Ammonia: The molecule for establishing N and N absolute shielding scales and a source of information on nuclear magnetic moments.

Multinuclear Nuclear Magnetic Resonance (NMR) studies of the gaseous mixtures He/NH and He/NH are reported. Precise analysis of the He, N, N, and H resonance frequencies show a linear dependence on the gas density. Extrapolation of these results to the zero-pressure limit gives ν(H), ν(N), and ν(N) resonance frequencies of the isolated ammonia molecule at 300 K. The analogous value for He atoms in gaseous mixtures ν(He) was measured as well. The application of a new scheme to introduce the most important electronic effects on NMR shieldings, together with highly accurate quantum chemical shielding calculations, allows the N and H shielding of the isolated ammonia molecule to be obtained with the greatest accuracy and precision. For the first time, these studies were carried out on ammonia within the so-called four-component relativistic framework. The NMR frequency comparison method provides an approach for determining the N and N nuclear magnetic moments. The new shielding parameters in ammonia were used for re-evaluation of the entire nitrogen absolute shielding scale. Additionally, the absolute shielding values of several gaseous compounds and secondary reference substances in liquids were presented. It was established that N and N absolute shielding constants in NH and NH are very similar and only differ by less than 0.01 ppm, which is not usually measurable in NMR experiments. Precise calculations of N and N dipole moments were also made from these accurate shielding values.