Spectroscopy and Dynamics of Cation Solvation in Clusters

化学中的许多重要反应，从工业过程（电池操作、金属薄膜的电化学电镀）到生物系统的功能（光合作用、细胞呼吸），都涉及溶解在水溶液中的带电离子。这些离子与水分子在原子和分子水平上的相互作用决定了它们在溶液中的移动速度和反应性，从而影响了所涉及的化学反应的结果和效率。由于离子与溶液中许多溶剂分子相互作用的复杂性，很难理解这些过程以优化它们或预测变化条件（如浓度或温度）的影响。在这个由化学部门化学结构动力学和机理（CSDM-A）项目资助的项目中，Michael Duncan教授使用新技术来制造和研究被称为“簇”的小分子组装，这些小分子组装包含嵌入在一些水溶剂分子中的单个离子。这些簇的组成是通过使用质谱仪选择附着在特定数量的水分子上的离子来控制的。这些离子的结构和围绕它们的溶剂网络是通过测量簇吸收的红外光的独特模式来确定的。对包含单个水分子到数十个或更多的簇的研究允许系统地研究离子周围的初始溶剂组织和最终发展的液体溶液的全部复杂性。此外，这项工作还训练学生设计和操作真空系统和质谱仪，以及使用激光进行光学实验。与埃默里大学（Emory University）的一名少数族裔教员合作，有利于招收少数族裔学生。该项目研究了阳离子溶剂簇，其中电荷载体要么是质子化的分子物种（例如，质子化的乙炔或二氧化碳），要么是过渡金属原子，溶剂通常是水。M+(H2O)n配合物是通过脉冲激光烧蚀产生的，而质子化离子是在脉冲放电中产生的。离子在分子束中通过超音速膨胀冷却，用飞行时间光谱仪进行质量分析和选择，并用红外激光光解光谱进行研究。将测量的振动模式与计算化学预测的不同异构体构型的振动模式进行比较，以确定这些簇的结构以及这种结构如何随溶剂分子数量的变化而变化。对非谐波振动理论方法的性能进行了评价。离子光碎片成像实验研究了这些体系中的键能学和电荷转移过程。对离子溶剂化的分子水平理解是化学和生物学的普遍挑战。分子离子的结构及其对溶剂化的反应是目前理论水平的严重障碍。从我们的实验基准数据是有价值的计算研究人员，因为他们验证和完善他们的高级非调和理论方法。邓肯实验室的结果也可能影响溶液或其界面的分子动力学模拟。小质子化离子在星际化学中也很有趣；数据可以指导星际离子识别所需的更高分辨率测量，并建议考虑新的离子。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。

Many important reactions in Chemistry, from industrial processes (battery operation, electrochemical plating of metal films) to the function of biological systems (photosynthesis, cell respiration) involve charged species known as ions dissolved in water solutions. The interactions between these ions and water molecules at the atomic and molecular level determine the rate at which they move in solution and their reactivity, which affect the outcomes and efficiency of the chemistry involved. Because of the complexity of ion interactions with the many solvent molecules in solution, it is difficult to understand these processes to optimize them or to predict the effects of changing conditions such as concentration or temperature. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Michael Duncan addresses these challenges using new techniques to make and study small molecular assemblies known as "clusters" containing individual ions imbedded in a few solvent molecules of water. The composition of these clusters is controlled by selecting ions attached to a specific number of water molecules using a mass spectrometer. The structure of these ions and the solvent network surrounding them is determined by measuring the distinctive patterns of infrared light that the clusters absorb. Studies on clusters containing a single water molecule up to dozens or more allow systematic investigation of the initial solvent organization around the ion and the eventual development of the full complexity of the liquid solution. Additionally, the work trains students in the design and operation of vacuum systems and mass spectrometers, as well as optical experiments using lasers. A partnership with a minority faculty member at Emory University facilitates the recruitment of minority students.The project investigates cation-solvent clusters in which the charge carrier is either a protonated molecular species (e.g., protonated acetylene or CO2) or a transition metal atom, and the solvent is usually water. M+(H2O)n complexes are produced by pulsed laser ablation, whereas protonated ions are produced in a pulsed discharge. The ions are cooled via supersonic expansion in a molecular beam, mass-analyzed and selected with a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy. The measured vibrational patterns are compared to those predicted by computational chemistry for different isomeric configurations to determine the structures of these clusters and how this changes with the number of solvent molecules present. The performance of anharmonic vibrational theory methods is also evaluated. Ion photofragment imaging experiments investigate the bonding energetics and charge transfer processes in these systems. The molecular level understanding of ion solvation is a universal challenge for Chemistry and Biology. The structures of molecular ions and how they respond to solvation represent severe obstacles for present levels of theory. Benchmark data from our experiments are of value to computational researchers as they validate and refine their high-level anharmonic theory methods. The results from the Duncan laboratory may also influence molecular dynamics simulations of solutions or their interfaces. Small protonated ions are also interesting for interstellar chemistry; data can guide higher resolution measurements needed for interstellar ion identification and suggest new ions for consideration.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.