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71.
72.
Matthias Albert Dieter Seebach Elke Duchardt Harald Schwalbe 《Helvetica chimica acta》2002,85(2):633-658
Oligomers of 3‐hydroxyalkanoic acids that contain two, three, and six residues with and without O‐terminal (tBu)Ph2Si and C‐terminal PhCH2 protection have been synthesized in such a way that the side chains on the oligoester backbone were those of the proteinogenic amino acids Ala (Me), Val (CHMe2), and Leu (CH2CHMe2). The enantiomerically pure 3‐hydroxyalkanoates were obtained by Noyori hydrogenation of the corresponding 3‐oxo‐alkanoates with [Ru((R)‐binap)Cl2](binap=2,2′bis(diphenylphosphanyl)‐1,1′‐binaphthalene)/H2 (Scheme 1), and the coupling was achieved under the conditions (pyridine/(COCl)2, CH2Cl2, −78°) previously employed for the synthesis of various oligo(3‐hydroxybutanoic acids) (Schemes 2 and 3). The Cotton effects in the CD spectra of the new oligoesters provided no hints about chiral conformation (cf. a helix) in MeOH, MeCN, octan‐1‐ol, or CF3CH2OH solutions (Figs. 1 and 2). Detailed NMR investigations in CDCl3 solution (Figs. 3–6, and Tables 1–5) of the hexa(3‐hydroxyalkanoic acid) with the side chains of Val (HC), Ala (HB), Leu (HH), Val, Ala, Leu (from O‐ to C‐terminus; 3 ) gave, on the NMR time‐scale, no evidence for the presence of any significant amount of a 21‐ or a 31‐helical conformation, comparable to those identified in stretched fibers of poly[(R)‐3‐hydroxybutanoic acid], or in lamellar crystallites and in single crystals of linear and cyclic oligo[(R)‐3‐hydroxybutanoic acids], or in the corresponding β‐peptide(s) (the oligo(3‐aminoalkanoic acid) analogs; 1 – 3 ). Thus, the extremely high flexibility (averaged or ‘random‐coil' conformation) of the polyester chain (CO−O rotational barrier ca. 13 kcal/mol; no hydrogen bonding), as compared to polyamide chains (CO−NH barrier ca. 18 kcal/mol; hydrogen bonding) has been demonstrated once again. The possible importance of this structural flexibility, which goes along with amphiphilic properties, for the role of PHB in biology, in evolution, and in prebiotic chemistry is discussed. Structural similarities of natural potassium‐channeling proteins and complexes of oligo(3‐hydroxybutanoates) with Na+, K+, or Ba2+ are alluded to (Figs. 7–9). 相似文献
73.
74.
75.
Synthesis and Structure of Mo2NCl7 The reaction of VN with MoCl5 at 175 °C in a sealed glass ampoule yields the molybdenum(V) nitride chloride Mo2NCl7 in form of air sensitive black crystals with the triclinic space group P1¯ and a = 905.7(8); b = 975.4((6); c = 1283.4(8) pm, α = 103.13(4)°; β = 109.83(5)° und γ = 98.58(5)°. The crystal structure is built up from dinuclear units [Mo2N2Cl7]3— and [Mo2Cl7]3+, which are connected by asymmetric nitrido bridges to form endless chains. Within both dinuclear units the Mo atoms are bridged by three Cl atoms resulting in a Mo‐Mo distance of 349.2(3) pm in the unit [Mo2N2Cl7]3—. In case of [Mo2Cl7]3+, however, a shorter Mo‐Mo distance of 289.4(3) pm is observed, which can be interpreted by a single bond. Correspondingly a reduced magnetic moment of 0.95 B.M. per Mo atom is observed. 相似文献
76.
