Thermodynamics of the hydroxyl radical addition to isoprene.

Authors:
Dr Marco A Allodi, PhD
Dr Marco A Allodi, PhD
The University of Chicago
Yen Postdoctoral Scholar
Laser Spectroscopy, Molecular Dynamics, Biophysics, Astrochemistry
Chicago, IL | USA

J Phys Chem A 2008 Jul 4;112(30):7064-71. Epub 2008 Jul 4.

Department of Chemistry, Center for Molecular Design, Hamilton College, 198 College Hill Road, Clinton, NY 13323, USA.

Oxidation of isoprene by the hydroxyl radical leads to tropospheric ozone formation. Consequently, a more complete understanding of this reaction could lead to better models of regional air quality, a better understanding of aerosol formation, and a better understanding of reaction kinetics and dynamics. The most common first step in the oxidation of isoprene is the formation of an adduct, with the hydroxyl radical adding to one of four unsaturated carbon atoms in isoprene. In this paper, we discuss how the initial conformations of isoprene, s-trans and s-gauche, influences the pathways to adduct formation. We explore the formation of pre-reactive complexes at low and high temperatures, which are often invoked to explain the negative temperature dependence of this reaction's kinetics. We show that at higher temperatures the free energy surface indicates that a pre-reactive complex is unlikely, while at low temperatures the complex exists on two reaction pathways. The theoretical results show that at low temperatures all eight pathways possess negative reaction barriers, and reaction energies that range from -36.7 to -23.0 kcal x mol(-1). At temperatures in the lower atmosphere, all eight pathways possess positive reaction barriers that range from 3.8 to 6.0 kcal x mol(-1) and reaction energies that range from -28.8 to -14.4 kcal x mol(-1).

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Source
http://dx.doi.org/10.1021/jp801869cDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2488963PMC

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July 2008
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