  Proton transfer via hydronium and hydroxide ions in water is ubiquitous. It
underlies acid-base chemistry, certain enzyme reactions, and even infection by
the flu. Despite two-centuries of investigation, the mechanism underlying why
hydronium diffuses faster than hydroxide in water is still not well understood.
Herein, we employ state of the art Density Functional Theory based molecular
dynamics, with corrections for nonlocal van der Waals interactions, and
self-interaction in the electronic ground state, to model water and the
hydrated water ions. At this level of theory, structural diffusion of hydronium
preserves the previously recognized concerted behavior. However, by contrast,
proton transfer via hydroxide is dominated by stepwise events, arising from a
stabilized hyper-coordination solvation structure that discourages proton
transfer. Specifically, the latter exhibits non-planar geometry, which agrees
with neutron scattering results. Asymmetry in the temporal correlation of
proton transfer enables hydronium to diffuse faster than hydroxide.
