Keywords

Criegee intermediate; ozone-assisted oxidation; ozonolysis; combustion; jet-stirred reactor (JSR); photonionization mass spectrometry.

Abstract

Criegee intermediates (CIs) are highly reactive species generated during the ozonolysis of alkenes, exerting a critical influence on atmospheric oxidation, combustion chemistry, and synthetic processes. Their reactivity, encompassing unimolecular and bimolecular pathways, shapes oxidation mechanisms and product distributions in these environments. A key determinant of CI behavior is the structural nature of the parent alkene. Acyclic alkenes typically yield intermediates with minimal steric hindrance, facilitating rapid reactions, whereas endocyclic alkenes produce geometrically constrained CIs with enhanced internal stabilization, redirecting reaction pathways and reducing overall reactivity. Despite their importance, mechanistic understanding remains limited due to experimental challenges in detecting CIs and tracking their transformations. This study investigates the ozone-assisted oxidation of C6 acyclic (trans-2-hexene) and endocyclic (cyclohexene) alkenes to elucidate how alkene structure governs CI stability and reactivity.

Experiments were performed in an atmospheric-pressure jet-stirred reactor coupled with molecular-beam mass spectrometry and tunable synchrotron vacuum-ultraviolet ionization, enabling direct identification of intermediates and products. Under conditions of 700 Torr and 2000 ppm O3, ozonolysis-initiated oxidation at a temperature significantly lower than oxygen-only reactions. Results reveal pronounced structural effects. Trans-2-hexene ozonolysis forms CIs such as acetaldehyde oxide (CH3CHOO) and butanal oxide (CH3CH2CH2CHOO), which undergo both unimolecular and bimolecular reactions, generating higher-molecular-weight compounds and complex networks. In contrast, cyclohexene produces adipaldehyde oxide (CHOCH2CH2CH2CH2CHOO), which primarily fragments and auto-oxidizes into smaller oxygenated species. These differences underscore how ring strain and stabilization in endocyclic alkenes limit reactivity and favor secondary rearrangements.

By clarifying structure–reactivity relationships, this work advances mechanistic understanding of CI chemistry, providing essential insights for refining atmospheric and combustion models. Accurate representation of these processes is vital for predicting oxidation behavior in both environmental and energy-related contexts.

Thesis Completion Year

2024

Thesis Completion Semester

Fall

Thesis Chair

Popolan-Vaida, Denisia

College

College of Sciences

Department

Chemistry

Thesis Discipline

Chemistry

Language

English

Access Status

Open Access

Length of Campus Access

None

Campus Location

Orlando (Main) Campus

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Rights Statement

In Copyright