The Influence of a Presence of a Heavy Atom on ¹³C Shielding Constants in Organomercury Compounds and Halogen Derivatives

Abstract

(13)C nuclear magnetic resonance shielding constants have been calculated by means of density functional theory (DFT) for several organomercury compounds and halogen derivatives of aliphatic and aromatic compounds. Relativistic effects have been included through the four-component Dirac-Kohn-Sham (DKS) method, two-component Zeroth Order Regular Approximation (ZORA) DFT, and DFT with scalar effective core potentials (ECPs). The relative shieldings have been analyzed in terms of the position of carbon atoms with respect to the heavy atom and their hybridization. The results have been compared with the experimental values, some newly measured and some found in the literature. The main aim of the calculations has been to evaluate the magnitude of heavy atom effects on the (13)C shielding constants and to check what are the relative contributions of scalar relativistic effects and spin-orbit coupling. Another object has been to compare the DKS and ZORA results and to check how the approximate method of accounting for the heavy-atom-on-light-atom (HALA) relativistic effect by means of scalar effective core potentials on heavy atoms performs in comparison with the more rigorous two- and four-component treatment.

Molecular modeling and experimental studies on structure and NMR parameters of 9-benzyl-3,6-diiodo-9H-carbazole

“…4 that the spin-orbit effects played a major role in the calculations of chemical shifts of carbon atoms directly attached to the halogen atom (here iodine). The observed HALA effects are very close to earlier results for halogen-substituted carbon atoms [15,35,42]. The other carbons, however, do practically not feel the presence of the heavy halogen atoms.…”

“…Ziegler, Autschbach and Arcisauskaite [35][36][37][38][39][40][41] reported on several relativistic calculations of NMR shieldings. In the work of Pecul et al [42] the importance of inclusion of the HALA effect on 13 C shielding constants in organomercury compounds and halogen derivatives was discussed. They observed large spin-orbit (SO) contributions to the HALA effects in the halogen substituted compounds, which were well reproduced using zeroth-order regular approximation with SO coupling (SO ZORA [43][44][45]) while these effects could not be recovered using relativistic effective core potentials (ECP [46]) on the halogen atoms.…”

DFT and experimental studies on structure and spectroscopic parameters of 3,6-diiodo-9-ethyl-9H-carbazole

“…The RMS is smaller than for the previously studied 9-benzyl-3,6-diiodo-9H-carbazole [58]. The observed HALA effects are very close to earlier results for halogen-substituted carbon atoms [35,41,66] and for our previous study of benzylcarbazole derivative (there the HALA effect was -42 ppm) [58]. The other carbons, especially atoms responsible for the rigidity and planarity Weighting scheme…”

“…Early relativistic calculations of NMR shieldings were reported by Malkin, Ziegler and Autschbach [35][36][37][38][39][40]. The importance of the HALA effect of mercury on the isotropic shielding of carbon atoms was reported by Wodyński et al [41]. They obtained significant HALA effects in halogen-substituted compounds, which were well reproduced using the zerothorder regular approximation with spin-orbit coupling (SO ZORA) [42][43][44].…”

Relativistic heavy atom effect on 13C NMR chemical shifts initiated by adjacent multiple chalcogens

“…The HALA effects induced by neighboring heavy atoms (HAs) on 13 C chemical shifts have been studied for a long time, though, the most part of works deals with the HALA effect induced by the heavy halogens or by neighboring heavy transition metals . At the present moment, with the exception for several casual communications, there is no systematic study of the HALA effect on carbon chemical shifts induced by heavy chalcogens.…”