Electron-Induced Formation of Ethyl Methyl Ether in Condensed Mixtures of Methanol and Ethylene

Electron-induced reactions in condensed mixtures of ethylene (C₂H₄) and methanol (CH₃OH) lead to the formation of ethyl methyl ether (EME, C₂H₅OCH₃), as shown by post-irradiation thermal desorption spectrometry (TDS). In contrast to the electron-induced reaction between water (H₂O) and C₂H₄, product formation as a consequence of proton transfer following electron attachment (EA) to C₂H₄ is not observed in the analogous reaction between CH₃OH and C₂H₄. However, a resonant process centered around 5.5 eV and a threshold-type increase of product yield starting at 9 eV is observed. On the basis of the presence and absence of particular side products after irradiation of the mixture as well as of the pure parent compounds, reaction mechanisms related to the two energy regimes are proposed. Below the ionization threshold of the reactants, dissociative electron attachment (DEA) to CH₃OH triggers the reaction sequence by producing reactive methoxy radicals, which attack neighboring C₂H₄ molecules. The resulting adduct then abstracts a hydrogen atom to yield EME. Above but near the ionization threshold, electron impact ionization (EI) produces primarily intact molecular cations, which drive the reaction by converting the repulsive Coulomb force between the high electron densities at the reactive sites of the two neutral parent species into an attractive force. This again induces the formation of an adduct between the two reactants that rearranges to the product EME. Fragmentation of the molecular CH₃OH^+• cation into CH₃O⁺, however, may provide an additional reaction pathway toward EME. In this scenario, CH₃O⁺ attacks a neighboring C₂H₄ molecule. The resulting adduct is then neutralized by a thermalized electron and abstracts a hydrogen atom from a nearby CH₃OH molecule to yield EME.