Isotropic chemical shifts and quadrupolar parameters for oxygen-17 using dynamic-angle spinning NMR

Several oxygen-17-enriched silicates were studied using dynamic-angle spinning (DAS) NMR spectroscopy at two magnetic field strengths. The DAS method averages second-order quadrupolar interactions by reorienting a sample about a time-dependent axis, thereby yielding high-resolution spectra for half-odd integer spin quadrupolar nuclei such as oxygen-17. A narrow spectral line is observed for each distinct oxygen site in a powdered sample at the sum of the isotropic chemical shift and the field-dependent isotropic second-order quadrupolar shift. Using equations for the total shift observed at two field strengths, the chemical shift is uniquely determined together with a product of the quadrupolar coupling constant (C_Q = e²qQ/h) and the quadrupolar asymmetry parameter (η). For one silicate, we demonstrate a computer program that uses the isotropic shifts and quadrupolar products as constraints and provides simulations of overlapped magic-angle spinning line shapes. In this way the quadrupolar parameters, C_Q and η, are determined separately for each crystallographic site. The silicates studied include the discrete orthosilicates larnite (Ca₂SiO₄) and forsterite (Mg₂SiO₄), as well as diopside (CaMgSi₂O₆), wollastonite (CaSiO₃), and clinoenstatite (MgSiO₃), which are minerals composed of chains of silicon-oxygen tetrahedra.