Using a the Magic-Angle Flipping approach with 29Si NMR, we can measure of the relative abundances of anionic species in silicate glasses. Measurements of the relative abundances of anionic species in silicate glasses are essential for any structure-based model of thermodynamic or transport properties of silicate liquids and magmas. Probably the most fundamental aspect of this speciation is the distribution of silicate tetrahedra with varying numbers of bridging oxygens, commonly described as Q(n) species, where n ranges from 0 to 4.
Under static conditions or off-magic angle spinning, each Q(n) species exhibits an anisotropic NMR lineshape as shown below due to the anisotropy (CSA) of the NMR chemical shift interaction. This interaction arises from the magnetic shielding produced by the electron cloud surrounding the nucleus and therefore reflects the nature and directionality of the bonding. Under magic-angle spinning, where anisotropic broadening is removed, those five units are mainly identified only by their isotropic position.
In order to quantify the relative Q(n) species populations in silicate glasses using one-dimensional (1D) MAS a least-squares analysis of the MAS lineshape is performed with the common assumption that the MAS lineshape is a superposition of Gaussian lineshapes, one for each Q(n) species present. Unfortunately, strong overlap of Q(n) resonances in the MAS spectra leads to large covariances between best fit intensity parameters and this results in significant uncertainties in the relative populations of Q(n) species. The separation of anisotropic lineshapes obtained in a two dimensional isotropic/anisotropic NMR spectrum provides a means of determining the distributions of Q(n) species without any a priori assumptions about the distribution, and also provides over an order of magnitude improvement in the precision of Q(n) species quantification in silicate glasses compared to previous methods.
Below is a comparison of the one-dimensional MAS Gaussian deconvolution with the decomposition of the MAS spectrum using the anisotropic cross-sections of the two-dimensional NMR spectrum above.
Silicon Site Distributions in an Alkali Silicate Glass Derived by Two-dimensional 29Si Nuclear Magnetic Resonance,