Investigation of Carbonate Substitution in Hydroxyapatite by Combining Solid-state NMR and DFT Calculations

Biological apatites (main constituent of natural bones) correspond to non-stoichiometric hydroxyapatite HAp, presenting a large variety of ions as substituents (CO₃²⁻, F⁻, SiO₄⁴⁻, Mg²⁺, Na⁺…). The precise location and configuration of ionic substitutes in the HAp matrix are generally difficult to identify and characterize. This contribution details the structural characterization based on NMR data of a particular case of hydroxyapatite substitution by carbonates. For this purpose, all substitution mechanisms proposed to our knowledge in the literature are modeled by DFT and the corresponding calculated NMR parameters allowed to propose or confirm some interpretations of a certain number of experimental observations to rationalize the dependencies of the ¹³C chemical shift and energy on these structural parameters. The presented results open the way for a fast interpretation of ¹³C NMR experiments on defective HAp materials and will allow to predict the most stable arrangement of CO₃²⁻ for a given family of defects.