Proton nmr studies of smectic phases of three n-4-n-alkyloxybenzylidene-4-n-alkylanilines n0 . msReportar como inadecuado




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Abstract : Proton NMR studies of the liquid crystals 50.6, 70.5, and 70.6 are presented. In all mesophases there is a clear proportionality between dipolar doublet splittings of the proton NMR spectra and the corresponding second moments indicating that the motions giving rise to the observed lineshapes remain fast on the NMR timescale even in the highly- ordered smectic-G phases and that no essential conformational changes occur. From the angular dependences of the lineshapes and second moments it could be concluded that the preferred direction of the sample, once aligned by the strong magnetic field in smectic-A, is preserved through all smectic phases though there are several changes from untilted to tilted phases. From linewidth measurements at the magic-angle orientation, activation energies for a thermally activated motional process could be derived in the low-temperature smectic phases B, F, G which differ for B- and F-G- phases, respectively. Simulations of the proton NMR lineshape confirm the existence of a twist of the aniline ring against the benzaldehyde — azomethine group plane as suggested by quantum-chemical considerations and X-ray investigations of related compounds. Besides one has to assume an additional motion decoupling the dipolar interaction between the azomethine proton and the protons of the neighbouring ring.

Keywords : molecular orientation NMR line breadth organic compounds proton magnetic resonance smectic liquid crystals





Autor: St. Limmer J. Schiffler M. Findeisen

Fuente: https://hal.archives-ouvertes.fr/



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