Nitrogen Nuclear Quadrupole Resonance Dips in the Proton Spin Relaxation Dispersion of Nematic and Smectic Thermotropic Liquid Crystals

Abstract

The Larmor frequency dependence of the proton spin relaxation time, obtained by means of the fast field-cycling NMR technique, has been used to study the ¹⁴N quadrupole coupling constant K and its asymmetry parameter η in the nematic and smectic phases of some liquid crystalline azoxybenzenes (PAA, BAB, HAB, HpAB), cyanobiphenyls (8CB, 9CB, 11CB) and oxycyanobiphenyls (9 OCB). Due to fast molecular reorientations, the effective quadrupole coupling constants are relatively small, whereas surprisingly the asymmetry parameters are rather large. The temperature dependence of both K and η within the mesophases, as well as their discontinuities at the different mesophase transitions, can be interpreted by the anisotropy of molecular rotations. It is found that temperature effects are significantly more pronounced for the (biaxial) smectic-C phase of the heptyloxyazoxybenzene (HpAB) than for the (uniaxial) smectic-A phase of the various investigated cyano- and oxycyanobiphenyls. As a rule, η turned out smaller in the smectic than in the nematic state, whereas K has similar values in both phases