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排序方式: 共有93条查询结果,搜索用时 15 毫秒
81.
Angelini G Coletti C De Maria P Ballini R Gasbarri C Fontana A Pierini M Siani G 《The Journal of organic chemistry》2012,77(2):899-907
The keto-enol tautomerism of some cyclic 2-nitroalkanones was studied in cyclohexane. Keto-enol equilibrium constants, K(T), at 25 °C were obtained from (1)H NMR spectra. The relative enol content for the investigated ketones as a function of ring size decreases in the order 6 > 7 > 11 > 12 > 15. This trend apparently is different from that observed in water. Density functional theory (DFT) calculations have been performed to rationalize the effects of ring size and of the solvent on tautomerism. The acidity constants, K(a)(KH), for the different keto tautomers were measured spectrophotometrically at 25 °C in buffered aqueous solutions. No simple correlations between K(a)(KH) and ring size was observed, and this is in agreement with a DFT analysis performed on the same compounds. 相似文献
82.
Angelini G Chiappe C De Maria P Fontana A Gasparrini F Pieraccini D Pierini M Siani G 《The Journal of organic chemistry》2005,70(20):8193-8196
Solute-solvent interactions on the keto-enol tautomerism of 2-nitrocyclohexanone in several organic solvents and room-temperature ionic liquids (RTILs) have been analyzed in terms of multiparameter equations. Permittivity and cohesive pressure values of the RTILs, unavailable by direct measurements, have been derived. 相似文献
83.
Adriana B. Pierini Mario G. Cardozo Aida A. Montiel Sem M. Albonico Maria T. Pizzorno 《Journal of heterocyclic chemistry》1989,26(4):1003-1008
We report a 1,3-dipolar cycloaddition reaction between a oxazolium 5-oxide derivative with chloroacrylonitrile or ethyl propiolate as dipolarophiles, in order to obtain substituted pyrrolizidines. Experimentally we found that the reaction is regiospecific with chloroacrylonitrile and regioselective with ethyl propiolate. The secondary attractive orbital interactions from the Frontier Molecular Orbital Theory, the differences in stability of the possible biradical intermediaries postulated for the reaction and some hindrance effects, explain the regioselectivity observed experimentally. 相似文献
84.
Gasparrini F Grilli S Leardini R Lunazzi L Mazzanti A Nanni D Pierini M Pinamonti M 《The Journal of organic chemistry》2002,67(9):3089-3095
Aryl benzyl oximes having the configuration Z give rise to stereolabile atropisomers when a halogen atom is present in the ortho position of the aryl moiety, as a consequence of the restricted aryl-CN bond rotation. By means of dynamic (1)H NMR spectroscopy it has been possible to determine the corresponding rotation barrier, hence the lifetime of the atropisomers that, in the case of the iodine derivative, was found sufficiently long as to allow a physical separation to be achieved on an appropriately cooled enantioselective HPLC column. Comparison of the barriers determined by dynamic NMR and dynamic HPLC proved the equivalence of the two techniques. When the iodine atom was substituted by an alpha-naphthyl group, two dynamic processes were observed. That with the lower barrier could be determined by NMR and that with the higher barrier by HPLC, thus outlining the complementarity of these two techniques. 相似文献
85.
Famiglini G Trufelli H Pierini E De Simoni E Mangani F Cappiello A 《Journal of AOAC International》2003,86(5):941-946
Our recent on-line solid-phase extraction (SPE) device for micro-liquid chromatography, known as micro-solid-phase extraction (microSPE), was compared with traditional SPE for the analysis, from aqueous samples, of 4 pesticides belonging to different classes. Two different kinds of adsorbents, C18 and graphitized carbon black, were tested. A 2-stage ion trap mass spectrometer, equipped with homemade microflow electrospray ion (ESI) source, was used. Detection limits with a signal-to-noise ratio of 3:1 for both extraction methods were in the range of 0.1 microg/L for all compounds. However, better recoveries were obtained when microSPE traps were used. 相似文献
86.
Phenyl telluride anion was prepared in liquid ammonia by reaction of dipheyyl ditelluride with two equivalents of sodium metal under nitrogen. This anion reacts with haloarenes under irradiation to form aryl phenyl tellurides, probably by the photo-SRN1 mechanism of aromatic substitution. 相似文献
87.
