NMR spectroscopy of oriented systems: studies at and near magic angle sample spinning

Abstract

The experiments described in this chapter are important in the context of applications mentioned in section 2.2, viz., (i) analysis of spectra of oriented molecules and (ii) use of liquid crystals for measurement of spinning angle in variable-angle sample spinning (VASS) experiments. The orientation of the director is found to be dependent on the spinning speed and the angle that the spinning axis makes with the magnetic field. By suitably adjusting these parameters, it is possible to orient the director either parallel or perpendicular to the spinning axis of sample rotation. These experiments lead to an understanding of the director dynamics when the samples are spun and to a proper interpretation of the observed spectra. In a nematic liquid crystal sample spinning in a magnetic field, for ? > 0 (?? < 0) and for ? > ?? (? < ??), the director is distributed about the plane perpendicular to the spinning axis. This distribution varies with the sample spinning frequency, which can be monitored from the ²H spectrum of a molecule oriented in the liquid crystal. Since the spectrum is symmetric about the center, the intensity pattern of the spinning sidebands is sensitive to whether the angle ? is greater or smaller than ?? (and hence to the ?? of the liquid crystal used).