Hydrogen transfer reactions of interstellar complex organic molecules

Abstract

Radical recombination has been proposed to lead to the formation of complex organic molecules (COMs) in CO-rich ices in the early stages of star formation. These COMs can then undergo hydrogen addition and abstraction reactions leading to a higher or lower degree of saturation. Here, we have studied 14 hydrogen transfer reactions for the molecules glyoxal, glycoaldehyde, ethylene glycol, and methylformate and an additional three reactions where CH n O fragments are involved. Over-the-barrier reactions are possible only if tunneling is invoked in the description at low temperature. Therefore the rate constants for the studied reactions are calculated using instanton theory that takes quantum effects into account inherently. The reactions were characterized in the gas phase, but this is expected to yield meaningful results for CO-rich ices due to the minimal alteration of reaction landscapes by the CO molecules.We found that rate constants should not be extrapolated based on the height of the barrier alone, since the shape of the barrier plays an increasingly larger role at decreasing temperature. It is neither possible to predict rate constants based only on considering the type of reaction, the specific reactants and functional groups play a crucial role. Within a single molecule, though, hydrogen abstraction from an aldehyde group seems to be always faster than hydrogen addition to the same carbon atom. Reactions that involve heavy-atom tunneling, e.g., breaking or forming a C-C or C-O bond, have rate constants that are much lower than those where H transfer is involved.

Formation of COMs through CO hydrogenation on interstellar grains

“…As mentioned earlier, the majority of our reaction rate constants are based on Álvarez- Barcia et al (2018). One of the most critical reactions to obtain agreement with the experiments is H+H 2 CO.…”

“…The overabundance in our model is hence either due to an overestimation of the branching ratio of HCO + CH 3 O − −− → HCOOCH 3 or due to missing reactions. From quantum chemistry calculations, we know that abstraction reactions such as HCOOCH 3 + H − −− → COOCH 3 + H 2 have very low rates (Álvarez-Barcia et al 2018). Other destruction reactions are not obvious based on the species available in our system.…”

“…The only radical-neutral reaction with a radical other than the H atom that we considered is H 2 CO + CH 3 O − −− → CH 3 OH + HCO. Although Butscher et al (2017) indicated that the reaction between the HCO radical and H 2 CO might also take place, in fact Álvarez- Barcia et al (2018) showed that reactions where heavy atoms play a role in the bond formation are not significantly enhanced by tunneling. In other words, the rate constants for the reactions H 2 CO + CH 3 O − −− → CH 3 OCH 2 O and H 2 CO + HCO − −− → (HCO)CH 2 O are below 10 −9 s −1 at low temperatures.…”

The ALMA-PILS survey: first detection of the unsaturated 3-carbon molecules Propenal (C2H3CHO) and Propylene (C3H6) towards IRAS 16293–2422 B

“…Formation enthalpies of the intermediate species are taken fromGoldsmith et al (2012). Energy barriers of hydrogenation reactions of HC(O)CHO, CH 2 (OH)CHO and CH 3 OCHO are based on studies ofColberg & Friedrichs (2006), Álvarez-Barcia et al (2018,Krim et al (2018) and our calculations. The reactions s-HCO + s-C 2 H 3 and s-HCO + s-C 2 H 5 that are removed in the runs D1 and D2 are shown inTable B.1.…”

Gas-phase reactivity of CH3OH toward OH at interstellar temperatures (11.7–177.5 K): experimental and theoretical study

“…More likely, reactions in the gas phase are H-abstraction reactions from H 2 or other organic species or by reactions on the surface of dust grains [see e.g. Álvarez-Barcia et al71 and references therein], forming CH 2 O.…”