Subnanoscale Order and Spin Diffusion in Complex Solids through the Processing of Cross-Polarization Kinetics

The advanced strategy of processing of NMR cross-polarization (CP) data has been developed separating the short-range order- and the bulk effects in the manifold of spin interactions. The effectiveness of such treatment have been demonstrated on 1 H– 31 P CP kinetics in the nanostructured calcium hydroxyapatite (nano-CaHA) and that containing amorphous phosphate phase (amorph-CaHA). This allowed to describe the oscillatory manner of CP kinetics, oscillation blurring phenomenon, and to determine the spin distribution at the short- and moderate internuclear distances. The contribution of the dipolar coupling with remote spins has been removed modeling their spatial distribution using a set of radial profiles. The short 31 P– 1 H contact peak at 0.24–0.27 nm has been well resolved in the spatial distribution profile of the nano-CaHA sample and it is hardly noticeable in the case of amorph-CaHA. It reflects the differences in surface organization in nanostructured and amorphous materials. The spin diffusion processes in both systems are practically unaffected upon MAS. The MAS decimates 31 P interactions with the protons on the surface layers and with the remote protons (>0.5 nm) that are plausibly involved in the H-bond network.