Silicon site distributions in an alkali silicate glass derived by two-dimensional Si-29 nuclear magnetic resonance

A common approach to quantify Q((n)) species in silicate glasses is to use Si-29 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and assume that the overlapping isotropic chemical shift distributions of Q((n)) species are Gaussian, We have shown that a two-dimensional isotropic/anisotropic Si-29 NMR experiment can not only determine the distributions of Q((n)) species without any a priori assumptions about the distribution, but can also provide over an order of magnitude improvement in the precision of Q((n)) species quantification in silicate glasses. Using this approach we have investigated an alkali silicate glass of composition 2Na(2)O . 3SiO(2) and have observed a small concentration of Q((4)) in a sample mainly having Q((2)) and Q((3)). We have found that the distribution oi isotropic chemical shifts for each of the Q((n)) is approximately Gaussian. The relative populations of Q((2)), Q((3)), and Q((4)) Obtained from these separated distributions give an equilibrium constant of 0.0129 +/- 0.0001 for the disproportionation reaction 2 Q((3)) reversible arrow Q((2)) + Q((4)). This value is slightly higher than what is obtained from analyzing the one-dimensional MAS spectrum alone, thus revealing a higher degree of disorder in speciation and configurational entropy for the glass.