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201.
Lusvardi G Malavasi G Menabue L Menziani MC Pedone A Segre U 《The journal of physical chemistry. B》2005,109(46):21586-21592
An automatic tool (named CLUSTER) for the prediction of the most probable crystal phases that can separate from glasses has been developed. The program analyzes the output of molecular dynamics simulations of glasses or glass ceramics, systematically sampling the ratios of the ions in different portions of the simulation box and comparing them to the stoichiometric ratio of compositionally equivalent crystalline phases retrieved from a crystal structure database. The efficacy of the similarity index elaborated has been judged by comparing the results obtained with the crystal phases identified by XRD analysis after thermal treatment in a series of multicomponent potential bioactive glasses and glass ceramics for which the advantages of rational-designed erosion-controlled release is straightforward. 相似文献
202.
Rosa Carballo Berta Covelo Ezequiel M. Vzquez‐Lpez Emilia García‐Martínez Alfonso Castieiras Juan Nicls 《无机化学与普通化学杂志》2005,631(4):785-792
Several new two‐ligand complexes of zinc(II) with the aromatic N, N‐donor ligands 2, 2′‐bipyridine or 1, 10‐phenanthroline and one of three different α‐hydroxycarboxylates (HL′) derived of the α‐hydroxycarboxylic acids (H2L′) (2‐methyllactic, H2mL; mandelic, H2M or benzilic, H2B) were prepared. The compounds of formula [Zn(HL′)2(NN)]·nH2O (HL′ = HM, HB) were isolated as white powders and characterized by elemental analysis, IR spectroscopy and thermogravimetric analysis. The complexes of general formula [Zn(HL′)(NN)2](HL′)·nH2O (HL′ = HmL, HM) and [Zn(HB)2(NN)2], were obtained as single crystals and were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis and X‐ray diffractometry. In all cases, the zinc atom is in a distorted octahedral environment. In [Zn(HL′)(NN)2](HL′)·nH2O the α‐hydroxycarboxylato ligands behave as bidentate chelating monoanion and an α‐hydroxycarboxylate as counterion is also present. In [Zn(HB)2(NN)2], the monoanionic benzilato ligand behaves as monodentate through one oxygen atom of the carboxylate function. The effect of the classical and no‐classical hydrogen bonding and of the π‐π and C‐H…π interactions in the 3D supramolecular arrangement of these molecular complexes is analyzed. 相似文献
203.
Ricard Herrera Lourdes Franco Alfonso Rodríguez‐Galn Jordi Puiggalí 《Journal of polymer science. Part A, Polymer chemistry》2002,40(23):4141-4157
Random copolyesters based on 1,4‐butanediol and different ratios between adipic and terephthalic units were synthesized from thermal polycondensation of the appropriate mixture of monomers or by melt transesterification of the mixture of homopolymers. 1H NMR spectroscopy makes feasible the study of the average block lengths of polymers once synthesized and after degradation in different media. Calorimetric data are reported, including those referred to the study of isothermal and nonisothermal crystallizations. Degradability of samples was evaluated by different methods including NMR and thermal analysis, evaluation of molecular weight by gel permeation chromatography or from intrinsic viscosity measures, scanning electron micrographs, and changes in mechanical properties. Distilled water at 70 °C acidic conditions provided by a pH 2.3 aqueous medium and enzymatic media containing lipases from Pseudomonas cepacia or Candida cylindracea were considered in this study. The degradability of the studied copolyesters strongly depends on the terephthalate content and the degradation media. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4141–4157, 2002 相似文献
204.