Alexander Donchev Andreas Schnepf Elke Baum Gregor Stßer Hansgeorg Schnckel 《无机化学与普通化学杂志》2002,628(1):157-161
[Ga6R8]2– (R = SiPh2Me): A Metalloid Cluster Compound with an Unexpected Ga6‐Frame The reaction of a metastable solution of GaBr with a solution of LiSiPh2Me in a toluene/THF mixture results in orange coloured crystals of [Ga6(SiPh2Me)8]2– · 2 [Li(THF)4]+ ( 1 ). The unexpected structure of the planar Ga6 frame (C2h) could also be realized with the help of DFT calculation. DFT calculations furthermore show that 1 is energetically favoured against an octahedral Ga6R62– species and R2. In contrast calculations for the similar Al and B species show that in these cases the octahedral entities are favoured. These results demonstrate that even for similar compounds of B, Al, and Ga Wade rules are too general and that they cannot predict the correct structure. Moreover the atomic arrangement within 1 shows that a structure is preferred which is also present in allotropic β‐Ga and that therefore clusters of this type should be called metalloid or more general elementoid. 相似文献
77.
78.
Manfredo Hrner Vanessa S. Carratu Jairo Bordinhao Angela Silva Elke Niquet 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(3):m140-m142
In the title compound, [Cd(C12H8F2N3)2(C5H5N)2], the Cd atom lies on a crystallographic twofold axis in space group Iba2. The coordination geometry about the CdII ion corresponds to a rhombically distorted octahedron, with two deprotonated 1,3‐bis(2‐fluorophenyl)triazenide ions, viz. FC6H4NNNC6H4F−, acting as bidentate ligands (four‐electron donors). Two neutral pyridine (py) molecules complete the coordination sphere in positions cis with respect to one another. The triazenide ligand is not planar (r.m.s. deviation = 0.204 Å), the dihedral angle between the phenyl rings of the terminal 2‐fluorophenyl substituents being 24.6 (1)°. The triazenide and pyridine Cd—N distances are 2.3757 (18)/2.3800 (19) and 2.3461 (19) Å, respectively. Intermolecular C—H⋯F interactions generate sheets of molecules in the (010) plane. 相似文献
79.
We consider the topological space of all weighted composition operators on weighted Bergman spaces of infinite order endowed
with the operator norm. We show that the set of compact weighted composition operators is path connected. Furthermore, we
find conditions to ensure that two weighted composition operators are in the same path connected component if the difference
of them is compact. Moreover, we compare the topologies induced by L(H∞) and L(H∞v) on the space of bounded composition operators and give a sufficient condition for a composition operator to be isolated. 相似文献
80.
Iwan Cornelius Susanna Guatelli Pauline Fournier Jeffrey C. Crosbie Manuel Sanchez del Rio Elke Bräuer‐Krisch Anatoly Rosenfeld Michael Lerch 《Journal of synchrotron radiation》2014,21(3):518-528
Microbeam radiation therapy (MRT) is a synchrotron‐based radiotherapy modality that uses high‐intensity beams of spatially fractionated radiation to treat tumours. The rapid evolution of MRT towards clinical trials demands accurate treatment planning systems (TPS), as well as independent tools for the verification of TPS calculated dose distributions in order to ensure patient safety and treatment efficacy. Monte Carlo computer simulation represents the most accurate method of dose calculation in patient geometries and is best suited for the purpose of TPS verification. A Monte Carlo model of the ID17 biomedical beamline at the European Synchrotron Radiation Facility has been developed, including recent modifications, using the Geant4 Monte Carlo toolkit interfaced with the SHADOW X‐ray optics and ray‐tracing libraries. The code was benchmarked by simulating dose profiles in water‐equivalent phantoms subject to irradiation by broad‐beam (without spatial fractionation) and microbeam (with spatial fractionation) fields, and comparing against those calculated with a previous model of the beamline developed using the PENELOPE code. Validation against additional experimental dose profiles in water‐equivalent phantoms subject to broad‐beam irradiation was also performed. Good agreement between codes was observed, with the exception of out‐of‐field doses and toward the field edge for larger field sizes. Microbeam results showed good agreement between both codes and experimental results within uncertainties. Results of the experimental validation showed agreement for different beamline configurations. The asymmetry in the out‐of‐field dose profiles due to polarization effects was also investigated, yielding important information for the treatment planning process in MRT. This work represents an important step in the development of a Monte Carlo‐based independent verification tool for treatment planning in MRT. 相似文献