Unimolecular Reactions of Dihydrated Alkaline Earth Metal Dications M2+(H2O)2, M = Be, Mg, Ca, Sr, and Ba: Salt-Bridge Mechanism in the Proton-Transfer Reaction M2+(H2O)2 → MOH+ + H3O+

Abstract

The unimolecular reactivity of M²⁺(H₂O)₂, M = Be, Mg, Ca, Sr, and Ba, is investigated by density functional theory. Dissociation of the complex occurs either by proton transfer to form singly charged metal hydroxide, MOH⁺, and protonated water, H₃O⁺, or by loss of water to form M²⁺(H₂O) and H₂O. Charge transfer from water to the metal forming H₂O⁺ and M⁺(H₂O) is not favorable for any of the metal complexes. The relative energetics of these processes are dominated by the metal dication size. Formation of MOH⁺ proceeds first by one water ligand moving to the second solvation shell followed by proton transfer to this second-shell water molecule and subsequent Coulomb explosion. These hydroxide formation reactions are exothermic with activation energies that are comparable to the water binding energy for the larger metals. This results in a competition between proton transfer and loss of a water molecule. The arrangement with one water ligand in the second solvation shell is a local minimum on the potential energy surface for all metals except Be. The two transition states separating this intermediate from the reactant and the products are identified. The second transition state determines the height of the activation barrier and corresponds to a M²⁺−OH^-−H₃O⁺ “salt-bridge” structure. The computed B3LYP energy of this structure can be quantitatively reproduced by a simple ionic model in which Lewis charges are localized on individual atoms. This salt-bridge arrangement lowers the activation energy of the proton-transfer reaction by providing a loophole on the potential energy surface for the escape of H₃O⁺. Similar salt-bridge mechanisms may be involved in a number of proton-transfer reactions in small solvated metal ion complexes, as well as in other ionic reactions.