Quantum chemical description of ions in solution

Abstract

The application of the MESQUAC-MO method, a mixed electrostatic quantum chemical approach [1, 2], allows the calculation of models simulating ions in solution in solution including one, two, three or more explicitly considered solvation shells, i.e. up to the level, where the region of the bulk solvent begins. Such calculations have been performed for most metal ions up to the atomic number 30 (Zinc). Since these calculations have been carried out mainly for aqueous solutions, values for binding energies of water molecules in the various hydration layers could be obtained, allowing a theoretical prediction of the size of the hydration spheres of these ions. The total binding energies can be compared well to the experimentally determined hydration energies and reflect well known effects as the ‘ligand field stabilization’ within the transition metal ions. A comparison of the calculated binding energies at various coordination numbers with experimental data for the kinetics of solvent exchange in the first hydration layer allowed to discuss some models for the mechanism of exchange, indicating am important role of the water molecules in the second layer, which is confirmed by some tentative calculations on the corresponding transition state. Finally, some first calculations on a model pathway for the ionic dissociation in solution have been carried out giving some theoretical background on the basis of quantum chemistry for this fundamental process of solution chemistry.