Elucidating the mechanism behind the stabilization of multi-charged metal cations in water: a case study of the electronic states of microhydrated Mg2+, Ca2+ and Al3+

Abstract

Metal atoms typically have second and higher ionization potentials (IPs) that are larger than the IP of water, resulting in the Coulombic explosion of the first few [M(H2O)n](+q) (q≥ 2) gas phase clusters as the M(+(q-1)) + (H2O)n(+) or MOH(+(q-1)) + H3O(+)(H2O)n-2 energy levels are energetically more stable than the M(q+) + (H2O)n ones for small n. We present a theoretical analysis of the various electronic states arising from the sequential hydration of the Ca(2+), Mg(2+) and Al(3+) cations with up to six water molecules. Our results quantify the relative shift of those electronic states with the degree of solvation, identify their complex interaction with other states arising from different dissociation channels and shed light on the mechanism behind the energetic stabilization of the multi-charged hydrated M(+q)(H2O)n complexes observed in aqueous solution with respect to the water ionization products.