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

Area of Science:

• Mineralogy and Materials Science
• Geochemistry and Environmental Science

Background:

• Nesquehonite (MgCO3·nH2O) is a magnesium carbonate mineral with implications for carbon capture and storage (CCS).
• The precise chemical formula of nesquehonite has been a subject of debate, with proposed formulas including MgCO3·3H2O and Mg(HCO3)OH·2H2O.
• Resolving this formula is critical for understanding its thermodynamic properties, phase behavior, and chemical reactivity in CCS applications.

Purpose of the Study:

• To definitively determine the chemical formula and atomic structure of nesquehonite.
• To elucidate the role of hydrogen bonding in the structural stability of nesquehonite.
• To showcase the utility of NMR crystallography in resolving mineralogical ambiguities.

Main Methods:

• Nuclear Magnetic Resonance (NMR) crystallography, specifically rotational-echo double-resonance (REDOR) 13C{1H} experiments, was employed to precisely measure 13C-1H distances.
• 13C static NMR lineshapes and density functional theory (DFT) calculations were utilized to model hydrogen atom positions.
• Neutron powder diffraction data was used to corroborate the structural assignments.

Main Results:

• The study confirms the formula of nesquehonite as MgCO3·3H2O.
• Precise determination of hydrogen atom positions and their bonding environment was achieved.
• Evidence from neutron diffraction supports the assignment of MgCO3·3H2O.

Conclusions:

• The chemical formula of nesquehonite is accurately represented as MgCO3·3H2O.
• Understanding hydrogen bonding is key to explaining nesquehonite's stability against dehydration.
• NMR crystallography is a powerful technique for resolving structural questions where X-ray diffraction is insufficient.