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   549篇
  免费   21篇
  国内免费   2篇
化学   427篇
晶体学   6篇
力学   8篇
数学   57篇
物理学   74篇
  2023年   6篇
  2022年   5篇
  2021年   12篇
  2020年   15篇
  2019年   5篇
  2018年   6篇
  2017年   6篇
  2016年   20篇
  2015年   23篇
  2014年   24篇
  2013年   47篇
  2012年   36篇
  2011年   46篇
  2010年   25篇
  2009年   15篇
  2008年   24篇
  2007年   28篇
  2006年   31篇
  2005年   29篇
  2004年   25篇
  2003年   17篇
  2002年   15篇
  2001年   4篇
  2000年   4篇
  1999年   2篇
  1998年   8篇
  1997年   3篇
  1996年   3篇
  1995年   2篇
  1994年   6篇
  1993年   3篇
  1992年   2篇
  1991年   2篇
  1990年   3篇
  1989年   2篇
  1988年   5篇
  1986年   2篇
  1985年   9篇
  1984年   5篇
  1983年   4篇
  1982年   4篇
  1981年   5篇
  1980年   6篇
  1979年   4篇
  1978年   2篇
  1944年   2篇
  1941年   3篇
  1932年   2篇
  1924年   1篇
  1922年   1篇
排序方式: 共有572条查询结果,搜索用时 0 毫秒
11.
12.
Despite its cultural and nutritional importance for local Aboriginal people, the unusual insect honey produced by Western Australian honeypot ant (Camponotus inflatus) has to date been rarely investigated. This study reports on the honey’s physicochemical properties, its total phenolic, major sugars and 5-hydroxymethylfurfural contents, and its antioxidant activities. The honey’s color value is 467.63 mAU/63.39 mm Pfund, it has a pH of 3.85, and its electric conductivity is 449.71 µSiemens/cm. Its Brix value is 67.00, corresponding to a 33% moisture content. The total phenolics content is 19.62 mg gallic acid equivalent/100 g honey. Its antioxidant activity measured using the DPPH* (2,2-diphenyl-1-picrylhydrazyl) and FRAP (ferric reducing–antioxidant power) assays is 1367.67 µmol Trolox/kg and 3.52 mmol Fe+2/kg honey, respectively. Major sugars in the honey are glucose and fructose, with a fructose-to-glucose ratio of 0.85. Additionally, unidentified sugar was found in minor quantities.  相似文献   
13.
The method of preparing protein mixtures for electrophoretic analysis of membrane-associated cell proteins was improved. By sonication, about one-half of the proteins of thyroid cells were released into the supernatant, while the other half preferentially comprising membrane proteins still remained in cell fragments, which could be sedimented by centrifugation. After sonication, even those proteins which remained in cell fragments, could completely be dissolved by free-flow isoelectric focusing media. They migrated through the free-flow electrophoresis chamber without forming precipitates. Because of these improvements, it was possible to show that the two thyroid cancer cell lines ML-1 and ONCO-DG1 express cytokeratin 8 at similar rates, but cytokeratins 7 and 18 differently. In addition, the presence of inorganic pyrophosphatase, tubulin-beta-5, and tubulin-beta-1 chains in human thyroid cells was proved for the first time.  相似文献   
14.
The experimental and theoretical spectroscopic and spectrometric elucidation in solid-state and gas-phase on the interacting ionic species of applied oriented synthetic derivatives on the base of the stilbazolium salts as molecular template was reported. The correlation between the molecular structure, and vibrational properties within THz-regime (10-0.3 THz) was performed. The collective vibrations, and gas-phase stabilized ionic species were comprehensive studied by the Raman spectroscopy and matrix-assisted laser desorption/ionization mass spectrometry, using the embedded organic dyes in host matrixes. The performed solid-state quantum chemical calculations contributed to further understanding of the nature of the guest–host interacting systems as well as to explain the observed optical phenomena within the THz-region.  相似文献   
15.
An asymmetric synthesis of densely functionalized 7–11‐membered carbocycles and 9–11‐membered lactones has been developed. Its key steps are a modular assembly of sulfoximine‐substituted C‐ and O‐tethered trienes and C‐tethered dienynes and their Ru‐catalyzed ring‐closing diene and enyne metathesis (RCDEM and RCEYM). The synthesis of the C‐tethered trienes and dienynes includes the following steps: 1) hydroxyalkylation of enantiomerically pure titanated allylic sulfoximines with unsaturated aldehydes, 2) α‐lithiation of alkenylsulfoximines, 3) alkylation, hydroxy‐alkylation, formylation, and acylation of α‐lithioalkenylsulfoximines, and 4) addition of Grignard reagents to α‐formyl(acyl)alkenylsulfoximines. The sulfoximine group provided for high asymmetric induction in steps 1) and 4). RCDEM of the sulfoximine‐substituted trienes with the second‐generation Ru catalyst stereoselectively afforded the corresponding functionalized 7–11‐membered carbocyles. RCDEM of diastereomeric silyloxy‐substituted 1,6,12‐trienes revealed an interesting difference in reactivity. While the (R)‐diastereomer gave the 11‐membered carbocyle, the (S)‐diastereomer delivered in a cascade of cross metathesis and RCDEM 22‐membered macrocycles. RCDEM of cyclic trienes furnished bicyclic carbocycles with a bicyclo[7.4.0]tridecane and bicyclo[9.4.0]pentadecane skeleton. Selective transformations of the sulfoximine‐ and bissilyloxy‐substituted carbocycles were performed including deprotection, cross‐coupling reaction and reduction of the sulfoximine moiety. Esterification of a sulfoximine‐substituted homoallylic alcohol with unsaturated carboxylic acids gave the O‐tethered trienes, RCDEM of which yielded the sulfoximine‐substituted 9–11‐membered lactones. RCEYM of a sulfoximine‐substituted 1,7‐dien‐10‐yne showed an unprecedented dichotomy in ring formation depending on the Ru catalyst. While the second‐generation Ru catalyst gave the 9‐membered exo 1,3‐dienyl carbocycle, the first‐generation Ru catalyst furnished a truncated 9‐membered 1,3‐dieny carbocycle having one CH2 unit less than the dienyne.  相似文献   
16.
