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Kinetics and Mechanism of the Tropospheric Oxidation of Vinyl Acetate Initiated by OH Radical: A Theoretical Study
Vinyl acetate [VA (CH<sub>3</sub>COOC<sub>2</sub>H<sub>3</sub>)]
is an important unsaturated and oxygenated volatile organic compound
responsible for atmospheric pollution. In this work, possible reaction
mechanisms for the degradation of OH-initiated atmospheric oxidation
of VA are investigated. The potential energy surfaces (PESs) for the
reaction of OH radical with VA in the presence of O<sub>2</sub> and
NO have been studied using the M06-2X/6-311++GÂ(d,p) method. The initial
addition reactions of more and less substituted ethylenic C-atoms
of VA are treated separately, followed by a conventional transition
state theory (TST) calculation for reaction rates. The direct H-abstraction
mechanism and kinetics have also been studied. The initial OH addition
occurs through a prereactive complex, and the calculated rate constants
in the temperature range 250–350 K for both the addition reactions
are found to have negative temperature dependence. The calculation
indicates that the reaction proceeds predominantly via the addition
of OH radical to the double bond rather than the direct abstraction
of H-atoms in VA. IM1 [CH<sub>3</sub>CÂ(O)ÂO<sup>•</sup>CHCH<sub>2</sub>OH] and IM2 [CH<sub>3</sub>CÂ(O)ÂOCHÂ(OH)<sup>•</sup>CH<sub>2</sub>], the OH adduct complexes formed initially, react with ubiquitous
O<sub>2</sub> followed by NO before their rearrangement. The formation
of the prereactive complex plays an important role in reaction mechanism
and kinetics. The calculated rate constant, <i>k</i><sub>298K</sub> = 1.61 × 10<sup>–11</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, is well harmonized with the
previous experimental data, <i>k</i><sub>298K</sub> = (2.48
± 0.61) × 10<sup>–11</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> (Blanco et al.) and <i>k</i><sub>298K</sub> = (2.3 ± 0.3) × 10<sup>–11</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> (Picquet-Varrult et al.). Additionally, consistent and reliable
enthalpies of formation at 298.15 K (Δ<sub>f</sub><i>H</i>°<sub>298.15</sub>) have been computed for all the species involved
in the title reaction using the composite CBS–QB3 method. The
theoretical results confirm that the major products are formic acetic
anhydride, acetic acid, and formaldehyde in the OH-initiated oxidation
of VA in the presence of O<sub>2</sub> and NO, which are in excellent
agreement with the experimental findings