全文获取类型
收费全文 | 179篇 |
免费 | 11篇 |
国内免费 | 3篇 |
专业分类
化学 | 143篇 |
晶体学 | 1篇 |
力学 | 4篇 |
数学 | 8篇 |
物理学 | 37篇 |
出版年
2023年 | 6篇 |
2022年 | 8篇 |
2021年 | 5篇 |
2020年 | 1篇 |
2019年 | 10篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 5篇 |
2014年 | 13篇 |
2013年 | 17篇 |
2012年 | 10篇 |
2011年 | 18篇 |
2010年 | 12篇 |
2009年 | 3篇 |
2008年 | 14篇 |
2007年 | 8篇 |
2006年 | 8篇 |
2005年 | 11篇 |
2004年 | 13篇 |
2003年 | 3篇 |
2002年 | 5篇 |
2001年 | 4篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有193条查询结果,搜索用时 15 毫秒
91.
Paul Coupillaud Julien Pinaud Nicolas Guidolin Joan Vignolle Maréva Fèvre Ellen Veaudecrenne David Mecerreyes Daniel Taton 《Journal of polymer science. Part A, Polymer chemistry》2013,51(21):4530-4540
Synthesis of novel poly(ionic liquid)s, namely, poly(1‐vinyl‐3‐alkylimidazolium hydrogen carbonate)s, denoted as poly([NHC(H)][HCO3])s or PVRImHCO3, where R is an alkyl group (R = ethyl, butyl, phenylethyl, dodecyl), is described. Two distinct synthetic routes were explored. The first method is based on the free‐radical polymerization (FRP) of 1‐vinyl‐3‐alkylimidazolium monomers featuring a hydrogen carbonate counter anion (HCO3?), denoted as VRImHCO3. The latter monomers were readily synthesized by alkylation of 1‐vinylimidazole (VIm), followed by direct anion exchange of 1‐vinyl‐3‐alkylimidazolium bromide monomers (VRImBr), using potassium hydrogen carbonate (KHCO3) in methanol at room temperature. Alternatively, the same anion exchange method could be applied onto FRP‐derived poly(1‐vinyl‐3‐alkylimidazolium bromide) precursors (PVRImBr). All PVRImHCO3 salts proved air stable and could be manipulated without any particular precautions. They could serve as polymer‐supported precatalysts to generate polymer‐supported N‐heterocyclic carbenes, referred to as poly(NHC)s, formally by a loss of “H2CO3” (H2O +CO2) in solution. This was demonstrated through selected organocatalyzed reactions of molecular chemistry, known as being efficiently mediated by molecular NHC catalysts, including benzoin condensation, transesterification and cyanosilylation of aldehyde. Of particular interest, recycling of the polymer‐supported precatalysts was possible by re‐carboxylation of in situ generated poly(NHC)s. Organocatalyzed reactions could be performed with excellent yields, even after five catalytic cycles. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4530–4540 相似文献
92.
Fèvre M Pinaud J Leteneur A Gnanou Y Vignolle J Taton D Miqueu K Sotiropoulos JM 《Journal of the American Chemical Society》2012,134(15):6776-6784
Anion metathesis of imidazol(in)ium chlorides with KHCO(3) afforded an easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO(3)]. In solution, these compounds were found to be in equilibrium with their corresponding imidazol(in)ium carboxylates, referred to as N-heterocyclic carbene (NHC)-CO(2) adducts. The [NHC(H)][HCO(3)] salts were next shown to behave as masked NHCs, allowing for the NHC moiety to be readily transferred to both organic and organometallic substrates, without the need for dry and oxygen-free conditions. In addition, such [NHC(H)][HCO(3)] precursors were successfully investigated as precatalysts in two selected organocatalyzed reactions of molecular chemistry and polymer synthesis, namely, the benzoin condensation reaction and the ring-opening polymerization of d,l-lactide, respectively. The generation of NHCs from [NHC(H)][HCO(3)] precursors occurred via the formal loss of H(2)CO(3)via a concerted low energy pathway, as substantiated by Density Functional Theory (DFT) calculations. 相似文献
93.
Combination of sugar analysis and stable isotope ratio mass spectrometry to detect the use of artificial sugars in royal jelly production 总被引:1,自引:0,他引:1
Marine Wytrychowski Ga?lle Daniele Hervé Casabianca 《Analytical and bioanalytical chemistry》2012,403(5):1451-1456
The effects of feeding bees artificial sugars and/or proteins on the sugar compositions and 13C isotopic measurements of royal jellies (RJs) were evaluated. The sugars fed to the bees were two C4 sugars (cane sugar and
maize hydrolysate), two C3 sugars (sugar beet, cereal starch hydrolysate), and honey. The proteins fed to them were pollen,
soybean, and yeast powder proteins. To evaluate the influence of the sugar and/or protein feeding over time, samples were
collected during six consecutive harvests. 13C isotopic ratio measurements of natural RJs gave values of around −25 ‰, which were also seen for RJs obtained when the bees
were fed honey or C3 sugars. However, the RJs obtained when the bees were fed cane sugar or corn hydrolysate (regardless of
whether they were also fed proteins) gave values of up to −17 ‰. Sugar content analysis revealed that the composition of maltose,
maltotriose, sucrose, and erlose varied significantly over time in accordance with the composition of the syrup fed to the
bees. When corn and cereal starch hydrolysates were fed to the bees, the maltose and maltotriose contents of the RJs increased
up to 5.0 and 1.3 %, respectively, compared to the levels seen in authentic samples (i.e., samples obtained when the bees
were fed natural food: honey and pollen) that were inferior to 0.2% and not detected, respectively. The sucrose and erlose
contents of natural RJs were around 0.2 %, whereas those in RJs obtained when the bees were fed cane or beet sugar were as
much as 4.0 and 1.3 %, respectively. The combination of sugar analysis and 13C isotopic ratio measurements represents a very efficient analytical methodology for detecting (from early harvests onward)
the use of C4 and C3 artificial sugars in the production of RJ. 相似文献
94.
