A Kinetic Study of Novel Gas-Phase Reaction Pathways for Pyruvic Acid and Hydrogen Halides with Hox Radicals.

Church, Jonathan Richard

Graduate Thesis Or Dissertation

A Kinetic Study of Novel Gas-Phase Reaction Pathways for Pyruvic Acid and Hydrogen Halides with Hox Radicals. Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/pk02c983n

Abstract

Hydroperoxy (HO₂) and hydroxyl (OH) radicals are known to play important roles in both combustion and atmospheric chemistry. Although less reactive than hydroxyl radicals, HO₂ has been found to be important to the ignition process in combustion systems and has been found in greater concentrations than that of OH in the troposphere. In this work, gas-phase reactions between HO₂ and hydrogen halides (HX, X=F, Cl, Br, I) are examined due to the catalytic effect halides can have in the ozone destruction process, as well as their ability to inhibit flame speeds under combustion settings. During the study of the abstraction reaction, a novel double exchange process between HO₂ and hydrogen halides was discovered. These new exchange reactions were found to have barriers significantly lower than the corresponding abstraction process due to a highly structured transition state which is characterized as a planar ring. The kinetics of the abstraction and the exchange process were also compared. It was found that due to a competition between entropic and energetic effects the abstraction process dominates at high temperatures and the exchange reaction is more significant at low temperatures.

The kinetics of gas-phase reactions between HO₂ and OH with pyruvic acid were also examined. Pyruvic acid is an important keto-acid intermediate formed from the oxidation pathway of isoprene. This molecule is unique due to slow oxidation rates by OH and the ability to absorb in the UV-vis region. These characteristics lead to photolysis being the primary channel for degradation in the troposphere. In this work, accurate rate coefficients are determined for use in atmospheric models and compared to experimental estimates. Due to the number of possible hydrogen transfer reactions in pyruvic acid, reactions with hydroperoxy radicals were also examined to see if barrier lowering effects could play a role under atmospheric conditions. It was found that although HO₂ mediation produced a catalytic effect on the reactions barriers, the high energy products of the reactions made this effect negligible on the rate constants.

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