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排序方式: 共有102条查询结果,搜索用时 31 毫秒
1.
The kinetics and the equilibrium constant of the chlorine transfer reaction between monochloramine NH2Cl and the amines: C2H5NH2, (CH3)2CHNH2, (CH3)2NH, and (C2H5)2NH are investigated by spectrophotometry in aqueous medium at 25°C, in the pH range from 8 to 13 and for an ionic strength equal to 1.03 ± 0.05M. For a concentration of total ammonia equal to 1M, the observed rate constant is pH independent below 8 and above 12.8 and reaches a maximum located between the pKas of NH4+ and RR'NH2+. From these results and those obtained earlier for NH2Cl and CH3NH2, the reaction is shown to involve an interaction between neutral molecules NH2Cl and RR'NH, subject to general acid catalysis. The ability of an interaction corresponding to a specific catalysis and involving NH3Cl+ and RR'NH rather than NH2Cl and RR'NH2+ is also discussed. The activation parameters are given for each reaction. 相似文献
2.
Dominique Dehareng Georges Dive Josette Lamotte-Brasseur Jean-Marie Ghuysen 《Theoretical chemistry accounts》1989,76(2):85-94
Summary The electrostatic interaction energy between methanol and the dyad water-imidazole has been computed numerically at three levels of approximation from 3D grids of the charge density of one partner and the electrostatic potential of the other. The minimum positions and energy values thus obtained compare well with those calculated analytically. The numerical procedure is especially interesting for the prediction of the stable conformers. 相似文献
3.
Frédérique-Marie Rattis Josette Péguet-Navarro Pascal Courtellemont Gérard Redziniak Daniel Schmitt 《Photochemistry and photobiology》1995,62(5):914-916
Abstract— -Urocanic acid (UCA) represents the major ultraviolet B (UVB, 290–320 nm)-absorbing component of the skin. Trans-UCA is naturally produced in the stratum corneum and converts to the cis isomer upon UVB irradiation. In this study, we examined the effect of purified cis -UCA (about 99% of cis isomer) on the human Langerhans cell (LC) allostimulatory function by using the mixed epidermal cell-lymphocyte reaction (MELR). We found that addition of increasing amounts (6.5–400 μg/mL) of purified cis-UCA or (rara-UCA did not modify the T-cell response supported by enriched LC (eLC: 8–25% LC) as well as purified LC (pLC: 70–90% LC) suspensions. Because cis-UCA had no effect on the allostimulatory function of untreated LC, we investigated whether this compound could modify T-cell proliferation induced by UVB-irradiated LC. The UVB exposure of eLC or pLC to 100 J/m2 significantly inhibited the capacity of both suspensions to mount a T-cell response. However, addition of cis- UCA did not potentiate this UVB-induced immunosuppression. The eLC or pLC were then incubated with cis-UCA for 18 h at 37°C and washed before adding to allogeneic T cells. The obtained proliferative response was similar to that induced by control LC incubated in medium alone, demonstrating that pretreatment with cis -UCA did not alter human LC function. In conclusion, these results strongly suggest that cis-UCA has no direct effect on human LC antigen-presenting function. 相似文献
4.
Josette Canceill Ludovic Jullien Liliane Lacombe Jean-Marie Lehn 《Helvetica chimica acta》1992,75(3):791-812
A new series of channel-type molecules presenting the features of potential transmembrane structures is described. They result from the grafting of amphiphilic side chains on a β-cyclodextrin derivative 3 that constitutes the organizing core. They belong to the ‘bouquet’ family (B). Compounds bearing poly(oxyethylene) side chains, B 16 and 17 , and their polymethylene analogues B 18 and 19 , were synthesized. The properties investigated emphasize the suitability of such molecules to be incorporated into lipid bilayer membranes. 相似文献
5.
Alain Ibanez Jean-Claude Jumas Josette Olivier-Fourcade Etienne Philippot 《Journal of solid state chemistry》1984,55(1):83-91
The crystal structures of Sn2SbX2I3, with X = S or Se, and Sn3SbSe2I5 are characterized by a statistical disorder of part of the [Sb] and [Sb, Sn] sites. All these crystal structures are built up from infinite ribbons (Sn2X4)n of SnX5 pyramids where X = ((S, Se) and I). The ribbons are weakly linked through Sn … I interactions to give infinite sheets. Between sheets are located [Sb] or [Sb, Sn] atoms in twinned sites. 相似文献
6.
7.
Josette Olivier-Fourcade Costana Mihaela Ionica-Bousquet Pierre-Emmanuel Lippens Jean-Claude Jumas 《Hyperfine Interactions》2012,207(1-3):33-36
Due to their high storage capacity Sn-based materials are of considerable interest as negative electrode for Li-ion batteries. However the strong volume change occurring during the alloy formation strongly limits the electrochemical performances (cycle and time life). Analysis by M?ssbauer spectroscopy using model compounds (Sn, Sn-Li and transition metal-Sn alloys) shows that the volume expansion is related to the structural change from a Sn based network to a Li based network. Two types of materials are proposed here to overcome this problem: tin dispersion in an electrochemically inactive oxide matrix with buffer role to absorb volume changes or tin alloying with an inactive transition metal to minimize the volume expansion. The use of M?ssbauer spectroscopy (in situ operando mode) allows a dynamic approach which is essential to understand the fundamental causes of ageing on cycling and to define then the key issues to be solved for material’s application. 相似文献
8.
The oxidation of several secondary-tertiary α diols by silver carbonate on Celite is described. In the steroid series, a 17α,20α diol leads to the related ketol, whereas its 17α,20β isomer is either not affected by the reagent, or slowly cleaved to give a 17-keto steroid. 相似文献
9.
G. Bastard 《Journal of luminescence》1985,30(1-4):488-501
Recent theoretical works on Coulombic bound states in semiconductor quantum wells (Q.W.) are reviewed. Due to carrier confinement along the growth axis the bound impurity or exciton states display enhanced binding energies over the bulk values. The presence of free carriers in modulation-doped quantum wells decreases the impurity binding energies. However the quasi-bidimensionality of the carrier motion prevents a complete vanishing of impurity bound states. The photoluminescence line of high-quality quantum well is often Stokes-shifted with respect to the absorption or excitation spectra. This Stokes shift can be correlated with interface defects in a qualitative fashion. 相似文献
10.
P. Voisin C. Delalande G. Bastard M. Voos L.L. Chang A. Segmuller C.A. Chang L. Esaki 《Superlattices and Microstructures》1985,1(2):155-159
We present optical absorption experiments performed at low temperature in very high quality GaSbAlSb superlattices grown by MBE. The absorption spectra exhibit pronounced exciton peaks separated by absorption plateaus. The data are interpreted within an effective mass theory, taking into account the effect of strains induced by the 0.65 % lattice mismatch of the hosts. From the number of observed transitions, we can determine quite accurately the band offsets in this system. The crossover of the quantized states originating from the Λ and L minima of the GaSb conduction band is also discussed. 相似文献