Concentrated acid solutions, particularly HCl, have been studied extensively to examine the proton hopping and infrared spectral signatures of hydronium ions. Much less attention has been given to the structural dynamics of concentrated HCl solutions. Here, we apply optical heterodyne detected–optical Kerr effect (OHD-OKE) measurements to examine HCl concentration-dependent dynamics from moderate (0.8 m) to very high (15.5 m) concentrations and compare the results to the dynamics of NaCl solutions, as Na⁺ is similar in size to the hydronium cation. Both HCl and NaCl OHD-OKE signals decay as triexponentials at all concentrations, in contrast to pure water, which decays as a biexponential. Two remarkable features of the HCl dynamics are the following: (1) the bulk viscosity is linearly related to the slowest decay constant, t₃, and (2) the concentration-dependent proton hopping times, determined by ab initio MD simulations and 2D IR chemical exchange experiments, both obtained from the literature, fall on the same line as the slowest structural dynamics relaxation time, t₃, within experimental error. The structural dynamics of hydronium/chloride/water clusters, with relaxation times t₃, are responsible for the concentration dependence of microscopic property of proton hopping and the macroscopic bulk viscosity. The slowest time constant (t₃), which does not have a counterpart in pure water, is 3 ps at 0.8 m and increases by a factor of ∼2 by 15.5 m. The two fastest HCl decay constants, t₁ and t₂, are similar to those of pure water and increase mildly with the concentration.

中文翻译：

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浓 HCl 溶液中的快速结构动力学：从质子跳跃到体积粘度

浓酸溶液，特别是 HCl，已被广泛研究以检查水合氢离子的质子跳跃和红外光谱特征。对浓 HCl 溶液的结构动力学的关注要少得多。在这里，我们应用光学外差检测-光学克尔效应 (OHD-OKE) 测量来检查从中等 (0.8 m) 到极高 (15.5 m) 浓度的 HCl 浓度依赖性动力学，并将结果与​​ NaCl 溶液的动力学进行比较，如Na ^{+ 的}大小与水合氢阳离子相似。 HCl 和 NaCl OHD-OKE 信号在所有浓度下均以三指数形式衰减，而纯水则以双指数形式衰减。 HCl 动力学的两个显着特征如下：(1) 体积粘度与最慢的衰减常数t ₃线性相关，(2) 浓度依赖性质子跳跃时间，由从头开始MD 模拟和 2D IR确定化学交换实验（均从文献中获得）与最慢的结构动力学弛豫时间t ₃落在同一直线上，且在实验误差范围内。具有弛豫时间t _{3 的}水合氢离子/氯化物/水簇的结构动力学决定了质子跳跃的微观性质和宏观体积粘度的浓度依赖性。最慢的时间常数 ( t ₃ ) 在纯水中没有对应的，在 0.8 m 处为 3 ps，并在 15.5 m 处增加约 2 倍。两个最快的 HCl 衰减常数t ₁和t ₂与纯水的衰减常数相似，并且随着浓度的增加而略有增加。