The eighteen new μ-alkylidene ruthenium complexes 5a–r and 5t are very easily and cleanly obtained along the diazoalkane or the hydrazone routes that involve treatment of the dinuclear, metal-metal doubly bonded precursor compound [(η5-C5H5)Ru(μ-NO)]2 (3) either with the diazoalkanes oxidizing agent (e.g., MnO2), with the respective hydrazones. Similarly, sulfur dioxide adds cleanly to the RuRu double bond of 3, thus giving the complex (μ-SO2)[(η5-C5H5)Ru(NO)]2 (5s). Regardless of the nature of the carbene bridge ligands, the dimetallacyclopropanes exhibit, in contrast to their iron analogues, exclusively terminal nitrosyl ligands. cis/trans-Isomerism with predominating amounts of the trans-isomers is observed for the derivatives that display unsymmetrically substituted carbene bridges.Treatment of the μ-methylene- and μ-ethylidene complexes (μ-CH2)[(η5-C5H5)Ru(NO)]2 (5a) and (μ-CHCH3)[(η5-C tetrafluoroboric acid or trifluoromethanesulfonic acid in diethyl ether yields, at ambient temperature, quantitatively the ionic complexes 6a,b and 7a,b, respectively, which were shown by 1H NMR spectroscopy to contain metal-metal bridging hydrogen functionalities. The reaction of hydrogen bromide with 5a under the same conditions gives the neutral bromo(methyl) complex 6d. This latter compound results from the isolable ionic intermediate of composition [(μ-CH2)(μ-H){(η5C5H5)Ru(NO)}2]+Br? (6c), which reaction stems from the nucleophilicity of the halide ion present in 6c.  相似文献   
17.
The diazoolefines of composition N2CCR2 (R/R = CH3/CH3 and(-CH2-)5) are suitable precursors of the corresponding vinylidene ligands CCR2. Thus, treatment of the RhRh complex [(η5-C5Me5)Rh(μ-CO)]2 (1) with the N-nitrosourethanes 2a and 2b, resp., in the presence of lithium t-butoxide yields the otherwise inaccessible μ-vinylidene complexes (μ-CCR2)[(η5-C5Me5)Rh(CO)]2 (R = CH3 (3a), R,R = (-CH2-)5 (3b)). The analogous cobalt compound (μ-CCMe2)[(η5-C5Me5)Co(CO)]2 (5a) is obtained similarly. This procedure extends the well-documented diazoalkane method for the synthesis of μ-alkylidene complexes to the less stable diazoalkenes. A single-crystal X-ray diffraction study of the dimethylvinylidene derivative 3a shows the CMe2 ligand to adopt an almost symmetrically metal-bridging position (d(RhC) 197.8(1) and 204.3(1) pm), with a rhodium-rhodium single bond completing a three-membered Rh2C-metallacycle (d(RhRh) 268.4(0) pm) analogous with cyclopropane.  相似文献   
18.
Poor aqueous solubility of active compounds is a major issue in today’s drug delivery. In this study the smartFilm-technology was exploited to improve the dermal penetration efficacy of a poorly soluble active compound (curcumin). Results were compared to the dermal penetration efficacy of curcumin from curcumin bulk suspensions and nanocrystals, respectively. The smartFilms enabled an effective dermal and transdermal penetration of curcumin, whereas curcumin bulk- and nanosuspensions were less efficient when the curcumin content was similar to the curcumin content in the smartFilms. Interestingly, it was found that increasing numbers of curcumin particles within the suspensions increased the passive dermal penetration of curcumin. The effect is caused by an aqueous meniscus that is created between particle and skin if the dispersion medium evaporates. The connecting liquid meniscus causes a local swelling of the stratum corneum and maintains a high local concentration gradient between drug particles and skin. Thus, leading to a high local passive dermal penetration of curcumin. The findings suggest a new dermal penetration mechanism for active compounds from nano-particulate drug delivery systems, which can be the base for the development of topical drug products with improved penetration efficacy in the future.  相似文献   
19.
The concurrent self-assembly of new 1,3,5-trisamide-cyclohexane-based low molecular weight hydrogelators and various surfactants in water leads to the formation of self-assembled fibrillar networks with encapsulated micelles. This prototype system presents an example of orthogonal self-assembly, that is, the independent formation of two different supramolecular structures, each with their own characteristics that coexist within a single system.  相似文献   
20.
Silica particles with different morphology have been functionalized with carbon shells by different synthetic procedures. In the key step, the bare silica particles are functionalized by a specific cationic surface polymerization with furfuryl alcohol (FA). The polyfurfuryl alcohol (PFA)/silica hybrid particles have been also post-functionalized with maleic anhydride (MSA) by a Diels Alder reaction. Simultaneously occuring cationic polymerization of FA and sol-gel process with TEOS has been used for producing interpenetrating carbon-silica hybrid materials. The thermal transformation of the PFA component on silica into the carbon phase has been carried out under argon atmosphere in a temperature range from 700°C to 900°C. The influence of the former morphology of the silica on the homogenity of the resulting carbon layer is shown by zetapotential measurements and electron microscopic investigations.  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