RUI: Microsolvation and halogen substitution effects in weakly bound complexes and carbocations

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In this project, funded by the Chemical Structure, Dynamics and Mechanism A (CSDM-A) program of the Chemistry Division, Professor Sean Peebles and Professor Rebecca Peebles of Eastern Illinois University are using microwave radiation and computational techniques to study how carbon dioxide (CO2) can, under certain conditions, act like a solvent in much the same manner that water can dissolve other molecules and ions. Highly compressed CO2 behaves as a fluid, and is more environmentally friendly and more versatile than many petroleum-derived solvents used in industrial processes. However, the reasons for CO2's versatility are not well understood. In the Peebles' laboratory, a single molecule of solute is isolated with one or a few CO2 solvent molecules. By probing these small clusters with microwave radiation, the arrangement of solvent molecules can be determined. Systematically varying the number of CO2 molecules builds understanding of its unique properties as a solvent. In a second related area of research, electrically charged molecules (carbocations) are generated and their structures are studied via interactions with microwave radiation. As with the CO2 clusters, this provides information about the arrangement of atoms, in this case helping answer questions about the specific steps by which atoms rearrange during a chemical reaction. Both aspects of this research also utilize computational studies to provide more information about the arrangement of electrons within the chemical structures. The results of this research may advance the use of CO2 as a solvent in important chemical processes.The project uses Fourier-transform microwave spectroscopy for investigating structures of unstable chemical species. A theme is the effect of halogen atom substitution within the chemical structures studied. In CO2 microsolvation studies, clusters are generated using a supersonic expansion into a vacuum chamber. Examination of clusters with varying amounts of fluorine substitution builds towards an understanding of why fluorinated species have unusually high solubility in supercritical CO2. Varying the number of CO2 molecules in a cluster allows observation of how weak intermolecular interactions vary as solvation occurs. The knowledge gained from these studies may also help in understanding and building CO2 sequestration frameworks using custom designed molecules. In the second part of the research, carbocations are generated using a pulsed discharge nozzle or electron gun together with the supersonic expansion. Probing with microwave radiation then provides structural information, helping scientists to understand whether carbocations exist in cyclic or noncyclic forms, and how structures change as halogen substitution is varied. In addition to implications for industrial use of CO2, broader impacts are hands-on education of undergraduate students in Chemistry and other STEM fields. Students gain significant practical, problem solving and computer skills and improve their critical thinking abilities.