Kinetics and Mechanism of the Tropospheric Oxidation of Vinyl Acetate Initiated by OH Radical: A Theoretical Study

Vinyl acetate [VA (CH3COOC2H3)] 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-2 and NO have been studied using the M06-2X/6-311++G(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 [CH3C(O)(OCHCH2OH)-C-center dot] and IM2 [CH3C(O)OCH(OH)(CH2)-C-center dot], the OH adduct complexes formed initially, react with ubiquitous O-2 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, k(298K) = 1.61 X 10(-11) cm(3) molecule(-1) s(-1), is well harmonized with the previous experimental data, k(298K) = (2.48 +/- 0.61) X 10(-11) cm(3) molecule(-1) s(-1) (Blanco et al.) and k(298K) = (2.3 +/- 0.3) X 10(-11) cm(3) molecule(-1) s(-1) (Picquet-Varrult et al.). Additionally, consistent and reliable enthalpies of formation at 298.15 K (Delta H-f degrees(298.15)) 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-2 and NO, which are in excellent agreement with the experimental findings.