Solid-State ¹⁷O NMR Spectroscopy of Hydrous Magnesium Silicates: Evidence for Proton Dynamics

First-principles calculations of ¹⁷O quadrupolar and chemical shift NMR parameters were performed using CASTEP, a density functional theory (DFT) code, to try and interpret high-resolution ¹⁷O NMR spectra of the humite group minerals hydroxyl-chondrodite (2Mg₂SiO₄·Mg(OH)₂) and hydroxyl-clinohumite (4Mg₂SiO₄·Mg(OH)₂), which are models for the incorporation of water within the Earth’s upper mantle. The structures of these humite minerals contain two possible crystallographically inequivalent H sites with 50% occupancy. Isotropic ¹⁷O multiple-quantum magic angle spinning (MQMAS) spectra were therefore simulated using the calculated ¹⁷O NMR parameters and assuming either a static or dynamic model for the positional disorder of the H atoms. Only the dynamic disorder model provided simulated spectra that agree with experimental ¹⁷O MQMAS spectra of hydroxyl-chondrodite and hydroxyl-clinohumite. Previously published ¹⁷O satellite-transition magic angle spinning (STMAS) spectra of these minerals showed significant dynamic line-broadenings in the isotropic frequency dimension. We were able to reproduce these line-broadenings with at least qualitative accuracy using a combination of the same dynamic model for the positional H disorder, calculated values for the change in ¹⁷O quadrupolar NMR parameters upon H exchange, and a simple analytical model for dynamic line-broadening in MAS NMR experiments. Overall, this study shows that a combination of state-of-the-art NMR methodology and first-principles calculations of NMR parameters is able to provide information on dynamic processes in solids with atomic-scale resolution that is unobtainable by any other method.