Ultrafast Proton Transport between a Hydroxy Acid and a Nitrogen Base along Solvent Bridges Governed by Hydroxide/Methoxide Transfer Mechanism

TL;DR

This study combines femtosecond spectroscopy and simulations to reveal a dominant hydroxide/methoxide transfer pathway in proton transport, confirming a long-standing acid-base neutralization mechanism.

Contribution

It provides the first experimental and theoretical evidence for hydroxide/methoxide transfer as the main proton transport pathway in mixed water-methanol solutions.

Findings

Hydroxide/methoxide transfer dominates proton transport in water-methanol mixtures.

Femtosecond pump-probe experiments support the transfer pathway.

Simulations confirm the hydrolysis/methanolysis mechanism.

Abstract

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.