A high-level theoretical study of the C₂H₄ + OH reaction

The reaction of ethylene (C₂H₄) with the hydroxyl radical (OH) plays a vital role in both hydrocarbon combustion and atmospheric chemistry. In this theoretical study, this reaction was investigated using the high-accuracy HEAT protocol combined with pragmatic two-dimensional master equation (2DME) analysis and semiclassical transition state theory (SCTST). The calculated HEAT reaction enthalpies show excellent agreement with benchmark ATcT values, falling well within the nominal uncertainty of ca. 0.2 kcal mol^-1 of the HEAT protocol for stable molecules. Two primary reaction mechanisms were confirmed: OH-addition and direct H-abstraction. Under ambient atmospheric conditions, OH-addition, yielding the thermalized stabilization product HOCH₂CH₂, is the major pathway, whereas direct H-abstraction dominates at combustion temperatures, with the transition found to occur around 800-850 K. By adjusting the HEAT-calculated energy of the inner transition state for the OH-addition step (TS1) by -0.32 kcal mol^-1, nearly all available experimental datasets for pressure-dependent falloff curves (1-1000 Torr) at room temperature, as well as the available pressure-dependent (0.01-100 atm) rate data as a function of temperature (600-2000 K), were successfully reproduced, generally within 20%. Vinyl alcohol produced via an addition/elimination mechanism was characterized under high-temperature flame conditions with an estimated yield of 10 ± 2%, which aligns well with literature values. Finally, production of the formaldehyde + CH₃ radical and the acetaldehyde + H-atom by fast tunneling was theoretically predicted and characterized for the first time as the principal-if not only-pathways at the very low temperatures of the interstellar medium.