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

Abstract

Several oxygen- 17-enriched silicates were studied using dynamic-angle spinning (DAS) NMR spectroscopy at two magnetic field strengths. The DAS method averages wnd-order quadrupolar interactions by reorienting a sample about a hedependent 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 (CQ = $qQ/h) and the quadrupolar asymmetry parameter (7). 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, CQ and q, are determined separately for each crystallographic site. The silicates studied include the discrete orthosilicates lamite (Ca2Si04) and forsterite (Mg2Si04), as well as diopside (CaMgSi206), wollastonite (CaSi03), and clinocnstatite ( MgSi03), which are minerals composed of chains of silicon-oxygen tetrahedra.