全文获取类型
收费全文 | 1871篇 |
免费 | 99篇 |
国内免费 | 22篇 |
专业分类
化学 | 1384篇 |
晶体学 | 17篇 |
力学 | 37篇 |
数学 | 304篇 |
物理学 | 250篇 |
出版年
2023年 | 19篇 |
2022年 | 34篇 |
2021年 | 44篇 |
2020年 | 53篇 |
2019年 | 50篇 |
2018年 | 41篇 |
2017年 | 52篇 |
2016年 | 88篇 |
2015年 | 75篇 |
2014年 | 80篇 |
2013年 | 131篇 |
2012年 | 134篇 |
2011年 | 164篇 |
2010年 | 103篇 |
2009年 | 83篇 |
2008年 | 123篇 |
2007年 | 108篇 |
2006年 | 106篇 |
2005年 | 83篇 |
2004年 | 78篇 |
2003年 | 48篇 |
2002年 | 46篇 |
2001年 | 22篇 |
2000年 | 17篇 |
1999年 | 24篇 |
1998年 | 10篇 |
1997年 | 12篇 |
1996年 | 28篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 13篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 9篇 |
1980年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1970年 | 3篇 |
1968年 | 2篇 |
1958年 | 6篇 |
1957年 | 2篇 |
排序方式: 共有1992条查询结果,搜索用时 31 毫秒
991.
Ľubomír Pikna Mária Heželová Silvia Demčáková Miroslava Smrčová Beatrice Plešingerová Michal Štefanko Mária Turáková Milan Králik Pavel Puliš Peter Lehocký 《Chemical Papers》2014,68(5):591-598
The effect of two types of catalysts on the activity of the catalytic hydrogenation of nitrobenzene was studied. Catalysts were prepared by the surface deposition of palladium hydroxide with a simultaneous reduction with formaldehyde in a basic environment and were characterised by X-ray powder diffraction, transmission electron microscopy, adsorption-desorption, and catalytic tests — hydrogenation of nitrobenzene in methanol. The influence of the supports’ (activated carbon and a mixture of activated carbon and multi-walled carbon nanotubes) surface area is discussed. Despite having a size comparable (4–5 nm) to crystallites of metallic palladium, the catalyst prepared on a mixture of activated carbon and nanotubes (Pd/C/CNT) was significantly less active than the catalyst prepared on pure activated carbon (Pd/C); the rate of this reaction was approximately 30 % lower than the initial reaction rate. This feature could be attributed to the lower specific surface area of the Pd/C/CNT (531 m2 g?1) in comparison with the Pd/C (692 m2 g?1). 相似文献
992.
Alexander Búcsi Janka Karlovská Michal Chovan Ferdinand Devínsky Daniela Uhríková 《Chemical Papers》2014,68(6):842-846
The pK a values of N-alkyl-N,N-dimethylamine-N-oxides were determined from the pH dependences of chemical shifts in 1H NMR and 13C NMR spectra. The dependences were measured at concentrations below and above the critical micelle concentration of the amine oxides. Above the critical micelle concentration, the plots of the peak positions vs. pH were either sigmoid (the groups close to the nitrogen) or “peak type” (farther groups). The sigmoidal behaviour was a direct result of the acid-base reaction; on the other hand, the “peak type” behaviour was probably an indirect consequence of the pH variation, mediated with the change in micelle size. 相似文献
993.
Direct arylation of adenine with fluoro- and chloronitrobenzenes leads to mixtures of N9 and N7 substituted adenines. After separation by column chromatography, the individual isomers can be efficiently hydrogenated on Pd to give the corresponding aminophenyladenines. A significant enhancement of the reaction rate by microwave irradiation was observed. This two-step procedure was found to be a feasible route to otherwise hardly available 7-aminophenyladenines. Correlation between calculated shielding constants and experimental values of chemical shifts in 13C and 15N NMR was used for assignment of the site of substitution.
[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.] 相似文献
994.
On the Role of Pre‐ and Post‐Electron‐Transfer Steps in the SmI2/Amine/H2O‐Mediated Reduction of Esters: New Mechanistic Insights and Kinetic Studies
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Michal Szostak Malcolm Spain Prof. Dr. David J. Procter 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(15):4222-4226
The mechanism of the SmI2‐mediated reduction of unactivated esters has been studied using a combination of kinetic, radical clocks and reactivity experiments. The kinetic data indicate that all reaction components (SmI2, amine, H2O) are involved in the rate equation and that electron transfer is facilitated by Brønsted base assisted deprotonation of water in the transition state. The use of validated cyclopropyl‐containing radical clocks demonstrates that the reaction occurs via fast, reversible first electron transfer, and that the electron transfer from simple Sm(II) complexes to aliphatic esters is rapid. Notably, the mechanistic details presented herein indicate that complexation between SmI2, H2O and amines affords a new class of structurally diverse, thermodynamically powerful reductants for efficient electron transfer to carboxylic acid derivatives as an attractive alternative to the classical hydride‐mediated reductions and as a source of acyl‐radical equivalents for C?C bond forming processes. 相似文献
995.
