  Aqueous proton transport plays a key role in acid-base neutralization, and
energy transport through biological membranes and hydrogen fuel cells.
Extensive experimental and theoretical studies have resulted in a highly
detailed elucidation of one of the underlying microscopic mechanisms for
aqueous excess proton transport, known as the von Grotthuss mechanism,
involving different hydrated proton configurations with associated high
fluxional structural dynamics. Hydroxide transport, with approximately two-fold
lower bulk diffusion rates than those of excess protons, has received much less
attention. We present femtosecond UV/IR pump-probe experiments and ab initio
molecular dynamics simulations of different proton transport pathways of the
bifunctional photoacid 7-hydroxyquinoline (7HQ) in water-methanol mixtures. For
7HQ solvent-dependent photoacidity, free energy-reactivity correlation
behaviour and QM/MM trajectories point to a dominant OH-/CH3O- transport
pathway, for all water-methanol mixing ratios investigated. This provides
conclusive evidence for the hydrolysis/methanolysis acid-base neutralization
pathway formulated by Manfred Eigen half a century ago.
