Unravelling the reaction kinetics of ethyl propiolate with OH radicals

Ethyl propiolate (EP) is released into the atmosphere primarily due to anthropogenic activities, particularly through its widespread use in laboratory and industrial syntheses of organic compounds. In this study, the kinetics of the gas-phase reaction between EP and OH radicals were investigated experimentally at 298 K and 700 mbar of N₂ using a relative rate technique. The measured rate constant was determined to be (2.34 ± 0.54) × 10^-13 cm³ per molecule per s. Theoretical calculations were performed to explore the temperature-dependent kinetics over the range of 200-800 K by employing the variational transition state theory (VTST) with high-level single-point energies (SPE). All possible addition and abstraction pathways were explored for two lowest-energy conformers, syn-gauche (R1) and syn-anti (R2). Quantum mechanical tunneling effects were accounted for using both Wigner and Eckart corrections. The computed rate constants exhibited a positive temperature dependence and were fitted to three-parameter Arrhenius expressions (cm³ per molecule per s): k _VTST/Wig = 4.1 × 10^-28 T ^5.2 exp[1230/T] and k _VTST/Eck = 1.3 × 10^-27 T ^5.0 exp[1020/T]. Kinetic analysis indicated that hydrogen abstraction from the secondary carbon (-CH₂-) dominated at lower temperatures, while at higher temperatures, OH addition to the terminal carbon and abstraction from the primary carbon (-CH₃) became competitive. Potential energy surface and thermochemical analyses supported these findings. Atmospheric assessment showed that OH-initiated oxidation was the primary global loss pathway for EP. The calculated global warming potentials (GWPs) indicated a stronger climate impact at shorter time horizons, which decreased at longer times due to the short atmospheric lifetime of EP.