Resolving the Chemical Formula of Nesquehonite via NMR Crystallography, DFT Computation, and Complementary Neutron Diffraction.

Nesquehonite is a magnesium carbonate mineral relevant to carbon sequestration envisioned for carbon capture and storage of CO . Its chemical formula remains controversial today, assigned as either a hydrated magnesium carbonate [MgCO  ⋅ 3H O], or a hydroxy- hydrated- magnesium bicarbonate [Mg(HCO )OH ⋅ 2H O]. The resolution of this controversy is central to understanding this material's thermodynamic, phase, and chemical behavior. In an NMR crystallography study, using rotational-echo double-resonance C{ H} (REDOR), C- H distances are determined with precision, and the combination of C static NMR lineshapes and density functional theory (DFT) calculations are used to model different H atomic coordinates. [MgCO  ⋅ 3H O] is found to be accurate, and evidence from neutron powder diffraction bolsters these assignments. Refined H positions can help understand how H-bonding stabilizes this structure against dehydration to MgCO . More broadly, these results illustrate the power of NMR crystallography as a technique for resolving questions where X-ray diffraction is inconclusive.