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141.
Thomas Kern Roland Fischer Michaela Flock Karl Hassler 《Phosphorus, sulfur, and silicon and the related elements》2013,188(7):931-940
The synthesis and purification of novel bi(silacyclohexyls) C5H10SiX-XSiC5H10 with X = H (1), Ph (2), F (3), Cl (4), Br (5), and I (6), which are composed of two silacyclohexyl rings connected by a silicon–silicon single bond, is described. Molecular structures for 2, 4, 5, and 6 were determined employing X-ray crystallography. Surprisingly, the SiSi bond lengths increase in the order I < Br < Cl, contrary to what is observed for methylated disilanes Me2XSiSiXMe2. The bond lengthening can be traced back to 1, 3 nonbonded interactions between the halogen atoms and the axial hydrogen atoms in the SiC5 ring, which also cause a decrease of the SiSiX bond angles with increasing size of the halogen atom. Both rings substitute each other in the equatorial position for 4, 5, and 6, but in the axial position for 2. 相似文献
142.
The Ramanujan Journal - We describe a new connection between the dilogarithm function and the solutions of Pell’s equation $$x^2-ny^2 = \pm 1$$ . For each solution x, y to... 相似文献
143.
Proton transfer and dissociation processes following excitation of the OH or NH stretching modes of the proton-bound complex GlyLysH(+) are studied by classical trajectories. "On the fly" simulations with the PM3 semiempirical electronic structure method for the potential surface are used. Initial conditions are sampled to correspond to the v=1 excited state of the OH or NH stretching modes. Five different conformers of the complex are studied as initial structures. The main findings are (1) Photoinduced proton transfer is on the picosecond time scale. (2) Proton transfer is much faster than the processes of dissociation. (3) Proton transfer involves different sites. Most trajectories show sequences of two proton transfer events. (4) The proton transfer events show high selectivity with regard to the initially excited vibration and the initial structure. (5) Photodissociation of the complex occurs on a typical time scale of 100 ps. (6) Conformational transitions are found to be often faster than proton transfer. These results have implications for the mass spectrometry of complexes, for dynamics of proton wires, and for proton migration in proteins. 相似文献
144.
Dynamic Structures of DNA Heptamers with Different Central Trinucleotide Sequences Studied by Electrochemical and Spectral Methods
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Iveta Pilarova Iva Kejnovska Michaela Vorlickova Libuse Trnkova 《Electroanalysis》2014,26(10):2118-2128
A short, single‐stranded DNA fragment d(GCGAAGC) and its analogs d(GCNNNGC) carrying different trinucleotide sequences in the center of the molecule (AAA,CCC, GGG or TTT) were investigated by spectral (CD and UV spectra) and electrochemical methods (voltammetry, polyacrylamide gel electrophoresis). Our results show that (i) the central triplet GAA or AAA dramatically stabilizes DNA hairpins, (ii) a stem‐loop configuration is not supported by CCC or TTT sequences, instead, these heptamers adopt bimolecular duplex forms. Finally (iii), in the case of GGG sequence a very stable supramolecular G‐quadruplex is formed. 相似文献
145.
5-Methylenolether derivatives of pyrrolo[3,4-a]carbazoles were obtained from cycloadditions between 3-(1-methoxyvinyl)-1-tosylindole and N-substituted maleimides. They
were transformed into the hydroxy derivatives by treatment with H2SO4, selectively reduced to the ether by H2/Pd–C, and in the imide moiety by L-Selectride?. From the analogous BOC protected indole derivative the parent α,β-unsaturated ketones were obtained, which were transformed into hydroxyimino compounds,
and which could be deprotected by heating to the melting point. Deprotection of the tosyl derivatives was not successful.
The imide part of the molecule was hydrolyzed using methanolic NaOH. The stereochemistry of all products was elucidated mainly
by spectroscopic methods, and compared with results of calculations. 相似文献
146.
147.
Tiberiu Freniu Ana-Maria Rusu Michaela Ponta Sorin D. Anghel E. A. Cordos 《Fresenius' Journal of Analytical Chemistry》1996,355(3-4):254-255
A low to medium power radiofrequency capacitively coupled plasma is characterized as spectral source for atomic emission. The signal to background ratio and the limits of detection were determined for 19 elements as a function of the plasma torch geometry and the observation point. 相似文献
148.
The regioselective nitration of 25,27-dipropoxythiacalix[4]arene was carried out as a key step in the synthesis of thiacalix[4]arene derivative bearing two arylureido functions on the upper rim. The preorganisation of ureido units using the thiacalix[4]arene/calix[4]arene moieties as a molecular scaffold gave novel anion receptors. These compounds, albeit based on hydrogen bonding interactions, show good complexation ability even in highly HB-competitive solvent, such as DMSO. Direct comparison of otherwise identical structures 6a and 7a revealed remarkable dominance of the thiacalix[4]arene derivative over its classical analogue in anion binding. 相似文献
149.
Alexander Jaekel Mo Legelli Michaela Wirtz Daniel Meyer Nicole Schräder Kevin Streckel Stefan Lamotte 《Journal of separation science》2023,46(20):2300204
In striving for the best possible separation, the selectivity of stationary phases as an optimization parameter is often underestimated although there are many ways to influence this powerful tool. This review serves to provide an insight into the various ways of adapting the selectivity of a separation in liquid chromatography. Approaches via temperature and flow rate tuning are discussed as a basis followed by focusing on the stationary phase as the superior optimization parameter. Highly selective stationary phases hereby provide an advantage for groups of similar analytes. For more complex mixtures, separations can be improved using mixed-mode technologies where different retention mechanisms are combined. Serial coupling, mixed-bed columns, and stationary phase optimized selectivity liquid chromatography provide solutions to various degrees. Finally, the advantages of stationary phase tuning over adaption of mobile phase and/or temperature are presented in terms of optimum application range. 相似文献
150.
We consider a flexible class of space-time point process models—inhomogeneous shot-noise Cox point processes. They are suitable for modelling clustering phenomena, e.g. in epidemiology, seismology, etc. The particular structure of the model enables the use of projections to the spatial and temporal domain. They are used to formulate a stepwise estimation method to estimate different parts of the model separately. In the first step, the Poisson likelihood approach is used to estimate the inhomogeneity parameters. In the second and third steps, the minimum contrast estimation based on K-functions of the projected processes is used to estimate the interaction parameters. We study the asymptotic properties of the resulting estimators and formulate a set of conditions sufficient for establishing consistency and asymptotic normality of the estimators under the increasing domain asymptotics. 相似文献