Chapter 9 Relativistic effects on NMR chemical shifts

Abstract

Abstract Our current understanding of relativistic effects on NMR chemical shifts is analyzed. After briefly reviewing four-, two-, and one-component quantum chemical formalisms to relativistically compute chemical shifts, a main focus of the article is placed on understanding spin-orbit effects that dominate the important „heavy-atom effects on shieldings of light atoms” (HALA). Many computational examples are provided, and an analogy between „spinorbit chemical shifts” and the FC mechanism of indirect spin-spin coupling is emphasized. This leads to a natural understanding of many periodic trends in chemical shifts, such as „normal” or „inverse” halogen dependence, or of the large sensitivity of hydrogen shifts to spin-orbit effects. Both shielding and deshielding spin-orbit effects may be understood, and long-range spin-orbit contributions follow the same pathways as spin-spin coupling constants. Scalar relativistic effects are important for both chemical shifts of nuclei neighboring heavy atoms, and for the heavy nuclei themselves. The dominant relativistic contributions to „heavy-atom effects on the shielding of the heavy atom itself (HAHA) appear to arise from an interesting cross term between kinematic corrections to the nuclear spin Zeeman term and the Fermi-contact operator. Most of the HAHA effects tend to cancel in relative chemical shifts and are thus less easily observed experimentally than the HALA effects.