¹⁵N NMR study of nitrate ion structure and dynamics in hydrotalcite-like compounds

We report here the first nuclear magnetic resonance (NMR) spectroscopic study of the dynamical and structural behavior of nitrate on the surface and in the interlayer of hydrotalcite-like compounds (¹⁵NO₃^--HT). Spectroscopically resolvable surface-absorbed and interlayer NO₃^- have dramatically different dynamical characteristics. The interlayer nitrate shows a well defined, temperature independent uniaxial chemical shift anisotropy (CS A) powder pattern. It is rigidly held or perhaps undergoes rotation about its threefold axis at all temperatures between -100 °C and +80 °C and relative humidities (R.H.) from 0 to 100% at room temperature. For surface nitrate, however, the dynamical behavior depends substantially on temperature and relative humidity. Analysis of the temperature and R.H. dependences of the peak width yields reorieritational frequencies which increase from essentially 0 at -100 °C to 2.6 × 10⁵ Hz at 60 °C and an activation energy of 12.6 kJ/mol. For example, for samples at R.H. = 33%, the surface nitrate is isotropically mobile at frequencies greater than 10⁵ Hz at room temperature, but it becomes rigid or only rotates on its threefold axis at -100 °C. For dry samples and samples heated at 200 °C (R.H. near 0%), the surface nitrate is not isotropically averaged at room temperature. In contrast to our previous results for ³⁵Cl^--containing hydrotalcite (³⁵Cl^--HT), no NMR detectable structural phase transition is observed for ¹⁵NO₃^--HT. The mobility of interlayer nitrate in HT is intermediate between that of carbonate and chloride.