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The 1H spin-lattice and spin-spin relaxation time as well as high data point density 1H–31P cross-polarization kinetics (static
and upon MAS) measurements have been carried out for calcium hydroxyapatite containing amorphous phosphate phase
(ACP-CaHA) and nano-structured hydroxyapatite (CaHA). The chosen setting of the sampling frequency of 5×104 s–1 allowed
to reveal all spin interactions having the dipolar splitting b≤25 kHz that in the case of 1H–31P interaction means the structures
with the inter-nuclear distances r≥0.125 nm could be resolved. The advanced processing of CP MAS kinetic data has been
developed introducing the variable cut-off distribution of the dipolar coupling. This procedure allows to describe the oscillatory
kinetics and CP curves in nano-structured 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 nano-cluster being within 0.3÷0.5 nm
has been determined for nano-structured CaHA. The 1H spin-lattice and spin-spin relaxation time measurements revealed
the fast spin motion takes place in ACP-CaHA. The effect of MAS rate on the 31P signal shape confirms that the correlation
time of this motion gets into the time scale of microseconds or even nanoseconds. Such fast dynamics can be attributed to the
rotational diffusion of adsorbed water molecules. The magnitude of the inhomogeneous anisotropic broadening of 1220±20
Hz determined for nano-structured sample is very close to 1185 Hz that corresponds to the maximum of Gauss distribution
of dipolar 1H–31P coupling obtained using 31P–1H CP MAS kinetics. The dynamics of this interactions runs in the time scale of
microseconds and it is much slower that in ACP-CaHA.
Vytautas Balevicius is a Physicist, Doctor of physical sciences and Professor. Currently he runs the Vilnius University Faculty of Physics dean's office.