O2-Dependence of reactions of 1,2-dimethoxyethanyl and 1,2-dimethoxyethanylperoxy isomers

Abstract

Reaction mechanisms of $\dot{R}$ and RO$\dot{O}$ radicals derived from low-temperature oxidation of 1,2-dimethoxyethane (CH₃O(CH₂)₂OCH₃) were investigated using speciation from multiplexed photoionization mass spectrometry (MPIMS) measurements via Cl-initiated oxidation, in conjunction with electronic structure calculations. The experiments were conducted at 5 bar, from 450 K – 650 K, and O₂ concentrations from 1 · 10¹⁴ cm^–3 – 6 · 10¹⁸ cm^–3 to probe the effects on competing reaction channels of 1,2-dimethoxyethanyl ($\dot{R}$) and 1,2-dimethoxyethanylperoxy (RO$\dot{O}$) isomers. Several species were detected with photoionization spectral fitting – ethene, formaldehyde, methyl vinyl ether, and 2-methoxyacetaldehyde – and, as determined by electronic structure calculations, may form via unimolecular decomposition of 1,2-dimethoxyethanyl or 1,2-dimethoxyethanylperoxy. O₂-dependent yield ratios show that the formation pathways for all species undergo a competition between O₂-addition and unimolecular decomposition. Adiabatic ionization energies were also calculated and utilized along with exact mass determinations to infer contributions for other species derived exclusively from first- and second-O₂-addition, including 1,2-dimethoxyethene, cyclic ethers, and dicarbonyls.

In addition to species formed from conventional low-temperature oxidation pathways, an important conclusion is derived from the detection of species produced from an O₂-addition step involving ĊH₂CH₂OCH₃ ($\dot{R}{}^{\prime }$), which forms via prompt dissociation of the primary 1,2-dimethoxyethanyl radical (ĊH₂O(CH₂)₂OCH₃). Species derived from $\dot{R}{}^{\prime }$ + O₂ – 1,3-dioxolane and methyl acetate – were detected at [O₂] = 1.2 · 10¹⁷ cm^–3 and formed on timescales parallel to the main $\dot{R}$ + O₂ reactions. In addition, ion signal at m/z 106 was detected and increased with O₂ concentration from which connections are drawn to ketohydroperoxides produced by $\dot{Q}{}^{\prime }$OOH + O₂. Detection of such species indicate that β-scission of 1,2-dimethoxyethanyl is sufficiently facile such that timescales of $\dot{R}{}^{\prime }$ + O₂ compete with conventional $\dot{R}$ + O₂ pathways.