Hydration of CH₃HgOH and CH₃HgCl compared to HgCl₂, HgClOH, and Hg(OH)₂: A DFT microsolvation cluster approach

We address the aqueous microsolvation of the CH₃HgCl and CH₃HgOH molecules using a stepwise hydration scheme including up to 33 water molecules and compare our results with the previously studied HgCl₂, HgClOH, and Hg(OH)₂ complexes. Optimized geometries and Gibbs free energies were obtained at the B3PW91/aug-RECP(Hg)-6-31G(d,p) level. At least 33 water molecules were required to build the first solvation shell around both methylmercury compounds. Optimized geometries were found having favorable interactions of water molecules with Hg, Cl, and the OH moiety. Born-Oppenheimer molecular dynamics simulations were performed on the largest CH₃HgX(X = Cl, OH)-(H₂O)₃₃ clusters at the same level of theory. Born-Oppenheimer molecular dynamics simulations at T = 300 K (ca. 0.62 kcal/mol) revealed the presence of configurations with hydrogen-bonded networks that include the OH moiety in CH₃HgOH and exclude both the Hg and Cl in CH₃HgCl, favoring a clathrate-type structure around the methyl moiety. The comparison to the microsolvated HgClOH, Hg(OH)₂, and HgCl₂ molecules showed that, in all cases, the water molecules easily move away from Cl, thus supporting the idea that HgCl₂ behaves as a non-polar solute. The theoretical (L_III edge) X-ray absorption near edge structure spectra are obtained and found in good agreement with experimental data, especially for the CH₃HgCl species.