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排序方式: 共有371条查询结果,搜索用时 15 毫秒
291.
292.
Paulo J. Amorim Madeira Ana Margarida Rosa Nuno M. Xavier Amélia P. Rauter M. Helena Florêncio 《Rapid communications in mass spectrometry : RCM》2010,24(7):1049-1058
Knowledge of the fragmentation mechanisms of lactones and their behaviour under electrospray ionization (ESI) conditions can be extended to larger and more complex natural products that contain an α,β‐unsaturated γ‐lactone moiety in their structure. Moreover, little is known about the gas‐phase behaviour of α,β‐unsaturated γ‐lactones linked or fused to sugars. Therefore, five α,β‐unsaturated γ‐lactones (butenolides) fused to a pyranose ring, recently synthesized compounds with potential relevance regarding their biological properties, were investigated using ESI‐MS and ESI‐MS/MS in both positive and negative ion modes. Their fragmentation mechanisms and product ion structures were compared. It was observed that two isomers could be unambiguously distinguished in the negative ion mode by the fragmentation pathways of their deprotonated molecules as well as in the positive ion mode by the fragmentation pathways of either the protonated or the sodiated molecule. Fragmentation mechanisms are proposed taking into account the MS/MS data and semi‐empirical calculations using the PM6 Hamiltonean. The semi‐empirical calculations were also very useful in determining the most probable protonation and cationization sites. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
293.
M.L. Afonso R.A.L. Silva M. Matos E.B. Lopes J.T. Coutinho L.C.J. Pereira R.T. Henriques M. Almeida 《Journal of Crystal Growth》2012,340(1):56-60
The conditions for the growth of (Perylene)2 [Pd(mnt)2] crystals either by chemical oxidation and electrochemical routes are described. The electrocrystallisation is limited by close proximity of the oxidation potentials of the perylene donor and [Pd(mnt)2]? anion, and depending on the experimental conditions different morphologies can be obtained. (Per)2 [Pd(mnt)2] crystals obtained by elecrocrystallisation were found to be mainly of the β-polymorph with properties comparable to the Cu, Ni and Pt analogues previously described at variance with those obtained by chemical oxidation which are mainly of the α-polymorph . 相似文献
294.
Corrigendum: Biosynthesis‐Assisted Structural Elucidation of the Bartolosides,Chlorinated Aromatic Glycolipids from Cyanobacteria
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295.
296.
M. Agostinha R. Matos Margarida S. Miranda Victor M. F. Morais Joel F. Liebman 《Molecular physics》2013,111(12):1833-1841
The standard molar enthalpy of formation in the gas phase of 2H-1,4-benzoxazin-3(4H)-one was derived from the standard energy of combustion determined by static bomb combustion calorimetry in oxygen atmosphere and from the standard sublimation enthalpy determined by Calvet microcalorimetry. In addition, we report the results of a systematic theoretical study of the keto and enol tautomers in benzoxazinones and diones using density functional theory. The keto tautomers are computed to be more stable than the enols. Tautomerization energies are reported. 相似文献
297.
Maria da Conceição Costa Pereira Tufic Madi Filho Margarida Mizue Hamada 《辐射效应与固体损伤》2013,168(12):921-928
Scintillation crystals have been used in various fields, such as high energy physics, nuclear instrumentation, radiation measurements, medical imaging, nuclear tomography, astrophysics and other fields of science and engineering. For these applications, the development of scintillation crystals with good performance is required. Scintillation crystals based on cesium iodide (CsI) matrix are matters with relatively low hygroscopicity, easy to handle and of low cost, characteristics that favor their use as radiation detectors. In this study, pure CsI crystal and lead doped CsI crystals were grown using the Bridgman vertical technique. The concentration of the lead doping element (Pb) was studied in the range of 10?2 to 5×10?4 M. The distribution of the doping element in the crystalline volume was determined by atomic absorption technique. The CsI:Pb crystal with nominal concentration of 10?3 M was cut into 14 slices of 6 mm. The results show a higher concentration at the top of the crystal with a decrease in the initial phase of growth. The dopant concentration of Pb showed good uniformity from the slice 2 to the slice 12: the region is, therefore, suitable for use as a radiation detector. The luminescence emission of these crystals was measured. A predominant luminescence band near 450 nm and a single broad band around 320 nm were found with the addition of the Pb2+ ions in the CsI matrix. Alpha particles spectrometry measurements were carried out to evaluate the developed scintillators.The resolution of 5.6% was obtained for the CsI:Pb 5×10?4 M crystal, when excited with alpha particles from a 241Am source, with energy of 5.54 MeV. 相似文献
298.
Mario F. S. Ferreira Margarida M. V. Facao Sofia C. V. Latas 《Fiber and Integrated Optics》2013,32(1):31-41
Stable soliton propagation in a system with linear and nonlinear gain and spectral filtering is investigated. Different types of exact analytical solutions of the cubic and the quintic complex Ginzburg-Landau equation (CGLE) are reviewed. The conditions to achieve stable soliton propagation are analyzed within the domain of validity of soliton perturbation theory. We derive an analytical expression defining the region in the parameter space where stable pulselike solutions exist, which agrees with the numerical results obtained by other authors. An analytical expression for the soliton amplitude corresponding to the quintic CGLE is also obtained. We show that the minimum value of this amplitude depends only on the ratio between the linear gain and the quintic gain saturating term. 相似文献
299.
Waller K. Dieterich R. Henriques und D. Holde 《Fresenius' Journal of Analytical Chemistry》1901,40(6):428-429
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300.