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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Hybrid Density Functional Theory Predictions of Low-Temperature Dimethyl Ether Combustion Pathways. II. Chain-Branching Energetics and Possible Role of the Criegee Intermediate
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Hybrid Density Functional Theory Predictions of Low-Temperature Dimethyl Ether Combustion Pathways. II. Chain-Branching Energetics and Possible Role of the Criegee Intermediate

机译：低温二甲基醚燃烧路径的混合密度泛函理论预测。二。分支链的能量学和Criegee中间体的可能作用

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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_2OCH_2OOH intermediate formed in the earlier chain-propagation step. Ideally, chain-branching leads to the formation of two highly reactive ·OH radicals from the ·OOCH_2OCH_2OOH 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_2OOH in a hydrogen-transfer isomerization step is unambiguous, the formation of the second ·OH from the remaining hydroperoxyformate (HPMF or HOOCH_2OC (=O)H) fragment is potentially very complicated. HPMF has many possible fates, including HCO + formic acid (HC(=O)OH) + ·OH; H_2O + formic acid anhydride (HC(=O)OC(=O)H); the Criegee intermediate (·CH_2OO·) + formic acid; peroxyformic acid (HC(=O)OOH)+H_2 + CO; dihydroxymethylformate (HO)_2HCOC(=O)H); ·OCH_2OC(=OH) + ·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_2OO·)-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_2OO· can potentially lead to ·OH formation. Thus, we propose ·CH_2OO· as a new intermediate that may significantly contribute to dimethyl ether's chain-branching mechanism.
机译：在第一部分中，我们讨论了低温（300-1000 K）二甲醚（DME）燃烧的链增长和可能的竞争机理。在这里，我们考虑导致爆炸燃烧的链支化机制，该机制是由O_2添加到在较早的链增长步骤中形成的·CH_2OCH_2OOH中间体而引发的。理想地，链支化从·OOCH_2OCH_2OOH前体形成两个高反应性·OH自由基。这两个·OH自由基中的每一个均可与另一个DME分子引发链反应“分支”，理想情况下，这会导致另外四个·OH的形成，依此类推。 ·OH浓度的这种指数增加导致DME氧化速率呈指数增加，从而导致爆炸性燃烧。在这里，我们表明，尽管在氢转移异构化步骤中从·OOCH_（2-）OCH_2OOH生成第一个·OH的途径是明确的，但从剩余的氢过氧甲酸酯（HPMF或HOOCH_2OC（= O） H）片段可能非常复杂。 HPMF有许多可能的命运，包括HCO +甲酸（HC（= O）OH）+·OH; H_2O +甲酸酐（HC（= O）OC（= O）H）; Criegee中间体（·CH_2OO·）+甲酸;过氧甲酸（HC（= O）OOH）+ H_2 + CO;二羟甲基甲酸酯（HO）_2HCOC（= O）H）; ·OCH_2OC（= OH）+·OH;还有其他可能。这些源自HPMF的产品的第一个和最后一个直接产生·OH，因此可以完成链支化步骤。需要42-44 kcal / mol的活化能来克服形成HPMF的两组产品的障碍。尽管这些途径直接形成·OH，但它们可能不是最有利的。 Criegee中间体（·CH_2OO·）-甲酸氢键加合物的形成所需的焓比直接生成·OH的途径少约15 kcal / mol。 Criegee中间体的形成以前从未被视为DME燃烧的中间体，但其形成（连同甲酸）似乎是HPMF分解的最有利的单分子途径。在大气化学中，振动激发的CH_2OO·的分解可能会导致·OH的形成。因此，我们提出·CH_2OO·作为一种可能对二甲醚的链支化机理有重要贡献的新中间体。

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