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Zagdoun A Casano G Ouari O Lapadula G Rossini AJ Lelli M Baffert M Gajan D Veyre L Maas WE Rosay M Weber RT Thieuleux C Coperet C Lesage A Tordo P Emsley L 《Journal of the American Chemical Society》2012,134(4):2284-2291
A new nitroxide-based biradical having a long electron spin-lattice relaxation time (T(1e)) has been developed as an exogenous polarization source for DNP solid-state NMR experiments. The performance of this new biradical is demonstrated on hybrid silica-based mesostructured materials impregnated with 1,1,2,2-tetrachloroethane radical containing solutions, as well as in frozen bulk solutions, yielding DNP enhancement factors (ε) of over 100 at a magnetic field of 9.4 T and sample temperatures of ~100 K. The effects of radical concentration on the DNP enhancement factors and on the overall sensitivity enhancements (Σ(?)) are reported. The relatively high DNP efficiency of the biradical is attributed to an increased T(1e), which enables more effective saturation of the electron resonance. This new biradical is shown to outperform the polarizing agents used so far in DNP surface-enhanced NMR spectroscopy of materials, yielding a 113-fold increase in overall sensitivity for silicon-29 CPMAS spectra as compared to conventional NMR experiments at room temperature. This results in a reduction in experimental times by a factor >12,700, making the acquisition of (13)C and (15)N one- and two-dimensional NMR spectra at natural isotopic abundance rapid (hours). It has been used here to monitor a series of chemical reactions carried out on the surface functionalities of a hybrid organic-silica material. 相似文献
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Joint Scalar versus Joint Velocity-Scalar PDF Simulations of Bluff-Body Stabilized Flames with REDIM
Two transported PDF strategies, joint velocity-scalar PDF (JVSPDF) and joint scalar PDF (JSPDF), are investigated for bluff-body
stabilized jet-type turbulent diffusion flames with a variable degree of turbulence–chemistry interaction. Chemistry is modeled
by means of the novel reaction-diffusion manifold (REDIM) technique. A detailed chemistry mechanism is reduced, including
diffusion effects, with N
2 and CO
2 mass fractions as reduced coordinates. The second-moment closure RANS turbulence model and the modified Curl’s micro-mixing
model are not varied. Radiative heat loss effects are ignored. The results for mean velocity and velocity fluctuations in
physical space are very similar for both PDF methods. They agree well with experimental data up to the neck zone. Each of
the two PDF approaches implies a different closure for the velocity-scalar correlation. This leads to differences in the radial
profiles in physical space of mean scalars and mixture fraction variance, due to different scalar flux modeling. Differences
are visible in mean mixture fraction and mean temperature, as well as in mixture fraction variance. In principle, the JVSPDF
simulations can be closer to physical reality, as a differential model is implied for the scalar fluxes, whereas the gradient
diffusion hypothesis is implied in JSPDF simulations. Yet, in JSPDF simulations, turbulent diffusion can be tuned by means
of the turbulent Schmidt number. In the neck zone, where the turbulent flow field results deteriorate, the joint scalar PDF
results are in somewhat better agreement with experimental data, for the test cases considered. In composition space, where
results are reported as scatter plots, differences between the two PDF strategies are small in the calculations at hand, with
a little more local extinction in the joint scalar PDF results. 相似文献
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A. Maas J. Wambach B. Grüter R. Alkofer 《The European Physical Journal C - Particles and Fields》2004,37(3):335-357
The infrared properties of the high-temperature limit of Landau-gauge Yang-Mills theory are investigated. In a first step the high-temperature limit of the Dyson-Schwinger equations is taken. The resulting equations are identical to the Dyson-Schwinger equations of the dimensionally reduced theory, a three-dimensional Yang-Mills theory coupled to an effective adjoint Higgs field. These equations are solved analytically in the infrared and ultraviolet, and numerically for all Euclidean momenta. We find infrared enhancement for the Faddeev-Popov ghosts, infrared suppression for transverse gluons and a mass for the Higgs. These results imply long-range interactions and over-screening in the chromomagnetic sector of high-temperature Yang-Mills theory while in the chromoelectric sector only screening is observed.Received: 5 August 2004, Published online: 21 September 2004 相似文献
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The collision-induced dissociation reactions of the monocarboxylate anions, generated from the dimethyl and diethyl esters of glutaric acid and its 3,3-dimetbyl analogue in a chemical ionization source, were studied as a function of the potential applied to the collision cell in combination with 2H labelling experiments. It was shown that many of the product anions are formed in consecutive steps. The mechanisms associated with these steps appear to be initiated by a functional group interaction between the carboxylate anion and the ester group, 1,5-hydrogen migrations both to the carboxylate anion and the uncharged ester group and charge remote fragmentation. In one of the collision-induced dissociation channels a methyl anion is generated as a granddaughter product anion, which contains the hydrogen atoms exclusively originating from positions 2 and 4 as shown by the applied 2H labelling. 相似文献