A natural abundance solid-state 25Mg NMR study of layered magnesium phosphates

Abstract

Natural abundance 25Mg solid-state magic-angle spinning and static NMR spectra of several representative layered magnesium phosphates were acquired at 21.1 and 9.4 T by using quadrupolar echo and double-frequency sweep quadrupolar Carr–Purcell–Meiboom–Gill pulse sequences. The spectra were dominated by the second-order quadrupolar interaction. The electric field gradient tensor parameters were extracted from the spectra via spectral simulations. These parameters, such as the quadrupolar coupling constant (CQ), appear to be sensitive to some parameters describing the distortion of the MgO6 octahedron in the layer. The empirical relationships between CQ and several structural parameters were established and used to obtain partial information on the Mg environment in a layered material (MgHPO4·1.2H2O) with unknown structure. Theoretical calculations suggest that the CQ values of the Mg sites are affected not only by the oxygen atoms in the first coordination sphere, but also by the spatial arrangements of the atoms in the second and third coordination spheres and beyond.