Rietveld refinement and NMR crystallographic investigations of multicomponent crystals containing alkali metal chlorides and urea

New mechanochemical preparations of three multicomponent crystals (MCCs) of the form MCl:urea·xH₂O (M = Li, Na and Cs) are reported. Their structures were determined by an NMR crystallography approach, combining Rietveld refinement of synchrotron powder X-ray diffraction data (PXRD), multinuclear (³⁵Cl, ⁷Li, ²³Na and ¹³³Cs) solid-state NMR (SSNMR) spectroscopy and thermal analysis. The mechanochemical syntheses of the three MCCs, two of which are novel, were optimized for maximum yield and efficiency. ³⁵Cl SSNMR is well suited for the structural characterization of these MCCs since it is sensitive to subtle differences and/or changes in chloride ion environments, providing a powerful means of examining H…Cl⁻ bonding environments. Alkali metal NMR is beneficial for identifying the number of unique magnetically and crystallographically distinct sites and enables facile detection of educts and/or impurities. In the case of NaCl:urea·H₂O, ²³Na magic-angle spinning NMR spectra are key, both for identifying residual NaCl educt and for monitoring NaCl:urea·H₂O degradation, which appears to proceed via an autocatalytic decomposition process driven by water (with a rate constant of k = 1.22 × 10⁻³ s⁻¹). SSNMR and PXRD were used to inform the initial structural models. Following Rietveld refinement, the models were subjected to dispersion-corrected plane-wave density functional theory geometry optimizations and subsequent calculations of the ³⁵Cl electric field gradient tensors, which enable the refinement of hydrogen-atom positions, as well as the exploration of their relationships to the local hydrogen-bonding environments of the chloride ions and crystallographic symmetry elements.