Molecular motion in thermotropic liquid crystals: Deuterium relaxation and spectral densities of motion for a highly ordered spin probe in 4-octyl-4′-cyanobiphenyl

Deuterium relaxation data obtained at 9.2 and 38.4 MHz are reported for a solution of p-diethynyl-d,d′-benzene (DEB) in 4-n-octyl-4′-cyanobiphenyl (8CB). The temperature and frequency dependences of the spectral densities of motion for this highly ordered spin probe in 8CB are markedly different from those observed for DEB in nematics without underlying smectic phases. J1(ω0) and J2(2ω0) are both frequency dependent and because of the high degree of ordering observed for the deuterons (Szz =0.28–0.67) we have interpreted the relaxation data in terms of ‘‘rotational diffusion in a cone.’’ Given the simplistic nature of this model, the semiquantitative agreement observed between theory and experiment is satisfactory.