Abstract Pyrolysis and low-temperature oxidation of dimethoxymethane (methylal, MeOCH2OMe) play an important role in the ignition of blended diesel fuels, but the underlying mechanisms are still debated. In these kinetic models, bimolecular hydrogen abstraction or unimolecular C–O bond fission are considered as the primary initial steps, while MeOCH2OMe isomerization is sometimes disregarded. In this work, we investigate the pyrolysis of MeOCH2OMe combining imaging photoelectron photoion coincidence spectroscopy with vacuum ultraviolet (VUV) synchrotron radiation and CBS-QB3 theoretical calculations to unveil reaction paths and energetics. In the mass spectrum of MeOCH2OMe, pyrolysis products and radical intermediates were observed at m/z 15 (CH3), 28 (CO), 29 (HCO), 30 (H2CO), 31 (CH2OH), 32 (CH3OH), 45 (CH3OCH2), and 75 (H-loss from methylal). Only the m/z 45 and 75 ions are found to be dissociative photoionization products of MeOCH2OMe, the other mass spectral peaks are attributed to ionization of the neutral MeOCH2OMe pyrolysis products. The m/z 31 peak was assigned to the methoxy radical in the previous studies. However, our photoion mass-selected threshold photoelectron spectrum (ms-TPES) confirms that it originates from dissociative photoionization of the primary pyrolysis fragment methanol. Based on the experimental and computational results, a thermal decomposition mechanism of MeOCH2OMe is proposed. Here, H-migration precedes the production of methoxymethylene (CH3OCH) and methanol, while dimethyl ether and formaldehyde are probably formed in multi-step processes, too. The sequential dissociation of CH3OCH and of dimethyl ether yields enhanced m/z 15, 28 and 29 signals at high temperature. Rate constants have been calculated to confirm the dominant role of MeOCH2OMe isomerization and to help improve predictive combustion models.

中文翻译：

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氢迁移作为二甲氧基甲烷热分解的潜在驱动力：来自热解成像光电子光离子符合光谱和计算的新见解

摘要 二甲氧基甲烷（甲缩醛，MeOCH2OMe）的热解和低温氧化在混合柴油的点火中起重要作用，但其潜在机制仍存在争议。在这些动力学模型中，双分子夺氢或单分子 C-O 键裂变被视为主要的初始步骤，而 MeOCH2OMe 异构化有时被忽略。在这项工作中，我们结合成像光电子光电重合光谱与真空紫外 (VUV) 同步辐射和 CBS-QB3 理论计算来研究 MeOCH2OMe 的热解，以揭示反应路径和能量学。在 MeOCH2OMe 的质谱图中，在 m/z 15 (CH3)、28 (CO)、29 (HCO)、30 (H2CO)、31 (CH2OH)、32 (CH3OH)、45 ( CH3OCH2）和 75（甲缩醛的 H-损失）。只有 m/z 45 和 75 离子被发现是 MeOCH2OMe 的解离光电离产物，其他质谱峰归因于中性 MeOCH2OMe 热解产物的电离。在之前的研究中，m/z 31 峰归属于甲氧基自由基。然而，我们的光离子质量选择阈值光电子谱 (ms-TPES) 证实它起源于初级热解碎片甲醇的解离光电离。基于实验和计算结果，提出了 MeOCH2OMe 的热分解机制。在这里，H 迁移先于甲氧基亚甲基 (CH3OCH) 和甲醇的产生，而二甲醚和甲醛也可能在多步过程中形成。CH3OCH 和二甲醚的连续解离产生增强的 m/z 15，28 和 29 信号在高温下。已计算速率常数以确认 MeOCH2OMe 异构化的主导作用并帮助改进预测燃烧模型。