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

Abstract

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.

Molecular synthesis in ices triggered by dissociative electron attachment to carbon monoxide

“…The electron energy-dependent formation of products upon electron irradiation of condensed molecular ices was monitored by post-irradiation thermal desorption spectrometry (TDS). The experimental procedures were described in detail previously [16][17][18][19][20][21][22][23]. Briefly, all experiments were performed in an ultrahigh vacuum chamber with a base pressure of 10 −10 mbar.…”

“…Molecular ices were prepared by leaking vapours of the reactants onto a Ta substrate (this work and Refs. [19][20][21][22][23]) or an Au substrate (Refs. [16][17][18][19]) held at ∼ 30-35 K either as a gas mixture or sequentially, the latter relying on rapid diffusion into pores of the first component.…”

“…The figure does not represent the relative yields of individual fragments from same molecule but merely serves to illustrate to resonance positions. Tick marks on the y-axis indicate the vertical offset [17], C 2 H 4 /H 2 O [18], as well as C 2 H 4 /CH 3 OH [21] ices. The actual stochiometric ratio of the two reactants in the ice can be deduced by integrating the area under Fig.…”

“…To this end, we present new results on mixed CO and NH 3 ices that reveal the role of DEA to CO in electron-induced chemistry by monitoring the formation of hydrogen cyanide HCN. Furthermore, we compare the results on CO/H 2 O and CO/CH 3 OH ices [22,23] with the data on mixed ices where CO is replaced by C 2 H 4 [18,21] to demonstrate that a resonance around 10 eV is only visible when CO is present. We thus show that DEA to CO plays an important role in electron-induced syntheses in molecular ices and propose mechanisms for the reactions that are initiated by the fragments resulting from DEA.…”

“…In an effort to unravel electron-induced chemistry in ices, we have recently conducted comprehensive studies by post-irradiation thermal desorption spectrometry (TDS) on thin layers of different pure and mixed ices [20][21][22][23]. In each case, the dependence of product formation on electron energy was monitored for several products.…”