Hydrogen Abstraction by the Nighttime Atmospheric Detergent NO3·: Fundamental Principles
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The nitrate radical (NO3·) was first identified as early as the 1881, but its role in atmospheric oxidation has only been identified within recent decades. Due to its high one-electron reduction potential and its reactivity toward a diverse set of substrates, it dominates nighttime atmospheric oxidation and has since been the subject of much work. Despite this, studies on NO3· hydrogen atom transfer reactions have been somewhat neglected in favor of its more reactive oxidative pathways. The first section of the dissertation will highlight the role of substrate structure, solvent effects, and the presence of a polar transition state on NO3· hydrogen abstractions from alcohols, alkanes, and ethers. In this work the acquisition of absolute rate constants from previously unexamined substrates was analyzed alongside a curated list of common organic pollutants degraded through hydrogen atom abstraction. It was found that NO3· reacts with low selectivity through an early polarized transition state with a modest degree of charge transfer. Compared to the gas-phase, condensed-phase reactions experience rate enhancement—consistent with Kirkwood theory—as a result of the polarized transition state. These insights are then applied to abstractions by NO3· from carboxylic acids in the next section. It was found that the rate constants for abstraction of α-carbons were diminished through induction by the adjacent carbonyl compared to the activation seen for the aforementioned substrates. The deactivation of abstraction by the carbonyl was found to be dramatically reduced as the substrate's alkyl chain was lengthened and/or branched. This apparent change in mechanism coincides with hydrogen abstraction of the alkyl chain for sufficiently large carboxylic acids and rules out the possibility of concerted bond breaking elsewhere in the molecule. Finally, the dissertation will cover some additional projects related to the overall nature of the work including examination of the kinetics of radical clock systems when complexed with metal ions and the examination of a highly oxidative biosourced monomer.
General Audience Abstract
The nitrate radical (NO3·) was first identified as early as the 1881, but its role in the breakdown of atmospheric pollutants has only been identified within recent decades. Operating primarily at night, NO3· serves as a major atmospheric oxidant—it breaks down pollutants by reactions that involve the removal of electrons from those substrates. This chemistry is particularly important in understanding the consequences of an increasingly industrialized world and the subsequent short-term health and environmental implications. Geographically, these reactions will occur in large concentrations near locations that contribute greatly to atmospheric pollution, such as above coal-powered plants, heavily industrialized areas, above the canopy of large forests, and immediately behind the engines of airplanes as they move through the sky. The proximity of these locations to large population centers has caused the pollutants to greatly impact human health. These contaminants have been linked to several of the leading global causes of death, such as ischemic heart disease, stroke, and respiratory illnesses. The first section of the dissertation will focus on the role of pollutant structure, the medium in which the reaction occurs, and the development of a charged complex when NO3· reacts with alcohols, alkanes, and ethers. These substrates are often found as the result of incomplete combustion when burning fuel or as products of even more sustainable biodiesels. In this work the exact rate constants were found for substrates that were previously unexamined and compared with similar known reaction rates. It was found that NO3· has a low preference for what it reacts with and passes through a modestly charged complex early in the reaction. Compared to gaseous reactions, reactions in a liquid environment proceed faster due to the formation of a charged complex. This was then applied to reactions with carboxylic acids in the next section. Carboxylic acids are often found in large concentrations above the canopy of large forests resulting from the oxidation of isoprenes that are naturally released from broad-leaf trees. It was found that these reactions were slower than reactions with alkanes as the development of the charged complex was inhibited due to the presence of an adjacent dipole. When the carboxylic acid was longer and/or more branched, the formation of the charged complex was no longer inhibited as the reaction site moved further from the dipole. A change in reaction pathway was observed when the acids were sufficiently large. This ruled out the possibility of the reaction occurring simultaneously with a fracturing and rearrangement elsewhere. Finally, the dissertation will cover some additional projects that share some overlap with the work already described including the study of the rates of radical clock systems in the presence of metal ions and the study of naturally sourced monomers that are prone to losing electrons.
- Doctoral Dissertations