Nature Abstract
Abstract
Compared to ohter ions, proton (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. In contrast,
hydroxide ion mobility in basic solutions has received far less
attention, 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
notion 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. However, recent studies
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. We find that the transport mechanism,
which differs significantly from the proton hole picture, involves
an interplay between previously identified hydration complexes
and is strongly influenced by nuclear quantum effects.