NMR Crystallography: Evaluation of Hydrogen Positions in Hydromagnesite by ¹³C{¹H} REDOR Solid‐State NMR and Density Functional Theory Calculation of Chemical Shielding Tensors

Solid‐state NMR measurements coupled with density functional theory (DFT) calculations demonstrate how hydrogen positions can be refined in a crystalline system. The precision afforded by rotational‐echo double‐resonance (REDOR) NMR to interrogate ¹³C‐¹H distances is exploited along with DFT determinations of the ¹³C tensor of carbonates (CO₃²¯). Here it is shown that nearby ¹H nuclei perturb the axial symmetry of the carbonate sites in the lattice of the hydrated carbonate mineral, hydromagnesite [4MgCO₃·Mg(OH)₂·4H₂O]. A match between the calculated structure and solid‐state NMR is found by testing multiple semi‐local and dispersion‐corrected DFT functionals, applying them to optimize the multiple hydrogen and other atom positions, starting from X‐ray diffraction (XRD) determined atomic coordinates. Optimized atomic coordinates are validated by comparing calculated to experimental ¹³C{¹H} REDOR and ¹³C chemical shift anisotropy (CSA) tensor values. The results demonstrate that the combination of solid‐state NMR, XRD, and DFT can significantly improve structure refinement for hydrated materials.

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