Role of Coordination Number, Geometry, and Local Disorder on 27Al NMR Chemical Shifts and Quadrupolar Coupling Constants: Case Study with Aluminosilicates
2017
Lam, Erwin | Comas-Vives, Aleix | Copéret, Christophe
²⁷Al solid-state NMR is a powerful tool for elucidating local geometries at Al sites in molecular and solid-state systems because they are typically associated with specific NMR signatures, namely, isotropic chemical shift (δᵢₛₒ) and quadrupolar coupling constant (CQ). Assignment is however mostly empirical; hence, obtaining a detailed understanding of the origins of the NMR parameters would be a valuable step toward a structure–property/reactivity relationship. Here, we investigate the origin of the ²⁷Al NMR signatures in aluminosilicates using DFT calculations on cluster models complemented by natural chemical shift (NCS) analysis. In particular, NCS analysis shows that the chemical shift of Al is mostly associated with the coupling Al–O σ and σ* orbitals for σ₁₁ leading to deshielding as the coordination number of Al decreases, allowing the distinction between tri-, tetra-, penta-, and hexacoordinated sites. In contrast, CQ can take a broad range of values (between 8.0 and 23.6 MHz) independently of the coordination number because it is greatly affected by slight variation of the bond distance of siloxane bonds coordinated to aluminum, which perturbs the electrostatic interaction with aluminum and thereby the CQ.
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