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

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<mark>Journal publication date</mark>8/01/2009
<mark>Journal</mark>The Journal of Physical Chemistry C
Issue number1
Volume113
Number of pages7
Pages (from-to)465-471
StatePublished
Early online date5/12/08
Original languageEnglish

Abstract

First-principles calculations of O-17 quadrupolar and chemical shift NMR parameters were performed using CASTEP, a density functional theory (I)FT) code, to try and interpret high-resolution O-17 NMR spectra of the humite group minerals hydroxyl-chondrodite (2Mg(2)SiO(4)center dot Mg(OH)(2)) and hydroxyl-clinohumite (4Mg(2)SiO(4)center dot 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 O-17 multiple-quantum magic angle spinning (MQMAS) spectra were therefore simulated using the calculated O-17 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 O-17 MQMAS spectra of hydroxyl-chondrodite and hydroxyl-clinohumite. Previously published O-17 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 O-17 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.