Date of Award

2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemistry

First Advisor

Marsha I. Lester

Second Advisor

Jessica M. Anna

Abstract

Direct detection of the carbon centered hydroperoxyalkyl radicals (•QOOH) transiently formed in the oxidation of volatile organic hydrocarbons in atmospheric and combustion environments has long been elusive. This thesis demonstrates the jet-cooled stabilization, infrared spectral signature, and energy-dependent unimolecular decay dynamics of a prototypical •QOOH, •CH2(CH3)2COOH, which would result from isobutane oxidation. •QOOH is generated in the laboratory by H atom abstraction from a methyl group of tert-butyl hydroperoxide ((CH3)3COOH, TBHP) using photolytically generated •Cl atoms. The infrared (IR) fingerprint of •QOOH is obtained by IR action spectroscopy utilizing OH laser induced fluorescence (LIF) detection. •QOOD (•CH2(CH3)2COOD), a partially deuterated analog, is also studied using partially deuterated TBHP, (CH3)3COOD, with OD LIF detection. Spectral features characterizing •QOOH/D are observed in the 2950-7050 cm-1 range at energies that lie below and above the transition state barrier leading to OH/D and cyclic ether products. The observed features include fundamental OH and CH stretch transitions, combination bands involving OH/D or CH stretch and a low frequency mode, and overtone OH/D and CH stretch transitions. Most of the observed •QOOH/D IR transitions are readily distinguished from TBHP, including the overtone OH stretch of TBHP characterized in this work. Energy-resolved rates for the unimolecular decay of •QOOH/D to OH/D, recorded by IR pump-UV probe time-delay measurements, are determined ranging from 3.2 ± 1.0 × 106 s-1 to ≥2.6 ± 0.5 × 108 s-1. The observed rates prompted state-of-the-art electronic structure calculations to characterize the transition state (TS) barrier region and bring computed Rice-Ramsperger-Kassel-Marcus rates in accord with experiment. A TS barrier of 10.3 kcal mol-1 is established for •QOOH with heavy atom tunneling along the reaction pathway enhancing the unimolecular dissociation rates. Partial deuteration in •QOOD results in a small increase in the TS barrier to 10.5 kcal mol-1 due to changes in zero-point energies. Comparison of experimental and theoretically predicted rates confirm that unimolecular decay is enhanced by heavy-atom tunneling along the reaction pathway for both •QOOH and •QOOD.

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