Experiment and theory on methylformate and methylacetate kinetics at high temperatures: Rate constants for H-atom abstraction and thermal decomposition

Abstract

The shock tube technique was used to study the high temperature thermal decomposition of methylformate (MF) and methylacetate (MA). The formation of H-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range of 1194–1371K at pressures ∼0.5atm. The H-atom profiles were simulated using a detailed chemical kinetic mechanism for MF and MA thermal decomposition. The simulations were used to derive rate constants for sensitive decomposition and H-abstraction reactions in MF and MA. In methylformate, the most sensitive reactions that determine H-atom profiles are: (A)CH3OC(O)H→HCO2+CH3(B)CH3OC(O)H+H→CH3OCO+H2 where H is formed from HCO2→H+CO2. In methylacetate the most sensitive reactions affecting H-atom formation are: (C)CH3OC(O)CH3→CH3+OC(O)CH3(D)CH3OC(O)CH3+H→CH2OC(O)CH3+H2 Minor sensitivity was observed for the energetically higher lying bond fission, (E)CH3OC(O)CH3→CH3+CH3OCO and H-atom abstraction from MA by CH3 through, (F)CH3OC(O)CH3+CH3→CH2OC(O)CH3+CH4(G)CH3OC(O)CH3+CH3→CH3OC(O)CH2+CH4 Unlike MF, where H-atoms are formed instantaneously at high-temperatures from (A), in MA, H-atoms form from the CH3 radicals (through CH3+CH3→C2H4+2H) generated primarily through the C–O bond fission channel (C) with minor contributions from (E). A master equation analysis was performed using CCSD(T)/cc-pv∞z//B3LYP/6-311++G(d,p) energetics and molecular properties for all thermal decomposition processes in MF and MA. The theoretical predictions were found to be in good agreement with the present experimentally derived rate constants for the bond fissions. TST calculations employing CCSD(T)/cc-pv∞z//MP2/aug-cc-pvtz energies and molecular properties for reactions (B) and (D) (the only sensitive abstraction processes in MF and MA) are in good agreement with the experimental rate constants. The theoretically derived rate constants for these processes can be represented by modified Arrhenius expressions for the bond fissions at 0.5atm over the T-range 1000–2000K and for the bimolecular abstractions over the 500–2000K regime. kA(T)=9.79×1068T-15.95exp(-57,434K/T)s-1kB(T)=5.67×10-19T2.50exp(-3188K/T)cm3molecule-1s-1kC(T)=1.42×1084T-19.60exp(-63,608K/T)s-1kD(T)=1.18×10-18T2.58exp(-3714K/T)cm3molecule-1s-1kE(T)=1.90×1082T-19.30exp(-64,724K/T)s-1 Our theoretical predictions for MA+CH3 give over the T-range 500–2000K, kF(T)=2.12×10-25T3.93exp(-4440K/T)cm3molecule-1s-1kG(T)=3.40×10-25T3.88exp(-4149K/T)cm3molecule-1s-1 To our knowledge this is the first study providing experimentally derived rate constant values for the primary bond fission and abstraction reactions in MF and MA.