Alfonso J. Chirinos-Padrn Jos Azcar A. Munoz-Escalona Alberto M. Fuentes 《Journal of polymer science. Part A, Polymer chemistry》1987,25(9):2469-2477
The changes in the molar mass of high molecular weight polyethylene, in o-dichlorobenzene, with time are examined using viscometry. The results show that the molar mass decreases with time and that the observed decay can be satisfactorily explained by a simple degradation model. Commercial antioxidants can inhibit the degradation process if the appropriate concentration is chosen. Thus, at very low concentrations a polynuclear phenol shows good antioxidant behavior but this decreases with an increase in concentration due to direct oxidation of the inhibitor. Conversely, zinc dimethyldithiocarbamate shows the best stabilizing activity which increases with concentration. The results are explained on the basis of the established mechanistic action of these compounds and show that viscometry is not an accurate method for measuring the molar mass of high molecular weight polyethylene samples unless the appropriate conditions are chosen. Even with the best inhibitor, some degradation is still observed. 相似文献
205.
We consider the semipositone problem $${\matrix {-\Delta u (x)= \lambda f (u(x))\ \ \; \ \ \ \ \ x \in \Omega \cr \qquad \qquad \qquad u(x)=0 \ \ \ \;\ \ \ \ x \in \partial \Omega \cr}}$$ where λ > 0 is a constant, Ω is a bounded region in Rn with a smooth boundary, and f is a smooth function such that f ′(u) is bounded below, f (0) < 0 and \({\rm lim}_{u \rightarrow}+\infty {f(u)\over u}=0. \) We prove under some additional conditions the existence of a positive solution (1) for λ ∈ I where I is an interval close to the smallest eigenvalue of —Δ with Dirichlet boundary condition and (2) for λ large. We also prove that our solution u for λ large is such that∥u∥ ? supx∈Ω ¦u(x)¦ → ∞ as A → ∞. Our methods are based on sub and super solutions. In particular, we use an anti maximum principle to obtain a subsolution for our existence result for λ ∈ I. 相似文献
206.
Tiziana Beringhelli Gianfranco Ciani Giuseppe DAlfonso Angelo Sironi Maria Freni 《Journal of organometallic chemistry》1982,233(3):C46-C50
The novel anions [Re3H3(CO)10(μ-O2CR)]? (R = H, CH3, CF3), obtained by reaction of [Re3H4(CO)10]? with the corresponding car?ylic acids, have been characterized by IR and NMR spectra and by X-ray analysis of the formate and trifluoroacetate derivatives. They contain a triangle of rhenium atoms, with the car?ylate group diaxially bridging on the shorter ReRe edge. 相似文献
207.
Several mass spectrometry methods were explored to determine the regiospecificity of deuterium substitutions in hydrocarbon mixtures. The case investigated in this work was that of ethane mixtures obtained by catalytic HD exchange between either C(2)H(6) and D(2) or C(2)D(6) and H(2) over platinum surfaces. A total of ten isotopologs are possible, and were indeed detected in all cases. Deconvolution of low-resolution mass spectra was found sufficient to determine the composition of the gas mixtures in terms of the total number of deuterium substitutions, but not to identify symmetric versus asymmetric substitutions in the C(2)D(2)H(4), C(2)D(3)H(3), and C(2)D(4)H(2) products. High-resolution mass spectrometry allowed the separation of the intensities due to C(2)X(4)(+) fragments from those from molecular C(2)X(6)(+) signals (X = H or D), and with that for a more accurate determination of the composition of the mixtures. Relative probabilities were determined for the symmetric versus asymmetric removal of X(2) from C(2)X(6)(+) ions and for isotope scrambling in the mass spectrometer, and with that information fairly good cracking patterns were then calculated for the C(2)X(4)(+) fragments produced by each individual pure C(2)X(6) isotopologue. However, total deconvolution of all ten components in the ethane mixtures obtained by HD exchange catalysis was beyond the experimental accuracy of the measurements. Tandem mass spectrometry/collision-induced decomposition mass spectrometry (MS/CID-MS) proved more useful for this task. In particular, it was possible to determine the proportion of symmetric versus asymmetric double HD exchange in samples for which the total ethane-d(2) (in the case of C(2)H(6) + D(2)) or ethane-d(4) (with C(2)D(6) + H(2)) amounted to only approximately 3% on the ethane mix. A comparison with other analytical methods, NMR in particular, is provided. 相似文献
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