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Horton-Garcia CF Pavlovskaya GE Meersmann T 《Journal of the American Chemical Society》2005,127(6):1958-1962
A wealth of information about porous materials and their void spaces has been obtained from the chemical shift data in (129)Xe NMR spectroscopy during the past decades. In this contribution, the only NMR active, stable krypton isotope (83)Kr (spin I = (9)/(2)) is explored as a novel probe for porous materials. It is demonstrated that (83)Kr NMR spectroscopy of nanoporous or microporous materials is feasible and straightforward despite the low gyromagnetic ratio and low abundance of the (83)Kr isotope. The (83)Kr line width in most of the studied cases is quadrupolar dominated and field-strength independent. A significant exception was found in calcium-exchanged zeolites where the field dependence of the line width indicates a distribution of isotropic chemical shifts that may be caused by long-range disorder in the zeolite structure. The (83)Kr chemical shifts observed in the investigated materials display a somewhat different behavior than that of their (129)Xe counterparts and should provide a great resource for the verification or refinement of current (129)Xe chemical shift theory. In contrast to xenon, krypton with its smaller atomic radius has been demonstrated to easily penetrate the porous framework of NaA. Chemical shifts and line widths of (83)Kr are moderately dependent on small fluctuations in the krypton loading but differ strongly between some of the studied samples. 相似文献
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In this paper a proof of existence and non-existence of theconically self-similar free-vortex solutions to the Navier-Stokesequations, originally found by Yih et al. (1982, Phys. Fluids.25, 2147-2158), is presented. This proof clearly establishesthat these solutions do not have any kind of singularity atthe symmetry axis. This analysis gives considerably improvedexistence and non-existence bounds and it is shown that thesebounds are close to optimal in the low-swirling limit. Thisapproach links the questions of existence and non-existencefor the swirling case and for the non-swirling case. The proof,which is an extension of techniques developed by Serrin (1972,Phil. Trans. R. Soc. Lond. 271, 325-360), is based on Schauder'sFixed Point Theorem and is, therefore, non-constructive. Therefore,the paper ends with a brief discussion of the question of howto compute the conically self-similar free-vortex solutionsto the Navier-Stokes equations. 相似文献
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