Journal
MAGNETIC RESONANCE IN CHEMISTRY
Volume 60, Issue 2, Pages 226-238Publisher
WILEY
DOI: 10.1002/mrc.5218
Keywords
Al-27; Keggin clusters; PFGSTE NMR
Funding
- IDREAM (Interfacial Dynamics in Radioactive Environments and Materials)
- DOE BES, Chemical Sciences, Geosciences, and Biosciences (CSGB)
- Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL) [DE AC05-76RL0-1830]
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This study utilized Al-27 NMR spectroscopy to investigate the translational diffusion of epsilon-Al-13 clusters, demonstrating that symmetric Al environments can be probed using this method.
Although nanometer-sized aluminum hydroxide clusters (i.e., epsilon-Al-13, [Al13O4(OH)(24)(H2O)(12)](7+)) command a central role in aluminum ion speciation and transformations between minerals, measurement of their translational diffusion is often limited to indirect methods. Here, Al-27 pulsed field gradient stimulated echo nuclear magnetic resonance (PFGSTE NMR) spectroscopy has been applied to the AlO4 core of the epsilon-Al-13 cluster with complementary theoretical simulations of the diffusion coefficient and corresponding hydrodynamic radii from a boundary element-based calculation. The tetrahedral AlO4 center of the epsilon-Al-13 cluster is symmetric and exhibits only weak quadrupolar coupling, which results in favorable T-1 and T-2 Al-27 NMR relaxation coefficients for Al-27 PFGSTE NMR studies. Stokes-Einstein relationship was used to relate the Al-27 diffusion coefficient of the epsilon-Al-13 cluster to the hydrodynamic radius for comparison with theoretical simulations, dynamic light scattering from literature, and previously published H-1 PFGSTE NMR studies of chelated Keggin clusters. This first-of-its-kind observation proves that Al-27 PFGSTE NMR diffusometry can probe symmetric Al environments in polynuclear clusters of greater molecular weight than previously considered.
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