Theoretical study of the models of Ca2+ and Mg2+ ions binding by the methylidene rhodanine neutral and anionic forms

The equilibrium geometry and energy parameters of the complexes of Ca2+ and Mg2+ with 5-methyl-2-thioxotiazolidin-4-one (methylidene rhodanine) and its anion in a 1:1 ratio in different conformations were calculated by the quantum-chemical method with the density functional theory on the level of hybrid functional B3LYP in the basis of atomic orbitals 6-31+G(d). The influence of metal ion size on the number of possible isomeric coordinations was indicated. The principles of stabilization and destabilization of the structures depending on their conformations al structure were described. Based on the calculated equilibrium geometry parameters of the complexes conformations the effect of complexation on the structure of rhodanine ligand was elucidated. In the framework of a polarizable continuum the relative stability of the possible tautomeric forms of methylidene rhodanine in water was investigated. A new structure of the methylidene rhodanine anion distinquished by a specific distribution of negative charge is suggested.