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• W2099056875 abstract "It has been shown that two nearby large lateral substituents can be introduced on the same side of a mesogenic core without destroying the nematic range, even if the pattern of substitution is not favourable. The newly synthesized compounds contain four rings in the main core, a lateral hexyloxy chain and a lateral aromatic branch. This aromatic ring itself is modified by different substituents at the meta or para position with respect to the core linkage. Two isomeric compounds containing a dodecyloxy chain in these positions have been studied by 13C NMR spectroscopy in the liquid crystalline phase. On entering the nematic phase, the 13C chemical shift showed a positive jump for the oxomethylenes attached to the core, but a negative jump for the oxomethylene on the aromatic lateral branch. It is found that the two lateral substituents are folded back along the core, implying a gauche conformation for the first fragment. However, in the dodecyloxy chain, the first and the subsequent methylene carbons have negative order parameters, showing a classical all-trans mean chain conformation. The values of the order parameters were derived from the transient oscillations observed during variable contact-time 1H-13C cross-polarization experiments. The order parameter values are not very dependent on the substitution position of the dodecyloxy chain. As a consequence, the dodecyloxy chain aligned in the nematic field affects the average molecular shape and the mean orientation of the whole molecule. Influence de substituants late′raux volumineux sur les proprie′te′s cristal liquide e′tudie′e par RMN 13C. Nous avons synthe′tise′ des nouveaux compose′s ne′matogènes contenant quatre cycles aromatiques dans le coeur central et posse′dant deux substituants late′raux adjacents. Le premier est une chaîne hexyloxy, le second contient un cycle aromatique lui-même substitue′ en para ou me′ta par des groupements rigides ou par une chaîne alkoxy. Les groupements rigides perturbent peu les propie′te′s me′sogènes de ces compose′s, à l'inverse de ce qui est observe′ pour les chaînes alkoxy. Deux compose′s isomères ayant une chaîne dode′cyloxy greffe′e sur le cycle late′ral ont e′te′ e′tudie′s par spectroscopie RMN 13C. Lors du passage dans la phase ne′matique, les de′placements chimiques pre′sentent un saut positif pour les oxome′thylènes directement accroche′s au coeur, tandis que l'oxome′thylène de la chaîne dode′cyloxy montre un saut ne′gatif. Ceci implique que les deux substituants late′raux sont globalement aligne′s le long du coeur me′sogène, impliquant une conformation gauche pour le premier fragment, tandis que la chaîne dode′cyloxy pre′sente une conformation moyenne trans, quelque soit le compose′. Les paramètres d'ordre des carbones des chaînes ont e′te′ de′termine′s à partir des oscillations dipolaires C–H observe′es lors des monte′es de polarization. Les valeurs des paramètres d'ordre sont peu de′pendantes de la position des carbones dans la chaîne dode′cyloxy. Ceci implique que cette chaîne est aligne′e dans le champ ne′matique, ce qui affecte la forme mole′culaire ainsi que l'orientation moyenne de la mole′cule." @default.
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• W2099056875 date "1998-01-01" @default.
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• W2099056875 title "Influence of large lateral substituents on the nematic liquid crystalline properties studied by 13C NMR" @default.
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• W2099056875 doi "https://doi.org/10.1039/a709247d" @default.
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