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排序方式: 共有1192条查询结果,搜索用时 15 毫秒
271.
Henriette Grger Christine Lode Heike Vollmer Harald Krautscheid Sergej Lebedkin 《无机化学与普通化学杂志》2002,628(1):57-62
Bromoplumbates with One‐dimensional Polymeric and Isolated Anions: (Bzl4P)2[Pb3Br8], (Bzl4P)2[Pb3Br8(dmf)2], (Bzl4P)[PbBr3], (Bzl4P)2[PbBr4], and (Bzl4P)4[Pb2Br6][PbBr4] PbBr2 reacts with LiBr and (Bzl4P)(PF6) (Bzl = CH2C6H5) in acetone to form a series of bromoplumbate complexes with compositions and structures depending on the conditions of reaction and crystallization. While the anions in (Bzl4P)2[Pb3Br8] ( 1 ) and (Bzl4P)[PbBr3] ( 2 ) are one‐dimensional polymers with penta‐ and hexacoordinated Pb atoms, the metal atoms in the mono‐ and dinuclear complex anions of (Bzl4P)2[PbBr4] · 2acetone ( 3 · 2acetone) and (Bzl4P)4[Pb2Br6][PbBr4] ( 4 ) bind to four bromo ligands. From DMF as a solvent (Bzl4P)2[Pb3Br8(dmf)2] ( 1 b ) crystallizes with the same bromoplumbate structure as in 1 a , but with dmf ligands occupying the coordination sites vacant in 1 a . Upon radiation of compound 3 with ultraviolet light greenish yellow photoluminescence (emssion maximum at 547 nm) is observed. Crystallographic details see “Inhaltsübersicht”. 相似文献
272.
Doris Kunz 《Chemie in Unserer Zeit》2009,43(4):224-230
Click‐Chemistry is a concept in organic synthesis that uses a limited amount of very reliable reactions on a broad variety of substrates. This leads within a few steps to molecules of high diversity which is important to accelerate drug‐discovery. Besides favourable thermodynamic requirements, the criteria for a click‐reaction include also simple reaction conditions and a quick and easy (almost unnecessary) workup. The most successful of these click‐reactions is the copper catalysed 1, 3‐dipolar cycloaddition of azides and alkynes, which in the meantime is used in most diverse areas of chemistry. The mechanistic ideas of this reaction is looked at in more detail. 相似文献
273.
Marinone MC Marque SM Suárez DA Diéguez Mdel C Pérez P De Los Ríos P Soto D Zagarese HE 《Photochemistry and photobiology》2006,82(4):962-971
This article explores the potential role of UV radiation (UVR) as an influence on zooplankton communities. In the first section we provide a general overview of UVR effects on freshwater zooplankton, with an emphasis on Argentine and Chilean environments. In the second section we present the results of a survey involving 53 temperate lakes across a gradient of UVR exposure to determine patterns of species richness and specific diversity. These community characteristics decreased at high potential UVR exposure (i.e. high mean water column irradiance or low lake optical density). A threshold value of mean water column irradiance of approximately 10% of the surface level seems to limit both richness and diversity to minimum values. On the basis of the collected evidence it is not possible to definitely conclude that UVR rather than another covarying factor is responsible for the decrease in specific diversity observed at the lowest end of lake optical depth. However, lakes with values above the previous threshold are likely to exhibit highly depauperate zooplankton communities regardless of the mechanism. As a cautionary note we suggest that changes in the optical characteristics (i.e. changes due to atmospheric conditions, precipitation patterns or vertical displacement of the tree line) may result in sudden shifts in zooplankton community structure. 相似文献
274.
Bradley CA Veiros LF Pun D Lobkovsky E Keresztes I Chirik PJ 《Journal of the American Chemical Society》2006,128(51):16600-16612
Treatment of the eta9,eta5-bis(indenyl)zirconium sandwich complex, (eta9-C9H5-1,3-(SiMe3)2)(eta5-C9H5-1,3-(SiMe3)2)Zr, with dialkyl ethers such as diethyl ether, CH3OR (R=Et, nBu, tBu), nBu2O, or iPr2O resulted in facile C-O bond scission furnishing an eta5,eta5-bis(indenyl)zirconium alkoxy hydride complex and free olefin. In cases where ethylene is formed, trapping by the zirconocene sandwich yields a rare example of a crystallographically characterized, base-free eta5,eta5-bis(indenyl)zirconium ethylene complex. Observation of normal, primary kinetic isotope effects in combination with rate studies and the stability of various model compounds support a mechanism involving rate-determining C-H activation to yield an eta5,eta5-bis(indenyl)zirconium alkyl hydride intermediate followed by rapid beta-alkoxide elimination. For isolable eta6,eta5-bis(indenyl)zirconium THF compounds, thermolysis at 85 degrees C also resulted in C-O bond cleavage to yield the corresponding zirconacycle. Both mechanistic and computational studies again support a pathway involving haptotropic rearrangement to eta5,eta5-bis(indenyl)zirconium intermediates that promote rate-determining C-H activation and ultimately C-O bond scission. 相似文献
275.
