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 ((NO3-)-N-15-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 (CSA) powder pattern. It is rigidly held or perhaps undergoes rotation about its threefold axis at all temperatures between -100 degrees C and +80 degrees 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 reorientational frequencies which increase from essentially 0 at -100 degrees C to 2.6 x 10(5) Hz at 60 degrees 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 10(5) Hz at room temperature, but it becomes rigid or only rotates on its threefold axis at -100 degrees C. For dry samples and samples heated at 200 degrees C (R.H. near 0%), th,,, surface nitrate is not isotropically averaged at room temperature. In contrast to our previous results for Cl-35(-)-containing hydrotalcite (Cl-35(-)-HT), no NMR detectable structural phase transition is observed for (NO3-HT)-N-15. The mobility of interlayer nitrate in HT is intermediate between that of carbonate and chloride.