首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   223篇
  免费   3篇
  国内免费   2篇
化学   163篇
晶体学   1篇
力学   3篇
数学   13篇
物理学   48篇
  2023年   4篇
  2022年   2篇
  2021年   6篇
  2020年   5篇
  2019年   9篇
  2017年   3篇
  2016年   4篇
  2014年   6篇
  2013年   9篇
  2012年   11篇
  2011年   21篇
  2010年   6篇
  2009年   8篇
  2008年   13篇
  2007年   9篇
  2006年   12篇
  2005年   11篇
  2004年   13篇
  2003年   13篇
  2002年   9篇
  2001年   3篇
  2000年   3篇
  1999年   2篇
  1998年   2篇
  1997年   3篇
  1996年   3篇
  1995年   1篇
  1994年   8篇
  1993年   3篇
  1992年   1篇
  1991年   1篇
  1987年   1篇
  1986年   2篇
  1985年   2篇
  1982年   2篇
  1981年   1篇
  1980年   3篇
  1978年   2篇
  1977年   1篇
  1975年   1篇
  1974年   1篇
  1973年   1篇
  1972年   1篇
  1969年   4篇
  1940年   2篇
排序方式: 共有228条查询结果,搜索用时 15 毫秒
11.
An original workflow allowing inline FAIMS separation, electrospray ionization, mass analysis and ion spectroscopy (IRMPD: InfraRed Multiple Photon Dissociation) is presented for multidimensional molecular analysis. This new instrument consists of an ultraFAIMS (Owlstone) device interfaced to a linear ion trap (LTQ XL Thermo Scientific) which was modified for IRMPD spectroscopy. Two modes of operation are demonstrated on an isomeric mixture of paracetamol and 2-phenylglycine. In the first mode a FAIMS (high-Field Asymmetric waveform Ion Mobility Spectrometry) separation of the isomers is performed with a static compensation field for mass- and isomer- selective ion spectroscopy. In the second mode, the compensation field is scanned while the ions are irradiated at a fixed wavenumber. The advantages of this workflow as compared to traditional FAIMS-MS and IRMPD spectroscopy are described. The potential of the two modes for molecular spectroscopy and analytical applications, in particular the new “omics” are discussed.  相似文献   
12.
The present study investigates the antioxidant activities of some Romanian plants, using different spectrophotometric methods (FRAP I, FRAP II, and CUPRAC). The plants investigated are hawthorn (Crataegus oxyacantha), bilberry (Vaccinium myrtillus L.), rosehip (Rosa canina), and chokeberries (Aronia melanocarpa). Hawthorn is used to treat a wide variety of inflammatory conditions, but the primary use is generally restricted for treating hypertension, ischemic heart disease, congestive heart failure, and arrhythmia. Investigations have proved the safe and reliable use of plant and plant extracts for treatment of cardiovascular disorders.  相似文献   
13.
Seven commercial Moroccan honeys were considered for chemical characterisation. Volatile fraction, total polyphenols content, antioxidant and antiradical activities were evaluated by employing different analytical methodologies. Several physical parameters such as refractive index, pH, water content, solids content and colour were measured. Volatile fraction revealed an abundant presence of cis- and trans-linalool oxide in the seven studied samples. The presence of high levels of compounds related to the Maillard reaction, like furfural and hydroxymethylfurfural, could be the result of thermal treatments used to liquefy commercial honeys or of long storage times. The CIE L*a*b*Cab*hab° chromatic coordinates confirmed the advanced stage of the Maillard reaction, showing L* values lower than the common values found for honey of similar typologies.  相似文献   
14.
Olive mill wastewater (OMW) contains valuable and interesting bioactive compounds, among which is hydroxytyrosol, which is characterized by a remarkable antioxidant activity. Due to the health claims related to olive polyphenols, the aim of this study was to obtain an extract from OMW with an increased level of hydroxytyrosol by means of microbial enzymatic activity. For this purpose, four commercial adsorbent resins were selected and tested. The beta-glucosidase and esterase activity of strains of Wickerhamomyces anomalus, Lactiplantibacillus plantarum, and Saccharomyces cerevisiae were also investigated and compared to those of a commercial enzyme and an Aspergillus niger strain. The W. anomalus strain showed the best enzymatic performances. The SP207 resin showed the best efficiency in selective recovery of hydroxytyrosol, tyrosol, oleuropein, and total phenols. The bioconversion test of the OMW extract was assessed by using both culture broths and pellets of the tested strains. The results demonstrated that the pellets of W. anomalus and L. plantarum were the most effective in hydroxytyrosol increasing in phenolic extract. The interesting results suggest the possibility to study new formulations of OMW phenolic extracts with multifunctional microorganisms.  相似文献   
15.
