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42568-GB6
Overtone-Initiated Chemistry of Atmospheric Hydroperoxides

Shizuka Hsieh, Smith College

Methyl and ethyl hydroperoxides undergo photodissociation to produce OH radicals in the atmosphere.  While their ultraviolet photochemistry has been well-characterized, less is known about the photochemical processes induced by weak absorption of visible and near-IR radiation.  Our lab has therefore pursued the visible and near-IR photochemistry of these two gas-phase hydroperoxides.  During this second grant period, we completed the first two goals outlined in the proposal for ethyl hydroperoxide.  The first goal entailed characterizing its absorption to vibrationally-excited states involving O-H stretch overtones.  The second entailed characterizing OH radical formation from the vibrationally-excited molecule.  In August 2007, we submitted a manuscript based on this work to the Journal of Physical Chemistry A titled "Vibrational-torsional excitation and direct overtone photodissociation of ethyl hydroperoxide at 5vOH."

Completing work from the previous summer (2006) required quantum mechanical calculations for O-H stretch overtone excitation spectra and statistical predictions of OH rotational distributions.  We compared these calculations against data collected mainly by Kristina Closser (Smith '07), and three main results emerged from this combination of theory and experiment.  First, the overtone excitation spectra were consistent with the presence of two ethyl hydroperoxide conformers in the gas-phase.  The more prominent had a trans conformation for the C-C-O-O backbone, while the higher energy form adopted a gauche geometry.  The estimated energy difference between the two was only ~300 cm-1, so any future atmospheric predictions for ethyl hydroperoxide should consider both conformers.  Second, the OH fragment's rotational quantum states matched predictions from statistical theories, indicating that relatively simple statistical calculations may also be adequate for obtaining kinetic constants necessary for any predictions of their atmospheric dissociation.  Third, we deduced an experimental O-O bond dissociation energy that was lower than values listed in current atmospheric chemistry databases.

Two undergraduate students, both previously funded by this ACS PRF grant, contributed to the submitted work.  The first author, Kristina Closser, started graduate school in Physical Chemistry at the University of California at Berkeley during the Fall of 2007.  The second author, Kristen Vogelhuber is continuing her graduate studies in Physical Chemistry at the University of Colorado at Boulder.  At the end of the second grant period, ACS PRF funding allowed for laser maintenance that will enable the continuation of experimental work involving two students, Geraldine Rodriquez (Smith '11) and Pyae Naing (Smith '10).  Geraldine is a Latina first-generation college student whom I am mentoring through the AEMES (Achieving Excellence in Mathematics, Engineering, and Science) program at Smith College.  Pyae is a second-year Chemistry major from Myanmar.

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