Research output: Contribution to Journal/Magazine › Journal article › peer-review
Research output: Contribution to Journal/Magazine › Journal article › peer-review
}
TY - JOUR
T1 - Dynamics on the microsecond timescale in hydrous silicates studied by solid-state H-2 NMR spectroscopy
AU - Griffin, John M.
AU - Miller, Andrew J.
AU - Berry, Andrew J.
AU - Wimperis, Stephen
AU - Ashbrook, Sharon E.
PY - 2010
Y1 - 2010
N2 - Solid-state H-2 NMR spectroscopy has been used to probe the dynamic disorder of hydroxyl deuterons in a synthetic sample of deuterated hydroxyl-clinohumite (4Mg(2)SiO(4)center dot 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 H-2 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 H-2 MAS NMR spectra, considerable line broadening is observed for hydroxyl-clinohumite and a H-2 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 H-2 (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 H-2 linewidths in variable-temperature MAS experiments, an activation energy for the exchange process was also determined. A simulated static H-2 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 H-2 DQMAS NMR spectrum of fluorine-substituted H-2-enriched hydroxyl-clinohumite shows how the dynamic exchange process is inhibited by O-D center dot center dot center dot F- hydrogen-bonding interactions.
AB - Solid-state H-2 NMR spectroscopy has been used to probe the dynamic disorder of hydroxyl deuterons in a synthetic sample of deuterated hydroxyl-clinohumite (4Mg(2)SiO(4)center dot 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 H-2 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 H-2 MAS NMR spectra, considerable line broadening is observed for hydroxyl-clinohumite and a H-2 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 H-2 (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 H-2 linewidths in variable-temperature MAS experiments, an activation energy for the exchange process was also determined. A simulated static H-2 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 H-2 DQMAS NMR spectrum of fluorine-substituted H-2-enriched hydroxyl-clinohumite shows how the dynamic exchange process is inhibited by O-D center dot center dot center dot F- hydrogen-bonding interactions.
KW - POWDER NEUTRON-DIFFRACTION
KW - MAS NMR
KW - 1ST-PRINCIPLES CALCULATIONS
KW - HUMITE MINERALS
KW - CRYSTAL-STRUCTURES
KW - MAGNESIUM SILICATES
KW - NATURAL CHONDRODITE
KW - MOLECULAR-DYNAMICS
KW - MULTIPLE-QUANTUM
KW - MANTLE OLIVINE
U2 - 10.1039/b924666e
DO - 10.1039/b924666e
M3 - Journal article
VL - 12
SP - 2989
EP - 2998
JO - Physical Chemistry Chemical Physics
JF - Physical Chemistry Chemical Physics
SN - 1463-9076
IS - 12
ER -