Applications of NMR Shielding Constant Calculations in Mineralogy and Geochemistry

Abstract

NMR has become a very powerful technique for the characterization of the local order present in amorphous solids, glasses and aqueous solutions. The NMR shielding constant depends upon the identity, number and distance(s) of nearest-neighbour atoms, upon the nature of the connection(s) between the nearest neighbour units and upon the identities, distances and angular relationships of the atoms in the second nearest-neighbour coordination sphere. Ab initio SCF calculations on carefully chosen molecular clusters have reproduced many of the trends in shielding constants observed in molecules and solids. For species for which no appropriate crystalline models exist, ab initio techniques have been used to calculate equilibrium structures and NMR shieldings have then been calculated at those structures, to assist in species identification. Changes in shielding constants have been correlated with changes in other electronic properties, so as to give a unified picture of the electronic structure of the species. Our calculations on O, Al, Si, P, Zn and Cd NMR shieldings, using both conventional common-origin coupled Hartree-Fock theory and the localized-orbital local-origin modification known as LORG, have reproduced many trends in both isotropic and anisotropic NMR shielding values and have, in some cases, substantiated qualitative interpretations of shielding trends. As examples of geochemical applications of such calculations, we consider the variation of Si NMR shieldings and O electric field gradients with <Si-O-Si in siloxanes and silicates, the change in Si shielding with coordination number, the anisotropy of the 29Si NMR shielding tensor in olivine (Mg2SiO4), Al NMR shieldings and electric field gradients for various Al fluorides and oxyfluorides in F-bearing aluminosilicate glasses, and Zn and Cd NMR shieldings for chlorides and bisulfides in aqueous solution. Erroneously large predicted values of O shieldings from conventional CHF theory are corrected by the LORG approach or by a simple core electron correction, while highly polar compounds, such as the Al fluorides, are shown to present problems for LORG in its conventional implementation. Our results emphasize the need for simultaneous study of other properties which complement the NMR results, for improvements in methodology and computer capabilites which will allow the study of more realistic model systems or real bulk solids and for further study of the relationship between NMR shielding constants and other electronic properties.