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61.
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The anomeric hydroxyl of lactose was carbamoylated selectively by treating the free sugar with alkylisocyanates, thus without preliminary protection of the other hydroxyls of the lactose molecule. Moreover, the readily available 1-aminolactose and 1-octyl-aminolactose reacted with isocyanates and isothiocyanates to afford selectively -lactosyl -alkylureas and -lactosyl -alkylthioureas, respectively. 相似文献
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Treatment of substituted 1,3-oxathiolanes with ethyl (triethylsilyl)diazoacetate in the presence of a copper catalyst effects one-carbon ring expansion to the corresponding 3-triethylsilyl-1,4-oxathiane-3-carboxylates. Subsequent desilylation can be brought about by treatment with tetrabutylammonium fluoride. 相似文献
65.
An instrumental determination of phosphorus in aluminium-silicon alloys by activation with 20-MeV helium-4 particles can be based on the 31P(α , n)34m Cl reaction. A mixture of aluminium powder and disodium hydrogenphosphate is used as a standard. For concentrations ranging from 1.7 to 25 μg g-1, the experimental standard deviation ranges from 1 to 24% with an average of 8%. The method was compared with thermal neutron activation analysis based on the 31P(n, γ)32P reaction. 32P was chemically separated by precipitation as ammonium molybdophosphate. This technique yielded results with a standard deviation between 0.3 and 7.5% with an average of 2%. The agreement between the two methods was satisfactory. The results were also compared with photometry and atomic absorption spectrometry. 相似文献
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E. J. Goethals E. Schacht D. Tack 《Journal of polymer science. Part A, Polymer chemistry》1972,10(2):533-539
3,3-Dimethyl selenetane has been polymerized under the influence of different cationic and anionic catalysts. Cationic polymerization led to limited conversions because of the occurrence of a termination reaction between the growing chains and the formed polymer. The obtained polymers had low molecular weights. Anionic polymerization gave polymers with molecular weight up to 35,000. The mechanisms of polymerization are believed to be analogous to the mechanisms proposed for the polymerization of thietanes. 相似文献
68.
Thanks to the use of localized orbitals and the subsequent possibility of neglecting long-range interactions, the linear-scaling methods have allowed to treat large systems at ab initio level. However, the limitation of the number of active orbitals in a complete active space self consistent-field (CASSCF) calculation remains unchanged. The method presented in this paper suggests to divide the system into fragments containing only a small number of active orbitals. Starting from a guess wave function, each orbital is optimized in its corresponding fragment, in the presence of the other fragments. Once all the fragments have been treated, a new set of orbitals is obtained. The process is iterated until convergence. At the end of the calculation, a set of active orbitals is obtained, which is close to the exact CASSCF solution, and an accurate CASSCF energy can be estimated. 相似文献
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