Testing the water

Spectroscopic data shows that a hydroxide ion can donate a hydrogen bond to water, supporting one of two previously proposed mechanisms for its unusually fast transport in aqueous solution.

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In comparison with other ions in aqueous solution, the transport of H⁺ and OH⁻ is anomalously quick. It is qualitatively accounted for by 'structural diffusion' — the hopping of ions through bond formation and cleavage. The mechanism of H⁺ ion diffusion has been well studied and excess proton transport is known to occur through the interconversion of Eigen [H₃O⁺(H₂O)₃] and Zundel [H₂O···H···OH₂]⁺ complexes.

The mechanism for hydroxide ion diffusion is less clear with scientists disagreeing over whether it occurs through a 'mirror image' of proton diffusion — where the hydroxide ion is treated as a 'proton hole', a water molecule with a missing proton — or through a model that predicts transport via a species that includes a hydrogen bond donated from the hydroxide ion to an adjacent water molecule. Now Bernd Winter of the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie and co-workers in Germany and Sweden have provided spectroscopic evidence¹ to support the latter theory.

By measuring the photoelectron spectrum of a hydroxide solution, Winter and colleagues have detected an energy-transfer process called intermolecular Coulombic decay (ICD). This occurs between a molecule that has been ionized by photoelecton emission and a neighbouring molecule — in this case, a hydroxide ion and water, respectively. ICD is not seen in the emission spectrum of the chemically similar fluoride ion, inferring that ICD occurs due to the presence of the hydroxide's hydrogen atom. Winter and co-workers argue that the energy transfer must take place through a hydrogen bond donated from the OH⁻ ion to the water molecule, supporting the theory that includes hydroxide hydrogen-bond donation and disagreeing with the 'proton hole' concept.