Effects of hydration on the molecular structure of atrazine dimers: A MOPAC (PM3) study

Abstract

A semi-empirical (PM3) study of atrazine dimer hydration in which the supermolecule approach is used to simulate a solvent sheath of water molecules varying from 7 to 24 H 2O's around the atrazine dimer. Enthalpies of complex formation and reaction are reported along with entropies of complex formation and free energies of reaction and formation. The hydration sphere around the atrazine dimer is reduced stepwise by removing the outermost water molecule followed by calculation of the new structure. In the case of the hydrated dimer with 21 water molecules, the removal of one water molecule results in rearrangement of the majority of the solvent sheath. This is accompanied by a change in the number of hydrogen bonds and the average hydrogen bond distance between water molecules, ring chlorines, a ring nitrogen and other water molecules. There are no additional major changes in the solvent sheath as one continues the stepwise removal of water In addition, reaction enthalpies and entropies show a systematic change at a point where the solvent sheath contains 17 water molecules. The atrazine rings and their attached nitrogens maintain their planarity during the solvation process, unlike the case where metal ion binding to the atrazine dimer occurs.