31p Solid-State NMR Relaxation in the Zirconium Phosphate Network in the Presence of Paramagnetic Centers: A Detailed Relaxation Study in Static and Rotating Samples of Layered Zirconium Phosphate Materials

The careful study of MAS effects on the spin lattice and spin spin phosphorus NMR relaxation in two layered zirconium phosphate materials, alpha-ZrP and mPEG-PO3/Zr(IV)/DOX/ZrP, has clearly shown that the fast spin-diffusion and limited spin diffusion mechanisms are completely negligible in spite of the presence of paramagnetic centers detected in these materials by EPR. The P-31 T-1 relaxation is always nonexponential in static and spinning samples as well. The P-31 T-1 relaxation in alpha-ZrP follows a stretched exponential law to show the presence of a P-31 T-1 distribution. The P-31 T-1 time remarkably depends on spinning rates only in a 7 mm NMR probe head due to the frictional heating effects in 7 mm rotors but not due to the spin-diffusion mechanism. In accord, the (PT1)-P-31 and T-2 times are independent of MAS rates in the experiments performed with 2.5 mm rotors. All the P-31 T-1 and T-2 relaxation data support well the interpretation of the P-31 T-1 times in the zirconium phosphate network by dipolar proton phosphorus interactions and justify the application of the P-31 T-1 and P-31 T-1/T-2 approaches to probing molecular mobility in zirconium phosphate materials.