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   139篇
  免费   2篇
化学   119篇
晶体学   6篇
数学   2篇
物理学   14篇
  2023年   1篇
  2022年   1篇
  2017年   2篇
  2015年   2篇
  2014年   3篇
  2013年   2篇
  2012年   4篇
  2011年   9篇
  2010年   6篇
  2009年   7篇
  2008年   7篇
  2007年   9篇
  2006年   5篇
  2005年   16篇
  2004年   6篇
  2003年   8篇
  2002年   8篇
  2001年   3篇
  2000年   3篇
  1999年   3篇
  1998年   1篇
  1997年   2篇
  1994年   2篇
  1993年   2篇
  1992年   3篇
  1991年   1篇
  1990年   2篇
  1988年   1篇
  1986年   2篇
  1985年   1篇
  1984年   3篇
  1983年   1篇
  1981年   2篇
  1980年   1篇
  1978年   1篇
  1975年   3篇
  1973年   1篇
  1972年   2篇
  1959年   1篇
  1957年   1篇
  1954年   3篇
排序方式: 共有141条查询结果,搜索用时 15 毫秒
101.
An alternative route to the parent nido-5,6-C2B8H12 dicarbaborane is reported together with a convenient synthesis of its carbon-substituted derivatives. The method is based on reactions between 4-(Me2S)-arachno-B9H13 and alkynes R1R2C2 (where R1R2=H,H; Me, Me; Ph,H, and Ph,Ph) in toluene at reflux. The characteristic reaction mode is a dicarbon insertion into the 9-vertex arachno cluster to produce a series of the 5,6-R1R2-nido-5,6-C2B8H10 species combined with concomitant elimination of one {BH} vertex. The products were characterised by high-field 1H and 11B NMR spectroscopy and mass spectrometry associated with [11B–11B]-COSY and 1H{11B(selective)} measurements that permitted complete assignments of all resonances to individual cluster {BH} units.  相似文献   
102.
Addition of PMe2Ph to fused-cluster syn-[(eta5-C5Me5)IrB18H20] 1 to give [(eta5-C5Me5)HIrB18H19(PMe2Ph)] 3 entails a diminution in the degree of intimacy of the intercluster fusion, rather than retention of inter-subcluster binding intimacy and a nido-->arachno conversion of the character of either of the subclusters. Reaction with MeNC gives [(eta5-C5Me5)HIrB18H19[C(NHMe)2]] 4 which has a similar structure, but with the ligand now being the carbene [:C(NHMe)2], resulting from a reductive assembly reaction involving two MeNC residues and the loss of a carbon atom.  相似文献   
103.
Summary Hydrogenation of vermistatin, a metabolite ofPenicillium vermiculatum, affordedcis- andtrans-3-(2-carboxy-4,6-dimethoxybenzyl)-6-propyltetrahydro-4H-pyran-4-one and 4,6-dimethoxy-3-(4-propylcyclopentanon-2-yl)phthalide. The latter compound was found to be the strongest inhibitor of RNA synthesis in P 388 leukemia cells of all compounds tested.
Hydrierung von Vermistatin
Zusammenfassung Bei der Hydrierung von Vermistatin, einem neuen sekundären Metabolit ausPenicillium vermiculatum, wurdencis- undtrans 3-(2-Carboxy-4,6-dimethoxybenzyl)-6-propyltetrahydro-4-pyran-4-on und 4,6-dimethoxy-3-(4-propylcyclopentanon-2-yl)phthalid erhalten und spektroskopisch charakterisiert. Die letztgenannte Verbindung zeigte von allen getesteten Verbindungen die stärkste Hemmwirkung auf die RNA Synthese in P 388 Leukämiezellen.
  相似文献   
104.
The paper deals with thermodynamic stability of hypothetical solid binaryM 4C3 carbides of gallium, indium and thallium. Heats of formation whose contribution to the stability of these compounds is dominant, were estimated by two independent methods: semiempirical theory ofMiedema and empirical trends in the heats of formation of nitrides and carbides of Group III elements. Entropies were calculated with the use of theCantor equation. The estimated values suggest thatM 4C3 carbides of gallium, indium and thallium are thermodynamically unstable with regard to their decomposition to elements.
