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Interaction between liquid water and hydroxide revealed by core-hole de-excitation


The hydroxide ion plays an important role in many chemical and biochemical processes in aqueous solution1. But our molecular-level understanding of its unusual and fast transport in water, and of the solvation patterns that allow fast transport, is far from complete. One proposal seeks to explain the properties and behaviour of the hydroxide ion by essentially regarding it as a water molecule that is missing a proton2, and by inferring transport mechanisms and hydration structures from those of the excess proton. A competing proposal invokes instead unique and interchanging hydroxide hydration complexes, particularly the hypercoordinated OH-(H2O)4 species and tri-coordinated OH-(H2O)3 that can form a transient hydrogen bond between the H atom of the OH- and a neighbouring water molecule3,4,5. Here we report measurements of core-level photoelectron emission and intermolecular Coulombic decay6,7,8 for an aqueous hydroxide solution, which show that the hydrated hydroxide ion is capable of transiently donating a hydrogen bond to surrounding water molecules. In agreement with recent experimental studies of hydroxide solutions9,10,11,12, our finding thus supports the notion that the hydration structure of the hydroxide ion cannot be inferred from that of the hydrated excess proton.

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Figure 1: Photoelectron, resonant Auger-electron and intermolecular Coulombic decay spectra of 4 molal NaOH aqueous solution.
Figure 2: Energy-level diagram of OH - (aq) and H 2 O(aq).

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Correspondence to Bernd Winter.

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Aziz, E., Ottosson, N., Faubel, M. et al. Interaction between liquid water and hydroxide revealed by core-hole de-excitation. Nature 455, 89–91 (2008).

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