Insight into the interaction of M2+(H2O)4-6 ions (M = Zn, Cd, and Hg) with the functional groups of thiol and hydroxy.

The interaction mechanisms of toxic metal ions with the dimercaprol drug and cellulose chain have not yet been fully elucidated. The interactions between M2+(H2O)4-6 ions (M = Zn, Cd, and Hg) and the thiol groups of thioalcohols as well as the hydroxy groups of alcohols were investigated using the M06-2X functional. The effects of the aqueous solvent were evaluated using the solvation model based on density (SMD). The two-layer ONIOM approach and molecular electrostatic potential (MEP) maps were employed for the hydroxyl groups of linear cellulose. The affinity of Zn2+(H2O)4-5 ions toward the dimercaprol molecule was probed at the M06-2X/6-311+G** level of theory. Furthermore, calculations using the B3LYP and B3LYP-D3BJ functionals were performed for comparison. The calculated results indicated that the adjacent dissociation Gibbs free energy changes (ΔrG0) of the metal complexes depend significantly on the inductive effects of substituent R, the molecular volume, the length of the linear cellulose, the chelation effect, and the hardness or softness of complexing elements. The affinity of M2+(H2O)5 ions toward the sixth water molecule and methanol molecule decreases in the order of Zn2+ > Cd2+ > Hg2+, whereas the reverse trend is observed for the methanethiol molecule. The C-O and C-S bond lengths in the metal complexes are longer than those of the free ligands. The computational results were in good agreement with the experimental data and with the hard and soft acids and bases (HSAB) theory.