Ab initio molecular dynamics simulation of the solvation and transport of H₃O⁺ and OH^- ions in water

Applying the ab initio molecular dynamics method, we have studied the solvation and dynamics of an excess proton and a proton hole in liquid water. We find for the H₃O⁺ ion a dynamic solvation complex which continuously fluctuates between a (H₅O₂)⁺ and (H₉O₄)⁺ structure as a result of proton transfer. The results of the simulation strongly suggest that the rate-limiting step for the migration of the excess proton is the concerted dynamics of the second solvation shell hydrogen bonded to the ligand H₂O molecules. The OH^- ion has a predominantly planar 4-fold coordination. Proton transfer is only observed when this (H₉O₅)^- complex is transformed into a tetrahedral (H₇O₄)^- configuration. The formation of this more open complex determines the OH^- diffusion rate.