Ammonia: The molecule for establishing 14N and 15N absolute shielding scales and a source of information on nuclear magnetic moments

Multinuclear Nuclear Magnetic Resonance (NMR) studies of the gaseous mixtures He-3/(NH3)-N-14 and He-3/(NH3)-N-15 are reported. Precise analysis of the He-3, N-14, N-15, and H-1 resonance frequencies show a linear dependence on the gas density. Extrapolation of these results to the zero-pressure limit gives nu(0)(H-1), nu(0)(N-14), and nu(0)(N-15) resonance frequencies of the isolated ammonia molecule at 300 K. The analogous value for He-3 atoms in gaseous mixtures nu(0)(He-3) 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-14/15 and H-1 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-14 and N-15 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-14 and N-15 absolute shielding constants in (NH3)-N-14 and (NH3)-N-15 are very similar and only differ by less than 0.01 ppm, which is not usually measurable in NMR experiments. Precise calculations of N-14 and N-15 dipole moments were also made from these accurate shielding values.