Mechanism and kinetics of the reaction CH₃ + CH₃CHO: Ab initio semiclassical transition state theory study

Abstract

The potential energy profile of the reaction between methyl radicals with acetaldehyde is theoretically investigated at different levels of theory prior to calculating the bimolecular rate constants of this reaction by semiclassical transition state theory (SCTST) and one‐dimensional master equation (1DME) modeling. The stationary points on the potential energy surface of the reaction CH3 + CH3CHO are characterized at the levels MP2/6‐311 + g(2d,2p) and CCD/6‐311 + g(2d,2p). To obtain more satisfying energies, single‐point calculations are performed at CCSD(T)/augh‐cc‐pVTZ+2df. It is shown that the title reaction proceeds via either hydrogen abstraction channels or the addition of methyl radical to acetaldehyde, forming an isopropoxy radical. Then, SCTST and 1DME modeling are used to compare the rate constants of distinct reaction channels. The aldehyde hydrogen atom abstraction by methyl radical producing CH4 + CH3CO is dominant both thermodynamically and kinetically. The calculated SCTST rate constants can be expressed as a function of temperature with cm3molecule−1s−1 (200‐3000 K). Hydrogen/deuterium (H/D) isotopic effects on the kinetics are computed for some specific abstraction channels, showing that tunneling indeed plays a vital role in the hydrogen abstraction process. Pressure dependencies of the addition reaction are followed using master equation calculations, illustrating that the stabilization of the isopropoxy radical is dominant at higher pressures and lower temperatures. In contrast, as pressure decreases and temperature increases, the energized radical prefers decomposing to new bimolecular sets.