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

Abstract

Two-dimensional magic angle flipping (MAF) was employed to measure the Q((n)) distribution in a (29)Si-enriched potassium disilicate glass (K(2)O.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 zeta = -74.9 +/- 0.2 ppm, eta = 0.03 +/- 0.01 for Q((3)) were observed in the potassium disilicate glass.

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

“…1 correlates a t 1 dimension containing both isotropic and anisotropic frequencies to a t 2 dimension containing only isotropic frequencies, as presented in previous MAF investigations of silicate glasses [12][13][14]. The 2D MAF spectra presented here, however, have been transformed to correlate a purely isotropic spectrum to a purely anisotropic spectrum through the application of an affine transformation [36], consisting of a shear parallel to t 2 using a shear factor of 1.…”

“…This approach applied to one-dimensional static sample NMR spectra, however, is limited by low sensitivity and strong overlap of anisotropic lineshapes for different Q (n) -species. In previous works [12][13][14], we demonstrated that Magic Angle Flipping (MAF) [15], which is a 2D NMR experiment capable of correlating the isotropic and anisotropic frequency contributions of the nuclear shielding along two orthogonal dimensions, provides an order of magnitude improvement in quantifying Q (n) -species compared to fitting 1D static or MAS NMR lineshapes alone. This 2D NMR approach has been successfully applied in quantifying the relative populations of Q (n) in silicate glasses of composition 2Na 2 O·3SiO 2 [12], K 2 O·2SiO 2 [13], and CaO·SiO 2 [14].…”

Role of alkaline cations and water content on geomaterial foams: Monitoring during formation

“…Thus, we can hypothesize that the following reaction (Eq. (2)) also occurs in the materials and provides other silica oligomers [34]:…”

TOP-PASS: A processing algorithm to reduce 2D PASS acquisition time

“…4 is an example of the application of the TOP processing applied to a 2D signal, S top ð; tÞ, constructed from the 29 Si NMR Hahn echo signal of polycrystalline Clinoenstatite. Clinoenstatite has two inequivalent tetrahedrally coordinated (i.e., Q (2) ) silicon sites, resolved at À85.2 ppm and À82.8 ppm in the isotropic dimension, and exhibit similar nuclear shielding anisotropies, as expected [24,25], in the sideband dimension.…”