Structural investigations of magnesium silicate glasses by 29Si 2D Magic-Angle Flipping NMR

Abstract

The two-dimensional Magic Angle Flipping Nuclear Magnetic Resonance (2D MAF NMR) experiment on 29Si nuclei is used to determine the distribution of Q(n) sites in two 29Si-enriched magnesium silicate glasses with compositions 2MgO·SiO2 and MgO·SiO2. A significant degree of polymerization is observed in the 2MgO·SiO2 glass, supporting previous studies using Raman and 29Si NMR spectroscopy. Relative abundances of 0.629±0.001 for Q(0) and 0.371±0.001 for Q(1) were obtained from spectral fits of the 2D MAF spectrum of the 2MgO·SiO2 glass. Mole fractions for the free oxygen anion and each Q(n)-species were calculated and used in a thermodynamic model of Q(n) disproportionation to calculate an equilibrium constant of k0=0.04±0.02 in 2MgO·SiO2. In the MgO·SiO2 glass relative abundance of 0.014±0.001 for Q(0), 0.191±0.003 for Q(1), 0.530±0.004 for Q(2), 0.252±0.003 for Q(3), and 0.014±0.001 for Q(4) were measured. The mole fractions for the free oxygen anion and each Q(n)-species in MgO·SiO2 were used to calculate corresponding disproportionation equilibrium constants of k1=0.19±0.02, k2=0.174±0.009, and k3=0.11±0.01. A comparison of k3 values from previous MAF studies of various alkali and alkaline earth silicate glasses indicate an exponential increase in k3 with the increasing modifying cation potential. Using the van't Hoff relation, we show that differences in both thermal history and modifier cation potential contribute to this spread in k3 values. Nuclear shielding tensor anisotropy, ζ, and asymmetry, η, values of ζ=0.0ppm and η=0.0 for Q(0) and ζ=33.0±0.1ppm, and η=0.4±0.1 for Q(1) in 2MgO·SiO2 glass were determined from its 2D MAF spectrum. These values were used in obtaining the remaining values of ζ=−36.0±0.5ppm and η=0.99±0.01 for Q(2), and ζ=−27.5±0.5ppm and η=0.45±0.11 for Q(3), ζ=0.0ppm and η=0.0 for Q(4) in the MgO·SiO2 glass from its 2D MAF spectrum. The magnitude of ζ values observed are lower than those reported in previous MAF studies of alkali and alkaline earth silicate glasses containing different modifier cations, consistent with previously reported trends in ζ versus modifying cation potential.