1,721,200 research outputs found
Conformational investigation of dibenzo-p-dioxin by proton magnetic resonance in a liquid crystalline solvent
No full textThe structure of dibenzo-p-dioxin has been studied by the nematic liquid crystal n.m.r. method. This molecule is solution adopts a folder conformation about the axis connecting the two heteroatoms with an angle of folding of 165.6 ± 2°
Conformational analysis of N-acetyldaunomycin in solution. A transient 1H nuclear Overhauser effect study of the glycosidic linkage geometry
No full textThe conformational properties of daunomycin (1) and several analogues (2)–(16) have been investigated by 1H n.m.r. in different solvents, CDCI3, D2O, DMSO, dioxane, and pyridine. From H,H three-bond and long-range coupling constants, the geometry of ring A in each solvent has been determined, through a Karplus–Altona equation, which includes a correction for the substiuent electronegativity. Conformers 9H8, 8H9, 9S, S9, and S8 have been found in solution, and in several cases, especially in DMSO, more than one conformer is present at equilibrium. The equilibrium is always fast compared with n.m.r. times and the relative populations of each conformer have been calculated from experimental and model coupling constants by using a least-squares procedure. Two factors have been recognized to be responsible for the shape of ring A: the intramolecular hydrogen-bond 9–OH O(7) and steric interactions between peri substituents on the A and B rings. Evidence for this hydrogen-bonding has been obtained by dilution experiments, and, for the peri interactions, by evaluating the substituent effects on the conformational preference. Other intramolecular hydrogen bonds have been proved not to exist in solution. The influence of the solvents on the shape of ring A has also been studied. All the results are discussed and compared with those obtained in the solid phase. Daunosamine moiety has also been analysed, and the conformation of the sugar ring is always 1′C4. (L) in all the solvents examined
Conformation of thianthren and 2,7-dichlorothianthren partially oriented in a nematic phase
No full textThe 1H n.m.r. spectra of thianthren and 2,7-dichlorothianthren dissolved in the Merck Nematic Phase IV were analysed and the structural parameters obtained. These molecules in solution adopt a folded conformation about the axis connecting the two sulphur heteroatoms. The angle of folding is 140.6° for thianthren and 139.8° for 2,7-dichlorothianthren. The correspondent standard deviations are ±0.4 and ±0.7°, respectively
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