Deuterium NMR Study of Molecular Order and Dynamics in 60CB

Abstract

To test models for molecular reorientation in liquid crystals, the biphenyl core of a perdeuterated nematic liquid crystal, 4-cyano-4′ −hexyloxy-d₁₃−biphenyl-d₈ (60CB-d₂₁) was studied using deuterium NMR spectroscopy. Two overlapped doublets, one from each deuterated phenyl ring, were seen in the deuterium spectrum of 60CB-d₂₁. An order parameter tensor of the molecular core was used to account for these doublet splittings. The asymmetry parameter η of the electric field gradient was found to be different for the deuterons on the two rings.

The spin-lattice relaxation times T_IZ and T_IQ were measured at 15.3 MHz for these rings, using a deconvolution routine that found the areas of two peaks in partially relaxed spectra. Four spectral densities of motion were derived as a function of temperature in the nematic phase. The small step rotational diffusion model or the “third rate anisotropic viscosity” model may be used to account for the measured spectral densities.

Key Words:

• nematic liquid crystals
• NMR relaxation
• molecular order
• dynamics