(31)p 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
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2017 American Chemical Society. The careful study of MAS effects on the spin-lattice and spin-spin phosphorus NMR relaxation in two layered zirconium phosphate materials, -ZrP and m-PEG-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 31P T1 relaxation is always nonexponential in static and spinning samples as well. The 31P T1 relaxation in -ZrP follows a stretched exponential law to show the presence of a 31P T1 distribution. The 31P T1 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 31P T1 and T2 times are independent of MAS rates in the experiments performed with 2.5 mm rotors. All the 31P T1 and T2 relaxation data support well the interpretation of the 31P T1 times in the zirconium phosphate network by dipolar proton-phosphorus interactions and justify the application of the 31P T1 and 31P T1/T2 approaches to probing molecular mobility in zirconium phosphate materials. (Graph Presented).
