Spectroscopy and Kinetics of Atmospheric Reservoir Species: HOONO, CH₃C(O)OONO₂, CH₃OOH, and HOCH₂OOH

Abstract

This thesis reports experimental and theoretical studies of the spectroscopy and kinetics of four atmospheric reservoir species: peroxynitrous acid (HOONO), peroxyacetyl nitrate (PAN, CH₃C(O)OONO₂), methyl hydroperoxide (MHP, CH₃OOH), and hydroxymethyl hydroperoxide (HMHP, HOCH₂OOH). Reservoir species are so named because they temporarily or permanently sequester reactive radicals (such as OH, HO₂, or NO₂), reducing the oxidative strength of the atmosphere and allowing transport of pollutants to remote regions. Two conformers, cis-cis and trans-perp HOONO, are identified in the 2vOH region by vibrational overtone initiated photodissociation spectroscopy, and the isomerization barrier from the less stable trans-perp to cis-cis HOONO is determined experimentally, statistically, and ab initio to be ~ 40 kJ/mol. This low barrier indicates that only cis-cis HOONO is atmospherically important. The complex vibrational spectroscopy of cis-cis HOONO is assigned with the aid of a simple two-dimensional OH-stretch/torsion coupling model of the planar, partially hydrogen-bound molecule. Combined with nonuniform quantum yield, this model explains the major features in the cis-cis HOONO spectrum. Its application to the fundamental region suggests an upward adjustment of the atmospherically important HOONO/HONO₂ product branching ratio in the OH + NO₂ association reaction. The rotational spectrum and dipole moment of cis-cis HOONO and DOONO are measured in the submillimeter region to characterize the molecular structure of HOONO and enable a quantitative atmospheric search. The overtone initiated photodissociation of PAN is studied in the 3vOH and 4vOH regions. No photodissociation is observed experimentally; statistical modeling is employed to estimate the importance of this process in PAN. The UV photodissociation of MHP and HMHP is studied in the 300 – 350 nm region and extrapolated to 400 nm to calculate total UV photolysis rates. The overtone initiated photodissociation of HMHP is studied in the 4vOH and 5vOH regions. The rich spectroscopy of this two-OH-chromophore molecule is assigned with the help of a one-dimensional anharmonic oscillator model on each OH stretch of three ab initio identified HMHP conformers. This modeling allows estimation of the (unknown) dissociation threshold for HMHP. Lastly, an atmospheric search for HOONO, likely the most atmospherically important of the four molecules studied herein, is proposed and outlined.