New Insights into the Structural Properties of Calcium Carbonate Hemihydrate by NMR Crystallography

While calcium carbonate is well known to exist in a range of different crystalline forms, including anhydrous polymorphs and hydrated phases, a new crystalline form─calcium carbonate hemihydrate (CCHH)─was reported in 2019 and has recently been observed in a biogenic material. The crystal structure of CCHH reported from diffraction studies is monoclinic, although a subsequent computational investigation suggested that an orthorhombic description of the structure may be more appropriate. Herein, we report experimental solid-state NMR characterization of CCHH, focused on solid-state 1H NMR and 13C NMR measurements, including analysis of 1H–13C heteronuclear correlation spectroscopy (HETCOR) and 13C chemical shift anisotropy (CSA) data, which reveals further insights into the structural and symmetry properties of this material. We demonstrate by means of DFT-GIPAW calculations that the monoclinic and orthorhombic descriptions of the crystal structure of CCHH are readily distinguishable on the basis of solid-state 1H and 13C NMR data. Our experimental solid-state NMR measurements are shown to support the orthorhombic description of the crystal structure rather than the monoclinic description.