Theoretical study on the combustion kinetics of trimethylamine and the key intermediate N-methylmethanimine

The combustion kinetics of trimethylamine (TMA) are systematically studied based on the high-level ab initio calculations. Reaction pathways include the direct C–N and C–H bond fissions, intramolecular H-shift, and H-abstraction by five small radicals (H/CH₃/OH/NH₂/HO₂) for TMA, as well as subsequent TMA radical isomerization and decomposition. The potential energy profiles are explored at the CCSD(T)/cc-pVxZ(x = T, Q) level and the results reveal that N-methylmethanimine (MMI) is the key intermediate for TMA decomposition. Therefore, the H-abstraction reaction kinetics of MMI with H/CH₃/OH/NH₂/HO₂ radicals are also investigated. The atomization method is further adopted to determine the standard enthalpy of formation of each species, showing good agreement with the available literature results. For the H-abstraction reactions of TMA/MMI + H/CH₃/OH/NH₂/HO₂, multi-structural variational transition state theory combined with small-curvature tunneling approximation (MS-CVT/SCT) is employed to obtain the high-pressure limit (HPL) rate constants and the conventional transition state theory (TST) rate constants in MS-CVT/SCT method are obtained by CCSD(T)/cc-pVxZ(x = T, Q) energies. For the unimolecular reactions of TMA decomposition, Rice-Ramsberger-Kassel-Marcus/Master-Equation (RRKM/ME) theory is used to obtain the pressure-dependent rate constants. The calculated rate constants are also in good agreement with the available experimental results and the comparison of rate constants for four MMI formation channels shows that the β-scissions of (CH₃)₂NCH₂ to form MMI + CH₃ and CH₃NCH₃ to form MMI + H are dominant. Due to the lack of experimental data for TMA under combustion, the performance of H-abstraction reactions of MMI + H/CH₃/OH/NH₂/HO₂ is evaluated by updating a dimethylamine kinetic model and then assessed against the measured ignition delay time. It shows that the updated kinetic model can better reproduce the experimental measurements. The comprehensive kinetic details presented are invaluable for the development of kinetic model for TMA and the refinement of kinetic models for other secondary and tertiary amines.