High-Frequency 13C and 29Si NMR Chemical Shifts in Diamagnetic Low-Valence Compounds of TlI and PbII: Decisive Role of Relativistic Effects

Abstract

The 13C and 29Si NMR signals of ligand atoms directly bonded to TlI or PbII heavy-element centers are predicted to resonate at very high frequencies, up to 400 ppm for 13C and over 1000 ppm for 29Si, outside the typical experimental NMR chemical-shift ranges for a given type of nuclei. The large 13C and 29Si NMR chemical shifts are ascribed to sizable relativistic spin-orbit effects, which can amount to more than 200 ppm for 13C and more than 1000 ppm for 29Si, values unexpected for diamagnetic compounds of the main group elements. The origin of the vast spin-orbit contributions to the 13C and 29Si NMR shifts is traced to the highly efficient 6p ' 6p∗ metal-based orbital magnetic couplings and related to the 6p orbital-based bonding together with the low-energy gaps between the occupied and virtual orbital subspaces in the subvalent TlI and PbII compounds. New NMR spectral regions for these compounds are suggested based on the fully relativistic density functional theory calculations in the Dirac-Coulomb framework carefully calibrated on the experimentally known NMR data for TlI and PbII complexes. © 2016 American Chemical Society.