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排序方式: 共有299条查询结果,搜索用时 15 毫秒
1.
2.
Inna Bumagina 《Geometriae Dedicata》2004,106(1):211-230
A group is said to be Hopfian if every surjective endomorphism of the group is injective. We show that finitely generated subgroups of torsion-free hyperbolic groups are Hopfian. Our proof generalizes a theorem of Sela (Topology
35 (2) 1999, 301–321). 相似文献
3.
Russian Chemical Bulletin - 相似文献
4.
Fireman-Shoresh S Popov I Avnir D Marx S 《Journal of the American Chemical Society》2005,127(8):2650-2655
Enantioselective surfactant-templated thin films were fabricated through the sol-gel (SG) process. The enantioselectivity is general in the sense that it discriminates between pairs of enantiomers not used for the imprinting process. The chiral cationic surfactant (-)-N-dodecyl-N-methylephedrinium bromide (1) was used as the surfactant template, and after its extraction chiral domains were created. The chiral discriminative feature of these films was examined by challenging with pure enantiomer solutions for rebinding. Selective adsorption was shown using (R)- and (S)-propranolol, (R)-2 and (S)-2, respectively, and (R)- and (S)-2,2,2-trifluoro-1-(9-anthryl)ethanol, (R)-3 and (S)-3, respectively, as the chiral probes. The selective adsorption was measured by fluorescence analysis, and the chiral selectivity factors were found to be 1.6 for 2 and 2.25 for 3. In both cases, (R)-enantiomer was adsorbed preferably. The resulting material was characterized by transmission electron microscopy, by diffraction, and by surface area measurements, and was found to be semicrystalline with short-range ordered domains (50 A) of hexagonal symmetry. 相似文献
5.
Nikolai N. Brandt Andrey Yu. Chikishev Inna K. Sakodinskaya 《Journal of Molecular Structure》2003,648(3):177-182
Raman spectroscopy is employed to study tris-(hydroxymethyl)aminomethane and its complexes with 18-crown-6. The results obtained are used to interpret the known effect of α-chymotrypsin activation by crown ether in organic solvents. Raman spectra of the samples lyophilized from aqueous solutions at various pH values are measured in solid state, acetonitrile and cyclohexane. 相似文献
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7.
V. Z. Sharf V. I. Isaeva A. N. Zhilyaev I. B. Baranovskii 《Russian Chemical Bulletin》1989,38(9):1796-1800
Binuclear hexafluoroacetylacetonate complexes of Rh(II) with axial ligands (Py, H2O) exhibit activity in hydrogenation and isomerization of allylbenzene, and the isomerization reaction also takes place in an atmosphere of Ar. The catalytic system [Rh2(hfacac)4(H2O)2]-Ph3P is much more active than Rh(II) hexafluoroacetylacetonate complexes in transformation of allylbenzene. Treatment of the acetate complex [Rh2(O2CCH2)4] with sodium borohydride significantly increases its activity, probably due to the formation of [Rh2(O2CCH3)3]+ and [Rh2(O2CCH2)2]2+ complexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1957–1961, September, 1989. 相似文献
8.
V. Z. Sharf L. Kh. Freidlin A. S. Nekrasov A. A. Nazaryan S. A. Ponomareva 《Chemistry of Heterocyclic Compounds》1971,4(4):457-458
In the presence of tricalcium phosphate, 3-oxa-, 3-aza-, and 3-thia-1, 5-pentanediols undergo intramolecular dehydration with the formation of heterocycles and also of unstable readily-polymerizing vinyl compounds. The total degree of conversion of the diols rises in the sequence diethyleneglycol <1, 5-pentanediol 相似文献
9.
Kocher N Henn J Gostevskii B Kost D Kalikhman I Engels B Stalke D 《Journal of the American Chemical Society》2004,126(17):5563-5568
The concept of hypervalency in molecules, which hold more than eight valence electrons at the central atom, still is a topic of constant debate. There is general interest in silicon compounds with more than four substituents at the central silicon atom. The dispute, whether this silicon is hypervalent or highly coordinated, is enlightened by the first experimental charge density determination and subsequent topological analysis of three different highly polar Si-E (E = N, O, F) bonds in a hexacoordinated compound. The experiment reveals predominantly ionic bonding and much less covalent contribution than commonly anticipated. For comparison gas-phase ab initio calculations were performed on this compound. The results of the theoretical calculations underline the findings of the experiment. 相似文献
10.