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991.
Double-well potential energy surfaces for hydrogen bonding in crystalline vitamin B6 have been associated with molecular environmental effects. New calculations, involving improved representations of a fragmentation model, include the introduction of a second pyridoxinium chloride system within the model in a “dimer-like” configuration. The new results confirm the double-well potential and the prediction of the experimentally observed position for the proton as being due essentially to environmental effects. Atomic difference maps are presented for the charge density distributions, which reflect the nature of the bonding as it depends on the proton position. Mulliken populations are examined particularly in relation to the “intermolecular” transfer of electrons. 相似文献
992.
L. C. Balbás J. A. Alonso G. Borstel 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1987,6(3):219-226
The non local weighted-density approximation (WDA) to the exchange-correlation potentialV xc (r) is used to compute electron densities, ionization potentials and electron affinities of several atoms and ions. Especial care is taken in calculating and discussing 〈r 2〉 in noble gas atoms. Good results are obtained for all those quantities, but it is critical that the WDA is applied in such a way that it preserves the correct larger behaviour ofV xc (r). A comparison of the calculated electron density with that of the Hartree-Fock method shows the effects of coulomb correlation. The density rearrangements upon the introduction of correlation agree qualitatively with the results of Configuration-Interaction calculations 相似文献
993.
J. T. Glass J. F. McCann D. S. F. Crothers 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1996,36(2):119-124
Starting with the impulse approximation, we analyse second-order effects in relativistic electron capture. The relation of this model with relativistic distorted-wave approximations is clarified. In particular it is shown that the second-order spin-coupling terms in the RCDW theory are consistent with the correct form given by perturbation theory. In the semirelativistic limit, the RCDW results are shown to accord with the formulae of Moiseiwitsch for flip and nonflip transitions in the ultra-relativistic limit. This confirms that the continuum-distorted-wave model generalises to relativistic spinors, and highlights the defects of scalar models. We also present a new symmetric eikonal theory which gives reliable results for capture without change of spin, but leads to a divergent total cross section for spin-flip transitions in the second-order term. This effect, which is quite distinct from the spurious spin-flip amplitudes of the scalar symmetric eikonal theory, is taken as further evidence that the eikonal approximation is not valid for magnetic transitions. 相似文献
994.
G. Solano S. A. Katz J. Holzbecher A. Chatt 《Journal of Radioanalytical and Nuclear Chemistry》1994,179(1):173-176
Reference materials for the speciation and quantification of chromium in contaminated soils were prepared by impregnating diatomaceous earth with BaCrO4 and Cr2O3. The chronium concentrations of these materials were confirmed to be 200 mg/kg both by atomic absorption spectrometry and by instrumental neutron activation analysis, but monthly assays over two calendar quarters of the reference material impregnated with BaCrO4 revealed the hexavalent chromium was not stable in this matrix. 相似文献
995.
A. J. Blotcky E. P. Rack R. R. Recker J. A. Leffler S. Teitelbaum 《Journal of Radioanalytical and Nuclear Chemistry》1978,43(2):381-388
A destructive neutron activation analysis procedure was developed for determining trace aluminum content in bone. It was found
that soil contamination can influence the aluninum bone levels in prehistoric bone specimens. These maximum aluninum content
values for prehistoric bone are larger than those of modern bone and comparable to aluminum levels present in bone from renal
patients. 相似文献
996.
E. M. Essassi J. P. Lavergne Ph. Viallefont J. Daunis 《Journal of heterocyclic chemistry》1975,12(4):661-663
The condensation of 3,4-diamino 1,2,4-triazole with ethyl aceloacelate gave 6-methyl-8,9-dihydro(7H)-s-triazolo[4,3-b]-1,2,4-triazepin-8-one ( 2 ); 2 has been caracterized by nmr spectroscopy and by comparison with its methylated derivative prepared in an unambigous manner. 相似文献
997.
Oligomers of butadiene have been prepared in hydrocarbons in the presence of butyllithium isomers. Nuclear magnetic resonance (NMR), gas chromatography and i.r. spectroscopy allowed the study of the microstructures of the products. Various parameters (nature of the deactivator, the solvent and the initiator as well as the monomer/initiator ratio) influence the microstructure. All the results may be explained on the basis of delocalized living species in anionic polymerization of conjugated dienes. 相似文献
998.
J R J Delben D R Chaves P D Candelorio A A S T Delben 《Journal of Thermal Analysis and Calorimetry》2006,83(2):411-414
New glasses have been synthesized in the system ZrF46BaF26PrF3. Glasses were prepared by conventional fusion method and the
vitreous domain was established. For vitreous samples the thermal stability
parameters were determined, so that the best compositions could be selected
according to a compromise between high glass forming ability and thermal stability
vs. crystallization. 相似文献
999.
Infrared laser ablation study of pressed soil pellets with inductively coupled plasma atomic emission spectrometry 总被引:1,自引:0,他引:1
Mikolás J Musil P Stuchliková V Novotný K Otruba V Kanický V 《Analytical and bioanalytical chemistry》2002,374(2):244-250
Potential of infrared laser ablation (LA) coupled with ICP-AES as a technique suitable for the determination of trace elements (Zn, Cu, Ni, Cr, and V) in agricultural soils was studied. Operating parameters such as laser beam energy, laser beam focusing with respect to the sample surface, and velocity of the sample translation in the plane perpendicular to the laser beam were optimized. Soil samples were mixed with powdered Ag as a binder, and an internal standard (GeO(2)), and pressed into pellets. Calibration samples were prepared by adding known amounts of oxides of elements of interest into soils of known elemental composition and then processed in the same way as the analyzed samples. Calibration curves were found to be linear at least up to several hundreds of mg kg(-1) for the elements of interest. The elemental contents obtained by using LA-ICP-AES were compared with those obtained by analysis using wet chemistry followed by ICP-AES with pneumatic nebulization (PN). The results were in good agreement. Accuracy was also tested using certified reference soils with a bias not exceeding 10% relative. 相似文献
1000.
The extraction separation of La and Sm by benzyldibutylamine (BDBA) from an aqueous solution containing 6 mol·dm–3 NaNO3 and 0.5 mol·dm–3 HNO3 is not very efficient. It may remarkably be improved by addition of a complexing agent, 2-hydroxyethylethylenediaminetriacetic acid (HEDTA). The effect of preferential La extraction into the organic phase together with the effect of preferential complexation of Sm in the aqueous phase may lead, by proper choice of the complexing agent concentration, to a 7–10 fold increase in La/Sm values. The procedure described here enables fairly good separation of a major from minor elements. 相似文献