Exploring the Kinetics and Reaction Dynamics of Peroxy Radical Unimolecular Decay

Dr. Michael F. Vansco¹, Assistant Professor

Co-PI(s): Dr. Rebecca L. Caravan², Dr. Stephen J. Klippenstein²

1: Coastal Carolina University, Conway, SC 29526

2: Argonne National Laboratory, Lemont, IL 60439

The objectives of this research are to implement innovative experimental and theoretical techniques to study the unimolecular decay of peroxy radicals. Furthermore, the research aligns with the Funding for Accelerated, Inclusive Research (FAIR) funding opportunity announcement goals by establishing a partnership between Argonne National Laboratory and Coastal Carolina University, a primarily undergraduate and emerging research institution. Peroxy radicals are reactive organic intermediates ubiquitous in the chemistry of oxygenated environments. The fate of peroxy radicals is highly dependent on the functionalization of the peroxy radical and the conditions of the local environment, such as temperature, pressure, and co-reactant concentrations. Hydrogen atom migration reactions are critical in the unimolecular decay of peroxy radicals: they impact the propensity for the recycling of hydroxyl radicals (an important oxidant), and play a central role in autoxidation, which leads to molecular weight growth and particle formation. Biomolecular reactions of peroxy radicals can compete with unimolecular decay, affecting tropospheric ozone concentrations and the formation of low-volatility molecules. The competition between peroxy radical reactivity between unimolecular decay and bimolecular reaction is sensitive to the rate of the hydrogen atom migration reactions, which highly depends on the functionalization of the peroxy radical. Recent theoretical investigations have shown that the H-atom migration rates of peroxy radicals with different functional groups can vary by as much as fifteen orders of magnitude. Such a vast range in unimolecular decay rates has potentially important implications for the relative importance of critical pathways in Earth's lower atmosphere and low-temperature combustion, such as autoxidation, particle formation, and autoignition. However, the predicted unimolecular decay rates of functionalized peroxy radical are largely uncharacterized due to experimental challenges. Understanding the unimolecular decay of functionalized peroxy radicals demands a holistic approach that uses multiple experimental and theoretical methods. Therefore, their chemistry will be investigated directly in separate complementary experiments at Coastal Carolina University using a new appartus to study the unimolecular decay of peroxy radicals under jet-cooled and collision-free conditions, and at Argonne National Laboratory under thermal conditions, in which the reactivity of peroxy radicals will be studied at a specific temperature and pressure. In both experiments, the peroxy radicals will be generated and stabilized under conditions that minimize unwanted side chemistry and allow direct observation of their unimolecular decay in fundamental pump-probe laser experiments. Peroxy radicals with carbonyl and carboxylic acid functionalization will be specifically targeted due to low energy barriers for their hydrogen atom migration reactions. The results from the experiments will be compared with state-of-the-art theoretical calculations of the predicted unimolecular decay reaction dynamics and kinetics using methods developed at Argonne National Laboratory. The combination of results from this multidisciplinary approach will provide a complete picture of the unimolecular decay of peroxy radicals and transform our understanding of their chemistry in oxygenated environments, such as the Earth's atmosphere and low-temperature combustion.

This research was selected for funding by the Office of Science Basic Energy Sciences

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