David L. Duewer Kenneth W. Pratt Chainarong Cherdchu Nongluck Tangpaisarnkul Akiharu Hioki Masaki Ohata Petra Spitzer Michal Máriássy Leoš Vyskočil 《Accreditation and quality assurance》2014,19(5):329-342
The key comparison (KC) studies of the Consultative Committee for Amount of Substance—Metrology in Chemistry help ensure the reliability of chemical and biochemical measurements relevant to international trade and environmental-, health-, and safety-related decision making. The traditional final evaluation of each measurement result reported by a KC participant is a “degree of equivalence” (DEq) that quantitatively specifies how consistent each individual result is relative to a reference value. Recognizing the impossibility of conducting separate KCs for all important analytes in all important sample matrices at all important analyte levels, emphasis is now shifting to documenting broadly applicable critical or “core” measurement competencies elicited through a series of studies. To better accomplish the necessary synthesis of results, data analysis and display tools must be developed for objectively and quantitatively combining individual DEqs. The information detailed in the 11 KCs of primary method pH measurements publically available as of 2013 provides an excellent “best case” prototype for such analysis. We here propose tools that enable documenting the expected primary pH measurement performance of individual participants between pH 1 and pH 11 and from 15 °C to 37 °C. These tools may prove useful for other areas where the uncertainty of measurement is a predictable function of the measured quantity, such as the stable gases. That results for relatively simple measurement processes can be combined using relatively simple analysis and display methods does not ensure that similarly meaningful summaries can be devised for less well understood and controlled systems, but it provides the incentive to attempt to do so. 相似文献
996.
Maximum Noble‐Metal Efficiency in Catalytic Materials: Atomically Dispersed Surface Platinum
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Albert Bruix Dr. Yaroslava Lykhach Dr. Iva Matolínová M. Sc. Armin Neitzel Dr. Tomáš Skála Dr. Nataliya Tsud Dr. Mykhailo Vorokhta M. Sc. Vitalii Stetsovych M. Sc. Klára Ševčíková Dr. Josef Mysliveček M. Sc. Roman Fiala M. Sc. Michal Václavů Dr. Kevin C. Prince Dr. Stéphanie Bruyère Dr. Valérie Potin Prof. Dr. Francesc Illas Prof. Dr. Vladimír Matolín Prof. Dr. Jörg Libuda Prof. Dr. Konstantin M. Neyman 《Angewandte Chemie (International ed. in English)》2014,53(39):10525-10530
Platinum is the most versatile element in catalysis, but it is rare and its high price limits large‐scale applications, for example in fuel‐cell technology. Still, conventional catalysts use only a small fraction of the Pt content, that is, those atoms located at the catalyst’s surface. To maximize the noble‐metal efficiency, the precious metal should be atomically dispersed and exclusively located within the outermost surface layer of the material. Such atomically dispersed Pt surface species can indeed be prepared with exceptionally high stability. Using DFT calculations we identify a specific structural element, a ceria “nanopocket”, which binds Pt2+ so strongly that it withstands sintering and bulk diffusion. On model catalysts we experimentally confirm the theoretically predicted stability, and on real Pt‐CeO2 nanocomposites showing high Pt efficiency in fuel‐cell catalysis we also identify these anchoring sites. 相似文献
997.
Time series data can provide valuable insight into the complexity of biological reactions. Such information can be obtained by mass-spectrometry-based approaches that measure pre-steady-state kinetics. These methods are based on a mixing device that rapidly mixes the reactants prior to the on-line mass measurement of the transient intermediate steps. Here, we describe an improved continuous-flow mixing apparatus for real-time electrospray mass spectrometry measurements. Our setup was designed to minimize metal–solution interfaces and provide a sheath flow of nitrogen gas for generating stable and continuous spray that consequently enhances the signal-to-noise ratio. Moreover, the device was planned to enable easy mounting onto a mass spectrometer replacing the commercial electrospray ionization source. We demonstrate the performance of our apparatus by monitoring the unfolding reaction of cytochrome C, yielding improved signal-to-noise ratio and reduced experimental repeat errors. 相似文献
998.
999.
Olga Trhlíková Jiří Zedník Pavel Matějíček Michal Horáček Jan Sedláček 《Polymer Degradation and Stability》2013
Molecular weight (MW) and configurational stabilities of a series of high-cis poly[(2,4-difluorophenyl)acetylene]s (PdFPhA, Mw from 2.2 × 104 to 9.7 × 105) exposed to the atmosphere and diffuse daylight are reported. Polymers dissolved in THF-d8 undergo simultaneous cis-to-trans isomerization and degradation under formation of a bimodal MW distribution. Degradation and isomerization rates are tightly connected and decrease with decreasing MW of the parent PdFPhAs. This finding may be interesting for the functional applications of substituted polyacetylenes. The partly aged PdFPhAs contain two separable fractions: higher-MW high-cis macromolecules containing a low amount of unpaired electrons and lower-MW deeply isomerized cis/trans macromolecules enriched in the unpaired electrons content. The difference in MW of these fractions reflects the various degradation rates of isomerized and high-cis macromolecules. Only a slow degradation without changes in the configurational structure proceeds if PdFPhA is allowed to age in the solid state. The suppression of the cis-to-trans isomerization reflects the suppression of motion of the polymer chains segments in the solid state. 相似文献
1000.
Zbigniew Bartczak Andrzej Galeski Marek Kowalczuk Michal Sobota Rafal Malinowski 《European Polymer Journal》2013
Polylactide (PLA), a biodegradable polymer, produced from annually renewable natural resources, has a glass transition temperature in the range of 50–60 °C and is stiff and brittle at room temperature. In this communication we demonstrate that blending of PLA with atactic poly([R,S]-3-hydroxy butyrate) (a-PHB) leads to significant improvement of drawability and impact strength. The blends of PLA with a-PHB are biodegradable, similarly to plain PLA, and can be considered as a potential material for packaging, especially for food. 相似文献