ANIONIC SPECIES DETERMINATION IN CASIO3 GLASS USING 2-DIMENSIONAL SI-29 NMR

The structure of the alkaline earth silicate glass CaSiO3 has been inv estigated using a two-dimensional Si-29 NMR experiment that correlates the isotropic magic-angle-spinning (MAS) spectrum with an anisotropic off-magic-angle-spinning spectrum. Although the one-dimensional magic -angle-spinning (MAS) spectrum is completely unresolved, all five type s of SiO4 tetrahedra (represented by the notation Q((n)), where n = 0- 4 representing the number of bridging oxygen) can be resolved and quan tified on the basis of the separated anisotropic line shapes in the 2D spectrum. The distribution of isotropic chemical shifts derived from the 2D spectrum suggests that in the case of CaSiO3 glass the conventi onal approach of fitting the one-dimensional MAS spectrum with overlap ping Gaussian line shapes would lead to significant errors in Q((n)), quantification. The equilibrium constants for the disproportionation r eaction Q((n)) reversible arrow Q((n-1)) + Q((n+1)) with n = 1, 2, and 3 were determined from the 2D spectrum to be 0.105 +/- 0.019, 0.156 /- 0.005, and 0.106 +/- 0.022, respectively. These results clearly ind icate a significantly greater deviation from a binary model of Q((n)), species disproportionation in alkaline earth silicate melts when comp ared to alkali silicate melts and thus suggest a relatively more disor dered structure.