Calcium phosphates: First-principles calculations vs. solid-state NMR experiments

Calcium phosphates (including hydroxyapatite) are inorganic components of numerous compounds such as bones and teeth. The in-depth characterization of their structures is of crucial importance for the understanding of the properties of biocompatible materials. Multinuclear solid-state NMR (including H-1, O-17 and P-31) appears as a valuable tool of investigation. In this paper, we show that full assignments of spectra were obtained through extensive use of first-principles calculations, based on the GIPAW (Gauge Included Projector Augmented Wave) approach [C.J. Pickard, F. Mauri, Phys. Rev. B 63 (2001) 245101]. H-1 and P-31 calculations (isotropic chemical shifts and CSA tensors) were validated by comparison with MAS experiments. In the case of H-1, full resolution was not obtained and subsequent assignment of resonances was obtained by taking into account the calculated isotropic chemical shifts. Interesting correlations involving delta(iso) (H-1) values and H-bond networks (characterized by intemuclear distances) were established, in good agreement with empirical data already published in the literature. H-1 CSA tensors were also analyzed. Furthermore, O-17 is a suitable spectroscopic target for the characterization of X-O-Y bonds, which may be present at bioinorganic interfaces. First-principles calculations showed that PO-, P-O-P, POH, CaO (and H2O) entities could be distinguished on the basis of O-17 chemical shifts and quadrupolar constants in MQ-MAS experiments.