A theoretical study on the isomerization and decomposition reaction kinetics of small unsaturated methyl esters: Methyl acrylate, methyl butenoate and methyl crotonate radicals

Reaction kinetics of radical isomerization and decomposition of three small unsaturated methyl esters of methyl acrylate (MA, C 4 H 6 O 2 ), methyl butenoate (MB, C 5 H 8 O 2 ) and methyl crotonate (MC, C 5 H 8 O 2 ) are systematically studied with high-level quantum chemistry computation. MB and MC are isomers with the C =C double bonds at different positions. The potential energy profiles for these reactions are obtained at the DLPNO-CCSD(T)/CBS(TQ)//M062X/ma-TZVP level of theory, and the thermodynamics of the involved species are derived at the CCSD (T)/CBS(T-Q) level of theory using the atomization enthalpy method. Our results show that isomerization reactions with stable cyclic transition states have lower reaction barrier heights, making them energetically and kinetically favored. For decomposition reactions, C - C beta - scission reactions are more energetically favorable than C - H beta - scission reactions, and the C - O beta - scission reactions to produce HCHO are the most energetically favorable. The related rate constants are calculated via solving the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation at 300 - 2500 K and over a pressure range of 0.01 - 100 atm along with the high-pressure limit. It is shown that isomerization reaction dominates at low and intermediate temperatures; while as the temperature increases, the importance of decomposition reaction becomes apparent. Pressure is found to have a significant impact on the studied reactions. Additionally, the computed rate constants of this work agree well with the available theoretical results but differ significantly from the estimated results in the literatures. Kinetic reaction mechanisms are further updated, and the results show that through incorporating the presently computed rate constants and thermodynamics, the experimental species concentration distributions can be better reproduced. This work provides the necessary rate constants and thermodynamics for the kinetic model construction of MA, MB, and MC and are expected to help establish more accurate chemical kinetic models of small unsaturated methyl esters, enriching our understanding of the combustion chemistry of biodiesel.