Insight into selected reactions in low-temperature dimethyl ether combustion from Born-Oppenheimer molecular dynamics

Dimethyl ether is under consideration as an alternative diesel fuel. Its combustion chemistry is as yet ill-characterized. Here we use Born-Oppenheimer molecular dynamics (BOMD) based on DFT-B3LYP forces to investigate the short-time dynamics of selected features of the low-temperature dimethyl ether (DME) oxidation potential energy surface. Along the chain propagation pathway, we run BOMD simulations from the transition state involving the decomposition of ^̇CH ₂OCH ₂OOH to two CH ₂=O and an ^̇OH radical. We predict that formaldehyde C-O stretch overtones are excited, consistent with laser photolysis experiments. We also predict that O-H overtones are excited for the ^̇OH formed from ^̇CH ₂OCH ₂OOH dissociation. We also investigate short-time dynamics involved in chain branching. First, we examine the isomerization transition state of ^̇OOCH ₂OCH ₂OOH → HOOCH ₂OĊHOOH. The latter species is predicted to be a short-lived metastable radical that decomposes within 500 fs to hydroperoxymethyl formate (HPMF; HOOCH ₂OC(=O)H) and the first ^̇OH of chain branching. The dissociation of HOOCH ₂OĊHOOH exhibits non-RRKM behavior in its lifetime profile, which may be due to conformational constraints or slow intramolecular vibrational energy transfer (IVR) from the nascent H-O bond to the opposite end of the radical, where O-O scission occurs to form HPMF and ^̇OH. In a few trajectories, we see HOOCH ₂OĊHOOH recross back to ^̇OOCH ₂OCH ₂OOH because the isomerization is endothermic, with only an 8 kcal/mol barrier to recrossing. Therefore, some inhibition of chain-branching may be due to recrossing. Second, trajectories run from the transition state leading to the direct decomposition of HPMF (an important source of the second ^̇OH radical in chain branching) to HĊO, ^̇OH, and HC(=O)OH show that these products can recombine to form many other possible products. These products include CH ₂OO + HC(=O)OH, H ₂O + CO + HC(=O)OH, HC(=O)OH + HC(=O)OH, and HC(=O)C(=O)H + H ₂O, which (save CH ₂OO + HC(=O)OH) are all more thermodynamically stable than the original HĊO + ^̇OH + HC(=O)OH products. Moreover, the multitude of extra products suggest that standard statistical rate theories cannot completely describe the reaction kinetics of significantly oxygenated compounds such as HPMF. These secondary products consume the second ^̇OH required for explosive combustion, suggesting an inhibition of DME fuel combustion is likely.