Nuclear magnetic resonance - Theoretical aspects

Nuclear magnetic resonance, or NMR, is one of the spectroscopic methods used to analyze matter. Compared with other spectroscopic methods, NMR is at the lowest end of the energy scale. As a result, NMR enables extremely detailed, non-destructive examination of the sample.

The basic physical phenomenon is nuclear magnetism. The vast majority of elements possess this property, at least in certain isotopic forms.

The field of NMR investigation is exceptionally broad. It can be used to study organic, biological and inorganic molecules, solids and living organisms. It is possible to examine a few milligrams or even micrograms of a molecule in solution, or a fragment of stone from a cathedral, or even a newborn child.

Why is NMR such a powerful means of investigation?

Information corresponding to nuclei of different types, e.g. ¹ H and ¹³ C or ¹ H and ² H, is completely distinct. In isotropic liquid media (high-resolution NMR), different nuclei of the same type, e.g. ¹ H or ¹³ C or ¹⁵ N, each give an individual response depending on the characteristics of their environment. In structural analysis, it is possible to identify the sequence of atoms and their relative positions in space. The proximity of nuclei across space, either within a molecule or between distinct entities, can be established. Numerous exchange phenomena are detected and analyzed. Molecular motion, whether global motion of a molecule, internal motion of part of a molecule or relative motion of distinct entities (diffusion), can be studied.

In anisotropic media, i.e. oriented liquids and solids, NMR presents additional constraints, due in particular to the reduced mobility of nuclei. Nonetheless, NMR has a number of advantages: it can be used to determine parameters not accessible in isotropic media, to observe insoluble substances, and to determine structural integrity when dissolution in a solvent does not allow it to be preserved. It goes far beyond X-ray studies, since the solid does not have to be a crystal.

In vivo NMR makes it possible to distinguish tissues according to their content of ¹ H nuclei (H ₂ O and/or other molecules), depending on parameters linked to the mobility of these molecules, and to obtain images of the various constituents of a structure such as a joint or blood vessels....