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排序方式: 共有181条查询结果,搜索用时 15 毫秒
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Wiedemann SH Lewis JC Ellman JA Bergman RG 《Journal of the American Chemical Society》2006,128(7):2452-2462
Evidence is presented for a proposed mechanism of C-H activation of 3-methyl-3,4-dihydroquinazoline (1) by (PCy(3))(2)RhCl. One intermediate (3), a coordination complex of 1 with (PCy(3))(2)RhCl, was identified along the path to the Rh-N-heterocyclic carbene product of this reaction (2). Isotopic labeling and reaction-rate studies were used to demonstrate that C-H activation takes place intramolecularly on the reaction coordinate between 3 and 2. Computational studies corroborate the proposed mechanism and suggest that the rate-limiting step is oxidative addition of the C-H bond to the metal center. The consequences of this mechanism for coupling reactions of N-heterocycles that occur via Rh-catalyzed C-H bond activation are discussed. 相似文献
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Wolfgang Weinmann Alexander Wiedemann Beatrix Eppinger Michaela Renz Michal Svoboda 《Journal of the American Society for Mass Spectrometry》1999,10(10):1028-1037
Factors influencing in-source collision-induced dissociation (ESI/CID) of organic molecules in a Perkin-Elmer/SCIEX ionspray source have been investigated. Breakdown curves of four drugs and organic compounds were acquired by monitoring the intensities of MH+ and specific fragment ions while ramping the orifice voltage. Haloperidol, diazepam, 1,4-acetamido-acetoxybenzene and diacetamido-1,2-benzene were found to be substances with characteristic breakdown curves, with maximums and points of intersection at orifice voltages between 20 and 70 V. The breakdown curves of haloperidol were used for comparison of ESI/CID with ionspray and turboionspray sources on three PE/SCIEX-API instruments. Using standardized source parameters and mass resolution, very similar fragmentation graphs were obtained for haloperidol with all instruments. Infusion of varying concentrations of haloperidol (0.1 to 10 micrograms/mL with ionspray, and 0.01 to 1 microgram/mL with turboionspray) yielded comparable breakdown curves. With turboionspray, a preconcentration of the aerosol occurred, yielding higher ion abundances. Solvent pH and the ratio of aqueous ammonium formate/acetonitrile had minor effects on the degree of fragmentation of haloperidol in a wide range. With these preconditions, a currently expanding mass spectral library of 400 drugs was set up by liquid chromatography/mass spectrometry with alternating orifice voltages (20, 50, and 80 V, respectively) in a looped experiment. An example of drug identification in a patient's serum with library search is shown. 相似文献
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Urs Achim Wiedemann 《Nuclear Physics A》2001,690(4):607-751
We study the gluon radiation spectrum off a hard in-medium produced quark in the multiple soft-rescattering formalism of Baier–Dokshitzer–Mueller–Peigné–Schiff and of Zakharov (BDMPS-Z). Its dependence on the quark and gluon energy, on the gluon transverse momentum, on the in-medium pathlength and on the rescattering properties of the nuclear medium is analyzed quantitatively. The two components of gluon radiation, the hard vacuum radiation associated to the quark production vertex, and the medium-induced rescattering contribution interfere destructively. For small spatial extensions of the medium, this destructive interference overcompensates the hard vacuum radiation, and the total medium-induced radiative energy loss decreases as ΔE∝− L3. Medium-induced gluon production dominates only above a finite critical length L>Lcrit which varies between 3 and more than 6 fm depending on the rescattering properties of the medium. Deviations from the BDMPS-L2-behaviour persist above Lcrit. The medium-dependence of the angular gluon distribution is dominated by transverse brownian k-broadening. This results in a depletion of the low transverse momentum part of both the hard and the medium-induced contribution. As a consequence, the medium-induced energy loss outside a finite angular cone size Θ can be more than a factor two larger than the total medium-induced radiative energy loss. We discuss implications of these results for the jet quenching signal in relativistic heavy-ion collisions at RHIC and LHC. 相似文献
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E. Wiedemann 《Fresenius' Journal of Analytical Chemistry》1879,18(1):582
Ohne Zusammenfassung 相似文献
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E. Wiedemann 《Analytical and bioanalytical chemistry》1879,18(1):582-582
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E. Wiedemann 《Fresenius' Journal of Analytical Chemistry》1876,15(1):435-436
Ohne Zusammenfassung 相似文献
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J. Unterzaucher A. Stoll E. Wiedemann A. Friedrich F. Blumer I. Marek W. R. Kirner H. A. Taylor H. E. Achilles S. Abe R. Hara H. Sinozaki D. T. Gibson Th H. Caulfield P. L. Kirk K. Dod und E. C. Kendall 《Fresenius' Journal of Analytical Chemistry》1937,110(7-8):291-298
Ohne Zusammenfassung 相似文献
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