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91.
Crystal Structures of KNdTe4, RbPrTe4, and RbNdTe4 — Investigations concerning the Thermal Stability of KNdTe4 as well as some Remarks concerning Additional Representatives of the Composition ALnTe4 (A = K, Rb, Cs and Ln = Rare Earth Metal) Of the compounds ALnQ4 (A = Na, K, Rb, Cs; Ln = Lanthanoid; Q = S, Se and Te) the crystal structures of the three new tellurides KNdTe4, RbPrTe4 and RbNdTe4 were determined by X‐ray single‐crystal structure analysis and of the three additional new ones KCeTe4, KPrTe4 and CsNdTe4 by X‐ray powder diffraction experiments. All six new compounds are isotypic with KCeSe4. Characteristic for the crystal structure of the compounds mentioned above are layers built from (Q2)2— dumbbells in form of 4.32.4.3 nets with embedded cations A+ and Ln3+ between them, which are coordinated eightfold in form of square‐shaped antiprisms by Q ions. The distances Te‐Te within the dumbbells were found to be 277.8(2) pm for all investigated tellurides. By combination of X‐ray diffraction and DTA measurements it was shown that the compound KNdTe4 is metastable at ambient temperature with a limited existence range between the temperatures 260 and 498 °C. 相似文献
92.
Daniel Obrecht Christian Lehmann Ruth Ruffieux Peter Schnholzer Klaus Müller 《Helvetica chimica acta》1995,78(6):1567-1587
A series of novel open-chain and cyclic conformationally constrained (R)- and (S)-α,α-disubstituted tyrosine analogues 1a–e were synthesized in good yields and high optical purities (Schemes 1 and 2). The absolute configurations of these tyrosine analogues were unambiguously determined based on the X-ray structures of the precursor diastereoisomeric peptides of type 4 and 5 . Four of these structures are described (Figs. 1–4), showing β-turn type-I geometries for dipeptides 4a, 5b , and 4c and an extended conformation for peptide 5c (Table 3). The conversion of the free amino acids 1a–c into suitably protected building blocks 11a–d and 15d,e for peptide synthesis is discussed (Schemes 3 and 4). 相似文献
93.
94.
1-Aminopyrroles (4) are obtained by reaction of 1,2-diazabuta-1,3-dienes (1) and ynamines (2). This reaction represents one of the very rare examples of 1,3-cycloaddition processes of alkynes and hetero-1,3-dienes. 相似文献
95.
96.
1,3-Oxazolidin-4-ones and 1,3-oxazin-4-ones were synthesized by formal cyclocondensation of imines with α- or β-hydroxy acids. 相似文献
97.
Klaus Rakus Sergej P. Verevkin Hans-Dieter Beckhaus Christoph Rüchardt 《欧洲无机化学杂志》1994,127(11):2225-2234
The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile ( 10a ) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile ( 10b ) decomposed heterolytically into carbenium ions and (CN)3C− anions, while 9-methyl-9-(tricyanomethyl)fluorene ( 10c ) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH ( 10c ) = 155· 2 kJ mol−1, ΔS ( 10c ) = 58· 5 J mol−1 K−1]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile ( 2 ) (422.45 kJ mol−1), 2,2-dicyanohexanenitrile ( 3 ) (349.74 kJ mol−1), 2,2-dicyano-3-phenylpropionitrile ( 4 ) (540.75 kJ mol−1), 2-butyl-2-methylhexanentrile ( 5 ) (-133.20 kJ mol−1), 2,2-dimethylpentanenitrile ( 6 ) (-45.78 kJ mol−1), and 2-methylbutyronitrile ( 7 ) (2.44 kJ mol−1) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1 RSE ( 1 ) = -18 kJ mol−1 was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol−1) and α,α-dicyanoalkyl (27 kJ mol−1) radicals, which were recalculated from bond homolysis measurements[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2). 相似文献
98.
Watson MD Jäckel F Severin N Rabe JP Müllen K 《Journal of the American Chemical Society》2004,126(5):1402-1407
Cyclophanes with the largest-to-date polycyclic aromatic hydrocarbon (hexa-peri-hexabenzocoronene, HBC) to be entrained in such a structural motif are reported. The two disks are covalently captured by intermolecular ring-closing olefin metathesis of dienes in good yield. DSC, optical microscopy, and WAXD show the new cyclophanes to self-assemble to thermotropic columnar liquid crystal mesophases similar to monomeric analogues. Solution spectroscopic studies reveal that the two disks within a single unit lie face-to-face, with a small average lateral offset. Self-assembly into two-dimensional crystals at a solid-liquid interface was visualized by STM, and the electrical properties of single molecules were assessed by scanning tunneling spectroscopy revealing a diode-like behavior which is similar to that previously reported for single HBC disks, laying the groundwork for future electrical interrogations of dynamic molecular complexes. 相似文献
99.
An all-atom molecular dynamics simulation of rhodopsin in a membrane environment has been carried out with lipid composition similar to that of the retinal membrane. The initial conformation of the protein was taken from the X-ray crystallographic structure (1F88), while those of the lipids came from a previous molecular dynamics simulation. During the course of the 12.5 ns simulation, the initially randomly placed lipids adopt an anisotropic solvation structure around the protein. The lipids, having one saturated stearic acid chain and one polyunsaturated docosohexaenoic acid chain with a zwitterionic phosphatidylcholine headgroup, arrange themselves to maximize contact between the polyunsaturated chain and the protein surface. This organization is driven by energetically favorable interactions between the transmembrance helices and the docosohexaenoyl chains that are largely of the van der Waals type. These observations are consistent with various experimental studies on rhodopsin and other G-protein coupled receptors and with the picture of extreme flexibility in polyunsaturated fatty acid chains that has arisen from recent NMR and computational work. 相似文献
100.
Klaus Merz Annegret Knüfer 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(3):o187-o188
The title compound, C6H13O3P, displays a crystallographic mirror plane. Bond lengths in the phosphonic acid moiety are P—O = 1.5557 (13) Å and P=O = 1.5089 (18) Å. The molecules are linked via intermolecular hydrogen bonding to form a one‐dimensional chain of fused rings. There are no significant contacts between planes. 相似文献