Like-charge ion pairs of hydronium and hydroxide in aqueous solution?

The like-charge ion pairings of hydronium and hydroxide were investigated using both ab initio cluster calculations and QM/MM-MD aqueous simulations. While only a two-water-bridged H₃O⁺(H₂O)₂H₃O⁺ is found in hydronium cluster calculations, three clusters of HO^-(H₂O)₂HO^-, HO^-(H₂O)₃HO^- and HO^-(H₂O)₄HO^- are stable dihydroxide aggregates. In addition, an interesting yet very stable parallelogram structure of [O-H⋯H-O]^2- without any bridging water was also discovered using QM/MM-MD simulations. According to our analysis, its unique structure reduces the electrostatic repulsion and allows stable coordination with solvents at the same time. In conclusion, hydroxide can form stronger like-ion pairs than hydronium in aqueous solution mostly due to its versatile coordination ability with solvents.