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.nH2O (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. Mechanochemical syntheses of the three MCCs, two of which are novel, are 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 MAS 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 = 1.22x10-3 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 DFT geometry optimizations and subsequent calculations of the 35Cl EFG 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.