Ab initio crystal structure prediction of magnesium (poly)sulfides and calculation of their NMR parameters

Ab initio prediction of sensible crystal structures can be regarded as a crucial task in the quickly-developing methodology of NMR crystallography. In this contribution, an evolutionary algorithm was used for the prediction of magnesium (poly)sulfide crystal structures with various compositions. The employed approach successfully identified all three experimentally detected forms of MgS, i.e. the stable rocksalt form and the metastable wurtzite and zincblende forms. Among magnesium polysulfides with a higher content of sulfur, the most probable structure with the lowest formation energy was found to be MgS₂, exhibiting a modified rocksalt structure, in which S²⁻ anions were replaced by S₂²⁻ dianions. Magnesium polysulfides with even larger fractions of sulfur were not predicted to be stable. For the lowest-energy structures, ²⁵Mg quadrupolar coupling constants and chemical shift parameters were calculated using the density functional theory approach. The calculated NMR parameters could be well rationalized by the symmetries of the local magnesium environments, by the coordination of magnesium cations and by the nature of the surrounding anions. In the future, these parameters could serve as a reference for the experimentally determined ²⁵Mg NMR parameters of magnesium sulfide species.