A theoretical kinetics study of the reactions of methylbutanoate with hydroperoxyl radical

Methyl butanoate (MB) is a representative surrogate fuel for biodiesel and its combustion chemistry has been gaining increasing interest in recent years. This work presents an ab initio chemical kinetics study of hydrogen abstraction reactions of MB with hydroperoxyl radical (HO₂). Important addition reactions followed by isomerization and β-scission reactions were also investigated. The geometry optimization and vibrational frequency calculation of all the molecular structures on the potential energy surface of MB + HO₂ were conducted with Density Functional Theory with B3LYP functional and 6-311++G(d, p) basis set. Higher level stationary point energies were obtained at the level of QCISD(T)/CBS. Intrinsic reaction coordinate (IRC) calculation was performed to validate all the connections between transition states and local minima. The transition state theory with the asymmetric Eckart tunneling correction was used to calculate the high-pressure limit rate constants over a board range of temperature (500-2500 K). Phenomenological rate coefficients for temperatureand pressure-dependent reactions were calculated by the time-dependent multiple-well master equation. Low-frequency torsional modes of reactants and transition states were replaced by one-dimensional hindered rotors. For the torsional modes that are strongly coupled to one another, Truhlar's internal-coordinate multi-structural method was employed to calculate the partition function. The predicted rate constants were compared with the available theoretical data and are expected to be useful in updating the present combustion modeling of methyl butanoate.