NMR studies of aD-phase compound

Proton NMR spectra and relaxation times (T₁s) 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{→}SC{→}D (or smectic-D){→}SA{→}isotropic. The spectrum narrows over two orders of magnitude upon heating into the D phase, then broadens again in the SA 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 SC and SA 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 194 ^∘{}C in the SC and SA phases. In the D phase, individual peaks have T₁s ranging from 0.18 to 1.1 s, with weighted averages agreeing with the trend of the data from the SC and SA phases. Measurements of spin echoes in the D phase without and with a field gradient gave values of about 50 ms for T₂ and 3×{}10^-11 m²/s for the diffusion coefficient. 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 SC and SA phases. This is accomplished by diffusion of molecules through regions having different directors.