A carbon-13 study of relaxation in the mesophases of 5O·7 and the 5CB homologous series

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Abstract
Carbon-13 spin-lattice relaxation times of the protonated ring carbons have been measured at 22·6 MHz in the nematic and all four smectic phases of 5O·7 (4-n-pentyloxybenzylidene-4′-n-heptylaniline). Dong has obtained the deuterium spectral densities J 1 and J 2 at 15·4MHz for the deuterated aniline ring of 5O·7-d 4, and has presented and applied a theory in which the spectral densities are expressed in terms of the diffusion constants D∥ and D⊊. His results are used to calculate 13C relaxation times from the spectral densities J 0, J 1 and J 2. The calculated 13C spin-lattice relaxation times are then compared with our experimental values to test the theory. The 13C spin-lattice relaxation times of all the resolved resonances in the various phases of the first four members of the 5CB homologous series have been published previously. Dong has also published an analysis of 5CB deuterium data, and we use his results for the diffusion constants D∥ and D⊊ to calculate 13C relaxation times of the protonated aromatic carbons of 5CB, 6CB, 7CB and 8CB. The 13C relaxation times of the unprotonated aromatic carbons of the 5CB series are calculated in the manner of Wittebort et al., but using the spectral density expressions developed by Dong. The calculated 13C spin-lattice relaxation times of the 5CB homologous series are then compared with our experimental values to test the theory for the protonated and unprotonated ring carbons.