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71.
Jean Rodolphe Chouna Pepin Alango Nkeng‐Efouet Bruno Ndjakou Lenta Jean Duplex Wansi Beate Neumann Hans‐Georg Stammler Samuel Fon Kimbu Norbert Sewald 《Helvetica chimica acta》2011,94(6):1071-1076
Two new endiandric acid derivatives, beilschmiedic acid F ( 1 ) and beilschmiedic acid G ( 2 ), together with three known constituents, beilschmiedic acid A, beilschmiedic acid C, and sitosterol 3‐β‐D ‐glucopyranoside, were isolated from the stem bark of Beilschmiedia anacardioides. Their structures were elucidated mainly by using a combination of 1D‐ and 2D‐NMR techniques. The structure and relative configuration of beilschmiedic acid G ( 2 ) was also confirmed by X‐ray crystallographic analysis. 相似文献
72.
We describe the catalytic activity of the first chimeric ligase containing a foldameric sequence of β- and γ-amino acids. The chimeric backbone provides for the spatial arrangement of all functional groups involved in the formation of the catalytic site to allow efficient catalysis to take place. Our finding indicates significant progress in the field of functionally active artificial motifs. 相似文献
73.
74.
Frank Strübe Jochen Mattay Beate Neumann Hans‐Georg Stammler 《Acta Crystallographica. Section C, Structural Chemistry》2011,67(1):o33-o36
Fulgides are a representative class of photochromic organic molecules which exhibit several interesting properties for diverse applications in fields such as data storage or high‐resolution spectroscopy. The crystal structures of three furyl fulgides with different steric constraints were determined and for two of the compounds both the E and Z isomer structures were defined. The compounds are 3‐[(E)‐1,3‐dimethyl‐4,5,6,7‐tetrahydro‐2‐benzofuran‐4‐ylidene]‐4‐isopropylidenetetrahydrofuran‐2,5‐dione, C17H18O4, (I‐E), 3‐[(E)‐1,3‐dimethyl‐5,6,7,8‐tetrahydro‐4H‐cyclohepta[c]furan‐4‐ylidene]‐4‐isopropylidenetetrahydrofuran‐2,5‐dione, C18H20O4, (II‐E), and the Z isomer, (II‐Z), and 3‐isopropylidene‐4‐[(E)‐1‐(5‐methoxy‐2‐methyl‐1‐benzofuran‐3‐yl)ethylidene]tetrahydrofuran‐2,5‐dione, C19H18O5, (III‐E), with two molecules in the asymmetric unit, and the Z isomer, (III‐Z). The structures of the E and Z isomers show only little differences in the bond lengths and angles inside the hexatriene unit. Because of the strained geometry there are deviations in the torsion angles. Furthermore, small differences in the distances between the bond‐forming C atoms in the electrocyclization process give no explanation for the unequal photochromic behaviour. 相似文献
75.
Wolfgang Drechsler 《Foundations of Physics》1993,23(2):261-276
Is Einstein's metric theory of gravitation to be quantized to yield a complete and logically consistent picture of the geometry of the real world in the presence of quantized material sources? To answer this question, we give arguments that there is a consistent way to extend general relativity to small distances by incorporating further geometric quantities at the level of the connection into the theory and introducing corresponding field equations for their determination, allowing thereby the metric and the Levi-Civita connection to remain classical quantities. The dualism between matter and geometry is extended to quantized fields with the help of a Hibert bundle ? raised over a Riemann-Cartan spacetime. Quantized subnuclear matter fields (generalized quantum mechanical wave functions) are sections on ? which determine generalized bilinear currents acting as sourc currents for the bundle geometry at small distances. The established dualism between matter and the underlying bundle geometry contains general relativity as a classical part. 相似文献
76.
77.
We study the regularity of solutions of interface problems for the Poisson equation in axisynunetric domains. The Fourier decomposition of the 3D-problem into a sequence of 2D-variational equations end uniform (with respect to the sequence parameter) a prior; estimates of their solutions are derived. Some non-tensorial singular functions describing the behaviour of the solution near interface edges are given and the smoothness of the stress intensity distribution as well as the tangential regularity are characterized in tenns of Sobolev spaces. In a forthcoming part II of this paper, the results will be applied to error estimates of the so-called Fourier-finite-element method for solving approximately elliptic interface problems in 3D. 相似文献
78.
79.
The influence of a coadsorbed submonolayer (carbon) on the diffusion of adatoms (lead) along a crystal surface (tungsten) is studied in a preliminary manner by a field electron microscope technique. Experiments show that the surface diffusion of lead is strongly affected by the coadsorption of carbon: (1) The diffusion of Pb which is easy around (110) and (111) and difficult around (100) on clean tungsten becomes easier around (100) than around (110) and (111). (2) The diffusion anisotropy on the stepped surface around (100) is changed by carbon adsorption. (3) C adatoms enhance the diffusion of Pb especially around (100). (4) The mean carbon coverage required to observe these effects is only a few hundredths of a monolayer. The dependence of the surface diffusion on such small impurity coverages is discussed. 相似文献
80.
F. S. Drechsler 《The Journal of the Operational Research Society》1968,19(4):409-419
Every system, left unto itself, tends towards disorder. Disorder frequently appears in the form of queues-machines, materials or customers waiting for service or waiting to be served. This article suggests a method of measuring disorder, and shows how this may be used in the analysis of complex decision type of systems. 相似文献