NUCLEAR MAGNETIC SHIELDING OF NITROGEN IN AMMONIA

The nitrogen shielding surface in ammonia is calculated using the localized orbital-local origin (LORG) method of Hansen and Bouman, in terms of the symmetry coordinates for the molecule. With respect to the inversion coordinate, the N shielding surface has a shape similar to the potential surface. Rovibrational averaging of the N shielding in NH3 and ND3 molecules is carried out using numerical wave functions which are solutions to the inversion potential which best fits the spectra of all isotopomers. The other coordinates are vibrationally averaged in the usual way, assuming small amplitude motions. The calculated temperature dependence of the N shielding due to inversion is in the opposite sense to that observed for a large number of molecules, and is nearly canceling the contributions from all the other coordinates. The temperature dependence of the nitrogen shielding in ammonia has been measured in the range 300-400 K in samples with densities in a hundredfold range (0.37-33 amagat). When the temperature-dependent intermolecular effects are separated out, the remaining temperature dependence is small and is consistent with the calculations. The inversion contribution to the deuterium-induced isotope shift is of opposite sign to the contributions from all other coordinates. The agreement with the experimental isotope shift in the liquid phase is satisfactory.