Relativistic effects play an important role in many areas of chemistry, such as reactivity, catalysis, and spectroscopic properties of molecules and materials. Because of the nature of nuclear magnetic resonance (NMR) spectroscopy, especially the involvement of the magnetic moment of the spectator nucleus in the interaction, the NMR parameters for compounds containing heavy element(s) are notably affected by the relativistic effects.

Spin-orbit coupling (SOC) may be the dominant relativistic effect for the NMR chemical shifts of nuclei around a heavy atom (HA). The SOC interaction induces at first sight a non-intuitive relativistic NMR effect resulting in exotic NMR chemical shifts of spectator (often light) atoms (LA), frequently beyond the typical NMR frequency ranges. This phenomenon has been termed Spin-Orbit (SO) Heavy-Atom (HA) effect on the Light-Atom (LA) NMR shift, abbreviated SO-HALA effect. It has been noted already about 50 years ago when the 1H NMR spectra for a series of halogen-containing compounds were measured. However, links between the SO-HALA chemical shifts and electronic structure have been revealed only gradually. Recent significant progress in quantum chemical methodology and tools for electronic structure analysis have enabled clear chemical interpretations of the experimental NMR observations in hitherto unknown depth. As the SO-HALA effects can affect the NMR resonances of heavy-atom compounds in many fields of research crucially, their understanding should be part of the knowledge of any scientist who needs to analyze the NMR spectra of such species.

We will discuss how SO-HALA dictates NMR chemical shift trends across the various groups and rows of the Periodic Table. We will demonstrate that the relativistic SO-HALA NMR chemical shifts and trends can reveal intricate details of the electronic structure of the heavy-atom compounds, such as the oxidation state and coordination number of the heavy atom, nature of the HA-LA bond, and involvement of the HA in weak interactions.

The article will be concluded with a short summary of possible applications and perspectives.

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