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 ⋅ 3H2O], or a hydroxy‐ hydrated‐ magnesium bicarbonate [Mg(HCO3)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.

Understanding Cu(i) local environments in MOFs via63/65Cu NMR spectroscopy

“…The EFG tensor parameters are sensitive to the local environment, which provides a metric to predict and optimize crystal structures. 86–92 In this section, we examine Cu( i ) structural insights obtained from plane-wave DFT calculations of 63/65 Cu EFG tensors in MOFs. It should be noted that the XRD crystal structures were obtained at low temperatures while our Cu NMR experiments were performed at room temperature, which could lead to discrepancies between calculated and experimental Cu NMR parameters due to issues such as temperature-dependent unit cell dimensions and dynamics.…”