Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai-cho, Nada, Kobe 657-8501, Japan.
Acc Chem Res. 2012 Nov 20;45(11):1982-91. doi: 10.1021/ar300017h. Epub 2012 Oct 30.
In liquid water, hydrogen bonds form three-dimensional network structures, which have been modeled in various molecular dynamics simulations. Locally, the hydrogen bonds continuously form and break, and the network structure continuously fluctuates. In aqueous solutions, the water molecules perturb the solute molecules, resulting in fluctuations of the electronic and vibrational states. These thermal fluctuations are fundamental to understanding the activation processes in chemical reactions and the function of biopolymers. In this Account, we review studies of the vibrational frequency fluctuations of solute molecules in aqueous solutions using three-pulse infrared photon echo experiments. For comparison, we also briefly describe dynamic fluorescence Stokes shift experiments for investigating solvation dynamics in water. The Stokes shift technique gives a response function, which describes the energy relaxation in the nonequilibrium state and corresponds to the transition energy fluctuation of the electronic state at thermal equilibrium in linear response theorem. The dielectric response of water in the megahertz to terahertz frequency region is a key physical quantity for understanding both of these frequency fluctuations because of the influence of electrostatic interactions between the solute and solvent. We focus on the temperature dependence of the three experiments to discuss the molecular mechanisms of both the frequency fluctuations in aqueous solutions. We used a biexponential function with sub-picosecond and picosecond time constants to characterize the time-correlation functions of both the vibrational and electronic frequency fluctuations. We focus on the slower component, with time constants of 1-2 ps for both the frequency fluctuations at room temperature. However, the temperature dependence and isotope effect for the time constants differ for these two types of fluctuations. The dielectric interactions generally describe the solvation dynamics of polar solvents, and hydrodynamic theory can describe the slow component for the electronic states. Compared with the slow component of the solvation dynamics, however, the picosecond component for the vibrational frequency fluctuations is less sensitive to temperature. Therefore, the slow component of the vibrational frequency fluctuation is determined by different underlying dynamics, which are important for the solvation dynamics of the electronic state. The time constant for the picosecond component for the vibrational frequency fluctuation does not significantly depend on the solute. We propose that the vibrational frequency fluctuates because of the constant structural changes in the hydrogen-bonding network of water molecules around the solute.
在液态水中，氢键形成三维网络结构，这已在各种分子动力学模拟中得到建模。在局部，氢键不断形成和断裂，网络结构不断波动。在水溶液中，水分子会干扰溶质分子，导致电子和振动状态的波动。这些热波动对于理解化学反应中的激活过程和生物聚合物的功能至关重要。在本综述中，我们使用三脉冲红外光子回波实验研究了水溶液中溶质分子振动频率波动。作为比较，我们还简要描述了用于研究水中溶剂化动力学的动态荧光斯托克斯位移实验。斯托克斯位移技术给出了一个响应函数，它描述了非平衡态中的能量弛豫，并且对应于线性响应定理中电子态在热平衡下的跃迁能量波动。在兆赫兹到太赫兹频率范围内，水的介电响应是理解这两种频率波动的关键物理量，因为它受到溶质和溶剂之间静电相互作用的影响。我们专注于这三个实验的温度依赖性，以讨论水溶液中频率波动的分子机制。我们使用具有亚皮秒和皮秒时间常数的双指数函数来描述振动和电子频率波动的时间相关函数。我们专注于较慢的分量，对于室温下的两种频率波动，其时间常数为 1-2 皮秒。然而，对于这两种波动，时间常数的温度依赖性和同位素效应是不同的。介电相互作用通常描述极性溶剂的溶剂化动力学，而流体力学理论可以描述电子态的慢分量。然而，与溶剂化动力学的慢分量相比，振动频率波动的皮秒分量对温度的敏感性较低。因此，振动频率波动的慢分量取决于不同的基础动力学，这对于电子态的溶剂化动力学很重要。振动频率波动的皮秒分量的时间常数与溶质的关系不大。我们提出，由于水分子氢键网络在溶质周围的不断结构变化，振动频率发生波动。
