Solid-State NMR Study of Hydroxyapatite Containing Amorphous Phosphate Phase and Nanostructured Hydroxyapatite: Cut-Off Averaging of CP-MAS Kinetics and Size Profiles of Spin Clusters

High data point density measurements of 1H–31P cross-polarization kinetics have been carried out for calcium hydroxyapatite containing amorphous phosphate phase (ACP-CaHA) and nanostructured hydroxyapatite (CaHA). The chosen setting of the sampling frequency of 5 × 104 s–1 allowed us to reveal all spin interactions having the dipolar splitting b ≤ 25 kHz, which in the case of 1H–31P interaction means the structures with the internuclear distances r ≥ 0.125 nm could be resolved. The advanced processing of CP-MAS kinetic data has been developed, introducing the variable cutoff distribution of the dipolar coupling. This procedure allows us to describe the oscillatory kinetics and CP curves in nanostructured materials over a wide range of contact time and to determine the characteristic size profile and composition of the spin clusters. The characteristic size of 31P–(1H) n spin nanocluster being within 0.3 to 0.5 nm has been determined for nanostructured CaHA. The nonclassical spin coupling model is more preferable to describe the CP kinetics in ACP-CaHA.