OH-initiated oxidation of vinyl butyrate: ab initio insights

Abstract

The widespread use of vinyl butyrate (CH₂CHOC(O)CH₂CH₂CH₃ or VB) in the polymer industry and daily-life materials inevitably results in its emission into the atmosphere. Therefore, understanding the mechanism and kinetics of the VB conversion is critical for evaluating its fate and environmental impacts. Herein, we theoretically investigate the chemical transformation of VB initiated by OH radicals in the atmosphere using the stochastic Rice–Ramsperger–Kassel–Marcus (RRKM)-based master equation kinetic model on the potential energy surface explored at the M06-2X/aug-cc-pVTZ level of theory. Showing excellent agreement with limited experimental kinetic data, the VB + OH kinetic model reveals that H-abstraction from C^β (i.e., –C^βH₂CH₃) prevails over the OH-addition to the double bond (CC), even at low temperatures. The detailed analyses, including those of the time-resolved species profiles, reaction rate, and reaction flux, reveal the reaction mechanism shift with temperature (causing the U-shaped temperature dependence of k(T, P)) and the noticeable pressure dependence of k(T, P) at low temperatures. The secondary chemistry under atmospheric conditions (namely, the reaction of the main product with O₂ and its subsequent reactions with NO) was then characterized within the same framework to reveal the detailed kinetic mechanism (e.g., [4-(ethenyloxy)-4-oxobutan-2-yl]oxidanyl (IM12) + NO₂ is the dominant channel under atmospheric conditions), suggesting VB is not a persistent organic pollutant and a new environmental concern regarding the formed NO₂. Also, the kinetic behaviors of vinyl butyrate and its oxidation products were extended from atmospheric to combustion conditions for further applications. Moreover, through TD-DFT calculations, it is shown that several related important species (i.e., 1-(ethenyloxy)-1-oxobutan-2-yl (P4), [4-(ethenyloxy)-4-oxobutan-2-yl]dioxidanyl (IM7), and IM12) can potentially undergo photolysis in the atmosphere.