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排序方式: 共有1305条查询结果,搜索用时 15 毫秒
941.
In the presence of excess H2O2, oxidation of dopamine was diverted from the usual pigment-forming pathway to afford 6-hydroxydopamine and then a colorless reaction mixture comprising a polar non-extractable product. The latter was obtained in 20% yield by oxidation of 6-hydroxydopamine and was tentatively formulated as the novel 5-(2-aminoethyl)-2-hydroxy-5-(3-hydroxy-2-oxotetrahydro-1aH-oxireno[2,3]cyclopenta[1,2-b]pyrrol-3a(4H)-yl)cyclohex-2-ene-1,4-dione by extensive spectral analysis and conversion to a tetraacetyl derivative. Mechanistic experiments suggested that formation of the product proceeds via 6-hydroxydopamine by H2O2-dependent epoxidation and cyclization steps followed by dimerization and ring contraction with decarboxylation. 相似文献
942.
Alessandra Napolitano Maria Grazia Corradini Giuseppe Prota 《Tetrahedron letters》1985,26(23):2805-2808
Reduction and subsequent acetylation of melanochrome, the last UV detectable intermediate in the biosynthesis of eunelanins, leads to the isolation of a crystalline derivative, leucomelanochrome A acetate, identified as 5,6,5′,6′-tetraacetoxy-2,2′-biindolyl (). 相似文献
943.
Riassunto L'origine dell'insegnamento pubblico per ingegneri e architetti a Ferrara; la formazione culturale dell'ingegnere nell'ambito
dell'Università (1771–1804); la Scuola di Idrostatica nel periodo napoleonico (1804–1815); La Scuola degli Ingegeri pontifici
(1817–1819); la Scuola speciale di Idraulica della Pontificia Università di Ferrara (1840–1859); la questione delle Scuole
per Ingegneri a Ferrara dopo l'unità d'Italia.
Lavoro eseguito con contributo del Ministero della Pubblica Istruzione. 相似文献
Résumé L'origine de l'enseignement publique pour les ingénieurs et les architectes à Ferrare, la formation culturelle des ingénieurs dans l'Université (1771–1804); la ?Scuola di Idrostatica? (1804–1815); la ?Scuola degli Ingegneri pontifici? (1817–1819); la ?Scuola speciale di Idraulica? (1840–1859); la question des écoles pour les ingénieurs à Ferrare après l'unité nationale.
Lavoro eseguito con contributo del Ministero della Pubblica Istruzione. 相似文献
944.
Alessandra Borrelli 《Annali dell'Universita di Ferrara》1979,25(1):143-155
Riassunto In questa nota si stabiliscono due teoremi di unicità della soluzione per il problema del moto di un fluido omogenco, incomprimibile,
perfetto, elettricamente conduttore attorno ad un ostacolo fisso, anch'esso elettricamente conduttore. Nelprimo teorema viene
trascurata la corrente di spostamento, mentre nel secondo si tiene conto anche di quest'ultima. Per la dimostrazione si utilizza
il “metodo della funzione peso” usato da G. P. Galdi e S. Rionero in un analogo teorema di unicità per magnetofluidi viscosi.
Summary The Author establishes two solution uniqueness theorems for the problem of homogeneous, incompressible, perfect, electrically conducting fluid motion around a fixed, electrically conducting solid body. In the first theorem displacemt current is disregarded, in the other one this current is considered. The proof is obtained by using the “weight function method” adopted by G. P. Galdi and S. Rionero in an analogous uniqueness theorem for viscous magnetofluids.相似文献
945.
Gabriele Albertin Stefano Antoniutti Alessandra Botter Jesús Castro 《无机化学与普通化学杂志》2016,642(3):250-254
Ethylene complexes [OsH(η2‐CH2=CH2)L4]Y ( 1 , 2 ) [L = PPh(OEt)2, P(OEt)3; Y = OTf, BPh4] were prepared by reacting the dihydride OsH2L4 first with methyl triflate CH3OTf and then with ethylene (1 atm). Alternatively, the compound [OsH(η2‐CH2=CH2){PPh(OEt)2}4]OTf was prepared by allowing the dinitrogen derivative [OsH(N2){PPh(OEt)2}4]OTf to react with ethylene. Acrylonitrile CH2=C(H)CN reacts with OsH(OTf)L4 [L = P(OEt)3] to give the complex [OsH{κ1‐NCC(H)=CH2}{P(OEt)3}4]BPh4 ( 3 ). The complexes were characterized spectroscopically (IR and 1H, 13C, 31P NMR) and by X‐ray crystal structure determination of the [OsH(η2‐CH2=CH2){PPh(OEt)2}4]BPh4 derivative. 相似文献
946.
