Microwave Studies of Prereactive Molecular Complexes

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In this project funded by the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) program in the Division of Chemistry, Professor Kenneth Leopold is using state-of-the-art instruments to study the forces that molecules exert on each other. These forces can influence the outcome of chemical reactions, especially for substances dissolved in water where many important chemical reactions take place. The work is accomplished by first studying pairs of gaseous molecules that could react with each other. Then, when additional molecules are added, their effect on the reactive pair can be observed. The instruments used measure how molecules absorb microwaves. The specific set of microwave frequencies that a molecule or cluster absorbs contains accurate information about its structure and about how atoms are bonded together. Beyond fundamental science, this work has potential applications to understanding how aerosol particles form in the Earth’s atmosphere and, thus, on our understanding of climate. Through this research project the students in Professor Leopold’s lab obtain valuable experience in advanced experimental methods and in the rigorous handling of highly accurate experimental data.Rotational spectroscopy is used to elucidate the molecular and electronic structure of bare and microsolvated clusters containing molecules on the verge of chemical change. This offers an interesting venue for studying the relationship between aggregation and reactivity. The rotational spectroscopy is implemented by exploiting both cavity-based and chirped-pulse microwave techniques. Accurate moments of inertia, hyperfine coupling constants, and tunneling frequencies provide information about structure and zero-point vibrational dynamics. The study of microsolvated clusters deepens our understanding of solvation, a critical factor influencing thermodynamics and reactivity in solution. It offers experimental benchmarks for the development of model potentials for theoretical treatments of condensed phase matter and provides chemical quality information that builds an intuitive understanding of the interactions of interest. Simple prototypical systems are investigated with strong focus on complexes subject to proton transfer or hydrolysis. The broader impact of the work is enhanced when model systems are chosen which are pertinent to problems in atmospheric chemistry, particularly the nucleation of atmospheric aerosols.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.