Low Temperature Chlorine-Initiated Oxidation of Small-Chain Methyl Esters: Quantification of Chain-Terminating HO2-Elimination Channels

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• 作者：Giel Muller ; Adam Scheer ; David L. Osborn ; Craig A. Taatjes ; Giovanni Meloni
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：March 17, 2016
• 年：2016
• 卷：120
• 期：10
• 页码：1677-1690
• 全文大小：819K
• ISSN：1520-5215

文摘

Cl-initiated oxidation reactions of three small-chain methyl esters, methyl propanoate (CH₃CH₂COOCH₃; MP), methyl butanoate (CH₃CH₂CH₂COOCH₃; MB), and methyl valerate (CH₃CH₂CH₂CH₂COOCH₃; MV), are studied at 1 or 8 Torr and 550 and 650 K. Products are monitored as a function of mass, time, and photoionization energy using multiplexed photoionization mass spectrometry coupled to tunable synchrotron photoionization radiation. Pulsed photolysis of molecular chlorine is the source of Cl radicals, which remove an H atom from the ester, forming a free radical. In each case, after addition of O₂ to the initial radicals, chain-terminating HO₂-elimination reactions are observed to be important. Branching ratios among competing HO₂-elimination channels are determined via absolute photoionization spectra of the unsaturated methyl ester coproducts. At 550 K, HO₂-elimination is observed to be selective, resulting in nearly exclusive production of the conjugated methyl ester coproducts, methyl propenoate, methyl-2-butenoate, and methyl-2-pentenoate, respectively. However, in MV, upon raising the temperature to 650 K, other HO₂-elimination pathways are observed that yield methyl-3-pentenoate and methyl-4-pentenoate. In each methyl ester oxidation reaction, a peak is observed at a mass consistent with cyclic ether formation, indicating chain-propagating OH loss/ring formation pathways via QOOH intermediates. Evidence is observed for the participation of resonance-stabilized QOOH in the most prominent cyclic ether pathways. Stationary point energies for HO₂-elimination pathways and select cyclic ether formation channels are calculated at the CBS-QB3 level of theory and assist in the assignment of reaction pathways and final products.