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

Abstract

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 O₂ 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)­O^•CHCH₂OH] and IM2 [CH₃C­(O)­OCH­(OH)^•CH₂], the OH adduct complexes formed initially, react with ubiquitous O₂ 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>_298K = 1.61 × 10^–11 cm³ molecule^–1 s^–1, is well harmonized with the previous experimental data, <i>k</i>_298K = (2.48 ± 0.61) × 10^–11 cm³ molecule^–1 s^–1 (Blanco et al.) and <i>k</i>_298K = (2.3 ± 0.3) × 10^–11 cm³ molecule^–1 s^–1 (Picquet-Varrult et al.). Additionally, consistent and reliable enthalpies of formation at 298.15 K (Δ_f<i>H</i>°_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 O₂ and NO, which are in excellent agreement with the experimental findings