(3,3′-aminophenyl)(fluoroalkyl)phosphine oxide by four-step synthesis, in the first stage of the reaction of perfluoro-1-iodohexane with triphenylphosphine does not proceed towards the formation of an intermediate quaternary phosphonium salt. Instead, the carbon chain of perfluoroalkyl iodide dimerizes to form perfluorododecane – Wurtz reaction product. We have proposed a new pathway for homocoupling of perfluoroalkylhalides into even-numbered perfluoroalkanes. 相似文献
72.
María P. Elizalde Sonia Gómez-Lavín Ane M. Urtiaga 《International journal of environmental analytical chemistry》2018,98(15):1423-1433
ABSTRACTMigration tests of perfluoroalkyl substances (PFASs) from a grease-proof paperbag used for packaging of pet food have been carried out. No migration of perfluorocaboxylic acids (PFCAs), from PFBA to PFTeDA, to the simulant Tenax® was found after 10 days at 40°C. However, the increase of temperature in the range 80–160°C gave rise to the migration of the PFCAs. Finally, the migration to real foods such as lyophilised whole and low-fat milk samples at 80 and 120°C was studied. The results indicate that the migration percentages of the PFCASs into food samples are much higher than those obtained into Tenax®. 相似文献
73.
《Journal of separation science》2018,41(6):1274-1286
New psychoactive substances are defined as compounds with consciousness‐changing effects and have been developed simultaneously with classical drugs. They arise through structural modifications of illegal substances and are mainly produced to circumvent laws. Availability is simple, since new psychoactive substances can be purchased from the Internet. Among them many chemical drug compound classes are chiral and thus the two resulting enantiomers can differ in their effects. The aim of this study is to develop a suitable chiral high‐performance liquid chromatography separation method for a broad spectrum of new psychoactive substances using cellulose tris(3,5‐dichlorophenylcarbamate) as a chiral selector. Experiments were performed by high‐performance liquid chromatography in normal‐phase mode under isocratic conditions using ultraviolet detection. Direct separation was carried out on a high‐performance liquid chromatography column (Lux® i‐Cellulose‐5, 3.5 μm, Phenomenex®), available since 2016. Excellent separation results were obtained for cathinones. After further optimization, even 47 instead of 39 out of 52 cathinones showed baseline separation. For amphetamine derivatives, satisfactory results were not achieved. Further, new psychoactive substances from other compound classes such as benzofuranes, thiophenes, phenidines, phenidates, morpholines, and ketamines were partially resolved, depending on the polarity and degree of substitution. All analytes, which were mainly purchased from the Internet, were proven to be traded as racemates. 相似文献
74.
75.
Schmid T Burkhard J Yeo BS Zhang W Zenobi R 《Analytical and bioanalytical chemistry》2008,391(5):1899-1905
Due to their direct influence on the stability of bacterial biofilms, a better insight into the nanoscopic spatial arrangement
of the different extracellular polymeric substances (EPS), e.g., polysaccharides and proteins, is important for the improvement
of biocides and for process optimization in wastewater treatment and biofiltration. Here, the first application of a combination
of confocal laser-scanning microscopy (CLSM) and atomic force microscopy (AFM) to the investigation of river-water biofilms
and related biopolymers is presented. AFM images collected at selected areas of CLS micrographs dramatically demonstrate the
heterogeneity of biofilms at the nanometer scale and the need for a chemical imaging method with nanoscale resolution. The
nanostructures (e.g., pili, flagella, hydrocolloids, and EPS) found in the extracellular matrix are classified according to
shape and size, which is typically 50–150 nm in width and 1–10 nm in thickness, and sets the demands regarding spatial resolution
of a potential chemical imaging method. Additionally, thin layers of the polysaccharide alginate were investigated. We demonstrate
that calcium alginate is a good model for the EPS architecture at the nanometer scale, because of its similar network-like
structure.
Figure CLSM-AFM allows imaging of nanometer-sized extracellular structures 相似文献
76.
Grabarczyk M 《Analytical and bioanalytical chemistry》2008,390(3):979-986
A simple and fast catalytic adsorptive stripping voltammetric procedure for trace determination of Cr(VI) in natural samples
containing high concentrations of humic substances has been developed. The procedure for chromium determination in the presence
of DTPA and nitrates was employed as the initial method. In order to enhance the selectivity vs. Cr(III) the measurements
were performed at 40°C. Interference from dissolved organic matter such as humic and fulvic acids was drastically decreased
by adding Amberlite XAD-7 resin to the voltammetric cell before the deaeration step. The whole procedure was applied to a
single cell, which allowed monitoring of the voltammetric scan. Optimum conditions for removing humic and fulvic acids due
to their adsorption on XAD-7 resin were evaluated. The use of XAD-7 resin also minimize interferences from various cationic,
anionic, and nonionic surfactants. The calibration graph for Cr(VI) for an accumulation time of 30 s was linear in the range
5 × 10−10 to 5 × 10−8 mol L−1. The relative standard deviation for determination of Cr(VI) at a concentration of 1 × 10−8 mol L−1 was 3.5% (n = 5). The detection limit estimated from 3 times the standard deviation for low Cr(VI) concentrations and an accumulation
time of 30 s was about 1.3 × 10−10 mol L−1. The proposed method was successfully applied to Cr(VI) determination at trace levels in soil samples. 相似文献
77.
