Étude en fonction de la température par R. M. M. du couplage quadripolaire de l'azote 14 dans un monocristal de nitrate de sodium

Abstract

Prior studies by Bernheim and Gutowsky, Andrew et al. and d'Alessio and Scott on the 14N quadrupole coupling constant (q. c. c.) of sodium nitrate showed a relationship with the phase transition at T03BB = 275 °C. We show the interest of studying the temperature dependence of the nitrogen q. c. c. This q. c. c. (745 kHz ± 2 % at 25 °C) could be measured between - 196 °C and + 285 °C by NMR on a single crystal. We observe that q. c. c. decreases more and more rapidly as the temperature increases up to T03BB, after which the q. c. c. continues to decrease. The intraionic contribution to the nitrogen q. c. c. in the nitrate ion is discussed in terms of the 03C3 and 03C0 orbital populations in the nitrogen, and of the electric charges in the oxygen atoms. Estimating the crys- talline contribution to the q. c. c. by two methods, we find that it is subtracting from the intraionic contribution, respectively, by an amount of 11 kHz or 38 kHz. Finally, the nitrogen q. c. c. decrease is well explained below room temperature by the Bayer- Kushida theory although its strong decrease at higher temperatures is not. This is in agreement with the recent theories about phase transition according to which NO3- ions undergo helicoïdal motions of high amplitude when the transition