Magic Angle Flipping NMR

Two-dimensional magic angle flipping (MAF) was employed to measure the Q(n) distribution in a 29Si enriched potassium disilicate glass (K2O · 2 SiO2). Relative concentrations of [Q(4)] = 7.2% ± 0.3%, [Q(3)] = 82.9% ± 0.1%, [Q(2)] = 9.8%± 0.6% were obtained. Using the thermodynamic model for Q(n) species disproportionation these relative concentrations yield an equilibrium constant k3 = 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 ppm, −91.32 ± 0.01 ppm, and −101.67± 0.02, and standard deviations of 3.27±0.03 ppm, 4.19±0.01 ppm, and 5.09± 0.03 for Q(2), Q(3), and Q(4), respectively. Additionally, nuclear shielding anisotropy values of ζ = −85.0 ± 1.3 ppm, η = 0.48 ± 0.02 for Q(2), and ζ = −74.9± 0.2 ppm, η = 0.03± 0.01 for Q(3) were observed in the potassium disilicate glass. ∗To whom correspondence should be addressed †OSU ‡CNRS Introduction Non-crystalline materials have the advantage that their properties are isotropic and vary continuously with the composition so any value of a particular property, within limits, can be obtained by adjusting the composition.1,2 Unfortunately, for many non-crystalline materials, a lack of sound theoretical models relating composition and structure to properties makes their design and preparation difficult. In non-crystalline silicates the relative abundance of anionic species are essential part of any structure-based model and one of the most fundamental aspects of this speciation is the distribution of silicate tetrahedra with varying numbers (n) of bridging oxygen, commonly described as Q(n) species. In earlier work3,4 we showed that an NMR method such as MagicAngle Flipping (MAF),5,6 which produces a two dimensional (2D) spectrum correlating isotropic and anisotropic nuclear shielding contributions to the solid-state spectrum, can be used to give over an order of magnitude improvement in quantifying Q(n) species concentrations when compared to conventional 29Si Magic-Angle Spinning (MAS) lineshape analysis. Additionally, this method does not require the assumption of a Gaussian distribution of isotropic 29Si chemical shifts for the different Q(n) species. Its accuracy and precision in quantifying Q(n) were demonstrated in a wellunderstood sodium silicate glass binary composi1 ha l-0 04 68 37 8, v er si on 1 30 M ar 2 01 0 Author manuscript, published in "Journal of Physical Chemistry A 114 (2010) 5503–5508" DOI : 10.1021/jp100530m