Mg-25 ultra-high field solid state NMRspectroscopy and first principles calculations of magnesium compounds

Abstract

Due to sensitivity problems, ²⁵Mg remains a largely under-explored nucleus in solid state NMR spectroscopy. In this work at an ultrahigh magnetic field of 21.1 T, we have studied at natural abundance the ²⁵Mg solid state (SS) NMR spectra for a number of previously unreported magnesium compounds with known crystal structures. Some previously reported compounds have been revisited to clarify the spectra that were obtained at lower fields and were either not sufficiently resolved, or misinterpreted. First principles calculations of the ²⁵Mg SS NMR parameters have been carried out using plane wave basis sets and periodic boundary conditions (CASTEP) and the results are compared with experimental data. The calculations produce the ²⁵Mg absolute shielding scale and give us insight into the relationship between the NMR and structural parameters. At 21.1 T the effects of the quadrupolar interactions are reduced significantly and the sensitivity and accuracy in determining chemicals shifts and quadrupole coupling parameters improve dramatically. Although T₁ measurements were not performed explicitly, these proved to be longer than assumed in much of the previously reported work. We demonstrate that the chemical shift range of magnesium in diamagnetic compounds may approach 200 ppm. Most commonly, however, the observed shifts are between −15 and +25 ppm. Quadrupolar effects dominate the ²⁵Mg spectra of magnesium cations in non-cubic environments. The chemical shift anisotropy appears to be rather small and only in a few cases could the contribution of the CSA be detected reliably. A good correspondence between the calculated shielding constants and experimental chemical shifts was obtained, demonstrating the good potential of computational methods in spectroscopic assignments of solid state ²⁵Mg NMR spectroscopy.