NMR Crystallography of Disordered Cocrystals

Abstract

Crystallographic disorder is common in the solid state but it is rarely investigated explicitly
despite having a fundamental impact on the solid-state structure of a material. In this work,
nuclear magnetic resonance (NMR) crystallography methods are utilised to achieve a
detailed understanding of the structure and dynamics of solid organic systems containing
disorder. Several new cocrystal systems are studied, each containing a topical drug
molecule (caffeine, naproxen or furosemide) and each serving to demonstrate how NMR
crystallography can be applied to a variety of structural questions. Hydrogen bonding
motifs are identified using single crystal X-ray diffraction experiments, where possible, and
are subsequently verified by solid-state NMR. Alternative hydrogen bonding models are
ruled out by comparison of experimental solid-state NMR data with density functional
theory calculated shieldings, and proton transfer can be investigated by monitoring the
energy of the system with respect to proton position. In a particularly challenging case, 2D
solid-state NMR experiments go some way to identify the hydrogen bonds in a system that
cannot be crystallised. Dynamic disorder of fragments and solvent molecules are
characterised by variable temperature solid-state NMR, including analysis of relaxation
times to establish energy barriers and rates of motion. A mechanism of motion is also
proposed for dynamic acetone molecules in a new cocrystal solvate, which is supported by good agreement between experimental and simulated 2H static NMR line shapes. Finally,
the current limit of NMR crystallography is identified with respect to the reproducibility of
calculated NMR parameters following geometry optimisation. It is shown that the
geometry optimisation protocol does not affect standard NMR crystallography
investigations pertaining to atom assignment, but it is significant for cases where very
subtle structural features are probed, such as NMR linewidths. Overall, NMR
crystallography investigations allow a deeper understanding of solid materials to be
achieved than would be possible with any single technique and this work highlights the
applicability of such methods to complex materials containing disorder.