Thermodynamic and topological investigation of the interaction between methimazole and Mz+ (Na+, Li+, Ca2+ and Mg2+)

Structures and thermodynamic properties of complexes formed from interaction between the methimazole (MMI) as an anti-thyroid drug and Mz+ (Ca2+, Mg2+, Na+ and Li+) were investigated at various levels of theory. We have found two types planar and non-planar MMI–Mz+ complexes on the potential energy surface. From the Gibbs free energy difference between the planar and non-planar forms of MMI–Mz+ complexes, it is found that the non-planar forms are the predominant ones. In addition, stability of MMI–Mz+ complexes increases in the following order: Na+ < Li+ < Ca2+ < Mg2+. NBO analysis confirms that the charge transfer takes place from MMI to metal cations. The AIM theory was applied to analyze the topological properties of the bond critical points (electron densities and their Laplacians) involved in the interaction between MMI and the metal cations. It was revealed that the SMz+ interactions in non-planar complexes of alkali and alkaline metal ions are electrostatic and partially covalent in nature, respectively. In addition, covalent nature of CS bond decreases upon complex formation.