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Liquid Crystals

Phase Transitions in a Discotic Liquid Crystal from 13C NMR

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Pages 115-130 | Received 16 Oct 1989, Published online: 22 Sep 2006
 

Abstract

13C CPMAS NMR spectra of benzene-hexa-n-heptanoate C6(OCOC6H13)6 have been studied from 150–360 K. Three broad regions corresponding to C˭O, aromatic carbon and the alkyl chain (with splittings due to solid state effects) show dramatic variations with temperature, paralleling the configurational and conformational changes and pinpoints the onset of successive group motions in the paraffinic chains. The NMR results are interpreted in terms of a rigid non-planar asymmetric short (C‒)H. O contact, the so-called hydrogen-bonded gauche structure involving CH2(2) and C˭O within a chain between 150–207 K. At 227 K, the CH3 group is free to rotate but breaks up the intramolecular (C‒H)… O «bond» to give a more symmetric structure with the onset of CH2(2) motion, corresponding to the first-order phase transition observed at 230 K from dTA. Finally at 287 K, an intermolecular (C‒)H… O «bond» between CH2(6) of one molecule and a C˭O group of an adjacent molecule stabilizes the columnar structure and leads to a virtually planar molecule (only carbonyl oxygens lying out of the plane) with onset of CH2(3) motion. This «hydrogen-bonded» structure is stable throughout the solid phase and the «bond» is broken only in the mesophase. CH2(4) motion is evident at 336 K from fine structure in the spectrum. Around 347–348 K, a strong pre-transitional effect viz., aromatic singlet abruptly gives a triplet along with CH2(5) motion. At 352 K, the aromatic signal is lost in the noise and solid state splittings tend to disappear. At the mesophase (353 K), only narrow lines from alkyl chains are observed with onset of CH,(6) motion. Interestingly, though shifts of the groups do not change from meso to isotropic phase, line intensities and widths reveal that the part gauche conformer changes to an extended trans conformation, whereupon the isotropic phase is reached at 359 K. In general, the groups are more deshielded in the solid state due to intermolecular interaction effects as compared to that in the isotropic phase.

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