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排序方式: 共有137条查询结果,搜索用时 31 毫秒
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Poly(dithiotriethylene terephthalate) (PSSTET), poly(dithiotriethylene adipate) (PSSTEA), poly(triethylene terephthalate) (PTET) and poly(triethylene adipate) (PTEA), these two last for comparison, were synthesized and characterized in terms of chemical structure and molecular weight. The thermal behaviour was examined by thermogravimetric analysis and differential scanning calorimetry. All the polymers showed a good thermal stability, even though lower for the sulphur-containing polyesters. At room temperature they appeared as semicrystalline materials, except PTEA, which was an oil; the effect of substitution of ether oxygen atoms with sulphur ones was found to be a lowering in the Tg value, an increment of the melting temperature and an increase of the crystallization rate. The results were explained as due to the presence of flexible C-S-C bonds in the polymeric chain. Lastly, the absence of a rigid-amorphous phase was evidenced in PSSTET and PTET. 相似文献
23.
Janet E. Del Bene Guy T. Worth Francis T. Marchese Michael E. Conrad 《Theoretical chemistry accounts》1975,36(3):195-206
Ab initio SCF calculations with a minimal STO-3G basis set have been performed to determine the equilibrium geometries of two series of carbonyl compounds, RCHO and R2 CO. For the mono-substituted compounds, R may be CH3, NH2, OH, F, CHO, and C2 H3. In the disubstituted compounds, R has been restricted to the isoelectronic saturated groups. The computed equilibrium geometries of these compounds are in satisfactory agreement with the experimentally-determined geometries, with an average difference of 0.021 Å between computed and experimental bond lengths, and 1.9 ° in corresponding bond angles. An analysis of the effect of the substituent on the electronic structure of the carbonyl group has also been made. The saturated groups are found to be electron-withdrawing groups relative to H, with the electron withdrawing ability increasing in the order CH3 2 2H2 are also electron withdrawing relative to H, and comparable to CH3 in acetaldehyde. Vertical ionization potentials andn* transition energies have also been calculated for these molecules at their optimized geometries, experimental geometries, and geometries given by a standard model. The effect of changes in molecular geometry on these computed properties has been analyzed. 相似文献
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Vito Fiandanese Giuseppe Marchese Francesco Naso 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-2):89-90
Abstract Synthetic, mechanistic, and stereochemical aspects of the reactions of cuprates with various systems are being investigated in these laboratories(1). The series of substrates includes several sulphonyl-activated compounds. 相似文献
26.
Dr. Fabio Carniato Dr. Lorenzo Tei Prof. Maurizio Cossi Prof. Leonardo Marchese Prof. Mauro Botta 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(35):10727-10734
Functionalised MCM‐41 mesoporous silica nanoparticles were used as carriers of GdIII complexes for the development of nanosized magnetic resonance imaging contrast agents. Three GdIII complexes based on the 1,4,7,10‐tetraazacyclododecane scaffold (DOTA; monoamide‐, DOTA‐ and DO3A‐like complexes) with distinct structural and magnetic properties were anchored on the silica nanoparticles functionalised with NH2 groups. The interaction between GdIII chelates and surface functional groups markedly influenced the relaxometric properties of the hybrid materials, and were deeply modified passing from ionic ? NH3+ to neutral amides. A complete study of the structural, textural and surface properties together with a full 1H relaxometric characterisation of these hybrid materials before and after surface modification was carried out. Particularly for the anionic complex 2 attached to MCM‐41, an impressive increase in relaxivity (r1p) was observed (from 20.3 to 37.8 mM ?1 s?1, 86.2 % enhancement at 20 MHz and 310 K), mainly due to a threefold faster water exchange rate after acetylation of the surface ? NH3+ ions. This high r1p value, coupled with the large molar amount of grafted 2 onto the silica nanoparticles gives rise to a value of relaxivity per particle of 29 500 mM ?1 s?1, which possibly allows it to be used in molecular imaging procedures. Smaller changes were observed for the hybrid materials based on neutral 1 and 3 complexes. In fact, whereas 1 shows a weak interaction with the surface and acetylation induced only some decrease of the local rotation, complex 3 appears to be involved in a strong interaction with surface silanols. This results in the displacement of a coordinated water molecule and in a decrease of the accessibility of the solvent to the metal centre, which is unaffected by the modification of ammonium ions to neutral amides. 相似文献
27.
Pascal Hubert Samuel Lelièvre Luca Marchese Corinna Ulcigrai 《Israel Journal of Mathematics》2018,225(2):553-607
Lagrange spectra have been defined for closed submanifolds of the moduli space of translation surfaces which are invariant under the action of SL(2, R). We consider the closed orbit generated by a specific covering of degree 7 of the standard torus, which is an element of the stratum H(2). We give an explicit formula for the values in the spectrum, in terms of a cocycle over the classical continued fraction. Differently from the classical case of the modular surface, where the lowest part of the Lagrange spectrum is discrete, we find an isolated minimum, and a set with a rich structure right above it. 相似文献
28.
Michele Lovallo Francesco Marchese Nicola Pergola Luciano Telesca 《Communications in Nonlinear Science & Numerical Simulation》2009,14(1):174-181
The time dynamics of long-term time series of satellite thermal signal, measured at Mount Etna, has been investigated. The signal has been analyzed by means of a recently proposed multi-temporal and robust technique (RST), which has already shown to be better capable to detect and monitor volcanic hotspots, compared to traditional satellite approaches. The temporal fluctuations of the thermal signal detected by RST over a long series (1995–2006) of advanced very high resolution radiometer (AVHRR) satellite data, have been investigated by means of the Fisher information measure, which is a powerful tool to investigate complex and nonstationary signals. The preliminary obtained results indicate that the proposed nonlinear approach can be used to dynamically characterize the volcanic phenomena and to recognize possible pre-eruptive temporal patterns. 相似文献
29.
Vito Fiandanese Daniela Bottalico Giuseppe Marchese Angela Punzi 《Journal of organometallic chemistry》2005,690(12):3004-3008
A straightforward synthesis of polyacetylenic montiporic acids A and B has been developed, based upon the selective and sequential substitution of the two trimethylsilyl groups of the readily available 1,4-bis(trimethylsilyl)-1,3-butadiyne. 相似文献
30.
The transfer and separation of Cu(II), Co(II), Ni(II) and Zn(II) ions across a hollow fiber supported liquid membrane containing LIX 864 as the mobile carrier dissolved in kerosene solvent has been investigated. The flux and selectivity for copper has been studied as a function of the feed flow, the carrier concentration in the liquid membrane and the extraction solution acidity. A maximum copper recovery at 30% of LIX (v/v) in the diluent was obtained. The permeation experiments showed that at pH 2 in the extraction solution a highly selective separation of Cu over the other cations can be achieved. Increasing the acidity of the extraction solution copper selectivity decrease and the grade of recuperation sequence is Cu>Co>Ni>Zn. These results suggest that in selected situations, this membrane system can be competitive with the conventional liquid-liquid extraction process, in particular in leaching solutions with low metal concentration. 相似文献