Aqueous proton transport involves the ultrafast interconversion of hydrated proton species that are closely linked to the hydrogen bond dynamics of water, which has been a long-standing challenge to experiments. In this study, we use ultrafast IR spectroscopy to investigate the distinct vibrational transition centered at 1750 cm^–1 in strong acid solutions, which arises from bending vibrations of the hydrated proton complex. Broadband ultrafast two-dimensional IR spectroscopy and transient absorption are used to measure vibrational relaxation, spectral diffusion, and orientational relaxation dynamics. The hydrated proton bend displays fast vibrational relaxation and spectral diffusion timescales of 200–300 fs; however, the transient absorption anisotropy decays on a remarkably long 2.5 ps timescale, which matches the timescale for hydrogen bond reorganization in liquid water. These observations are indications that the bending vibration of the aqueous proton complex is relatively localized, with an orientation that is insensitive to fast hydrogen bonding fluctuations and dependent on collective structural relaxation of the liquid to reorient. We conclude that the orientational relaxation is a result of proton transfer between configurations that are well described by a Zundel-like proton shared between two flanking water molecules.

中文翻译：

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超快速红外光谱揭示了水中的皮秒质子转移动力学

质子水传输涉及与水的氢键动力学密切相关的水合质子物质的超快速互变，这一直是实验的长期挑战。在这项研究中，我们使用超快红外光谱研究以1750 cm ^–1为中心的独特振动跃迁。在强酸溶液中，这是由水合质子配合物的弯曲振动引起的。宽带超快二维红外光谱和瞬态吸收用于测量振动弛豫，光谱扩散和取向弛豫动力学。水合的质子弯曲显示出快速的振动弛豫和200-300 fs的光谱扩散时标；但是，瞬态吸收各向异性在非常长的2.5 ps时标上衰减，这与液态水中氢键重组的时标相匹配。这些观察结果表明，质子水溶液复合物的弯曲振动是相对局部的，其取向对快速的氢键波动不敏感，并且取决于液体的整体结构松弛以进行重新取向。