The nature and transport mechanism of hydrated hydroxide ions in aqueous solution

Compared to other ions, protons (H⁺) and hydroxide ions (OH⁻) exhibit anomalously high mobilities in aqueous solutions. On a qualitative level, this behaviour has long been explained by 'structural diffusion' - the continuous interconversion between hydration complexes driven by fluctuations in the solvation shell of the hydrated ions. Detailed investigations have led to a clear understanding of the proton transport mechanism at the molecular level^2-8. In contrast, hydroxide ion mobility in basic solutions has received far less attention^{2, 3, 9, 10}, even though bases and base catalysis play important roles in many organic and biochemical reactions and in the chemical industry. The reason for this may be attributed to the century-old notion¹¹ that a hydrated OH⁻ can be regarded as a water molecule missing a proton, and that the transport mechanism of such a 'proton hole' can be inferred from that of an excess proton by simply reversing hydrogen bond polarities^11-18. However, recent studies^{2, 3} have identified OH⁻ hydration complexes that bear little structural similarity to proton hydration complexes. Here we report the solution structures and transport mechanisms of hydrated hydroxide, which we obtained from first-principles computer simulations that explicitly treat quantum and thermal fluctuations of all nuclei^19-21. We find that the transport mechanism, which differs significantly from the proton hole picture, involves an interplay between the previously identified hydration complexes^{2, 3} and is strongly influenced by nuclear quantum effects.