Session: Physical Chemistry Poster:

The ozonolysis of unsaturated hydrocarbons is known to form ozonides, which are in chemical equilibrium with the corresponding combinations of carbonyl and Criegee intermediate products. Criegee intermediates formed by ozonolysis are a major pathway by which alkenes and alkynes in the atmosphere—especially anthropogenic and biogenic alkenes—are mineralized to inorganic compounds. Ozonolysis of these compounds also results in the formation of hydroxyl radicals, which allow for the chemical breakdown of other pollutants present in the atmosphere. The main objective of the current research is to determine the statistical branching of the ozonide into its possible Criegee intermediates and their subsequent decay to the final ozonolysis products. This provides insight into the true yield of hydroxyl radicals in the atmosphere from unsaturated hydrocarbons.

Firstly, the study of the ozonolyses of ethylene, propylene, and isoprene was conducted to explore alkyl-substituent effects on the product branching, as well as the comparison of the effects of syn- and anti-isomers of Criegee intermediates on the overall yield of the hydroxyl radical. Syn-alkyl-substituted Criegee intermediates can undergo proton transfer, which may then decay to the hydroxyl and corresponding alkenoxy radicals. In some cases, the anti-isomer is not able to form a quasi-cyclic transition state in the same way as the syn-isomer, resulting in no proton transfer. Additionally, this proton transfer may be realized via proton tunneling. This drove the development of a computational method introducing proton tunneling in classical dynamics models, accounting for significant tunneling effects of such intermediates. The Criegee intermediate resulting from ethylene undergoes a different mechanistic pathway via dioxirane.

Finally, the ozonolyses of α- and β-pinene were studied to compare the dynamics of traditional and endocyclic Criegee intermediates. This is particularly important as the ozonolysis of α-pinene gives an endocyclic Criegee intermediate with more internal energy than a traditional Criegee intermediate. Initial computational results also point to the possibility of some ozonolysis reactions occurring in sequence rather than in concert, as it is typically considered.

This work aims to systematically explore the unimolecular decay dynamics of Criegee intermediates with increasing molecular complexity to determine dominant geometric and electronic effects associated therewith.

Mr. Spencer J Leger

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