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

The Gibbs energy ΔG _b 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 ΔG _b 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 ΔG _b was calculated using quantum-chemical modeling. The major contribution to ΔG _b 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 ΔG _b were also of importance.