MODELING MOLECULAR ORDER AND DYNAMICS OF A LIQUID-CRYSTAL BY DEUTERON NMR

The quadrupolar splittings in deuterated 4-n-hexyloxy-4'-cyanobiphenyl (60CB) have recently been reassigned with some certainty. This has necessitated a re-evaluation of the fitting of these splittings with the molecular mean field theory. The corresponding spectral densities of motion in this compound could not be successfully modeled before, partly because of the incorrect peak assignments to carbon positions 3 and 4 in the hexyl chain. This difficulty caused us to remeasure these spectral densities as a function of temperature with an improved signal to noise ratio at 15.1 and 46 MHz. In modeling the NMR observables, we found that the presence of an oxygen atom at the top of the hexyl chain has changed the molecular geometry and Ideal energies in the hexyl chain of 60CB. The observed static and dynamic NMR properties depend on all configurations which are generated using a realistic geometry for the molecule and the rotational isomeric state model of Flory. The additive potential method is employed to construct the potential of mean torque in the nematic phase, which allows the quadrupolar splittings to be calculated. The decoupled model is employed to describe correlated internal modes of motion by means of a master rate equation. Nordio's model of rotational diffusion is used to describe the overall molecular reorientation. Our relaxation data support the recent peak assignments [Liq. Cryst. 17, 303 (1994)] for deuterated 60CB. Furthermore, we were able to explain the quadrupolar splittings and spectral densities at various carbon sites of 60CB in a self-consistent manner.