Quantum mechanical calculation of 23Na NMR shieldings in silicates and aluminosilicates |
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| Authors: | J A Tossell |
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| Institution: | (1) Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA, US |
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| Abstract: | To assist in the assignment and interpretation of 23Na NMR spectra in silicate and aluminosilicate minerals and glasses we have calculated the 23Na NMR shieldings and the electric field gradients (EFG) at the Na for a number of Na-containing species. Included are Na(OH2)
n
+, n = 1, 2, 4, 5, 6 and 8, and Na+ complexes with SiH3OH, SiH3ONa and O(SiH3)2. We have also evaluated shieldings and EFGs for Na-centered clusters extracted from crystalline Na2SiO3 and anhydrous sodalite, Na6AlSiO4]6. Using 6-31G* SCF optimized geometries and the GIAO method with a 6-31G* basis set and 6-311(2d,p) bases for the smaller
clusters] we find a calculated increase in shielding with coordination number (CN) for the Na(OH2)
n
+, n = 4, 6, 8 series that agrees reasonably well with experimental trends. Calculated changes in the Na shielding as water is
replaced by bridging or nonbridging silicate O atoms are also consistent with experimental observations. The deshielding of
Na (with respect to gas-phase Na+) which is produced by an O-containing ligand is a strongly decreasing function of the R(Na–O) and a weakly decreasing function
of the underbonding or free valence of the O. Deshielding contributions to the isotropic shielding from different ligands
are additive to good approximation for low CN species, so that the total deshielding can be calculated accurately by summing
the contributions from the individual ligands. However for the larger CN species the directly calculated deshieldings are
substantially smaller than those obtained using such an additivity approximation. We further test this approximation by calculating
the deshieldings for Na in 12 different sites in silicate and aluminosilicate minerals which have recently been studied experimentally,
using our calculated deshielding contributions for individual O-containing ligands and experimental values for the Na–O distances.
Correlation coefficients between the experimental shifts and the calculated deshieldings are around 0.9 and the slope of the
correlation is almost 1.0 . Calculations on large Na-centered clusters extracted from the crystal structures of Na2SiO3 and anhydrous sodalite reproduce the experimental values for both NMR shieldings and electric field gradients but at considerable
computational cost. Comparison with recent 23Na NMR studies on hydrous albite glasses indicates that coordination of either H2O or OH− to the Na could give the magnitude of deshielding observed, depending upon the detailed Na–O distances within the hydrous
glass.
Received: 31 December 1998 / Revised, accepted: 11 May 1999 |
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