27 Al NMR diffusometry of Al 13 Keggin nanoclusters.

Although nanometer-sized aluminum hydroxide clusters (i.e., ϵ-Al ₁₃ , [Al ₁₃ O ₄ (OH) ₂₄ (H ₂ O) 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, ²⁷ Al pulsed field gradient stimulated echo nuclear magnetic resonance (PFGSTE NMR) spectroscopy has been applied to the AlO ₄ core of the ϵ-Al ₁₃ cluster with complementary theoretical simulations of the diffusion coefficient and corresponding hydrodynamic radii from a boundary element-based calculation. The tetrahedral AlO ₄ center of the ϵ-Al ₁₃ cluster is symmetric and exhibits only weak quadrupolar coupling, which results in favorable T ₁ and T ₂ ²⁷ Al NMR relaxation coefficients for ²⁷ Al PFGSTE NMR studies. Stokes-Einstein relationship was used to relate the ²⁷ Al diffusion coefficient of the ϵ-Al ₁₃ cluster to the hydrodynamic radius for comparison with theoretical simulations, dynamic light scattering from literature, and previously published ¹ H PFGSTE NMR studies of chelated Keggin clusters. This first-of-its-kind observation proves that ²⁷ Al PFGSTE NMR diffusometry can probe symmetric Al environments in polynuclear clusters of greater molecular weight than previously considered.
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