Cavity Ringdown Spectroscopy, Kinetics, and Quantum Chemistry of Atmospherically Relevant Reactions

Abstract

This thesis describes laboratory experiments and electronic structure calculations on three chemical systems relevant to tropospheric ozone chemistry: peroxynitrous acid (HOONO), hydroxymethylperoxy radical formed from HO2 + HCHO (R1), and products of alkoxy isomerization (R2). R1 and R2 were studied experimentally using a gas flow cell that combined UV photolysis with cavity ringdown spectroscopy (CRDS). All chemical systems were studied using electronic structure calculations and kinetics modeling. HOONO is one of the products of the reaction OH + NO2, and acts as a temporary reservoir for HOx and NOx in the atmosphere. Torsional excitation of HOONO will break its internal hydrogen bond, leading to sequence band formation in the OH stretch spectrum. Chapter 3 describes a calculated 3-dimensional potential energy surface to examine torsional mode coupling and sequence band formation. We apply these results to previous CRDS kinetics studies of HOONO. The reaction of HO2 with carbonyls is believed to be a major sink of HOx and carbonyl compounds at reduced temperatures. R1 is the simplest of these reactions. Despite numerous previous studies, considerable uncertainty exists on the activation energy and rate constant of R1. Chapters 4-6 describe CRDS and electronic structure studies on the isomerization product, hydroxymethylperoxy. CRDS was used to make the first measurements of the OH stretch and A-X electronic spectra, and the kinetics of hydroxymethylperoxy chemistry. Electronic structure calculations were used to simulate the spectroscopic bands and examine the conformers of hydroxymethylperoxy and 2-hydroxyisopropylperoxy. Atmospheric alkoxy radicals can isomerize or react with O2, and each pathway has a different impact on ozone chemistry. Chapters 7-10 describe cavity ringdown spectroscopy, kinetics, and electronic structure calculations on the n-butoxy and 2-pentoxy isomerization products, specifically δ-HOC4H8•, δ-HOC4H8OO•, δ-HO-1-C5H10•, and δ-HO-1-C5H10OO•. CRDS was used to make the first measurements of the A-X electronic spectrum of δ-HOC4H8OO• and clean OH stretch spectra of all four radicals. Relative kinetics data previously obtained using CRDS were reanalyzed to include the effects of additional alkoxy reactions. Electronic structure calculations were performed to explain the observations that the OH stretch absorption cross section differs between HOR• and HOROO•.