NMR crystallography for validating organic crystal structures

Developments in Density Functional Theory codes means that it is now relatively straightforward to calculate Nuclear Magnetic Resonance (NMR) parameters, such as chemical shifts, for a given crystal structure. In recent work examining repeat structure determinations in the Cambridge Structural Database, we have found many cases where additional experimental evidence, e.g. from NMR, is needed to distinguish between alternative solutions derived from diffraction studies. Given the variety of NMR nuclei, the variety of NMR observables (e.g. isotropic chemical shifts, quadrupolar couplings etc.), and the variety of 1D and 2D NMR experiments available it is not obvious, however, which NMR experiments distinguish most effectively between correct and incorrect structures.

A number of approaches to validating crystal structures will be evaluated, ranging from simple measures of deviations between experimental and calculated shifts to more sophisticated methods e.g. Bayesian inference to determine quantitatively the information provided by different data sets. We will also exploit local expertise in machine learning approaches to identify patterns in results, since it is likely that different situations will require different experimental inputs.