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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry