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