Methoxymethanol formation starting from CO hydrogenation

¹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: he@mpia.de
³ Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
⁴ Research Laboratory for Astrochemistry, Ural Federal University, Kuibysheva St. 48, 620026 Ekaterinburg, Russia
⁵ Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
⁶ Current address: Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands
⁸ School of Electronic Engineering and Computer Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK
⁹ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁰ National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
¹¹ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA

Received: 11 October 2021
Accepted: 11 December 2021

Abstract

Context. Methoxymethanol (CH₃OCH₂OH) has been identified through gas-phase signatures in both high- and low-mass star-forming regions. Like several other C-, O-, and H-containing complex organic molecules (COMs), this molecule is expected to form upon hydrogen addition and abstraction reactions in CO-rich ice through radical recombination of CO hydrogenation products.

Aims. The goal of this work is to experimentally and theoretically investigate the most likely solid-state methoxymethanol reaction channel – the recombination of CH₂OH and CH₃O radicals – for dark interstellar cloud conditions and to compare the formation efficiency with that of other species that were shown to form along the CO-hydrogenation line. We also investigate an alternative hydrogenation channel starting from methyl formate.

Methods. Hydrogen atoms and CO or H₂CO molecules were co-deposited on top of predeposited H₂O ice to mimic the conditions associated with the beginning of “rapid” CO freeze-out. The formation of simple species was monitored in situ using infrared spectroscopy. Quadrupole mass spectrometry was used to analyze the gas-phase COM composition following a temperature-programmed desorption. Monte Carlo simulations were used for an astrochemical model comparing the methoxymethanol formation efficiency with that of other COMs.

Results. The laboratory identification of methoxymethanol is found to be challenging, in part because of diagnostic limitations, but possibly also because of low formation efficiencies. Nevertheless, unambiguous detection of newly formed methoxymethanol has been possible in both CO+H and H₂CO+H experiments. The resulting abundance of methoxymethanol with respect to CH₃OH is about 0.05, which is about six times lower than the value observed toward NGC 6334I and about three times lower than the value reported for IRAS 16293B. Astrochemical simulations predict a similar value for the methoxymethanol abundance with respect to CH₃OH, with values ranging between 0.03 and 0.06.

Conclusions. We find that methoxymethanol is formed by co-deposition of CO and H₂CO with H atoms through the recombination of CH₂OH and CH₃O radicals. In both the experimental and modeling studies, it is found that the efficiency of this channel alone is not sufficient to explain the observed abundance of methoxymethanol with respect to methanol. The rate of a proposed alternative channel, the direct hydrogenation of methyl formate, is found to be even less efficient. These results suggest that our knowledge of the reaction network is incomplete or involving alternative solid-state or gas-phase formation mechanisms.