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71.
Absorption and fluorescence spectra of Sm3+ were measured in GeO2, ternary germanate, and borate glasses. From these the intensity parameters were calculated by use of the Judd-Ofelt formula.Visible emission and decay times from the4G5/2 level and its relative quantum efficiencies were measured The quantum efficiencies (QE) of the fluorescence in ternary germanate was higher by a factor of 20 than in GeO2, The small QE in GeO2, is explained by cross-relaxation between neighboring Sm3+ ions. The later process in hindered by the change in glass structure in the presence of modifier ions. A similar effect is observed in other glasses such as borate, where the addition of modifiers increases the QE of fluorescence. 相似文献
72.
A theoretical study has been undertaken of the relative rates and the regio- and stereo-chemistry of ring closure of a variety of alkenyl, alkenylaryl, alkenylvinyl and similar radicals. The method involves the application of MM2 force-field calculations to model transition structures for which the dimensions of the arrays of reactive centres have been obtained by MNDO-UHF techniques. The results, which generally accord with guidelines based on stereochemical considerations, show excellent qualitative and satisfactory quantitative agreement with experimental data. The method has been successfully applied to complex systems including ring closure of alkylperoxy radicals, and formation of the triquinane system by three consecutive cyclisations. 相似文献
73.
The three main types of hydroxyl groups on a silica surface are classified as isolated, hydrogen bonded, and inaccessible. The isolated and hydrogen bonded groups are the most important as these readily exchange with D(2)O and thus are exposed to reactant molecules. However, it has generally been accepted that the inaccessible groups do not participate in surface reactions as only a small fraction of these groups exchange with D(2)O. It is shown that the inaccessible hydroxyl groups on nonporous fumed silica and mesoporous MCM-48 silica powders and films fully exchange with D(2)O and are reactive with octadecylydimethylchlorosilane when supercritical CO(2) is used as the solvent. Furthermore, it is found that the CO(2) penetrating the regions containing the inaccessible groups is not removed by simple evacuation but rather slowly diffuses from the silica over periods of months. 相似文献
74.
Richard A. Bartsch David A. Babb Bronislaw P. Czech Dhimant H. Desai Eddy Chapoteau Carl R. Gebauer Wolodymyr Zazulak Anad Kumar 《Journal of inclusion phenomena and macrocyclic chemistry》1990,9(2):113-123
Four novel chromogenic benzocryptands and one side-armed chromogenic cryptand have been synthesized and their complexing abilities for alkali metal cations are described. 相似文献
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Ruthenium(II) complexes of monodonor ligands: efficient reagents for asymmetric ketone hydrogenation
Xu YA Clarkson GC Docherty G North CL Woodward G Wills M 《The Journal of organic chemistry》2005,70(20):8079-8087
[Chemical reaction: See text] A series of BINOL-derived ligands have been prepared and incorporated into ruthenium(II) complexes containing a diamine ligand. The complexes have proven to be excellent catalysts for the asymmetric hydrogenation of ketones, giving reduction products with enantiomeric excesses of up to 99%. 相似文献
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An investigation to determine the compounds present in the Ln2O3Nb2O5 systems (Ln = La, Gd, and Y) and attempts to activate such compounds with Ti, In, Sb, Bi, Eu, and Tb revealed that only the orthoniobate (LnNbO4) compound was a suitable host lattice and LnNbO4:Bi under 2537 Å excitation produced the best phosphors. GdNbO4:Bi is a brighter phosphor than the La and Y analogs, emitting at slightly higher energies. The position of the GdNbO4:Bi peak emission at 4500 Å is independent of the activator concentration whereas peak emissions for LaNbO4:Bi and YNbO4:Bi move to lower energies with increasing Bi concentration. 相似文献