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   16篇
  免费   0篇
化学   7篇
数学   1篇
物理学   8篇
  2016年   1篇
  2013年   1篇
  2012年   1篇
  2008年   2篇
  2007年   2篇
  2006年   2篇
  2003年   1篇
  2000年   1篇
  1996年   2篇
  1991年   1篇
  1933年   1篇
  1881年   1篇
排序方式: 共有16条查询结果,搜索用时 31 毫秒
1.
We present new hairy black hole solutions of SU(N) Einstein-Yang-Mills (EYM) theory in asymptotically anti-de Sitter (AdS) space. These black holes are described by N+1 independent parameters and have N-1 independent gauge field degrees of freedom. Solutions in which all gauge field functions have no zeros exist for all N, and for a sufficiently large (and negative) cosmological constant. At least some of these solutions are shown to be stable under classical, linear, spherically symmetric perturbations. Therefore there is no upper bound on the amount of stable gauge field hair with which a black hole in AdS can be endowed.  相似文献   
2.
3.
Isodesmic and homodesmic equations at the B3LYP/6-311+G(d,p)+ZPVE level of theory have been used to estimate strain for the homologous series of cyclic allenes and cyclic butatrienes. A simple fragment deformation approach also has been applied and appears to work better for the larger rings. For the cyclic allene series, estimates for allene functional group strain (kcal/mol) include: 1,2-cyclobutadiene, 65; 1,2-cyclopentadiene, 51; 1,2-cyclohexadiene, 32; 1,2-cycloheptadiene, 14; 1,2-cyclooctadiene, 5; 1,2-cyclononadiene, 2; 1,2,4-cyclohexatriene, 34; and bicyclo[3.2.1]octa-2,3-diene, 39. For cyclic butatrienes, functional group strain estimates include: 1,2,3-cyclobutatriene, >100; 1,2,3-cyclopentatriene, 80; 1,2,3-cyclohexatriene, 50; 1,2,3-cycloheptatriene, 26; 1,2,3-cyclooctatriene, 17; and 1,2,3-cyclononatriene, 4. Barriers to interconversion of enantiomers in cyclic allenes are reduced with increasing strain. Newly predicted values include: 1,2-cyclopentadiene <1 kcal/mol and bicyclo[3.2.1]octa-2,3-diene, 7.4 kcal/mol. Estimated levels of strain parallel the known reactivity of these substances.  相似文献   
4.
This paper reports a series of chloride anion receptors containing two catechol head groups connected through their ortho-positions via a spacer chain. The linking group chosen to attach the spacer chain to the catechol units has a major impact on the anion-binding potential of the receptor. Linking groups that are capable of forming stable six-membered intramolecular hydrogen-bonded rings with the catechol O-H groups significantly inhibit the ability of the catechol units to hydrogen bond to chloride anions. However, where the linking groups are only capable of forming five- or seven-membered intramolecular hydrogen-bonded rings, then anion binding via hydrogen bonding through the catechol O-H groups becomes a possibility. This process is solvent dependent; the presence of competitive solvent (e.g., DMSO-d6) disrupts the intramolecular hydrogen-bonding pattern and enhances anion binding relative to simple unfunctionalized catechol. The most effective receptor is that in which the hydrogen-bonding linker (-CH2CONH-) is most distant from the catechol units and can only form a seven-membered intramolecular hydrogen-bonded ring. In this case, the receptor, which contains two catechol units, is a more effective chloride anion binder than simple unfunctionalized catechol, demonstrating that the two head groups, in combination with the N-H groups in the linker, act cooperatively and enhance the degree of anion binding. In summary, this paper provides insight into the hydrogen-bonding patterns in ortho-functionalized catechols and the impact these have on the potential of the catechol O-H groups to hydrogen bond to a chloride anion.  相似文献   
5.
The synthesis of dyes based on a highly substituted terephthalic acid core is described, starting from readily available 2,5-dihydroxy-terephthalic acid diethyl ester. The dyes are highly colored, soluble in organic solvents and reasonably fluorescent in solution and in the solid state. The maxima for absorption and emission are around 402 and 502 nm, respectively. The fluorophores are readily cyclized to generate compounds which comprise the basic 6,13-dihydroxy-chromeno[2,3-b]xanthene-7,14-dione unit. These new derivatives are nonfluorescent.  相似文献   
6.
7.
Anion binding by catechols--an NMR, optical and electrochemical study   总被引:1,自引:0,他引:1  
The X-ray structure of the ClC chloride channel made it clear that O-H...chloride interactions play a key role in important biological membrane-bound systems, however, surprisingly this type of interaction has only been rarely exploited for the development of synthetic anion receptors. This paper therefore reports the anion binding strengths and selectivities of some simple commercially available bis-phenols. In particular, we compare catechol (1,2-dihydroxybenzene) and resorcinol (1,3-dihydroxybenzene) which show interesting and different selectivities between the halide anions in acetonitrile solution. Catechol binds tetrabutylammononium (TBA) chloride almost 30 times more strongly than TBA bromide, whilst for resorcinol, this difference drops to a factor of ca. 3.5. It is suggested that this is a consequence of the bite angle of the chelating hydrogen bonding groups of catechol being particularly appropriate for effective binding of the smaller anion. The oxidation of catechol to ortho-quinone is perturbed by the addition of chloride anions, as probed via cyclic voltammetry, and this compound can therefore be considered to act as an electrochemical sensor for chloride. Nitrocatechol is able to bind chloride anions more strongly than catechol as a consequence of its enhanced acidity and hence greater hydrogen bond donor character. Furthermore, nitrocatechol senses the bound anion via changes in its UV-visible spectrum. Notably, binding still occurs even in the presence of small amounts of competitive solvents (e.g. water). This observation has biomimetic importance as wet acetonitrile has some similarity in terms of overall polarity and hydrogen bond competition to the solvent shielded interiors of biological macromolecules and membranes--such as the environment within the ClC chloride channel itself. Finally, we report that catechol undergoes a unique colorimetric response on the addition of basic anions, such as fluoride. We can assign this response as being due to oxidative degradation of catechol catalysed by the basic anions (which bind to, and deprotonate, the catechol). This process is somewhat analogous to the well-known metal catalysed oxidation of catechol which can take place in aqueous solution. The speed of response and easily monitored and distinctive colour change induced by fluoride anions indicates this may be a useful mechanism for exploitation in the development of selective fluoride sensors.  相似文献   
8.
9.
10.
The Einstein-conformally coupled scalar field system is studied in the presence of a cosmological constant. We consider a massless or massive scalar field with no additional self-interaction, and spherically symmetric black hole geometries. When the cosmological constant is positive, no scalar hair can exist and the only solution is the Schwarzschild–de Sitter black hole. When the cosmological constant is negative, stable scalar field hair exists provided the mass of the scalar field is not too large.  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