Methanol Formation via Oxygen Insertion Chemistry in Ices

Abstract

We present experimental constraints on the insertion of oxygen atoms into methane to form methanol in astrophysical ice analogs. In gas-phase and theoretical studies this process has previously been demonstrated to have a very low or non-existent energy barrier, but the energetics and mechanisms have not yet been characterized in the solid state. We use a deuterium UV lamp filtered by a sapphire window to selectively dissociate O 2 within a mixture of O 2 :CH 4 and observe efficient production of CH 3 OH via O( 1 D) insertion. CH 3 OH growth curves are fit with a kinetic model, and we observe no temperature dependence of the reaction rate constant at temperatures below the oxygen desorption temperature of 25K. Through an analysis of side products we determine the branching ratio of icephase oxygen insertion into CH 4 : ∼65% of insertions lead to CH 3 OH with the remainder leading instead to H 2 CO formation. There is no evidence for CH 3 or OH radical formation, indicating that the fragmentation is not an important channel and that insertions typically lead to increased chemical complexity. CH 3 OH formation from O 2 and CH 4 diluted in a CO-dominated ice similarly shows no temperature dependence, consistent with expectations that insertion proceeds with a small or nonexistent barrier. Oxygen insertion chemistry in ices should therefore be efficient under low-temperature ISM-like conditions, and could provide an important channel to complex organic molecule formation on grain surfaces in cold interstellar regions such as cloud cores and protoplanetary disk midplanes.

ALMA Detection of Interstellar Methoxymethanol (CH3OCH2OH)

“…Electronically excited species like O( 1 D) can form in cosmic ray-irradiated dust-grain ice-mantles, where they quickly react with a neighboring species or are quenched by the solid . Indeed, Bergner et al (2017) recently found evidence that an analogous reaction,…”

A theoretical investigation of the reaction between the amidogen, NH, and the ethyl, C2H5, radicals: a possible gas-phase formation route of interstellar and planetary ethanimine

“…In all cases, a possible conclusion is that, if no other, yet unexplored, formation routes of ethanimine are actually contributing, the observed ethanimine is mostly produced in the gas-phase (where the process is controlled by kinetics and provides almost identical populations of the two isomers), with a much smaller contribution from the grain-surface (where the thermal population of the ethanimine adsorbed on ice would be E-/Z-about 10^5 at 30 K). Alternatively, non-thermal ice chemistry could be at work (see, for instance,Frigge et al 2018 andBergner et al 2017). More constraints on the observation and additional experimental or theoretical data on the possible formation routes of the two isomers (such as the determination of the population of the two isomers in hydrogenation experiments of acetonitrile) are strongly needed to finally solve the puzzle.The results obtained for the title reaction are also of interest in the chemistry of the atmosphere of Titan, the massive moon of Saturn, which is characterized by a large fraction of nitrogen and a small percentage of methane and higher hydrocarbons.…”

Unusual Chemical Processes in Interstellar Chemistry: Past and Present

“…There are two values of R comp R photo N(i) for each molecule i undergoing photodissociation to form O ( 1 D): one for external radiation and the other for cosmicray-induced radiation (Carder et al, 2021). The particular reaction with methane (CH 4 ) has been studied in the laboratory, and produces methanol and formaldehyde (Bergner et al, 2017). The results can be reproduced assuming that after photodissociation, the newly formed metastable oxygen instantaneously reacts with methane lying adjacent to or under the O ( 1 D) to form methanol and formaldehyde (Carder et al, 2021).…”