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Organic compounds; Diffusion; Nuclear magnetic resonance; Liquid crystals; Molecular spectra
Abstract:
Deuterium nuclear magnetic resonance (NMR) spectra are presented for homologues of two series of pyramidic liquid crystals, viz., hexaalkyloxytribenzocyclononatriene, I‐n, and hexaalkanoyloxytribenzocyclononatriene, II‐n, (where n represents the number of carbon atoms per side chain). The homologues of series I‐n exhibit a single hexagonal columnar mesophase PA, while those of series II‐n exhibit one or two columnar mesophases (depending on n); a low‐temperature biaxial phase PD, and a high‐temperature hexagonal phase PC. NMR measurements were performed in the mesophase region on compounds specifically deuterated at the crown methylene groups of the tribenzocyclononatriene core. The spectra in both systems exhibit dynamic features typical of molecules reorienting about their C3 axes within the columnar structures. The experimental spectra were compared with simulated line shapes calculated on the basis of two reorientation mechanisms, (i) symmetric threefold jumps and (ii) planar diffusion about the molecular symmetry axis. It was found that the two compounds of series I‐n studied, I‐7 and I‐10, exhibit spectral line shapes which fit best the planar diffusion model, as was found earlier for the I‐8 homologue of this series, while the spectra of the three compounds of series II‐n, II‐13, II‐14, and II‐15 are only consistent with the symmetric threefold jump model. Possible reasons for the difference in behavior between the two series are discussed. In an Appendix the asymptotic method for calculating frequency‐domain spectra is described.
