First-principles-derived kinetics of the reactions involved in low-temperature dimethyl ether oxidation

Spin-polarised density functional theory (DFT-B3LYP) energies, harmonic vibrational frequencies, and moments of inertia are used to construct modified Arrhenius rate expressions for elementary steps in chain-propagation and chain-branching pathways for dimethyl ether combustion. Barrierless reactions were treated with variational RRKM theory, and global kinetics were modeled using master equation and perfectly stirred reactor simulations. Our kinetics analysis suggests that the bottleneck along the chain propagation path is the isomerisation of CH3OCH2OO, contrary to earlier interpretations. Comparing the rate constants for competing decomposition pathways of the key chain-branching intermediate hydroperoxymethyl formate (HPMF), we find that formation of formic acid and the atmospherically relevant Criegee intermediate (CH2OO) via a H-bonded adduct may be more favourable than O-O bond scission. Since the latter forms a source of a second OH radical beyond that supplied in chain propagation, which is necessary for explosive combustion, the more favourable pathway to formic acid may inhibit autoignition of the fuel. We predict that the HPMF O-O scission product, OCH2OC(=O)H, most likely directly dissociates to HCO + HC(=O)OH. This implies an overabundance of CO at 550-700 K, since HC(=O)OH is a fairly stable product in this temperature range and facile H abstraction from HCO leads to CO. We find that CO2 product yields are sensitive to the creation of CH2OO and that creation of CH2OO is competitive with the O-O scission reaction.