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241.
The interaction of Cd2+ with uracil,2-thiouracil,4-thiouracil and 2,4-dithiouracil have been investigated by the density functional theory(DFT) calculations.For uracil and 2,4-dithiouracil,where the two basic sites are the same,Cd2+ attachment to the heteroatom at position 4 is preferred.However,for the systems where both types of basic centers,a carbonyl and a thiocarbonyl groups,are present,Cd2+ association with sulfur is favorable.The enhanced stability of these enolic and thiol forms comes from Cd2+ interaction with two basic sites simultaneously,which thereby triggers a significant aromatization of the ring.More significantly,the Cd2+ binding energy with uracil and its thio-derivatives is larger than the tautomerization barriers connecting the diketo-like forms with the corresponding enolic-like tautomers.Consequently,when associated with Cd2+,all tautomers are energetically accessible and should be observed in the gas phase. 相似文献
242.
Yu. V. Zavgorodnev K. A. Prokhorov L. A. Novokshonova T. M. Ushakova E. E. Starchak V. G. Krasheninnikov G. Yu. Nikolaeva E. A. Sagitova P. P. Pashinin M. A. Guseva P. Donfack B. von der Kammer A. Materny 《Laser Physics》2012,22(4):730-737
Ethylene/1-hexene copolymers, which differ significantly in 1-hexene content (from 1.9 to 37.0 mol %), are studied by Raman spectroscopy. The copolymer spectra are compared with the Raman spectra of polyethylene at elevated temperatures. We have found that the phase and conformational order of the copolymer macromolecules depends strongly on the 1-hexene content. In particular, an increase in 1-hexene content leads to a decrease in content of the PE-like orthorhombic crystalline phase and in content of trans-conformers in the amorphous phase. The Raman signature at about 800 cm?1 provides information about the end conformers for the ethylene/1-hexene copolymers with high 1-hexene content. 相似文献
243.
Lyudmila Novokshonova Natalia Kovaleva Irina Meshkova Tatiana Ushakova Vadim Krasheninnikov Tatiana Ladygina Ilia Leipunskii Alexey Zhigach Michail Kuskov 《Macromolecular Symposia》2004,213(1):147-156
Heterogenized activators - “support-H2O/AlR3” (where R=Me, iBu, support=montmorillonite, zeolite), synthesized directly on the support, form with metallocenes metal alkyl complexes highly active in olefin polymerization without the use of commercial methylaluminoxane (MAO). It was shown by the method of temperature programmed desorption with the application of mass-spectrometry (TPD-MS) that the aluminumorganic compound in support-H2O/AlR3 is in general similar to the structure of commercial MAO. The heterogenization of Zr-cenes on support-H2O/AlR3 is accompanied by the appearance of the energy non-uniformity of active sites. The activation energy of thermal destruction of active Zr-C bonds in the active sites of prepared catalysts changes in the range from 25 to 32 kcal/mol. 相似文献