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

Abstract

Nesquehonite is a magnesium carbonate mineral relevant to carbon sequestration envisioned for carbon capture and storage of CO₂. Its chemical formula remains controversial today, assigned as either a hydrated magnesium carbonate [MgCO₃)OH ⋅ 2H₂O]. 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 ¹³C{¹H} (REDOR), ¹³C−¹H distances are determined with precision, and the combination of ¹³C static NMR lineshapes and density functional theory (DFT) calculations are used to model different H atomic coordinates. [MgCO₃ ⋅ 3H₂O] 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 MgCO₃. More broadly, these results illustrate the power of NMR crystallography as a technique for resolving questions where X-ray diffraction is inconclusive.