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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry