Abstract

Ab initio NMR calculations at the Hartree-Fock GIAO level with 6-311+G(2df,p) basis sets have been performed on many Na-centered clusters with different O-ligands. Several structural factors that can affect the 23Na NMR parameters have been carefully investigated. The accuracy of our calculations is checked by comparing calculated 23Na NMR shieldings for crystalline materials with experimental results. The largest Na-O-ligand distance that contributes to the deshielding of the Na atom (i.e., the reduction of the Na shielding from its free ion value) is found to be about 3.5 Å. Quantitative contributions to the Na deshielding from different types of O-ligands are emphasized. These contributions are carefully compared to establish the order of decreasing deshielding as:

 
\[OH,\ NBO(Al)\ {>}\ Al-O-Si\ {>}\ NBO(Si)\ {>}\ Al-OH\ {>}\ H_{2}O\ {>}\ Si-O-Si\ {>}\ Si-OH\]

Based on these results we find there is no single formula that is useful for relating sodium NMR shifts to structural parameters. Na-clustering effects from other nearby cations are also found to tremendously increase the shielding of Na. Finally, the relationships established between local structure and O-ligand identity and the 23Na NMR shieldings have been used to explain the Na NMR data for hydrous albite glasses.

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