NMR crystallography

Traditionally, the structural characterization of crystalline solids has been the domain of diffraction techniques. These had achieved some measure of maturity at the time NMR spectroscopy came into common use, some 50–60 years ago. Solid-state NMR tended to be of the “broadline” variety where line shapes arising from abundant spin-1/2 nuclei were examined and analyzed in terms of dipolar couplings. For powders, the observation of fine structure was relatively rare and recourse was usually taken to second moment analysis to obtain structural and dynamic information. Some applications definitely did qualify to be considered as early versions of NMR crystallography, such as the measurement of inter-proton distances in the water molecules of crystal hydrates, the determination of the orientation of the dipolar coupling tensor in water molecules of single crystal hydrates, distinguishing water molecules from hydroxyls in inorganic compounds, etc.

An examination of the titles in this themed issue shows clearly why modern NMR crystallography is an attractive and powerful approach for characterizing structure and dynamics in solids. The versatility of the approach is evident from the materials studied and includes simple salts, complex organic solids, nanoporous frameworks and self-assembled supramolecular solids. Nuclei used in these studies cover the breadth of the periodic table and include not only those for which the correlations of structure with NMR tensors are well established but also those for which these relationships are still being explored. The field is dynamic as amply demonstrated by the ever increasing complexity of pulse sequences combined with advances in hardware such as fast MAS and microcoils.

It is also worth noting the pragmatic nature of NMR crystallography, as useful information obtained from a variety of techniques is incorporated in the methods used to solve specific problems. Not surprisingly, diffraction techniques are a major source of such information; therefore, NMR crystallography should generally be considered as a complementary tool. It is quite remarkable to see the importance that computational methods have assumed. Calculated magnetic shielding tensors and quadrupolar coupling parameters have become almost a routine input in defining and refining structures that are in some cases claimed to be more accurate than structures from diffraction.

There is no doubt that the future is bright for NMR crystallography as development continues at a rapid pace. However, there are also signs of some maturity, as there is an interest in standardizing and streamlining the approach for obtaining structures of some classes of materials.

We thank the authors for contributing their work to this themed issue. We also thank the editorial team for their dedication and assistance in the efficient handling of manuscript submission and review. We hope that this collection of articles will play a role in stimulating further interest and advances on the fundamental aspects and new applications of this rapidly advancing field.

---