The thermodynamic characteristics of formation of organic molecule complexes with the magnesium ion in water: The results of quantum-chemical modeling

Abstract

The Gibbs energy ΔGb of formation of organic molecule complexes with the Mg2+ ion in water was calculated on the basis of a two-stage scheme for the complex formation reaction. The first stage is ligand transfer from infinity into the second coordination sphere of the Mg2+ ion, and the second stage is the dissociation of bonds between water molecules and the Mg2+ ion and the formation of bonds between the ligand and Mg2+. The contribution of the first reaction stage to ΔGb was calculated on the assumption that the ligand was a solid body with a charge or dipole moment (if the ligand was neutral). The contribution of the second stage to ΔGb was calculated using quantum-chemical modeling. The major contribution to ΔGb was made by a change in the internal energy of the complex as a result of the dissociation/formation of coordination bonds and a change in the electric component of the Gibbs energy of interaction between the magnesium ion and molecule with water when they formed a complex. The contribution of the nonpolar component of complex interaction with water was comparatively small. Accurate calculations of the contribution of vibrational degrees of freedom to ΔGb were also of importance.