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941.
M. I. Prudêncio J. M. P. Cabral 《Journal of Radioanalytical and Nuclear Chemistry》1988,123(1):309-320
Eight rare-earth elements (REE), namely La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu, as well as other elements (Na, K, Sc, Cr, Fe, Co, Rb, Cs, Ba, Hf, Ta, and Th), have been determined in fifteen cretaceous clay samples of continental facies by instrumental neutron activation analysis. It was found that the REE contents are variable in absolute and relative values, but the means of these values are similar to those of European, shales. Analyses have also been made of the fractions <38 m and <2 m (clay-sized). The mineral contents of the clay-sized fraction were determined semi-quantitatively by X-ray diffraction. The results suggest the preferential presence of REE, Hf, and Th in fractions 2–38 m, which can be explained by the presence of apatite, monazite, and zircon. A correlation study of chemical and mineralogical data of the clay-sized fraction showed that kaolinite is correlated with REE, specially the lighter ones; illite with K, Rb, and Cs; and smectite with Na. 相似文献
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946.
Calculations of the 13C NMR chemical shifts for the methoxy and ethoxy groups adsorbed on Y and ZSM-5 zeolites were computed at GIAO/B3LYP/6-31+G*//MM+ level of theory, using a cluster representing a real part of the zeolites. The Y zeolite was represented by a cluster with 168 atoms, while ZSM-5 was represented by a cluster with 144 atoms. The calculated chemical shifts agreed well with reported experimental values, showing that the difference in chemical shifts is associated with differences in the geometry of the alkoxides on the two zeolites. 相似文献
947.
[reaction: see text] Molecular modeling was used to determine the structure of lithium vinylcarbenoids in the gas phase and in THF solution. Solvent effects were modeled by microsolvation with explicit THF ligands on each of the lithium atoms. The carbenoid geometries are dependent on the heteroatom and on solvation. The calculations predict 1-chlorovinyllithium and 1-bromovinyllithium to be a mixture of monomer and dimer at 200 K and mostly monomer at higher temperatures, whereas the 1-fluoro-, 1-methoxy-, and 1-dimethylaminovinyllithium are predicted to be dimeric in solution. 相似文献
948.
This study relates regional and seasonal UV index (UVI) variations, number of skin cancer cases and population skin-color distribution in Brazil. UVI calculations were performed using the UV Global Atmospheric Model (UVGAME), whose characteristics and validations are provided in thiis article. Health and racial data sets are based on the health and census data collected by Brazilian governmental agencies in the past. The discussion covers cultural customs and details of health and educational campaigns in Brazil. Despite lower UV levels in the South and Southeast regions, the results show a number of nonmelanoma skin cancer (NMSC) cases regions, where the white population is predominant. In general, in the southern regions about 50 new NMSC cases per 100000 inhabitants have been diagnosed each year. These rates decrease almost 40% in the Central-North regions and more than 80% in Northeast region, where miscegenation is common. In addition, the UVI evaluation is extended to other South American sites with singular characteristics, e.g. populous cities located in high altitudes or those affected by the Antarctic ozone hole in the extreme south of the continent. 相似文献
949.
The mechanism of the solid state displacement reaction NaCl + KF → NaF + KCl was investigated, employing diffusion couples (single crystals), in air at 550°C.The product layer obtained was formed with NaF and (Na, K)Cl solid solution.From cation concentration profiles, photometrically determined, for the NaClz.sfnc;KF and NaCl|KCl systems after annealing at 550°C, and from X-ray diffraction analysis on product layer surfaces for the NaCl|KF system, it was possible to state that the overall process is governed by a cation-counterdiffusion mechanism.A comparison between the diffusion coefficient evaluated through the rate constant and that calculated by means of the Boltzmann-Matano analysis for the system NaClKCl, allows one to deduce that the cations Na+ and K+ are transported in the (Na, K)Cl solid solution. 相似文献
950.
The kinetics of the solid state displacement reaction TlCl + KI → TlI + KCl was investigated by the diffusion couple method (using single crystal disks and pellets) in the temperature range 215–300°C. Two distinct layers are present: the first, in contact with TlCl, formed by a solid solution of TlI in TlCl, grows with a linear rate; the second, in contact with KI, formed by two solid solutions of TlCl in TlI and in KCl, respectively, grows according to the parabolic law. From marker experiments and X-ray analyses on product layer surfaces parallel to the original interface, along with a comparison of the rate constants and the diffusion coefficients, it was possible to deduce that the overall process is governed by diffusion of Tl+ and Cl? in the iodide-rich solid solution Tl(I, Cl) and that the rate-determining step is the diffusion of Tl+. 相似文献