27Al NMR diffusometry of Al13 Keggin nanoclusters

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.

Automated summary

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.