Dynamics of Ions and Molecules in Concentrated Electrolyte and Acid Solutions

水是地球上最重要的液体。它涉及大量的技术应用，同时也是生物学的基础。我们对水的了解，至少是对水的纯粹形式的了解，是相当广泛的。水分子通过氢键相互作用，一个分子上部分带正电的氢原子被另一个分子上部分带负电的氧原子吸引。氢键使液态水具有明显的有序结构，甚至在某些地方像冰一样。大多数时候，在自然界和人类创造的技术中，水都含有其他化学物质，当这些化学物质的浓度很高时，我们所知道的纯水就不再适用了。水分子不再相互连接，而是被盐或酸的正离子和负离子所吸引。在这个由化学系化学结构、动力学和机理a项目资助的项目中，斯坦福大学的Michael Fayer教授和他的学生正在使用二维红外激光光谱技术（2D-IR）来探索含有高浓度盐和酸的水溶液的结构和动力学。红外光谱学揭示了分子的振动运动。2D-IR揭示了一个分子的振动是如何受到附近分子振动或附近离子运动的影响的。该项目使用超快激光脉冲，以万亿分之一秒的速度，对盐和酸溶液的动力学和结构进行直接测量。超快2D-IR还可以揭示水分子在遇到其他分子或离子时如何重新定位。尽管浓酸和浓盐溶液的结构和动力学对生命系统和许多工业技术都非常重要，但关于它们还有很多需要了解的地方。两名研究生参与了这个研究项目。除了获得先进激光技术方面的经验外，他们还接受理论和计算技术方面的培训，以帮助解释实验数据。电解质水溶液在化学、生物和工业应用（如燃料电池、海水淡化和电池技术）中无处不在。离子溶剂化结构、离子聚类和动力学以及水氢键网络的动力学是盐溶液中有趣的方面。浓缩电解质溶液有时被称为“盐中水”。在这些溶液中，水分子之间的氢键网络受到严重干扰。离子不能被水分子完全溶剂化，形成对、溶剂介导对和簇。拥挤的离子环境会产生强大的电场，从而抑制水分子的运动。本项目利用各种新型超快二维红外（2D IR）光谱和从头算分子动力学模拟，对浓盐溶液和酸溶液中离子与水的动力学、结构和相互作用进行实验和理论研究。以往关于浓离子溶液中水动力学的研究都是用稀离子溶液的氧-氘（OD）延伸作为振动探针。然而，较短的振动寿命（1.8皮秒）使得不可能观察到在浓盐溶液中发生的相对缓慢的过程。本研究采用了长寿命的振动探针，例如甲基硫氰酸酯的CN延伸，其相对较窄的吸收光谱使得获得酸和盐溶液的详细信息成为可能。正在对浓盐和酸溶液进行二维红外实验。二维红外化学交换光谱直接测量质子在酸溶液中的跳跃时间。偏振选择性二维红外（PS2DIR）实验测量了水分子的取向弛豫。新的实验与高水平从头算分子动力学模拟相结合，这是与教授合作进行的。斯坦福大学的托马斯·马克兰和华盛顿州立大学的奥罗拉·克拉克。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Water is the most important liquid on Earth. It is involved in a vast number of technological applications as well as being fundamental to biology. Our understanding of water, at least in its pure form, is quite extensive. Water molecules interact with each other via hydrogen bonding, where a partially positively charged hydrogen atom on one molecule is attracted to a partially negatively charged oxygen on another molecule. Hydrogen bonding causes liquid water to have a significantly ordered structure, even ice-like in places. Most of the time in nature and human-conceived technologies, water has other chemicals in it, and when the concentration of these chemicals is high, much of what we know about pure water no longer applies. Instead of connecting with each other, water molecules become attracted to the positive and negative ions of the salt or acid species. In this project funded by the Chemical Structure, Dynamics and Mechanisms-A Program of the Division of Chemistry, Professor Michael Fayer and his students at Stanford University are employing a two-dimensional infrared laser spectroscopy technique (2D-IR) to explore the structure and dynamics of aqueous solutions containing high concentrations of salts and acids. Infrared spectroscopy reveals the vibrational motions of molecules. 2D-IR reveals how one molecule’s vibrations are affected by vibrations of nearby molecules, or the motions of nearby ions. This project uses ultrafast laser pulses, a tenth of a trillionth of a second, to make direct measurements on the dynamics and structure of salt and acid solutions. Ultrafast 2D-IR can also reveal how water molecules reorient themselves when they encounter other molecules or ions. Although the structure and dynamics of concentrated acid and salt solutions are very important to living systems and many industrial technologies, there is a great deal that remains to learn about them. Two graduate students are involved in this research project. In addition to gaining experience in advanced laser techniques, they are also being training in theory and computational techniques that aid in the interpretation of the experimental data.Aqueous electrolyte solutions are important do to their ubiquity in chemistry, biology, and industrial applications such as fuel cells, water desalination and battery technology. Ion solvation structure, ion clustering and dynamics, and the dynamics of the water hydrogen bond networks are among the interesting aspects of salt solutions. Concentrated electrolyte solutions are sometimes referred to as “water-in-salt.” In these solutions, the hydrogen bond network among water molecules is severely disturbed. The ions cannot be fully solvated by water molecules and form pairs, solvent mediated pairs, and clusters. The crowded ionic environment, which produces strong electric fields, will restrain the motions of water molecules. This project involves experimental and theoretical investigations of the dynamics, structure, and interaction of ions and water in concentrated aqueous salt solutions and acid solutions using various types of new ultrafast two-dimensional infrared (2D IR) spectroscopies and ab-initio molecular dynamics simulations. Previous studies of water dynamics in concentrated ionic solutions used the oxygen-deuterium (OD) stretch of dilute HOD as the vibrational probe. However, the short vibrational lifetime (1.8 picoseconds) makes it impossible to observe relatively slow processes that occur in concentrated salt solutions. This study employs long-lived vibrational probes, e.g., the CN stretch of methylthiocyanate, whose relatively narrow absorption spectra makes it possible to obtain detailed information on acid and salt solutions. 2D IR experiments are being conducted on both concentrated salt and acid solutions. 2D IR chemical exchange spectroscopy directly measures the proton hopping time in acid solutions. Polarization selective 2D IR (PS2DIR) experiments measure water molecule orientational relaxation. The new experiments are being combined with high level ab initio molecular dynamics simulations, which are being performed in collaboration with Profs. Thomas Markland of Stanford University and Aurora Clark of Washington State University.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.