Variational Effect and Anharmonic Torsion on Kinetic Modeling for Initiation Reaction of Dimethyl Ether Combustion

The reaction of dimethyl ether (DME) with molecular oxygen has been considered to be the dominant initiation pathway for DME combustion compared to the C-O bond fission. This work presents a detailed mechanism and kinetics investigation for the O₂ + DME reaction with theoretical approaches. Using the CCSD(T)/6-311+G(2df,2pd) potential energy surface with the M06-2X/MG3S gradient, Hessian, and geometries, rate constants are evaluated by multistructural canonical variational transition-state theory (MS-CVT) including contributions from hindered rotation and multidimensional tunneling over the temperature range 200-2800 K. The CCSD(T) and QCISD(T) with 6-311+G(2df,2pd) calculations predict a barrier of 190-194 kJ mol^-1 for the O₂ + DME reaction based on the optimized structures at various levels. It is proposed that there exists a weakly interacting adducts on the product side with subsequent dissociation to the separate HO₂ and CH₃OCH₂ radicals. Torsions in transition state are found to be significantly coupled to generate four conformations whose contributions do influence the rate constant predictions. Variational effects are observed to be significant at high temperatures, while tunneling effect quickly becomes insignificant with temperature. Finally, four-parameter Arrhenius expression 9.14 × 10¹³(T/300)^-3.15 exp[-184.52(T + 110.23)/(T² + 110.23²)] cm³ mol^-1 s^-1 describes the temperature dependence of MS-CVT rate constants with small-curvature tunneling correction.