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

M. C. Davis, D. C. Kaseman, S. M. Parvani, K. J. Sanders, P. J. Grandinetti, D. Massiot, P. Florian
062 - J. Phys. Chem. A: 114 , 5503-5508 (2010).

# Abstract

Two-dimensional magic angle flipping (MAF) was employed to measure the $Q^{(n)}$ distribution in a $^{29}$Si enriched potassium disilicate glass (\mbox{K$_2$O $\cdot$ 2 SiO$_2$}). Relative concentrations of $[Q^{(4)}] = 7.2\% \pm 0.3\%$, $[Q^{(3)}] = 82.9\% \pm 0.1\%$, $[Q^{(2)}] = 9.8\% \pm 0.6\%$ were obtained. Using the thermodynamic model for $Q^{(n)}$ species disproportionation these relative concentrations yield an equilibrium constant $k_3 =0.0103 \pm 0.0008$, 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 $-82.74 \pm 0.03$ ppm, $-91.32 \pm 0.01$ ppm, and $-101.67 \pm 0.02$, and standard deviations of $3.27 \pm 0.03$ ppm, $4.19 \pm 0.01$ ppm, and $5.09 \pm 0.03$ for $Q^{(2)}$, $Q^{(3)}$, and $Q^{(4)}$, respectively. Additionally, nuclear shielding anisotropy values of $\zeta=-85.0 \pm 1.3$ ppm, $\eta= 0.48 \pm 0.02$ for $Q^{(2)}$, and $\zeta= -74.9 \pm 0.2$ ppm, $\eta= 0.03 \pm 0.01$ for $Q^{(3)}$ were observed in the potassium disilicate glass.