Temperature and Pressure Dependent Rate Coefficients for the Reaction of C₂H₄ + HO₂ on the C₂H₄O₂H Potential Energy Surface

The potential energy surface (PES) for reaction C₂H₄ + HO₂ was examined by using the quantum chemical methods. All rates were determined computationally using the CBS-QB3 composite method combined with conventional transition state theory­(TST), variational transition-state theory (VTST) and Rice–Ramsberger–Kassel–Marcus/master-equation (RRKM/ME) theory. The geometries optimization and the vibrational frequency analysis of reactants, transition states, and products were performed at the B3LYP/CBSB7 level. The composite CBS-QB3 method was applied for energy calculations. The major product channel of reaction C₂H₄ + HO₂ is the formation C₂H₄O₂H via an OH^···π complex with 3.7 kcal/mol binding energy which exhibits negative-temperature dependence. We further investigated the reactions related to this complex, which were ignored in previous studies. Thermochemical properties of the species involved in the reactions were determined using the CBS-QB3 method, and enthalpies of formation of species were compared with literature values. The calculated rate constants are in good agreement with those available from literature and given in modified Arrhenius equation form, which are serviceable in combustion modeling of hydrocarbons. Finally, in order to illustrate the effect for low-temperature ignition of our new rate constants, we have implemented them into the existing mechanisms, which can predict ethylene ignition in a shock tube with better performance