Solid-state oxygen-17 nuclear magnetic resonance spectroscopic studies of alkaline earth metasilicates

We have obtained high-field (11.7 T, 67.8 MHz) ¹⁷O nuclear magnetic resonance (NMR) spectra of a series of alkaline earth metasilicates (clinoenstatite, MgSiO₃; diopside, CaMgSi₂O₆; pseudowollastonite, α-CaSiO₃; strontium metasilicate, α-SrSiO₃; and barium metasilicate, BaSiO₃). Values of the nuclear quadrupole coupling constants (e²qQ/h), electric field gradient tensor asymmetry parameters (η), and isotropic chemical shifts (δⁱ) have been deduced for all the types of oxygen sites encountered. The e²qQ/h values for the nonbridging oxygens (Si-O-M, M = divalent cation) are in the range 1.6-3.2 MHz and increase with increasing cation electronegativity, the asymmetry parameters vary from 0 to 0.1, and the isotropic chemical shifts become more deshielded with increasing cation radius. The total range of isotropic chemical shifts for the nonbridging oxygens is 127 ppm. For the bridging oxygens (Si-O-Si) in the metasilicates, the e²qQ/h values are in the range from 3.7 to 5.1 MHz and the asymmetry parameters vary from 0.2 to 0.4. The isotropic chemical shifts again become more deshielded with increasing cation radius, the total range of isotropic chemical shifts being 25 ppm. The chemical shifts for both bridging and nonbridging oxygens in metasilicates indicate that deshielding may be correlated with the presence of large cations, as found previously for ¹⁷O NMR spectra of the group IIA (2) and group IIB (12) oxides.