Molecular Orientation in High-Field High-Resolution NMR

Abstract

At high magnetic fields a special feature of high-resolution NMR becomes important. It originates from the tendency of molecules, having anisotropic magnetic properties, to orient in the magnetic field of the spectrometer. As a result, anisotropic nuclear interactions are incompletely averaged and become manifest in the spectra of solutes in the mobile phases, which therefore may show additional fine structure. At currently available high fields, the effects of magnetic dipolar and electric quadrupolar spin couplings can be quite pronounced. NMR spectroscopists should therefore be aware of these alignment effects when interpreting their spectra. The study of magnetic field induced molecular orientation in NMR offers some interesting applications. Thus, information may be obtained about molecular susceptibility anisotropies and asymmetries, intermolecular interactions, aromaticity, solution conformations and quadrupole coupling constants. These possibilities in combination with the inherent simplicity of the technique make it an attractive tool for investigating properties and interactions of molecules in liquid solutions. It is expected that the use of supercritical fluid solvents and the future availability of higher magnetic fields in NMR, will greatly enlarge the range of possibilities of the method.