Ultrafast Proton Transport between a Hydroxy Acid and a Nitrogen Base along Solvent Bridges Governed by the Hydroxide/Methoxide Transfer Mechanism
Ultrafast Proton Transport between a Hydroxy Acid and a Nitrogen Base along Solvent Bridges Governed by the Hydroxide/Methoxide Transfer Mechanism
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DOI：
10.1021/jacs.9b03471
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发表时间：
2019-09-18
影响因子：
15
通讯作者：
Sebastiani, Daniel
中科院分区：
文献类型：
--
作者：
Ekimova, Maria;Hoffmann, Felix;Sebastiani, Daniel
质子在水溶液中的传输在生物膜和氢燃料电池的酸碱中和和能量传输中起着关键作用。广泛的实验和理论研究已经导致了一个非常详细的阐明水过量质子运输，被称为冯Grotthuss机制，涉及不同的水合质子配置与相关的高流动性结构动力学的基本微观机制之一。氢氧化物的运输，约2倍低的体积扩散率相比，过量的质子，已收到少得多的关注。我们提出飞秒紫外/红外泵浦-探测实验和从头算分子动力学模拟不同的质子传输路径的双功能光酸7-羟基喹啉（7 HQ）在水/甲醇混合物。对于7 HQ溶剂依赖性的光酸性，自由能反应性相关行为和量子力学/分子力学（QM/MM）轨迹点占主导地位的OH-/CH 3 O-运输途径的所有水/甲醇的混合比调查。我们的联合超快红外光谱和从头算分子动力学研究提供了确凿的证据，水解/甲醇分解酸碱中和途径，制定了曼弗雷德艾根半世纪前。我们的研究结果的芳香族羟基和芳香族氮官能团的明显不同的酸碱反应性建议这些自由能反应性的相关性作为溶剂极性的函数进一步探索的有用性。最终，溶剂依赖性酸度的测定将有助于更好地理解质子在弱极性表面和跨膜质子泵蛋白质或氢燃料电池材料的极性或离子区域附近的质子传输机制。
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 2-fold-lower bulk diffusion rates compared to 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 bifunctional photoacid 7-hydroxyquinoline (7HQ) in water/methanol mixtures. For 7HQ solvent-dependent photoacidity, free-energy-reactivity correlation behavior and quantum mechanics/molecular mechanics (QM/MM) trajectories point to a dominant OH-/CH3O- transport pathway for all water/methanol mixing ratios investigated. Our joint ultrafast infrared spectroscopic and ab initio molecular dynamics study provides conclusive evidence for the hydrolysis/ methanolysis acid-base neutralization pathway, as formulated by Manfred Eigen half a century ago. Our findings on the distinctly different acid-base reactivities for aromatic hydroxyl and aromatic nitrogen functionalities suggest the usefulness of further exploration of these free-energy-reactivity correlations as a function of solvent polarity. Ultimately the determination of solvent-dependent acidities will contribute to a better understanding of proton-transport mechanisms at weakly polar surfaces and near polar or ionic regions in transmembrane proton pump proteins or hydrogen fuel cell materials.