Spectroscopy and Kinetics of Atmospheric Reservoir Species: HOONO, CH₃C(O)OONO₂, CH₃OOH, and HOCH₂OOH
<p>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). Reser...
| Summary: | <p>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.</p>
<p>Two conformers, cis-cis and trans-perp HOONO, are identified in the 2<sub>vOH</sub> 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.</p>
<p>The overtone initiated photodissociation of PAN is studied in the 3<sub>vOH</sub> and 4<sub>vOH</sub> regions. No photodissociation is observed experimentally; statistical modeling is employed to estimate the importance of this process in PAN.</p>
<p>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.</p>
<p>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.</p>
<p>Lastly, an atmospheric search for HOONO, likely the most atmospherically important of the four molecules studied herein, is proposed and outlined.</p>
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