25Mg Solid-State NMR of Magnesium Phosphates: High Magnetic Field Experiments and Density Functional Theory Calculations

Natural-abundance Mg-25 solid-state NMR data obtained using very high magnetic fields of 17.6, 20.0, and 30.0 T are reported for a series of magnesium phosphate compounds, some of which are of potential biomedical interest. The Mg-25 NMR parameters have been calculated by using the DFT PAW and GIPAW methods, for both the experimental and DFT atomic position optimized structures For most of the studied compounds, the geometry optimization step improves significantly the accuracy of the calculations and good correlations between experimental and calculated Mg-25 chemical shifts and quadrupolar coupling constants were achieved showing that this approach can be used to obtain unambiguous assignments of the Mg-25 resonances in more complex phosphate compounds. The possibility of recording natural abundance Mg-25 NMR spectra in materials with very low Mg content is illustrated for a similar to 10% Mg-substituted hydroxyapatite sample. In this case, the distribution of Mg-25 quadrupolar coupling measured experimentally has been compared with values previously calculated for several structural models. The results suggest that more complex structural models must be developed to improve the understanding of the Ca/Mg substitution on the basis of Mg-25 NMR data.