Angular and frequency dependent spin relaxation study of liquid crystalline cyanobiphenyls

NMR field-cycling measurements of the Larmor frequency (nu) and angular (Delta) dependences of the longitudinal proton spin relaxation time T-1 for the nematic liquid crystals 5CB and 8CB allow a more detailed analysis of the underlying molecular motions than data available previously. All T-1(nu, Delta) dispersion profiles essentially distinguish three frequency ranges where T-1 is governed by either local field effects, collective motions (director order fluctuations), or rotational and translational diffusion of individual molecules or molecular groups, respectively. The angular dependence supports and extends previous conclusions about the significance of the order fluctuation term at low (kHz) and high (MHz) Larmor frequencies; in addition it is the basis for the disentanglement of local held effects, which involve Jeener's dipolar relaxation, and of the sophisticated rotational relaxation models suggested in the literature by Dong, Nordio and Void. It is found that Void's third rate concept gives the best explanation of the measurements. The results on the rotational diffusion processes essentially agree with deuteron studies from the literature, but also reveal clear distinctions with regard to the anisotropy parameter sigma, essentially due to the improved separation from the order fluctuation contribution.