Q(n) Species Distribution in K2O•2SiO2 Glass by 29Si Magic Angle Flipping NMR

Two-dimensional magic angle flipping (MAF) was employed to measure the Q((n)) distribution in a Si-29-enriched potassium disilicate glass (K2O center dot 2SiO(2)). Relative concentrations of [Q((4))] = 7.2 +/- 0.3%, [Q((3))] = 82.9 +/- 0.1%, and [Q((2))] = 9.8 +/- 0.6% were obtained. Using the thermodynamic model for Q((n)) species disproportionation, these relative concentrations yield an equilibrium constant k(3) = 0.0103 +/- 0.0008, indicating, as expected, that the Q((n)) species distribution is close to binary in the potassium disilicate glass. A Gaussian distribution of isotropic chemical shifts was observed for each Q((n)) species with mean values of -82.74 +/- 0.03, -91.32 +/- 0.01, and -101.67 +/- 0.02 ppm and standard deviations of 3.27 +/- 0.03, 4.19 +/- 0.01, and 5.09 +/- 0.03 ppm for Q((2)), Q((3)), and Q((4)), respectively. Additionally, nuclear shielding anisotropy values of zeta = -85.0 +/- 1.3 ppm, eta = 0.48 +/- 0.02 for Q((2)) and = -74.9 +/- 0.2 ppm eta = 0.03 +/- 0.01 for Q((3)) were observed in the potassium disilicate glass.