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Proton NMR spectra and relaxation times (Tis) at 60 MHz were obtained for the thermotropic compound 4'-n-hexadecyloxy-3'-nitro-biphenylcarboxylic acid, which, upon heating, has the sequence of phases: crystal~S&~D (or smectic-D)~S&~isotropic. The spectrum narrows over two orders of magnitude upon heatiqg into the D phase, then broadens again in the S& phase. The spectrum in the D phase is composed of a number of chemical shift peaks, the same as in the isotropic phase, except that the carboxylic acid peak is shifted further downfield in the D phase, evidence of intermolecular hydrogen bonding. In the smectic phases S|and S& the spectrum is super-Lorentzian with a width of around 2 kHz, considerably narrowed compared with the 36-kHz-wide spectrum of the solid. The relaxation times increase smoothly from 0.35 s at 124'C to 0.73 s at T2 and 3)(10 " m /s for the diffusion coeScient. The data indicate that, in the D phase, the molecules sample enough orientations on the time scale of the NMR experiments to average out the dipolar couplings left after the motional narrowing that takes place in the S& and S& phases. This is accomplished by diffusion of molecules through regions having different directors.

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