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941.
Lead(II) alkanoates with even chain lengths from octanoate to octadecanoate have been investigated by Raman spectroscopy. In the low frequency region, transverse and longitudinal acoustical modes (TAM, LAM) have been assigned. It was shown that LAM-1 is the vibration of the double chain with the node of the vibration in the Pb2+ layer. A fully extended conformation of the chains in the low temperature phase was confirmed. The frequencies and intensities of the LAMs as compared with those of the alkanes and the fatty acids led to an estimate of the force constant and polarizability of the Pb2+ -COO- bond relative to the C-C bond. The defects at the chain ends were investigated in the ρ(CH3) and v(CC) region. For the intermediate (CM) phase, both the Pb2+ layer distance reduction and the chain length independent enthalpy contributions can be attributed mainly to defects at the chain ends. 相似文献
942.
The toxicity of inorganic trivalent arsenic for living organisms is reduced by in vivo methylation of the element. In man, this biotransformation leads to the synthesis of monomethylarsonic (MMA) and dimethylarsinic (DMA) acids, which are efficiently eliminated in urine along with the unchanged form (Asi). In order to document the methylation process in humans, the kinetics of Asi, MMA and DMA elimination were studied in volunteers given a single dose of one of these three arsenicals or repeated doses of Asi. The arsenic methylation efficiency was also assessed in subjects acutely intoxicated with arsenic trioxide (As2O3) and in patients with liver diseases. Several observations in humans can be explained by the properties of the enzymic systems involved in the methylation process which we have characterized in vitro and in vivo in rats as follows: (1) production of Asi metabolites is catalyzed by an enzymic system whose activity is highest in liver cytosol; (2) different enzymic activities, using the same methyl group donor (S-adenosylmethionine), lead to the production of mono- and di-methylated derivatives which are excreted in urine as MMA and DMA; (3) dimethylating activity is highly sensitive to inhibition by excess of inorganic arsenic; (4) reduced glutathione concentration in liver moderates the arsenic methylation process through several mechanisms, e.g. stimulation of the first methylation reaction leading to MMA, facilitation of Asi uptake by hepatocytes, stimulation of the biliary excretion of the element, reduction of pentavalent forms before methylation, and protection of a reducing environment in the cells necessary to maintain the activity of the enzymic systems. 相似文献
943.
E. M. S. Castanheira J. M. G. Martinho 《Journal of photochemistry and photobiology. A, Chemistry》1994,80(1-3):151-156
Solvatochromic shifts of pyrene (Py) and naphthalene (Np) excimers were obtained in polar and non-polar solvents. The observed shifts for both excimers are explained by changes in the polarizability between the excimer and the dissociative ground state. The magnitudes of the shifts in the pyrene excimer are larger, indicating that the pyrene excimer is more polarizable than the naphthalene excimer. 相似文献
944.
Blends of polyarylate of bisphenol A, PAr, with two commercial main chain liquid crystalline polymers, Vectra A950 and Vectra B950, are studied. From dynamic viscoelastic measurements it is deduced that both systems (PAr/Vectra A950 and PAr/Vectra B950) are immiscible and scanning electron microscopy (SEM) micrographs show the presence of spherical domains of the liquid crystalline polymer when PAr constitutes the matrix. Extrusion capillary measurements reveal that, under conditions of temperature and shear rate similar to those of processing, the viscosity is reduced to approximately 10% of its value when the content of liquid crystalline polymer is only 20%. This great improvement of the rheological properties is observed in both PAr/Vectra A950 and PAr/Vectra B950 blends. The effect of draw ratio on Young's modulus for different compositions is also analyzed, pointing out the reinforcing action of both liquid crystalline polymers on polyarylate: for instance, 20% of Vectra B950 in the blend gives rise to a 700% increase of the modulus of fibres prepared at a draw ratio of 50. SEM of the extrudates reveals that the spherical domains are elongated at the entrance of the capillary giving rise to a microfibrillar morphology which is related to the excellent rheological and mechanical properties of the blends. 相似文献
945.
946.
Reduced charge montmorillonites (RCM) were prepared by the thermal treatment of lithiumsaturated montmorillonite. Samples prepared by mild thermal treatment with lithium contained more water sorbed than the original montmorrilonite. When RCMs were prepared, part of the lithium cations reacted with hydroxyl groups in the octahedral sheet and released protons, which reacted with the structure. Acid treatment probably enhanced the surface area. which was reflected in the amount of water sorbed. Deprotonation of hydroxyl groups was proved by the measurements of the ignition loss. The heating of lithium saturated montmorillonite at higher temperatures brough about the collapse of the interlayers and a decrease in the amount of water sorbed. 相似文献
947.
Polypropylenes of differing morphologies were examined by differential thermal analysis. The problems of reordering during
the programmed heating are discussed. Four potential procedures by which the reordering could be inhibited are considered,
and three of these are experimentally assessed. Their advantages and deficiencies are identified. Two pretreatments, an etching
and a crosslinking, are recognised as able to qualitatively characterise the initial morphology, with a preference for the
former. A test to identify the occurrence of reordering is identified.
In celebration of the 60th birthday of Dr. Andrew K. Galwey 相似文献
948.
N-7, O-Disilylation of 4(3H)-oxo-7H-pyrrolo[2,3-d]pyrimidines provides a convenient method for activation of these compounds toward regiospecific halogenation at C-5. The sequence is conveniently carried out without isolation of the silylated derivatives and desilylation is spontaneous upon addition of water to the reaction mixture. 相似文献
949.
J. M. Godoy M. L. Godoy Z. L. Carvalho D. C. Lauria 《Journal of Radioanalytical and Nuclear Chemistry》1994,183(1):19-26
A short overview of HPLC column packings is presented. The properties of chromatographic carriers and the possibilities to combine the solid matrices with organic polymeric stationary phases are elucidated in detail. The latest achievements and anticipated future developments in the area are outlined. 相似文献
950.
O. Kindel F. J. Herrmann L. Schmidt 《Journal of Radioanalytical and Nuclear Chemistry》1994,183(2):219-224
By simulation experiments with a 10–5 mol/l solution of iodododecane labeled with131I in n-dodecane the influence of various materials and conditions, which are possible in nuclear fuel reprocessing, has been investigated. The formation of decomposition products was detected via HPLC with a radioactivity monitor. By means of252Cf plasma-desorption mass spectrometry (PDMS) the decomposition products were identified. It was found that a temperature of 100°C favored the formation of iodoalkanes with chain lengths of C1 to C11. The presence of TBP(tri-n-butyl-phosphate) accelerated the decomposition of iodododecane. In pure TBP only iodobutane was formed as a decomposition product. 相似文献