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1.
A critical review of known results about the permanence conditions for the null character of the solutions to the (vacuum) Maxwell equations, is presented. Concomitants of the electromagnetic field and the metric tensor are constructed, which give the principal directions of the field in covariant form. The known permanence conditions are generalized in order to includeall the (local) null fields; the above concomitants allow these conditions to be explicitly formulated in terms of the electromagnetic field.Supported in part by Conselleria de Cultura, Educació i Ciència de la Generalitat Valenciana. 相似文献
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Lavilla R Kumar R Coll O Masdeu C Spada A Bosch J Espinosa E Molins E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(10):1763-1772
The natural tendency of 1,4-dihydropyridines to undergo "biomimetic" oxidation to afford pyridinium salts can be switched off and, through the use of reagents that interact electrophilically with the enamine moiety present in the heterocyclic system, it is possible to promote alternative oxidations. In this way, efficient regio- and stereocontrolled synthetic methods have been developed that lead to diversely substituted di- and tetrahydropyridines. These include iodoazidation, diamination, bis-sulfonamidation, sulfonylation, sulfinylation, thiocyanation, sulfanylation, bis-sulfanylation, and halo-phosphonylation processes. 相似文献
4.
Coll R. Saurina J. Escoda L. Suñol J. J. 《Journal of Thermal Analysis and Calorimetry》2018,131(2):1277-1286
Journal of Thermal Analysis and Calorimetry - This paper reports the effects of welan gum on the hydration and hardening behaviors of Portland cement (PC) by using X-ray diffractometry, mercury... 相似文献
5.
Martín A. Iglesias Arteaga Roxana Pérez Gil Vivian Leliebre Lara Carlos S. Pérez Martinez Francisco Coll Manchado Arístides Rosado Pérez 《合成通讯》2013,43(8):1381-1386
The synthesis of a plant growth promoter furostanol which bears the characteristic functionality of teasterone on rings A and B is described. 相似文献
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B. Cantó C. Coll E. Sánchez S. C. Cardona J. Navarro-Laboulais 《Journal of mathematical chemistry》2014,52(4):1023-1035
The dynamics of the composition of chemical species in reacting systems can be characterized by a set of autonomous differential equations derived from mass conservation principles and some elementary hypothesis related to chemical reactivity. These sets of ordinary differential equations (ODEs) are basically non-linear, their complexity grows as much increases the number of substances present in the reacting media and can be characterized by a set of phenomenological constants (kinetic rate constants) which contains all the relevant information about the physical system. The determination of these kinetic constants is critical for the design or control of chemical systems from a technological point of view but the non-linear nature of the ODEs implies that there are hidden correlations between the parameters which maybe can be revealed with a identifiability analysis. 相似文献
8.
In a sodium dodecyl sulphate (SDS) micellar solution, the rate of coupling of a diazonium ion with N-(1- naphthyl)ethylenediamine (NED) increases greatly, the protonation of the resulting azo dyes takes place at higher pH values and the dyes are more soluble. These favourable features were applied to the development of a simple flow-injection spectrophotometric procedure for the determination of diazotizable substances of pharmaceutical interest. Limits of detection in the range 0.2–0.5 μg ml?1 (signal-to-noise ratio=3), with relative standard deviation of 0.7–3% (n=3) for 5 μg ml?1 standards, were obtained. 相似文献
9.
Hctor Novoa de Armas Oswald M. Peeters Norbert M. Blaton Camiel J. De Ranter Ramn Poms Hernndez Jos L. Mola Carlos Serafín Prez Leticia Surez García Martín A. Iglesias Francisco Coll Manchado 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(1):78-79
In the title compound, C24H36O6, the ester linkage in ring A is equatorial. The six‐membered rings A, B and C have chair conformations. The five‐membered ring D adopts a 13β,14α‐half‐chair conformation and the E ring adopts an envelope conformation. The A/B, B/C and C/D ring junctions are trans, whereas the D/E junction is cis. 相似文献
10.
Preparation of potential insect antijuvenile hormone agents 2,2-dimethyl-7-(2,2,2-trifluoroethoxy)-2H-chromene (), 6-methoxy-2,2-dimethyl-7-(2,2,2-trifluoroethoxy)-2-chromene () and 7-methoxy-2,2-dimethyl-6--(2,2,2-trifluoroethoxy)-2-chromene () and the corresponding 3,4-epoxides and is reported. 相似文献