N.M.R. study of the temperature dependence of 14N quadrupole coupling in a single crystal of sodium nitrate

After recalling the studies conducted by Bernheim and Gutowsky, Andrew et al., and d'Alessio and Scott on the quadrupolar coupling (q.c.) of sodium in relation to the phase transition at Tλ = 275 °C, we demonstrate the interest of studying the q.c. of nitrogen as a function of temperature to clarify the behavior of nitrate ions as the temperature increases. This q.c. (745 kHz ± 2 % at 25 °C) was measured by R. M. N. on a single crystal, between -196 °C and +285 °C. It decreases increasingly rapidly as the temperature rises to Tλ and, beyond that, it continues to decline. The intra-ionic contribution to the q.c. of nitrogen from the nitrate ion is then discussed in terms of the populations of the σ and π orbitals on nitrogen and the charges carried by the oxygens. Two distinct evaluations provide the order of magnitude of the crystalline contribution (11 kHz or 38 kHz depending on the calculation) and show that it subtracts from the intra-ionic contribution. Finally, the decrease of the q.c. of nitrogen at temperatures below ambient is well interpreted by the Bayer-Kushida theory; however, this theory does not account for the significant decrease of the q.c. above ambient, which can be explained by recent theories regarding the existence of large amplitude helical movements of NO3- ions accompanying the phase transition.