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51.
When the Hammett-Taft equation log (k/ko)=ρq · σ is applied to the solvolysis of the 3-substituted propyl bromides 6a-6i in ethanol/water 4:1 (v/v) log k correlates linearly with σ except in cases where R exerts an anchimeric effect. The reaction constant ρq for 6 is ? 0.12 and is typical for a nucleophilic solvent-assisted ks process at a primary C-atom. The tertiary halides 1 and 3 , however, which react with little or no nucleophilic solvent assistance, i.e. by kc processes, lead to larger ρq values of ?0.71 and ?1.14, respectively. The reaction constant pq is therefore a sensitive gauge for charge development in the transition state for solvolysis of saturated compounds. 相似文献
52.
53.
Eshelby J Goessman M Parsons PJ Pennicott L Highton A 《Organic & biomolecular chemistry》2005,3(16):2994-2997
The construction of the fourteen membered ring present in galbonolide B 1 is reported. The 10,11-diene system present in the southern portion of has been constructed using an ester enolate rearrangement/silicon mediated fragmentation cascade, whilst the macrocycle has been synthesised following a Johnson rearrangement/mercury assisted ring closure protocol. 相似文献
54.
M. I. Bîrjega N. Popescu-Pogrion C. A. Constantin 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,20(1):369-371
The structure of vacuum annealed amorphous sputtered Cr and CrNi (55:45) thin films has been studied by transmission electron microscopy (TEM) and electron diffraction (TED). The thin films with thicknesses ranging from 150 to 600 Å were annealed while still on their alkali halide substrates under high vacuum at temperatures ranging from 200 to 370°C. The TEM and TED data permitted to establish the annealing conditions (temperature, duration), which lead to the nucleation and growth of nanosized crystals in the Cr and CrNi icosahedral glasses. 相似文献
55.
Crespo A Martí MA Estrin DA Roitberg AE 《Journal of the American Chemical Society》2005,127(19):6940-6941
A novel technique for computing free energy profiles in enzymatic reactions using the multiple steering molecular dynamics approach in the context of an efficient QM-MM density functional scheme is presented. The conversion reaction of chorismate to prephenate catalyzed by the Bacillus subtilis enzyme chorismate mutase has been chosen as an illustrative example. 相似文献
56.
Fundueanu G Constantin M Bortolotti F Ascenzi P Cortesi R Menegatti E 《Macromolecular bioscience》2005,5(10):955-964
Despite the large number of publications and patents concerning pH/thermoresponsive polymers, few data are available concerning the preparation of thermoresponsive cross-linked microspheres from preformed polymers. Therefore, N-isopropylacrylamide-co-acrylamide-co-(2-hydroxyethyl acrylate) copolymers were obtained as a new thermoresponsive material with a lower critical solution temperature (LCST) around 36 degrees C, in phosphate buffer at pH 7.4, and with a cross-linkable OH group in their structure. The LCST value was determined both by UV spectroscopy and microcalorimetric analysis. These copolymers were solubilised in acidified aqueous solution below their LCST, dispersed in mineral oil, and transformed into stable microspheres by cross-linking with glutaraldehyde. The thermoresponsive microspheres were characterised by optical and scanning electron microscopy, degree of swelling, and water retention. The pore dimensions of the microspheres and the retention volumes of some drugs and typical compounds were evaluated at different temperatures by liquid chromatography. Indomethacin, as a model drug, was included in the microspheres by the solvent evaporation method. Finally, the influence of temperature and of temperature cycling on drug release was investigated. 相似文献
57.
Constantin Bănică 《Archiv der Mathematik》1984,43(3):250-257
Sans résumé 相似文献
58.
An improvement of the signal-to-noise ratio in atomic absorption spectroscopy may be obtained by each of the following three methods of signal averaging: increase of the time constant digital signal averaging true integration of the signal The most adequate of these methods appears to be true integration. Two examples are presented to demonstrate the improved signal-to-noise ratio due to the application of this method: trace analysis of mercury in water by means of flameless AAS. The detection limit appears to be 0.2 ng which corresponds to a concentration of 2 · 10?13 g/ml. trace analysis of copper in human urine by means of flame AAS: 1,3 · 10?8 g/ml copper could be detected without any solvent extraction process. 相似文献
59.
Horgan A Saunders B Vincent B Heenan RK 《Journal of colloid and interface science》2003,262(2):548-559
A series of water-soluble, amphiphilic graft copolymers has been prepared by free-radical copolymerization of methoxypoly(ethylene glycol) macromonomers, with either methyl methacrylate or butyl methacrylate as the comonomers, in water/ethanol solvent mixtures. Lower molecular weight copolymers were obtained by increasing the concentration of the initiator, azobisisobutyronitrile (AIBN), used in the polymerization reaction. However, the route used also led to the formation of significant quantities of tetramethylsuccinodinitrile, a toxic byproduct resulting from the cage reaction of AIBN. Static fluorescence measurements using pyrene as a probe, along with 1H NMR experiments, showed that the graft copolymers form aggregates in water at very low concentrations (approximately 0.01 g l(-1)) with the pendant hydrophilic graft chains forming a stabilizing shell around the hydrophobic backbone. An increase in the hydrophile-lipophile balance of the graft copolymers was found to lead to smaller aggregates with lower aggregation numbers and highly swollen hydrophilic shells, as revealed by small angle neutron scattering (SANS). 相似文献
60.