Multinuclear NMR Investigations of the Oxygen, Water, and Hydroxyl Environments in Sodium Hexaniobate

Solid-state ¹H, ¹⁷O MAS NMR, ¹H- ⁹³Nb TRAPDOR NMR, and ¹H double quantum 2D MAS NMR experiments were used to characterize the oxygen, water, and hydroxyl environments in the monoprotonated hexaniobate material, Na₇[HNb ₆O₁₉]-15H₂O. These solid-state NMR experiments demonstrate that the proton is located on the bridging oxygen of the [Nb ₆O₁₉]^8- cluster. The solid-state NMR results also show that the NbOH protons are spatially isolated from similar protons, but undergo proton exchange with the water species located in the crystal lattice. On the basis of double quantum ¹H MAS NMR measurements, it was determined that the water species in the crystal lattice have restricted motional dynamics. Two-dimensional ¹H-¹⁷O MAS NMR correlation experiments show that these restricted waters are preferentially associated with the bridging oxygen. Solution ¹⁷O NMR experiments show that the hydroxyl proton is also attached to the bridging oxygen for the compound in solution. In addition, solution ¹⁷O NMR kinetic studies for the hexaniobate allowed the measurement of relative oxygen exchange rates between the bridging, terminal, and hydroxyl oxygen and the oxygen of the solvent as a function of pH and temperature. These NMR experiments are some of the first investigations into the proton location, oxygen and proton exchange processes, and water dynamics for a base stable polyoxoniobate material, and they provide insight into the chemistry and reactivity of these materials.