Angular dependence of 2H-NMR longitudinal spin relaxation in aligned nematic 4-n-pentyl-4'-cyanobiphenyl: molecular rotation and director fluctuations

The longitudinal (spin-lattice) relaxation in nuclear magnetic resonance (NMR) experiments on 4-n-pentyl-4'-cyanobiphenyl (5CB) mesogen, deuteriated in the alpha-position, has been investigated by changing the orientation of the phase director with respect to the instrumental magnetic field through the application of a competing alternating electric field. The angular-dependent profiles of the corresponding rotational spectral densities were fitted and interpreted by invoking a model for single-molecule reorientational dynamics and fluctuations of the local director with respect to the average direction of alignment. The relative contribution of the two processes to the longitudinal relaxation rate has been estimated, and values of the principal components of the rotational diffusion tensor of the 5CB molecule were obtained from the data fit. The work focuses mainly on the methodological grounds, by pointing out the increase in the extent of achievable information when passing from standard NMR experiments performed at the canonical orientation of the sample (that is an alignment collinear to the magnetic field) to measurements on arbitrarily oriented samples.