Job D. Guzmán Marc Mangnus Peter Aerts Edwin Mes Joachim Kaschta Pete Pierini 《Rheologica Acta》2008,47(2):213-221
We have found discrepancies between the predictions of two double-reptation models and the corresponding linear viscoelasticity
measurements for commercial, polydisperse polystyrene samples with weight-average molecular weights ranging from 140 to 300kg/mol.
The accuracy of the experimental data has been confirmed by conducting viscoelasticity measurements in different laboratories
using different types of rheometers and by verifying that small distortions in chromatographic measurements cannot account
for the discrepancies seen in the viscoelastic response. In addition, we show that the discrepancies between predictions and
measurements are systematic, suggesting that gaps in the theory are responsible for the mismatch. Is it concluded that commercial
polystyrene resins may pose additional challenges for rheological modeling because of their relatively high polydispersity
and low degree of entanglement. The experimental data given here can be used to validate future modeling efforts. 相似文献
88.
S. Pierini 《Il Nuovo Cimento C》1986,9(6):1045-1061
Summary An implicit, three-time level, finite-difference scheme is presented which solves the initial-boundary-value problem for a
regularized Kadomtsev-Petviashvili equation, modelling the evolution of a variety of wave systems for which weak dispersive,
nonlinear and three-dimensional effects are taken into account. Linear and nonlinear tests are performed to test the scheme
and the numerical results are found to compare accurately with known theoretical solutions. The evolution of a localized weakly
three-dimensional initial condition in a channel is then studied. In all cases the numerical experiments show that the wave
tends to a two-dimensional state: for initial waves with positive net volume two-dimensional solitons rapidly emerge. This
behaviour agrees with laboratory experiments performed by different authors. Moreover, the radiation yields three-dimensional
patterns similar to Genus 2, solutions of the KP equation.
Riassunto Si presenta uno schema alle differenze finite implicito e a tre livelli temporali per una versione regolarizzata dell'equazione Kadomtsev-Petviashvili, descrivente la dinamica di una vasta classe di sistemi d'onda per i quali si hanno deboli effetti dispersivi, non lineari e tridimensionali. Una serie di test lineari e non lineari dà risultati numerici coincidenti con le corrispondenti soluzioni teoriche. Si analizza quindi l'evoluzione in un canale rettangolare di condizioni iniziali tridimensionali localizzate. In tutti i casi si osserva un processo di aggiustamento bidimensionale. Solitoni bidimensionali emergono rapidamente da onde iniziali positive. Ciò concorda con esperimenti di laboratorio. Inoltre il campo di radiazione si presenta sotto forma di onde tridimensionali simili alle solizioni di Genus 2 dell'equazione KP.相似文献
89.
A. R. Mastrogiacomo M. F. Ottaviani E. Pierini M. Cangiotti M. Mauro F. Mangani 《Chromatographia》2002,55(5-6):345-348
Summary Some of the most commonly employed carbon blacks (Carbograph 1,2,4,5 from Lara and Carbopack X form Supelco), were evaluated
by the BET surface characterization method for pore size distribution, porosity and specific surface. This information is
often incomplete as furnished by manufacturers and is needed for a deeper understanding of carbon adsorption properties in
chromatography.
The data on surface characterization were obtained before and after graphitisation treatment (except for Carbopack X which
could not be obtained in non-graphitized form). The nature of the active sites present on these carbons was investigated by
BET analysis, atomic absorption and FT-IR.
Dedicated to the memory of F. Bruner and G. Crescentini, pioneers in the study of graphitized carbon blacks for the sampling
and analysis of environmental pollutants [1–4]. 相似文献
90.
Evaluation of a new graphitized carbon black employed in sampling volatile organic compounds 总被引:1,自引:0,他引:1
A. R Mastrogiacomo E. Pierini L. Sampaolo F. Bruner 《Journal of chromatography. A》1998,810(1-2):131-139
A new adsorbent, high-surface-area graphitized carbon black (HSGCB), with a surface area of 560 m2/g, superior to that of commonly traded graphitized carbons, was studied. The evaluation was performed in terms of breakthrough volumes and thermal desorption recoveries of substances representative of the main classes of organic compounds. HSGCB was found to be an interesting alternative to activated charcoal for volatile compounds in that it can be used in thermal desorption. 相似文献