199Hg NMR Shielding and Chemical Shifts of 2-, 3-, and 4-Coordinate Hg(II)-Thiolate Species

Abstract

Spectroscopic characterization of Hg(II)-thiolate complexes is central to the bioinorganic chemistry of Hg(II). Interpretation of ¹⁹⁹Hg NMR usually relies on data recorded for reference compounds; however, here, it is demonstrated that ¹⁹⁹Hg NMR chemical shifts may be calculated within 40 ppm of experimental values for 2-, 3-, and 4-coordinate Hg(II)-thiolate complexes, using the PBE0 functional, the QZ4P basis set on Hg and S and TZP on all other atoms, and SO-ZORA to account for relativistic effects. The chemical shift is particularly sensitive to the Hg–S bond length (ca. 300 ppm/0.1 Å per Hg–S bond), while it is less sensitive to changes of S–Hg–S angles (up to 40 ppm/10 degrees ¹). Rigid rotation of the methyl groups around the Hg–S axis in model complexes, [Hg(SCH₃)_n]^2–n (n = 2, 3, 4), may give rise to changes of up to 100 ppm. Finally, calculations of the ¹⁹⁹Hg NMR chemical shift for a model system of the Hg(II) binding site in the MerR protein demonstrate that experimental ¹⁹⁹Hg NMR chemical shift data in combination with DFT calculations may be used as a constraint in the optimization of Hg(II) sites in proteins.