Sarah K?hler Tim Liebert Thomas Heinze Antje Vollmer Petra Mischnick Eugen M?llmann Winfried Becker 《Cellulose (London, England)》2010,17(2):437-448
Water-soluble hydroxyalkyl cellulose with a molar degree of substitution of up to 2.79 was prepared under completely homogeneous
reaction conditions in various ionic liquids without addition of inorganic bases. In acetate containing solvents the IL acts
as a catalyst. The substitution patterns of the cellulose ethers were analyzed by 13C NMR spectroscopy, 1H NMR spectroscopy after peracetylation and GLC/MS after permethylation and depolymerization. A diminished tendency towards
the formation of side chains compared to heterogeneously prepared hydroxyalkyl celluloses in the presence of inorganic bases
was found. 相似文献
276.
Kornherr A Vogtenhuber D Ruckenbauer M Podloucky R Zifferer G 《The Journal of chemical physics》2004,121(8):3722-3726
We present a model combining ab initio concepts and molecular dynamics simulations for a more realistic treatment of complex adsorption processes. The energy, distance, and orientation of water molecules adsorbed on stoichiometric and reduced rutile TiO(2)(110) surfaces at 140 K are studied via constant temperature molecular dynamics simulations. From ab initio calculations relaxed atomic geometries for the surface and the most probable adsorption sites were derived. The study comprises (i) large two-dimensional surface supercells, providing a realistically low concentration of surface oxygen defects, and (ii) a water coverage sufficiently large to model the onset of the growth of a bulk phase of water on the surface. By our combined approach the influence of both, the metal oxide surface, below, and the bulk water phase, above, on the water molecules forming the interface between the TiO(2) surface and the water bulk layer is taken into account. The good agreement of calculated adsorption energies with experimental temperature programmed desorption spectra demonstrates the validity of our model. 相似文献
277.
Li W Shavel A Guzman R Rubio-Garcia J Flox C Fan J Cadavid D Ibáñez M Arbiol J Morante JR Cabot A 《Chemical communications (Cambridge, England)》2011,47(37):10332-10334
An oriented attachment and growth mechanism allows an accurate control of the size and morphology of Cu(2-x)S nanocrystals, from spheres and disks to tetradecahedrons and dodecahedrons. The synthesis conditions and the growth mechanism are detailed here. 相似文献
278.
Kern T Hediger S Müller P Giustini C Joris B Bougault C Vollmer W Simorre JP 《Journal of the American Chemical Society》2008,130(17):5618-5619
Solid-state NMR spectroscopy is applied to intact peptidoglycan sacculi of the Gram-negative bacterium Escherichia coli. High-quality solid-state NMR spectra allow atom-resolved investigation of the peptidoglycan structure and dynamics as well as the study of protein-peptidoglycan interactions. 相似文献
279.
Doris Ellmer Manuel Brehs Mahmood Haj‐Yahya Hilal A. Lashuel Christian F. W. Becker 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(6):1630-1634
A variety of methods have been employed to study the impact of posttranslational modifications on Tau protein function. Here, a semisynthesis strategy is described that enables selective modification within the central repeat domain of Tau4 (residues 291‐321), comprising a major interaction motive with tubulin as well as one of the key hexapeptides involved in Tau aggregation. This strategy has led to the preparation of four semisynthetic Tau variants with phosphoserine residues in different positions and one with a so far largely ignored carboxymethyllysine modification that results from a non‐enzymatic posttranslational modification (nPTM). The latter modification inhibits tubulin polymerization but exhibits an aggregation behavior very similar to unmodified Tau. In contrast, phosphorylated Tau variants exhibit similar binding to tubulin as unmodified Tau4 but show lower tendencies to aggregate. 相似文献
280.
Abstract Prolonged treatment of tetra-O-acetyl-1, 5-anhydro-hex-1-enitols (“tetra-O-acetyl-hydroxy-glycals”) 3 and 5 with BF3 in CH2Cl2 at RT lead to anomeric mixtures of the title compounds 2 and 4a, the α-anomer 4a dominating. Reaction of 5 gave the higher yields of 4a (71%) and 2 (12%), the results being accounted mechanistic grounds. The same reaction performed in an aromatic solvent, like toluene, gave rise to competing C-alkylation., The ortho and para-tolyl derivatives 6 and 7, also with enone structure, were isolated in a combined maximum yield of 40% from 5. β-Enone 2 was also prepared in moderate yield by thermolysis of β-d-glucopyranose pentaacetate (1). In this case no α-anomer 4a was detected. 相似文献