Hybrid Density Functional Theory Predictions of Low-Temperature Dimethyl Ether Combustion Pathways. II. Chain-Branching Energetics and Possible Role of the Criegee Intermediate

In part I, we discussed the chain-propagating and possible competing mechanisms of low-temperature (300−1000 K) dimethyl ether (DME) combustion. Here we consider the chain-branching mechanism that results in explosive combustion, initiated by O 2 addition to the ·CH 2 OCH 2 OOH intermediate formed in the earlier chain-propagation step. Ideally, chain-branching leads to the formation of two highly reactive ·OH radicals from the ·OOCH 2 OCH 2 OOH precursor. Each of these two ·OH radicals can initiate a chain-reaction “branch” with another DME molecule, which, ideally, leads to the formation of four more ·OH, and so on. This exponential increase in ·OH concentration causes an exponential increase in the DME oxidation rate, leading to explosive combustion. Here we show that although the pathway to create the first ·OH from ·OOCH 2 OCH 2 OOH in a hydrogen-transfer isomerization step is unambiguous, the formation of the second ·OH from the remaining hydroperoxyformate (HPMF or HOOCH 2 OC( O)H) fragment is potentially very complicated. HPMF has many possible fates, including HĊO + formic acid (HC( O)OH) + ·OH; H 2 O + formic acid anhydride (HC( O)OC( O)H); the Criegee intermediate (·CH 2 OO·) + formic acid; peroxyformic acid (HC( O)OOH) + H 2 + CO; dihydroxymethylformate ((HO) 2 HCOC( O)H); ·OCH 2 OC( O)H + ·OH; and quite possibly others. The first and last of these products derived from HPMF directly produce ·OH and thus can complete the chain-branching step. Activation energies of 42−44 kcal/mol are needed to overcome barriers to form these two sets of products from HPMF. While these pathways directly form ·OH, they may not be the most favorable. The formation of a Criegee intermediate (·CH 2 OO·)−formic acid hydrogen-bonded adduct requires ∼15 kcal/mol less enthalpy than paths directly producing ·OH. Formation of the Criegee intermediate has never been considered as an intermediate in DME combustion before, but its formation (along with formic acid) appears to be the most favorable unimolecular path for HPMF decomposition. In atmospheric chemistry, decomposition of vibrationally excited ·CH 2 OO· can potentially lead to ·OH formation. Thus, we propose ·CH 2 OO· as a new intermediate that may significantly contribute to dimethyl ether's chain-branching mechanism.