NMR in Liquid Crystals Spinning at and Near Magic Angle

Abstract

NMR spectra of liquid crystalline phases and the molecules dissolved therein, spinning at and near the magic angle provide information on the director dynamics and the order parameter. The studies on the dynamics of the liquid crystal director for sample spinning near magic angle in mesophases with positive and negative diamagnetic susceptibility anisotropies (Δχ) and their mixtures with near-zero macroscopic diamagnetic susceptibility anisotropies have been reported. In systems with weakly positive Δχ, the director has been observed to switch from an orientation parallel to the spinning axis at low rotational speeds to one perpendicular to the spinning axis at high rotational speeds, when the angle θ, the axis of rotation makes with the magnetic fieid is smaller than the magic angle θm. For systems with a small negative Δχ, similar director behaviour has been observed for θ greater than θm. At magic angle, the spectra under slow spinning speeds exhibit a centre band and side bands at integral values of the spinning speeds. The intensities of the spinning side bands have been shown to contain information on the sign and the magnitude of the order parameter(s). The results are discussed with illustrative examples. Results on the orientation of the chemical shielding tensor obtained from a combination of the NMR studies in the solid and the liquid crystalline states, have been described.