A series of new complexes with mixed ligands of the type [ML(C3H3O2)2nH2O (((1) M=Mn, n=1; (2) M=Co(II), n=2; (3) M=Ni(II), n=4; (4) M=Cu(II), n=1.5; (5) M=Zn(II), n=0; L=3-amino-1,2,4-triazole and (C3H3O2)=acrylate anion) were synthesized and characterised by chemical analysis and IR data. In all complexes the 3-amino-1,2,4-triazole acts as bridge while the acrylate acts as bidentate ligand except for complex (5) where it is found as unidentate. The thermal behaviour steps were investigated in nitrogen flow. The thermal transformations are complex processes according to TG and DTG curves including dehydration, acrylate ion and 3-amino-1,2,4-triazole degradation respectively. The final products of decomposition are the most stable metal oxides, except for complex (4) that leads to metallic copper.  相似文献   
16.
A method is given to determine whether or not the distribution functions describing the two spin measurements in the spin-s Einstein-Podolsky-Rosen experiment are compatible with the existence of distributions describing three spin measurements (not all of which can actually be performed). When applied to the spin-1/2 case the method gives the results of Wigner, or of Clauser, Holt, Horne, and Shimony, depending on whether or not the two-spin distributions are assumed to have the forms given by the quantum theory. Generalizations of the conditions of Wigner or of Clauser et al. to the spin-1 case are explicitly calculated. The spin-3/2 case is examined in some simple geometries to show that an apparently monotonic trend toward local realism as s increases from1/2 to1 is, in fact, violated when s increases from1 to3/2. The analysis is based on a novel representation of the modulus squared of a rotation matrix element. The structure of that matrix element responsible for the restoration of local realism in the classical (large s) limit is identified, but a rigorous treatment of the classical limit is not attempted. The higher-spin results are significantly stronger than those given by Mermin's spin-s Bell inequality.  相似文献   
17.
Microcionamides A (1) and B (2) have been isolated from the Philippine marine sponge Clathria (Thalysias) abietina. These new linear peptides are cyclized via a cystine moiety and have their C-terminus blocked by a 2-phenylethylenamine group. Their total structures, including absolute stereochemistry, were determined by a combination of spectral and chemical methods. Compound 1 was shown to slowly isomerize about the C-36/C-37 double bond when stored in DMSO. Microcionamides A (1) and B (2) exhibited significant cytotoxicity against the human breast tumor cells lines MCF-7 and SKBR-3 and displayed inhibitory activity against Mycobacterium tuberculosis H(37)Ra.  相似文献   
18.
19.
For plane-wave and many-spiral states of the experimentally based Luo-Rudy 1 model of heart tissue in large (8 cm square) domains, we show that a space-time-adaptive time-integration algorithm can achieve a factor of 5 reduction in computational effort and memory-but without a reduction in accuracy-when compared to an algorithm using a uniform space-time mesh at the finest resolution. Our results indicate that such an algorithm can be extended straightforwardly to simulate quantitatively three-dimensional electrical dynamics over the whole human heart.  相似文献   
20.
Reaction of a mixture of bicyclic phosphorus sulfide selenide iodides α‐P4SnSe3−nI2 (n = 0–3) with PriNH2 and Et3N gave corresponding diamides α‐P4SnSe3−n(NHPri)2 (n = 0–3) and imides α‐P4SnSe3−n(μ‐NPri) (n = 2–3), identified in solution by 31P NMR. In one isomer of α‐P4S2Se(μ‐NPri), the C2 symmetry of imides such as α‐P4S3(μ‐NPri) was broken, allowing relative assignment of 2J NMR couplings to the PNP bridge and the PSP bridge opposite to it. The coupling through the sulfur bridge was found to be reduced to ca. zero, in contrast to previous assumptions for this class of compounds. Ab initio models were calculated at the MPW1PW91/svp level for the sulfide selenide imides and for a selection of bond rotamers of the diamides, and at the MPW1PW91/LanL2DZ(d) level for the sulfide selenide diiodides. Different skeletal isomers were prevalent for the mixed chalcogenide diamides than for the diiodides, showing that exchange of chalcogen between skeletal positions took place in the amination reaction even at room temperature. Similar differences to those observed were predicted by the models, suggesting that equilibrium was attained.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号