N.M.R. study of collective molecular motions in smectogen liquid crystals

The proton spin relaxation dispersion, T 1(v), was studied by field-cycling techniques over a broad Larmor frequency range in the nematic and smectic phases of several liquid crystals (4,4′-bis-heptyloxyazoxybenzene, 4-cyano-4′-8-alkylbiphenyl, 4-cyano-4′-9-alkylbiphenyl, 4-cyano-4′-11-alkylbiphenyl and 4-cyano-4′-9-alkoxybiphenyl) with different sites of the nitrogen atom. The results can be explained quantitatively in terms of nematic and smectic order fluctuations (T 1 ∝ v 1/2, T 1 ∝ v 1), molecular self-diffusion, molecular rotations and up to six cross-relaxation resonances due to a nitrogen-proton coupling The order fluctuations contribution and the transition from the v 1/2 to the v 1 dispersion profile occurs always at Larmor frequencies in the kilohertz range. Some additional measurements of the frequency dependence of the dipolar relaxation time T 1D(v), are not in accord with existing theories.