78.
Study of alloxydim photodegradation in the presence of natural substances: Elucidation of transformation products 总被引:1,自引:0,他引:1
Beatriz Sevilla-Morn Pilar Sandín-Espaa M. Jesús Vicente-Arana Jos L. Alonso-Prados Jos M. García-Baudín 《Journal of photochemistry and photobiology. A, Chemistry》2008,198(2-3):162-168
A photodegradation study of alloxydim was performed under simulate solar irradiation (Suntest apparatus) at different irradiation intensities. Moreover, indirect photolysis of the tested herbicide was investigated under the presence of various concentrations of humic acids (HA), nitrate and Fe (III) ions. The photodegradation of alloxydim follows a first-order reaction kinetics in all cases. HA inhibited the photolysis kinetic whereas rate constants measured in the presence of nitrate ions indicated no effect on degradation. On the contrary, Fe (III) ions enhanced the photodegradation rate of alloxydim. Kinetics experiments were monitored by HPLC–DAD and the half-lives ranged from 165.78 to 4.63 min for different intensities in direct photolysis and from 104.81 to 1.14 min for indirect photolysis. The study of transformation products have been investigated by HPLC coupled to quadrupole time-of-flight mass spectrometry (QTOF-MS) employing the electrospray technique.The most important transformation process was found to be the cleavage of the O–N bond of the oxime moiety. Minor photo-isomerization to Z-isomer was also observed. The appearance of these degradation products is reported in aqueous media for the first time. 相似文献
79.
Gómez-González Y Surratt JD Cuyckens F Szmigielski R Vermeylen R Jaoui M Lewandowski M Offenberg JH Kleindienst TE Edney EO Blockhuys F Van Alsenoy C Maenhaut W Claeys M 《Journal of mass spectrometry : JMS》2008,43(3):371-382
In the present study, we have characterized in detail the MS(2) and MS(3) fragmentation behaviors, using electrospray ionization (ESI) in the negative ion mode, of previously identified sulfated isoprene secondary organic aerosol compounds, including 2-methyltetrols, 2-methylglyceric acid, 2-methyltetrol mononitrate derivatives, glyoxal and methylglyoxal. A major fragmentation pathway for the deprotonated molecules of the sulfate esters of 2-methyltetrols and 2-methylglyceric acid and of the sulfate derivatives of glyoxal and methylglyoxal is the formation of the bisulfate [HSO(4)](-) anion, while the deprotonated sulfate esters of 2-methyltetrol mononitrate derivatives preferentially fragment through loss of nitric acid. Rational interpretation of MS(2), MS(3) and accurate mass data led to the structural characterization of unknown polar compounds in K-puszta fine aerosol as organosulfate derivatives of photooxidation products of unsaturated fatty acids, i.e. 2-hydroxy-1,4-butanedialdehyde, 4,5- and 2,3-dihydroxypentanoic acids, and 2-hydroxyglutaric acid, and of alpha-pinene, i.e. 3-hydroxyglutaric acid. The deprotonated molecules of the sulfated hydroxyacids, 2-methylglyceric acid, 4,5- and 2,3-dihydroxypentanoic acid, and 2- and 3-hydroxyglutaric acids, showed in addition to the [HSO(4)](-) ion (m/z 97) neutral losses of water, CO(2) and/or SO(3), features that are characteristic of humic-like substances. The polar organosulfates characterized in the present work are of climatic relevance because they may contribute to the hydrophilic properties of fine ambient aerosol. In addition, these compounds probably serve as ambient tracer compounds for the occurrence of secondary organic aerosol formation under acidic conditions. 相似文献
80.
Marina Briand Elsa Anselmi Guillaume Dagousset Emmanuel Magnier 《Chemical record (New York, N.Y.)》2023,23(9):e202300114
Over the last years, methods devoted to the synthesis of asymmetric molecules bearing a perfluoroalkylated chain have been limited in number. Among them, only a few can be used on a large variety of scaffolds. This microreview aims at summarizing these recent advances in enantioselective perfluoroalkylation (−CF3, −CF2H, −CnF2n+1) and highlights the need for new enantioselective methods to easily synthesize chiral fluorinated molecules which would be useful for the pharmaceutical and agrochemical industries. Some perspectives are also mentioned. 相似文献