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Urea Group (urea + group)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Optimization of the separation conditions of tetracyclines on a preselected reversed-phase column with embedded urea group

JOURNAL OF SEPARATION SCIENCE, JSS, Issue 7 2006
Leila Kallel
Abstract The use of a C12 stationary phase with embedded polar group has been investigated for the separation of seven tetracyclines. The influence of pH, organic modifier, buffer, and temperature on the peak shape and analyte separation was discussed. It appears that all the chromatographic conditions had a great effect on both the resolution and peak shape whereas the elution order was not affected. The baseline separation with symmetrical peaks of the seven tetracyclines can be obtained with a mobile phase containing either 5 mM phosphate buffer pH 2.5/ACN (84 : 16 v/v) or 5 mM perchlorate buffer pH 2.5/ACN (75 : 25 v/v) at a temperature not exceeding 20°C. This study reveals that the retention mechanism is ion-pairing. [source]

Impact of polyurea structure on grease properties

LUBRICATION SCIENCE, Issue 9 2010
L. Liu
Abstract The thickener structure of polyurea grease has a crucial effect on its properties. Therefore, it is very important to study the relationship between a thickener structure and physical and performance properties. In this study, polyurea greases were synthesised, having a different number of urea groups in per polyurea thickener molecule. Properties such as dropping point, penetration, oil separation, shear stability and four-ball wear and extreme pressure (EP) were evaluated. The result shows that properties of polyurea grease vary with the number of urea group per polyurea molecule. Based on molecular theory, the mechanisms explaining these effects are briefly discussed. Copyright © 2010 John Wiley & Sons, Ltd. [source]

Tuning the Photoluminescence of Silsesquioxanes with Short Substituted Urea Bridges

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 6 2008
María L. Gómez
Abstract The polycondensation of a precursor synthesized by the reaction of 3-(anilinepropyl)trimethoxysilane with 3-(isocyanatopropyl)triethoxysilane led to a silsesquioxane bearing a substituted urea group in the short organic bridge. The self-assembly of organic bridges, analyzed by SAXS and FTIR spectra, could be controlled by varying the conditions of the synthesis. Depending on the size of organic clusters, the silsesquioxane exhibited photoluminescence either in the green or red regions of the spectra, or an emission that could be tuned in the whole visible region by the excitation wavelength. [source]

Non-racemic atropisomeric (thio)ureas as neutral enantioselective anion receptors for amino-acid derivatives: Origin of smaller Kass with thiourea than urea derivatives

CHIRALITY, Issue 9 2006
Christian Roussel
Abstract The synthesis of a limited series of non-racemic atropisomeric 1-(2-(4-methyl-2-thioxothiazol-3(2H)-yl)phenyl)-3-(hetero)aryl-(thio)ureas is described. Using NMR titration experiments monitoring the shift of the two NH of the (thio)urea and the C-5 hydrogen of the heterocycle, the binding constants for some optically pure (thio)-ureas with the enantiomers of N-protected amino acid tetrabutylammonium salts were determined in CD3CN. The obtained enantioselectivities were modest. Contrary to what was expected on the basis of the NH acidity in thiourea versus urea group, the association constants were smaller with the thiourea than with the corresponding urea. X-ray data, DFT calculations, and NMR provided the explanation of that unexpected behavior: the urea presents a pre-organized (Z,Z) conformation suitable for a double hydrogen bond with the carboxylate anion, the thiourea presents a (Z,E) conformation, which must be reorganized in a constrained (Z,Z) conformation in the complex. An intramolecular hydrogen bond between one NH and the thiocarbonyl group of the heterocycle, which is present in the thiourea and absent in the urea, might also contribute to the smaller Kass for the thiourea. The possible implication of these observations in the field of bifunctional organocatalysis is briefly discussed. Chirality, 2006. © 2006 Wiley-Liss, Inc. [source]