Modeling of Heavy‐Atom–Ligand NMR Spin–Spin Coupling in Solution: Molecular Dynamics Study and Natural Bond Orbital Analysis of HgC Coupling Constants

Abstract

Ab initio molecular dynamics (MD) and relativistic density functional NMR methods were applied to calculate the one-bond Hg-C NMR indirect nuclear spin-spin coupling constants (J) of [Hg(CN)(2) ] and [CH(3) HgCl] in solution. The MD averages were obtained as J((199) Hg-(13) C)=3200 and 1575 Hz, respectively. The experimental Hg-C spin-spin coupling constants of [Hg(CN)(2) ] in methanol and [CH(3) HgCl] in DMSO are 3143 and 1674 Hz, respectively. To deal with solvent effects in the calculations, finite "droplet" models of the two systems were set up. Solvent effects in both systems lead to a strong increase of the Hg-C coupling constant. From a relativistic natural localized molecular orbital (NLMO) analysis, it was found that the degree of delocalization of the Hg 5d(σ) nonbonding orbital and of the HgC bonding orbital between the two coupled atoms, the nature of the trans Hg-C/Cl bonding orbital, and the s character of these orbitals, exhibit trends upon solvation of the complexes that, when combined, lead to the strong increase of J(Hg-C).

Relativistic calculations of magnetic resonance parameters: background and some recent developments

“…The effects on J-coupling were in line with those previously observed in DFT calculations. The finite-nucleus effects on heavy-atom shielding constants were similar in magnitude to those determined in a benchmark study of 199 Hg NMR shielding [105]. It appears that the effect on the nuclear spin perturbation operator is secondary to the effect from the modification of the electron density at the heavy nucleus due to the finite-nucleus potential.…”

“…Scalar relativistic NESC calculations have been used to evaluate 199 Hg hfc constants of a number of radicals containing mercury [210]. NR results were also provided, showing relativistic increases by factors of about 2-3 in HF calculations, consistent with the effects reported in the past for 199 Hg NMR J-couplings [38,40,211].…”

“…Orbital energies can be improved substantially by applying the scaled ZORA corrections, as has been shown again recently by Saue [5]. In ZORA NMR calculations, the absolute shielding is adversely affected by errors from core orbitals, as has been demonstrated in a comparative benchmark study of 199 Hg shielding constants [105]. However, the errors cancel when the chemical shift is evaluated, as was shown by a comparison of Hg NMR shifts of HgX 2 (X = Cl, Br, I) obtained with the ZORA implementation in the ADF code and 4c results calculated with the Dirac package, with Hg(CH 3 ) 2 as the reference.…”

“…The experimental coupling is 5778 in methanol. The difference between the relativistic calculations for an isolated complex and experimental data was later shown to be due to missing solvent effects [199].…”

“…This is indeed the case. Consider the comparison of point-nucleus versus finite-nucleus 199 Hg hfc constants in the HgF and HgAg radicals listed in table 3. The data were selected from larger collections reported in [145,[212][213][214].…”