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.