31P NMR Relaxation and Motions of Phosphate Groups in Layered Zirconium Phosphate Materials

There is little systematic understanding of the motions of phosphate groups in layered zirconium phosphate (α-ZrP) materials applied in chemistry, industry, and nanomedicine for drug delivery. In this paper, we show how to quantitatively characterize correlation times of motions in the zirconium phosphate network on the basis of NMR 31P relaxation time measurements. By studies of α-ZrP phosphates and their intercalated and decorated derivatives, we have established that room-temperature 31P relaxation is governed by rotations of HPO4– groups with correlation times of (1.1–4.7) × 10–8 s restricted by H-bonding with water molecules in ordered and locally disordered areas. The disordered areas have been found by the 1H spin-lock proton–phosphorus cross-polarization 31P{1H} MAS NMR experiments.