Rattling in the Cage: Ions as Probes of Sub-picosecond Water Network Dynamics
Rattling in the Cage: Ions as Probes of Sub-picosecond Water Network Dynamics
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DOI：
10.1021/ja9083545
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发表时间：
2009-12-30
影响因子：
15
通讯作者：
Havenith, Martina
中科院分区：
文献类型：
--
作者：
Schmidt, Diedrich A.;Birer, Oezguer;Havenith, Martina
我们提出了太赫兹（THz）测量的盐溶液，揭示了新的光的争议是否盐作为kosmotropes（结构制造商）或离液剂（结构破坏者），这提高或降低溶剂顺序，分别。我们对15种碱金属卤化物的溶剂化盐在85 cm（-1）（2.5 THz）附近的浓度依赖性THz吸收系数进行了精确测量。此外，我们记录了概览光谱之间的30和300厘米（-1）使用太赫兹傅里叶变换光谱仪六碱金属卤化物。对于所有的解决方案，我们发现了一个线性增加的太赫兹吸收相比，纯水（太赫兹过量），随着溶质浓度的增加。这些结果表明，离子可以被视为简单的缺陷在氢键网络。因此，它们不能被表征为亲液剂或离液剂。低于200 cm（-1），观察到的所有盐的太赫兹过量可以通过水吸收和归因于水网络内离子的振动运动的额外吸收的线性叠加来描述。通过提供一组全面的数据，不同的盐溶液，我们发现，所有的解决方案可以很好地描述了一个模型，其中包括阻尼谐波振荡的阴离子和阳离子的水网络。我们发现，该模型预测的主要特征的太赫兹光谱的各种盐溶液。六种碱金属卤化物的太赫兹傅里叶变换光谱支持亚皮秒时间尺度上存在这些离子晃动运动的假设。超过200 cm（-1）的过量被解释为水网络振动模式翼的变化。伴随的分子动力学模拟使用的TIP 3 P水模型支持我们的结论，并表明，快速的亚皮秒运动的离子和他们的周围环境几乎是解耦的。这些研究结果提供了一个完整的描述溶质诱导的变化，在太赫兹溶剂化动力学的调查盐。我们的研究结果表明，太赫兹光谱是一个强大的实验工具，建立一个新的观点的贡献的阴离子和阳离子的结构水。
We present terahertz (THz) measurements of salt solutions that shed new light on the controversy over whether salts act as kosmotropes (structure makers) or chaotropes (structure breakers), which enhance or reduce the solvent order, respectively. We have carried out precise measurements of the concentration-dependent THz absorption coefficient of 15 solvated alkali halide salts around 85 cm(-1) (2.5 THz). In addition, we recorded overview spectra between 30 and 300 cm(-1) using a THz Fourier transform spectrometer for six alkali halides. For all solutions we found a linear increase of THz absorption compared to pure water (THz excess) with increasing solute concentration. These results suggest that the ions may be treated as simple defects in an H-bond network. They therefore cannot be characterized as either kosmotropes or chaotropes. Below 200 cm(-1), the observed THz excess of all salts can be described by a linear superposition of the water absorption and an additional absorption that is attributed to a rattling motion of the ions within the water network. By providing a comprehensive set of data for different salt solutions, we find that the solutions can all be very well described by a model that includes damped harmonic oscillations of the anions and cations within the water network. We find this model predicts the main features of THz spectra for a variety of salt solutions. The assumption of the existence of these ion rattling motions on sub-picosecond time scales is supported by THz Fourier transform spectroscopy of six alkali halides. Above 200 cm(-1) the excess is interpreted in terms of a change in the wing of the water network librational mode. Accompanying molecular dynamics simulations using the TIP3P water model support our conclusion and show that the fast sub-picosecond motions of the ions and their surroundings are almost decoupled. These findings provide a complete description of the solute-induced changes in the THz solvation dynamics for the investigated salts. Our results show that THz spectroscopy is a powerful experimental tool to establish a new view on the contributions of anions and cations to the structuring of water.