Spectroscopy and Dynamics of Cation Solvation in Clusters

University Of Georgia Research Foundation Inc, Athens GA

Investigators

Abstract

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.