Lluveras-Tenorio A Andreotti A Bonaduce I Boularand S Cotte M Roqué J Colombini MP Vendrell-Saz M 《Chemistry Central journal》2012,6(1):45-18
ABSTRACT: BACKGROUND: Establishing the distribution of materials in paintings and that of their degradation products by imaging techniques is fundamental to understand the painting technique and can improve our knowledge on the conservation status of the painting. The combined use of chromatographic-mass spectrometric techniques, such as GC/MS or Py/GC/MS, and the chemical mapping of functional groups by imaging SR FTIR in transmission mode on thin sections and SR XRD line scans will be presented as a suitable approach to have a detailed characterisation of the materials in a paint sample, assuring their localisation in the sample build-up. This analytical approach has been used to study samples from Catalan paintings by Josep Maria Sert y Badia (20th century), a muralist achieving international recognition whose canvases adorned international buildings. RESULTS: The pigments used by the painter as well as the organic materials used as binders and varnishes could be identified by means of conventional techniques. The distribution of these materials by means of Synchrotron Radiation based techniques allowed to establish the mixtures used by the painter depending on the purpose. CONCLUSIONS: Results show the suitability of the combined use of SR FTIR and SR XRD mapping and conventional techniques to unequivocally identify all the materials present in the sample and their localization in the sample build-up. This kind of approach becomes indispensable to solve the challenge of micro heterogeneous samples. The complementary interpretation of the data obtained with all the different techniques allowed the characterization of both organic and inorganic materials in the samples layer by layer as well as to establish the painting techniques used by Sert in the works-of-art under study. 相似文献
95.
Hong Bing LIU Cheng Bin CUI * Bing CAI Qian Qun GU Dong Yun ZHANG Qing Chun ZHAO Hua Shi GUAN Key Laboratory of Marine Drugs Ministry of Education China Marine Drug Food Institute Ocean University of China Qingdao Tianjin Institute for Biomedicinal Research 《中国化学快报》2005,16(2):215-218
Pterocarya tonkinesis(Franch.)Dode.(Juglandaceae)forms a typical community ofriverside season rain forest in Xishuangbanna area of Yunnan province1.The same genusplant P.stenoptera C.DC.,has long been used as a traditional Chinese herbal medicine2and P.tonkinesis,itself,is also used as a folk medicine for the treatment of certain cancersin partial area of China.However,no report had so far been seen on the chemicalconstituents and their biological activities of the title plant yet.In the … 相似文献
96.
97.
Producing clear and intelligible layouts of hierarchical digraphs knows a renewed interest in information visualization. Recent
experimental results show that metaheuristics are well-adapted methods for this problem. In this paper, we develop a new Hybridized
Genetic Algorithm for arc crossing minimization. It follows the basic scheme of a GA with two major differences: problem-based
crossovers adapted from ordering GAs are combined with a local search strategy based on averaging heuristics. Computational
testing was performed on a set of 180 random hierarchical digraphs of standard sizes with various structures. Results show
that the Hybridized Genetic Algorithm significantly outperforms Tabu Search—which is one of the best known methods for this
problem- and also a multi-start descent except for highly connected graphs. 相似文献
98.
Etchart I Bérard M Laroche M Huignard A Hernández I Gillin WP Curry RJ Cheetham AK 《Chemical communications (Cambridge, England)》2011,47(22):6263-6265
We report efficient white upconversion luminescence in Yb(3+)-, Er(3+)- and Tm(3+)-doped monophasic and biphasic Y(2)BaZnO(5) phosphors under 977 nm near-infrared excitation and at low excitation power densities (down to ~25 mW mm(-2)). 相似文献
99.
100.
Arnaud De Luca Karine Bernardo Marine Frasquet-Darrieux Patricia Christin Anne-Marie Schiphorst Mathilde Grand 《Isotopes in environmental and health studies》2013,49(4):385-393
ABSTRACTObesity increases protein metabolism with a potential effect on nitrogen isotope fractionation. The aim of this study was to test the influence of obesity on human milk extracted protein 15N natural isotope abundance (NIA) at one month post-partum and to compare human milk extracted protein 15N NIA and bulk infant hair 15N NIA. This cross-sectional observational study involved 16 obese mothers (body mass index (BMI)?≥?30?kg?m?2 before pregnancy) matched with 16 normal-weight mothers (18.5?kg?m?2?≤?BMI?<?25?kg?m?2) for age and pregnancy characteristics. Human milk extracted protein and bulk infant hair 15N NIA were determined by isotope ratio monitoring by mass spectrometry interfaced to an elemental analyser (IRM-EA/MS). No significant difference was found in human milk protein 15N NIA values between obese and normal-weight mothers (8.93?±?0.48?‰ vs. 8.95?±?0.27?‰). However, human milk protein 15N NIA was significantly lower than bulk infant hair 15N NIA: 8.94?±?0.38?‰ vs. 9.66?±?0.69?‰, respectively. On the basis of these results, it is concluded that human milk protein 15N NIA measured at one month post-partum is not influenced by maternal obesity. These findings suggest that 15N NIA may be exploited to study metabolism without considering maternal obesity as a confounder. 相似文献