Chapter 12 - Anisotropic NMR

Abstract

Many of the earliest nuclear magnetic resonance (NMR) experiments were performed on solid molecular systems. The high-resolution NMR of isotropic liquids gives very sharp spectra with fine structure dominated by isotropic spin–spin couplings J and chemical shifts. Unfortunately, only the relative signs of the J couplings can be determined from such spectra. The high-resolution NMR of ordered liquids can give information about: anisotropies in shielding constants, molecular structural parameters, the absolute signs of isotropic spin coupling constants, anisotropies in spin coupling constants, nuclear quadrupole coupling constants, and the mean orientation of a molecule in an anisotropic environment. Only low orientations are achievable using electric or magnetic fields to align the small molecules whose NMR spectrum is tractable to solution. The liquid-crystal solvent has a large average orientation, and the solute experiences the liquid-crystal field and becomes relatively well ordered. The spectra obtained are high-resolution and the sign of J can be related to the sign of the orientation order.