Infrared Spectroscopy and Unimolecular Dynamics of Transient Criegee Intermediates and Hydroperoxyalkyl Radicals Yujie Qian

Format:

Book

Thesis/Dissertation

Author/Creator:

Qian, Yujie, author.

Contributor:

University of Pennsylvania. Chemistry., degree granting institution.

Language:

English

Subjects (All):

0431.

0485.

0490.

0494.

Local Subjects:

0431.

0485.

0490.

0494.

Physical Description:

1 electronic resource (552 pages)

Contained In:

Dissertations Abstracts International 87-07B

Place of Publication:

Ann Arbor : ProQuest Dissertations and Theses, 2025

Language Note:

English

Summary:

Alkene ozonolysis and alkane oxidation are major degradation pathways for atmospheric volatile organic compounds (VOCs), which proceed through highly reactive, transient intermediates. This thesis presents a direct spectroscopic and kinetic investigation of two critically important classes of these intermediates: zwitterionic Criegee intermediates (CI, R1R2C=O+O-) formed in alkene ozonolysis, and carbon-centered hydroperoxyalkyl radicals (⦁QOOH) generated in hydrocarbon oxidation in the atmosphere and low-temperature combustion (LTC). Laboratory detection of these elusive species is achieved using new synthetic approaches combined with supersonic jet-cooling. Infrared (IR) action spectroscopy is employed to obtain the spectral fingerprints and energy- and time-resolved unimolecular decay dynamics of the CI and ⦁QOOH intermediates with detection of hydroxyl (OH) radical products, which are probed by ultraviolet (UV) laser-induced fluorescence (LIF). The two CIs studied, formaldehyde oxide (CH2OO) and 2-butenal oxide, which are generated via photolysis of diiodo precursors in the presence of O2, undergo unimolecular decay via novel reaction mechanisms with extremely different rates. Vibrational activation of CH2OO in the overtone CH stretch region results in remarkably slow unimolecular decay to OH products at energies below the transition state (TS) barrier for the rate-limiting 1,3 ring-closure step. The observed pronounced nonstatistical kinetic behavior is attributed to slow intramolecular vibrational energy redistribution (IVR) and quantum mechanical (QM) tunneling through the TS barrier. In contrast, IR excitation of 2-butenal oxide in the fundamental CH stretch region promotes extremely rapid, QM tunneling-enhanced allylic 1,6 H-transfer via a 7-membered ring TS, again yielding OH products. The ⦁QOOH intermediates studied, originating from cyclohexane, cyclopentane, and isopentane oxidation, are generated by H-atom abstraction from corresponding organic hydroperoxide precursors and are directly observed. Spectral features associated with OH stretch, CH stretches, and combination bands are identified with vibrational activation, in each case inducing unimolecular decay to OH products. The isomer- and energy-resolved unimolecular decay rates provide the first experimental test of the structure, stability, and TS barriers associated with these ⦁QOOH intermediates and are in good accord with complementary high-level calculations. These direct observations provide fundamental insights into the reaction dynamics of transient CI and ⦁QOOH intermediates. The experimentally measured IR spectral signatures and energy-dependent unimolecular decay rates offer a deeper understanding of their reaction pathways and TS properties, providing critical benchmarks for state-of-the-art theoretical calculations and enhancing our knowledge of atmospheric and LTC chemistry

Notes:

Advisors: Lester, Marsha I. Committee members: Francisco, Joseph S.; Saven, Jeffery G.; Baumgart, Tobias

Source: Dissertations Abstracts International, Volume: 87-07, Section: B.

Ph.D. University of Pennsylvania 2025

Vendor supplied data

Local Notes:

School code: 0175

ISBN:

9798276007441

Access Restriction:

Restricted for use by site license