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排序方式: 共有698条查询结果,搜索用时 171 毫秒
151.
152.
Thomas Criton Dr. Debora Vilona Dr. Guy Jacob Dr. Maurice Médebielle Prof. Elise Dumont Dr. Lionel Joucla Dr. Emmanuel Lacôte 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(11):3670-3674
Polyazanes (i.e., higher nuclearity homologues of hydrazines) with increasing numbers of bound nitrogen atoms (from 3 to 5), including the first pentazane ever described, were prepared by the addition of lower-order polyazanes to diazo reagents. A structure was obtained. It was shown that the polynitrogen chains adopt a helical conformation. DFT modeling shows that the arrangement persists in solution. Although the polyazanes are all reducing agents, they become less so as the number of nitrogens increases. 相似文献
153.
ABSTRACT The preparation of an iodinated derivative of D-glucose where a β-iodoethyl moiety has been introduced at O-5 is presented. In such an analogue the existence of pyranose forms is precluded and the iodinated tag lies in a region of the carbohydrate not judged essential for recognition by the glucose transport protein; also noteworthy in such a compound is the stability of the carbon-iodine bond, a prerequisite for its potential use in SPECT medical imaging. 相似文献
154.
An efficient parallel spectral method for direct numerical simulations of transitional and turbulent flows is described in this paper. The parallelization is classically based on a bidimensional domain decomposition, but has been specifically developed for a solenoidal Fourier–Chebyshev spectral approximation where in one Fourier direction, the number of modes is very large compared with the two other directions. The approach therefore differs from classical libraries developed for cubic Fourier boxes. The strategy uses message‐passing interface (MPI) for message‐passing among nodes and is fairly portable. One of the originalities of this paper is the use of an efficient hybrid programming with MPI for internodes communications and a coarse grain parallelism using OpenMP for core shared‐memory computation, instead of the classical hybrid programming with MPI and a fine granularity parallelism at the loop level with OpenMP directives. This hybrid parallelism has been tested on the recent generation of high‐performance parallel supercomputers involving a few tens of cores per node. Performances are evaluated on different low‐frequency and high‐frequency processors massively parallel platforms. We demonstrate that spectral methods, which are known to be inherently ill‐fitted for the new generation of high‐performance distributed‐memory computers, can be implemented efficiently using this hybrid programming with good scalability and a very fast wall‐clock time per iteration. New numerical experiments are therefore now accessible on petascale computers, while keeping the attractive features of spectral methods such as accuracy, exponential convergence, computational efficiency and conservative properties. This is illustrated by a direct numerical simulation of the transition of the boundary layers developing from the entrance section of a plane channel and interacting to merge into a fully turbulent flow. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
155.
Amandine Fabrello Chiara Dinoi Lionel Perrin Philippe Kalck Laurent Maron Martine Urrutigoity Odile Dechy‐Cabaret 《Magnetic resonance in chemistry : MRC》2010,48(11):848-856
103Rh NMR represents a powerful tool to assess the global electronic and steric contribution of diphosphine ligands on [Rh(COD)(diphosphine)]+ complexes. In the case of DIOP, BINAP and MeDUPHOS, this approach proved to be more informative than classical CO‐stretching frequency measurements. After validation, this method has been extended to a set of seven diphosphines. 103Rh NMR measurements on [Rh(COD)(diphosphine)]PF6 lead to the following order of donor properties: dppe > MeBPE > MeDUPHOS > dppb > DIOP > BINAP > Tol‐BINAP. This trend has been validated by DFT in the case of DIOP, BINAP and MeDUPHOS. In conjunction, 31P NMR chemical shift has been shown to reflect the ring constraints of the Rh‐diphosphine scaffold. This contribution is a step towards a mechanistic investigation of the catalytic hydrogenation of unsaturated substrates by 103Rh NMR and DFT. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
156.
Ha-Thi MH Shafizadeh N Poisson L Soep B 《Physical chemistry chemical physics : PCCP》2010,12(45):14985-14993
The time evolution of electronically excited heme (iron II protoporphyrin IX, [Fe(II) PP]) and its associated salt hemin (iron III protoporphyrin IX chloride, [Fe(III) PP-Cl]), has been investigated for the first time in the gas phase by femtosecond pump-probe spectroscopy. The porphyrins were excited at 400 nm in the S(2) state (Soret band) and their relaxation dynamics was probed by multiphoton ionization at 800 nm. This time evolution was compared with that of the excited state of zinc protoporphyrin IX [Zn PP] whose S(2) excited state likely decays to the long lived S(1) state through a conical intersection, in less than 100 fs. Instead, for [Fe(II) PP] and [Fe(III) PP-Cl], the key relaxation step from S(2) is interpreted as an ultrafast charge transfer from the porphyrin excited orbital π* to a vacant d orbital on the iron atom (ligand to metal charge transfer, LMCT). This intermediate LMCT state then relaxes to the ground state within 250 fs. Through this work a new, serendipitous, preparation step was found for Fe(II) porphyrins, in the gas phase. 相似文献
157.
158.
Lionel Schwartz 《Comptes Rendus Mathematique》2009,347(11-12):599-602
One constructs minimal injective resolutions for certain unstable modules that appear to be the mod 2 cohomology of Thom spectra. The terms of the resolution are tensor products of Brown–Gitler modules and Steinberg modules introduced by S. Mitchell and S. Priddy. A combinatorial result of Andrews shows that the alternating sum of the Poincaré series of the considered modules is zero. One gives homotopical applications of this result. To cite this article: D.H.H. Nguyen et al., C. R. Acad. Sci. Paris, Ser. I 347 (2009). 相似文献
159.
Allouche L Marquis A Lehn JM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(28):7520-7525
The translational diffusion coefficients of various helicates have been determined by using NMR diffusion spectroscopy (Diffusion Ordered SpectroscopY, DOSY), in order to investigate the individual behaviour of the helicates according to their length (different nuclearities; 1-5 metals), to the nature of the metal involved (CuI or AgI), as well to their bulkiness due to the presence of substituents on the periphery of the assembly. Furthermore, the spectrum of a mixture of helicates belonging to the same series, but with different lengths and nuclearities, showed the signals of each component, with no observable cross-linking, confirming the self-recognition properties of the helicates. 相似文献
160.
Chemical retention, i.e., partition of the element between aqueous solution and mineral surface, is a key phenomenon for assessing the safety of possible nuclear waste disposal. For this purpose, the sorption of Eu(III) onto a model mineral-alpha-alumina-is studied here, including the effects of groundwater chemistry: pH and concentrations of small organic and inorganic ligands (acetate, oxalate, and carbonate anions). This work presents some experimental evidence for a synergic mechanism of sorption of europium-ligand complexes onto the alumina. Only cationic complexes were necessary to consider to model experimental results. Using the ion-exchange theory (IET) and a corresponding restricted set of parameters-exchange capacities and thermodynamic equilibrium constants-the whole set of sorption experiments of Eu(III) cationic species onto the alpha-alumina was modeled under various chemical conditions. 相似文献