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、电镀金属膜)到生物系统的功能(光合作用、细胞呼吸)都涉及到被称为离子的带电物种溶解在水溶液中。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移动的速度和它们的反应性，这影响了所涉及的化学的结果和效率。由于离子与溶液中许多溶剂分子相互作用的复杂性，很难理解这些过程来优化它们，也很难预测浓度或温度等变化条件的影响。在这个由化学系化学结构动力学和机理(CSDM-A)计划资助的项目中，Michael Duncan教授使用新技术来解决这些挑战，以制造和研究被称为“簇”的小分子组件，该组件包含嵌入在几个水溶剂分子中的单个离子。这些团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。这些离子和它们周围的溶剂网络的结构是通过测量这些团簇吸收的红外光的独特模式来确定的。对含有几十个或更多单个水分子的团簇的研究，可以系统地研究离子周围的初始溶剂组织，并最终发展出液体溶液的全部复杂性。此外，这项工作还培训学生真空系统和质谱仪的设计和操作，以及使用激光的光学实验。与埃默里大学少数族裔教员的合作关系促进了少数族裔学生的招募。该项目研究的是阳离子-溶剂簇，其中的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，而溶剂通常是水。脉冲激光烧蚀产生M+(H2O)n络合物，而脉冲放电产生质子化离子。离子在分子束中通过超声膨胀冷却，用飞行时间光谱仪进行质量分析和选择，并用红外激光光解离光谱进行研究。将测量的振动模式与计算化学预测的不同同分异构体的振动模式进行比较，以确定这些团簇的结构以及这种结构如何随存在的溶剂分子的数量而变化。并对非简谐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成键能级和电荷转移过程。分子水平上对离子溶剂化的理解是化学和生物学面临的普遍挑战。分子离子的结构和它们如何对溶剂化反应是目前理论水平的严重障碍。我们实验的基准数据对于计算研究人员来说是有价值的，因为他们验证和改进了他们的高级非谐理论方法。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。小的质子化离子对星际化学也很有趣；数据可以指导星际离子识别所需的更高分辨率的测量，并建议考虑新的离子。这一奖项反映了NSF的法定任务，并通过使用基金会的智力优势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.