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 ¥CH2OCH2OOH to two CH2dO 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 ¥CH2OCH2OOH dissociation. We also investigate short-time dynamics involved in chain branching. First, we examine the isomerization transition state of ¥OOCH2OCH2OOH f HOOCH2OCœ HOOH. The latter species is predicted to be a short-lived metastable radical that decomposes within 500 fs to hydroperoxymethyl formate (HPMF; HOOCH2OC(dO)H) and the first ¥OH of chain branching. The dissociation of HOOCH2OCœ HOOH 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 HOOCH2OCœ HOOH recross back to ¥OOCH2OCH2OOH because the isomerization is endothermic, with only an 8 kcal/mol barrier to recrossing. Therefore, som