Über die Instabilität von Galliumgruppenkarbiden des TypsM 4C3
Zusammenfassung Es wird die thermodynamische Stabilität von hypothetischen Gallium-, Indium- und Thalliumkarbiden diskutiert. Zur Abschätzung von Bildungswärmen, deren Beitrag zur Stabilität dieser Verbindungen dominant ist, wurden zwei unabhängige Methoden verwendet: die semiempirischeMiedema-Theorie und empirische Trends der Bildungswärmen bei den Nitriden und Karbiden der Elemente der III. Gruppe. Die Entropiewerte wurden mittels derCantor-Gleichung berechnet. Aus den abgeschätzten Werten geht hervor, daß die Galliumgruppenkarbide bezüglich der Zersetzung in die Elemente thermodynamisch instabil sind.
  相似文献   
105.
The neutral azatricarbaborane 7-t-Bu-arachno-7,1,5,12-NC(3)B(8)H(12), isolated as a side product (yield 2%) from the new synthesis of 7-t-BuNH2-nido-7,8,9-C(3)B(8)H(10) (yield 70%), can be easily converted to the first parent representatives of the 10-vertex nido family of tricarbaboranes, [5,6,9-C(3)B(7)H(10)]- and 5,6,9-C(3)B(7)H(11).  相似文献   
106.
Reaction of the [arachno-4-NB(8)H(12)](-) anion with [RhCl(2)(eta(5)-C(5)Me(5))](2) in CH(2)Cl(2) at room temperature affords a mixture of red '6,9' isomer [9-(eta(5)-C(5)Me(5))-nido-6,9-NRhB(8)H(11)] () and its yellow '6,8' isomer, [8-(eta(5)-C(5)Me(5))-nido-6,8-NRhB(8)H(11)] (). Under the same conditions, reactions of with [IrCl(2)(eta(5)-C(5)Me(5))](2) and [RuCl(2)(eta(6)-MeC(6)H(4)-4-(iso)Pr)](2) give the '6,8' isomers, yellow [8-(eta(5)-C(5)Me(5))-nido-6,8-NIrB(8)H(11)] () and red [8-(eta(6)-MeC(6)H(4)-4-(iso)Pr)-nido-6,8-NRuB(8)H(11)] (), respectively. In contrast, [IrCl(PPh(3))(3)] yields orange [9,9-(PPh(3))(2)-9-H-nido-6,9-NIrB(8)H(11)] (), which exhibits the '6,9' configuration. Compound isomerizes quantitatively in solution to give . At high temperatures, compound gives the yellow '6,8' species, [8,8-(PPh(3))(2)-8-H-nido-6,8-NIrB(8)H(11)] (), in low yields. Possible mechanisms for the unprecedented 6,9 --> 6,8 isomerization are discussed.  相似文献   
107.
Nitrophenols are important environmental pollutants and their monitoring is important because of their genotoxic and ecotoxic properties. Easy electrochemical reduction of nitro groups can be used for their voltammetric determination using mercury based electrodes. However, requirements of green analytical chemistry prompted us to investigate a novel type of silver porous electrode (AgPE) prepared by powder metallurgy compatible with both “green” and “white” analytical chemistry requirements. In this paper, AgPE was for the first time successfully used for differential pulse voltammetric determination of micromolar concentrations of 2-nitrophenol (NP), 2,4-dinitrophenol (DNP), and 2,4,6-trinitrophenol (TNP) in aqueous media. The main advantage of the novel method is the possibility to use small sample volume (down to 25 μL) and to work in the presence of oxygen when using supporting electrolyte of pH 3. This advantage partially compensates the fact that the obtained sensitivity and limit of detection are not better than with the previously investigated electrodes.  相似文献   
108.
109.
Poly(methyl methacrylate)s with terminal bromine atom, prepared by bromination of anionically polymerized MMA, were used as ATRP macroinitiators giving di- and triblock copolymers with MMA, styrene and butyl acrylate blocks. Multifunctional ATRP macroinitiators were synthesized by introducing bromomethyl or 2-bromoacyloxy groups onto the main chain of polystyrene or poly(4-methyl styrene) and used for ATRP grafting of tert-butyl acrylate leading to densely grafted copolymers with more or less uniform grafts.  相似文献   
110.
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