UsingNMR shielding calculations to help determine local and midrange order in amorphous materials

Abstract

Solid-state NMR spectroscopy has become a critically important method for the determination of local order in amorphous materials, both natural (such as silicate glasses) and man-made (such as alkali Si sulfide fast ion conductors). Although assignment of NMR features can sometimes be made by comparison with crystalline analogs, there are often unique structural units in glasses which are not found in crystals. To characterize such units, direct ab initio calculations of NMR shieldings using the RPALORC, method have proven very valuable. In addition to considerations of method choice and basis-set selection which are important in calculating NMR shieldings for gas-phase molecules, calculation of NMR shieldings for species in glasses involves a choice of the model used to characterize the local or midrange environment. There is often fragmentary information on the structure and other spectral properties of such species which can be used in combination with the NMR spectra to obtain a firm characterization. Some example materials discussed will be (1) B2O3 glass, for which boroxol ring and nonring sites NMR properties have recently been determined by dynamic angle spinning techniques, (2) F-containing aluminosilicate glasses, where a 5-coordinate Al complex with both oxygen and fluorine ligands has been established, (3) CO2 saturated aluminosilicate glasses, where 13C-NMR reveals a number of complexes of CO2 with aluminosilicate fragments of both “CO2” and “CO3” type, (4) silicon sulfide glasses, which show both corner and edge-sharing oligomeric units, and (5) Si3N4, where the shielding of a central N is found to be strongly affected by the orientation of the neighboring N lone pairs. © 1995 John Wiley & Sons, Inc.