Catia de Bonis Cataldo Simari Vasiliki Kosma Barbara Mecheri Alessandra D’Epifanio Valentina Allodi Gino Mariotto Sergio Brutti Sophia Suarez Kartik Pilar Steve Greenbaum Silvia Licoccia Isabella Nicotera 《Journal of Solid State Electrochemistry》2016,20(6):1585-1598
An organically functionalized titania, TiO2-RSO3H, was evaluated as filler in sulfonated polyetheretherketone (sPEEK)-based composite membranes for application in high temperature direct methanol fuel cells. The presence of propylsulfonic acid groups covalently bound onto the TiO2 surface and the nanometric nature of the additive were analyzed by Raman spectroscopy and transmission electron microscopy, respectively. The properties of the sPEEK/TiO2-RSO3H composite membranes were compared with those of the pure sPEEK membranes and those of the sPEEK/TiO2 composite membranes containing pristine titania nanoparticles at same filler content. Water and methanol transport properties were investigated by NMR methods, including relaxation times and self-diffusion coefficients as function of temperature (up to 130 °C), and pressure (from 0 up to 2 kbar). The incorporation of the nanoadditivies in the sPEEK polymer demonstrates considerable effects on the morphology and stiffness of the membranes, as well as on the transport properties and barrier effect to the methanol crossover. In particular, the functionalization by propylsulfonic acid groups promotes a higher reticulation between the polymeric chains, increasing the tortuosity of the methanol diffusional paths, so reducing the molecular diffusion, while the proton mobility increases being favored by the Grotthus-type mechanism. Conductivity measurements point out that the filler surface functionalization avoids the reduction of the overall proton conduction of the electrolyte due to the embedding of the low-conducting TiO2. Finally, remarkable improvements were found when using the sPEEK/TiO2-RSO3H composite membrane as electrolyte in a DMFC, in terms of reduced methanol crossover and higher current and power density delivered. 相似文献
947.
948.
Although lipase from Pseudomonas cepacia (PCL) shows high enantioselectivity towards many secondary alcohols, it usually exhibits only low to moderate enantioselectivity towards primary alcohols. To increase this enantioselectivity, we optimised the reaction conditions for the PCL-catalysed hydrolysis of esters of three chiral primary alcohols: 2-methyl-3-phenyl-1-propanol 1, 2-phenoxy-1-propanol 2 and solketal 3. The enantioselectivity towards 1-acetate increased from E=16 to 38 upon changing the solvent from ethyl ether/phosphate buffer to 30% n-propanol in phosphate buffer and increased again to E ≥190 upon changing the substrate from 1-acetate to 1-heptanoate. The same changes increased the enantioselectivity towards alcohol 2 from E=17 to 70, but did not significantly increase the enantioselectivity towards alcohol 3. The best solvent was similar to the solvent used to crystallise the open form of PCL and likely stabilises the open form of PCL. This stabilisation may increase the enantioselectivity by removing kinetic contributions from a non-enantioselective lid-opening step. We determined the kinetic contribution of the lid-opening step by measuring the interfacial activation of PCL. The activation energy for the PCL-catalysed hydrolysis of ethyl acetate was at least 2.6 kcal/mol lower in the presence of a water–organic solvent interface. 相似文献
949.
Tisato F Refosco F Porchia M Bolzati C Bandoli G Dolmella A Duatti A Boschi A Jung CM Pietzsch HJ Kraus W 《Inorganic chemistry》2004,43(26):8617-8625
The nature of the heteroatom X incorporated in the five-membered PXP-diphosphine bridging chain was found to play a primary unit role both in the overall stability and in the stereochemical arrangement of nitrido-containing [M(N)(PXP)](2+) metal fragments (M = Tc, Re). Thus, by mixing PXP ligands with labile [Re(N)Cl(4)](-) and Tc(N)Cl(2)(PPh(3))(2) nitrido precursors in CH(2)Cl(2)/MeOH mixtures, a series of neutral M(N)Cl(2)(PXP) complexes (M = Tc, 1-5; M = Re, 8, 9) was collected. In the resulting distorted octahedrons, PXP adopted facial or meridional coordination, and combination with halide co-ligands produced three different stereochemical arrangements, that is, fac,cis, mer,cis, and mer,trans, depending primarily on the nature of the diphosphine heteroatom X. When X = NH, mer,cis-Tc(N)Cl(2)(PNP1), 1, was the only isomer formed. Alternatively, when a tertiary amine nitrogen (X = NR; R = CH(3), CH(2)CH(2)OCH(3)) was introduced in the bridging chain, fac,cis-M(N)Cl(2)(PN(R)P) complexes (M = Tc, 2, 3; M = Re, 8f) were obtained. Isomerization into the mer,cis-Re(N)Cl(2)(PN(R)P), 8m, species was observed only in the case of rhenium when the tertiary amine group carried the less encumbering methyl substituent. fac,cis-Tc(N)Cl(2)(PSP), 4f, was isolated in the solid state when X = S, but a mixture of fac,cis-Tc(N)Cl(2)(PSP) and mer,trans-Tc(N)Cl(2)(PSP), 4m, isomers was found in equilibrium in the solution state. A similar equilibrium between fac,cis-M(N)Cl(2)(POP) (M = Tc, 5f; M = Re, 9f) and mer,trans-M(N)Cl(2)(POP) (M = Tc, 5m; M = Re, 9m) species was detected in POP-containing complexes. The molecular structure of all of these complexes was assessed by means of conventional physicochemical techniques including multinuclear NMR spectroscopy and X-ray diffraction analysis of representative mer,cis-Tc(N)Cl(2)(PN(H)P), 1, fac,cis-Tc(N)Cl(2)(PSP), 4f, and mer,cis-Re(N)Cl(2)(PN(Me)P), 8m, compounds. 相似文献
950.
Tullio AD Angelis FD Reale S Grasso DA Visicchio R Castracani C Mori A Moli FL 《Rapid communications in mass spectrometry : RCM》2003,17(18):2071-2074
The Dufour's gland content of workers of two ant species of the genus Messor has been analyzed by solid-phase microextraction and gas chromatography with mass spectrometry (GC/MS). The structures of the compounds in the pheromonal mixtures have been determined. In both cases only one intact gland, inserted in a properly dimensioned capillary vial, is sufficient to produce a clean and fully interpretable GC/MS profile. It is worth noting that, for the first time in Messor ants, farnesol has been detected as a minor component of glandular secretion in Messor capitatus. 相似文献