Experimental and Theoretical Evidence of Spin-Orbit Heavy Atom on the Light Atom 1 H NMR Chemical Shifts Induced through H⋅⋅⋅I- Hydrogen Bond

Abstract

Spin-orbit (SO) heavy-atom on the light-atom (SO-HALA) effect is the largest relativistic effect caused by a heavy atom on its light-atom neighbors, leading, for example, to unexpected NMR chemical shifts of ¹ H, ¹³ C, and ¹⁵ N nuclei. In this study, a combined experimental and theoretical evidence for the SO-HALA effect transmitted through hydrogen bond is presented. Solid-state NMR data for a series of 4-dimethylaminopyridine salts containing I^- , Br^- and Cl^- counter ions were obtained experimentally and by theoretical calculations. A comparison of the experimental chemical shifts with those calculated by a standard DFT methodology without the SO contribution to the chemical shifts revealed a remarkable error of the calculated proton chemical shift of a hydrogen atom that is in close contact with the iodide anion. The addition of the relativistic SO correction in the calculations significantly improves overall agreement with the experiment and confirms the propagation of the SO-HALA effect through hydrogen bonds.

Keywords: DFT calculations; NMR spectroscopy; hydrogen bonding; relativistic effects; spin-orbit coupling.