Liquid Crystals As Solvents in NMR Spectroscopy: Current Developments in Structure Determination

Nuclear Magnetic Resonance (NMR) spectroscopy of molecules aligned in liquid crystalline media provides information on molecular structure and order parameters. The partial alignment of the molecules in the anisotropic phase of the liquid crystals gives rise to residual intramolecular dipolar couplings which are dependent on the internuclear distances and therefore yield information on the geometry of molecules. The spectra of these aligned molecules become rapidly complex with the increase in the number of interacting spins and with decrease in the symmetry of the molecules. The spectra of these molecules are generally strongly coupled and are complex. Thus the first order analysis of the spectra as applicable to liquid state is generally not applicable and one should resort to numerical analysis using computers. In this report problems associated with the analyses of such complex spectra, methods developed to aid such analyses and the precision of the structural parameters derived are discussed. Use of mixed liquid crystals of opposite diamagnetic susceptibility anisotropies to extract information which is otherwise not possible to derive using single liquid crystals is highlighted. Current developments in the field with special emphasis on the discovery of a novel liquid crystal with low order parameter and the use of natural abundance deuterium NMR spectra of the probe molecules for the analyses of complex spectra are discussed.