High-field multinuclear solid-state nuclear magnetic resonance (NMR) and first principle calculations in MgSO4 polymorphs

A combination of solid-state nuclear magnetic resonance (NMR) and first principles calculations was applied to obtain 17O, 25Mg, and 33S NMR parameters for two polymorphs of anhydrous magnesium sulfate. Working at the very high magnetic field of 21.14 T results in a dramatic improvement of resolution through a reduction of the effects of quadrupolar interactions and significant improvement in sensitivity. Experimental 25Mg and 33S spectra are dominated by quadrupolar interactions with quadrupolar parameters unique for each polymorph. In the case of 17O, there is a substantial contribution of the chemical shift anisotropy. The use of multiple-quantum magic-angle spinning (MQMAS) experiments allows the resolution of distinct oxygen species and assignment of signals in the experimental 17O spectrum. Chemical shielding constants and quadrupolar parameters for all three nuclei were calculated using plane wave pseudopotential density functional theory as implemented in the CASTEP computational package. The calculated NMR parameters are in very good agreement with the experimental results and help in signal assignment of the 17O spectrum. The results suggest applicability of such a combined computational and experimental solid-state NMR approach for the refinement of crystallographic data.