Dynamics on the microsecond timescale in hydrous silicates studied by solid-state ²H NMR spectroscopy

Solid-state ²H NMR spectroscopy has been used to probe the dynamic disorder of hydroxyl deuterons in a synthetic sample of deuterated hydroxyl-clinohumite (4Mg₂SiO₄·Mg(OD)₂), a proposed model for the incorporation of water within the Earth's mantle. Both static and magic angle spinning (MAS) NMR methods were used. Static ²H NMR appears to reveal little evidence of the dynamic process, yielding results similar to those obtained from deuterated brucite (Mg(OD) ₂), where no dynamics on the relevant timescale are expected to be present. However, in ²H MAS NMR spectra, considerable line broadening is observed for hydroxyl-clinohumite and a ²H double-quantum (DQ) MAS NMR spectrum confirms that this is due to motion on the microsecond timescale. Using a model for dynamic exchange of the hydroxyl deuterons between two sites identified in previous diffraction studies, first-principles density functional theory (DFT) calculations of ²H (spin I = 1) quadrupolar NMR parameters, and a simple analytical model for dynamic line broadening in MAS NMR experiments, we were able to reproduce the observed motional line broadening and use this to estimate a rate constant for the dynamic process. From analysis of the observed ²H linewidths in variable-temperature MAS experiments, an activation energy for the exchange process was also determined. A simulated static ²H NMR lineshape based on our dynamic model is consistent with the observed experimental static NMR spectrum, confirming that the motion present in this system is not easily detectable using a static NMR approach. Finally, a ²H DQMAS NMR spectrum of fluorine-substituted ²H-enriched hydroxyl-clinohumite shows how the dynamic exchange process is inhibited by O-D⋯F^- hydrogen-bonding interactions.