Thiols in Natural Organic Matter: Molecular Forms, Acidity, and Reactivity with Mercury(II) from First-Principles Calculations and High Energy-Resolution X-ray Absorption Near-Edge Structure Spectroscopy

Thiol functional groups in natural organic matter form strong complexes with Hg(II) and other soft metal cations and therefore are an important component of sulfur and metal cycling in the environment. However, characterizing thiol reactivity is difficult because natural organic molecules are complex both in composition and molecular structure. Here, reactivity was assessed by calculating the Gibbs free energies of thiolation and thiol deprotonation reactions for model structures considered to form during abiotic sulfurization of natural organic matter. Gaussian calculations were performed at the CCSD(T) level of theory. Thiol addition is predicted to be faster by as much as 8 orders of magnitude on unsaturated cyclic structures rich in ketone and ether linkages than on open-chain carbonyl structures. The RIC, values of thiols added to carboxyl-rich alicyclic molecules are predicted to be above 10, whereas plc values of thiols bonded to lignin-derived polyhydroxyphenols are predicted to be between 2 and 6. Reactivity of thiols with Hg(II) was evaluated by monitoring the kinetics of transformation of Hg(SR)(2) complexes to nanoparticulate metacinnabar in dissolved organic matter with different chemical compositions under oxic conditions using high-energy-resolution X-ray absorption near-edge structure spectroscopy (HR-XANES). The most rapid transformation occurred in the material with the greatest quinone carbon (ketone) and polyphenol contents, consistent with the predicted low pK(a) values of thiolated ketones near phenolic hydroxyl groups. The slowest transformation occurred in the material with the least amount of cyclic ketone structures despite having the highest amount of thiols. These results indicate that the sulfur atoms in thiolated ketones are those that are most reactive and removed during the alkyl transfer reaction considered to nucleate metacinnabar and that the concentration of the low-pK(a) thiols controls the transformation rate.