Hydrogenation of small aromatic heterocycles at low temperatures

Miksch, April M.; Riffelt, Annalena; Oliveira, Ricardo R.; Kästner, Johannes; Molpeceres, Germán

doi:10.1093/mnras/stab1514

Cited by 19 publications

(26 citation statements)

References 57 publications

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“…The computational setting for this work follows similar protocols that some other works by us (Miksch et al 2021). Briefly, we have determined the activation energies (∆U a , including zeropoint energy contributions) and reaction energies (∆U e ) for a set of reactions of significance for the formation of ethyl and vinyl bearing radicals and starting from (a) acetylene (C 2 H 2 ), (b) ethylene (C 2 H 4 ), and (c) ethane (C 2 H 6 ).…”

Section: Methodsmentioning

confidence: 99%

“…The latter are determined below the crossover temperature using conventional instanton theory and reduced instanton theory (McConnell & Kästner 2017) above it. Crossover temperatures (T c ) are defined as the temperatures below which quantum effects dominates over thermal effects and they are defined as in our previous works (Miksch et al 2021;Molpeceres et al 2021), following the formulation of Gillan (1987):…”

Section: Methodsmentioning

confidence: 99%

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Radical Addition and H Abstraction Reactions in C2H2, C2H4 and C2H6: A Gateway for Ethyl and Vinyl Bearing Molecules in the Interstellar Medium

Molpeceres,

Rivilla

2022

Preprint

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Context. Recent interstellar detections include a significant number of molecules containing vinyl (C 2 H 3 ) and ethyl (C 2 H 5 ) groups in their structure. For several of these molecules, there is not a clear experimental or theoretical evidence that support their formation from simpler precursors Aims. We carried out a systematic search of viable reactions starting from closed shell hydrocarbons containing two carbon atoms (ethane, C 2 H 6 ; ethylene, C 2 H 4 ; and acetylene, C 2 H 2 ) with the goal of determining viable chemical routes for the formation of vinyl and ethyl molecules on top of interstellar dust grains. Methods. We used density functional theory calculations in combination with semiclassical instanton theory to derive the rate coefficients for the radical-neutral surface reactions. The effect of a surface was modelled through an implicit surface approach, profiting from the weak interaction between the considered hydrocarbons and the dust surfaces. Results. Our results show that both H and OH radicals are key in converting acetylene and ethylene into more complex radicals that are susceptible to continue reacting and forming interstellar complex organic molecules. The relevant reactions, for example OH additions, present rate constants above 10 1 s −1 that are likely competitive with OH diffusion on grains. Similarly, H atom addition to acetylene and ethylene is a very fast process with rate constants above 10 4 s −1 in all cases, and greatly enhanced by quantum tunneling. Hydrogen abstraction reactions are less relevant, but may play a role in specific cases involving the OH radical. Reactions with other radicals NH 2 , CH 3 are likely to have a much lesser impact in the chemistry of ethyl and vinyl bearing molecules. Conclusions. The effective formation at low temperatures of four radicals (C 2 H 3 , C 2 H 5 , C 2 H 2 OH, and C 2 H 4 OH) through our proposed mechanism opens the gate for the formation of complex organic molecules and indicates a potential prevalence of OH bearing molecules on the grain. Following our suggested reaction pathway, we explain the formation of many of the newly detected molecules, and propose new molecules for detection. Our results reinforce the recent view on the importance of the OH radical in the interstellar surface chemistry.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Radical Addition and H Abstraction Reactions in C2H2, C2H4 and C2H6: A Gateway for Ethyl and Vinyl Bearing Molecules in the Interstellar Medium

Molpeceres,

Rivilla

2022

Preprint

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“…We employed theoretical methods with an accurate treatment of quantum tunneling (Meisner et al 2017;Oba et al 2018;Álvarez-Barcia et al 2018;Miksch et al 2021; to investigate H abstraction reactions from FA and TFA, both by H and D atoms. Moreover, the effect of the binding of the acids with the surface in the context of their chemistry was elucidated.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen abstraction reactions in formic and thioformic acid isomers by hydrogen and deuterium atoms

Molpeceres

Jiménez-Serra

Oba

et al. 2022

A&A

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Context. The isomerism of molecules in the interstellar medium and the mechanisms behind it are essential questions in the chemistry of organic molecules in space. In the particular case of simple formic and thioformic acids, the low temperatures found in molecular clouds indicate that cis-trans isomerization in the gas-phase must be impeded. Reactions taking place on top of interstellar dust grains may explain the isomer interconversion at low temperatures. Aims. We studied the isomerization processes of formic and thioformic acid that are likely to take place on the surface of interstellar dust grains after being initiated by H abstraction reactions. Similarly, deuterium enrichment of the acids can occur by the same mechanism. Our objective is to shed light on both topics to expand our understanding of the key precursors of organic molecules in space. Methods. We determined the rate constants for the H abstraction reactions as well as the binding energies for the acids on water ice using ab initio calculations and the instanton method for calculating the rate constants, including quantum tunneling. In addition, we tested the viability of the deuteration of formic acid with tailored experiments and looked for it on the L1544 source. Results. For formic acid, there is a clear dependence of the H abstraction reactions on the isomer of the reactant, with rate constants at ~50 K that differ by five orders of magnitude. Correspondingly, we did not observe the trans-cis reaction in our experiments. In the case of thioformic acid, a very similar cis-trans reactivity is found for abstraction reactions at the thiol (-SH) group in contrast to a preferential reactivity that is found when abstractions take place at the -CH moiety. We found comparable binding energies for both isomers with average binding energies of around −6200 and −3100 K for formic and thioformic acid, respectively. Our binding energy calculations show that the reactions are precluded for specific orientations, affecting the overall isomerization rate. For H abstractions initiated by deuterium atoms, we found very similar trends, with kinetic isotope effects varying in most cases between 13 and 20. Conclusions. Our results support the cis-trans interconversion of cis-formic acid on dust grains, suggesting that such an acid should not withstand the conditions found on these objects. On the other hand, the trans isomer is very resilient. Both isomers of thioformic acid are much more reactive. A non-trivial chemistry is behind the apparent excess of its trans isomer that is found in cold molecular clouds and star-forming regions due to a subtle combination of preferential reactivity and binding with the surface. In light of our results, all the deuterated counterparts of thioformic acid are viable molecules to be present on the ISM. In contrast, only the trans isomer of deuterated formic acid is expected, for which we provide upper bounds of detection. Given the mechanisms presented in this paper, other mechanisms must be at play to explain the tiny fraction of cis-formic acid observed in interstellar cold environments, as well as the current trans-DCOOH and trans-HCOOD abundances in hot-corinos.

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“…We employed theoretical methods with an accurate treatment of quantum tunneling (Meisner et al 2017;Oba et al 2018;Álvarez-Barcia et al 2018;Miksch et al 2021;Molpeceres et al 2021a) to investigate H abstraction reactions from FA and TFA, both by H and D atoms. Moreover, the effect of the binding of the acids with the surface in the context of their chemistry was elucidated.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen abstraction reactions in formic and thioformic acid isomers by hydrogen and deuterium atoms. Insights on isomerism and deuteration

Molpeceres,

Jiménez-Serra,

Oba

et al. 2022

Preprint

Self Cite

View full text Add to dashboard Cite

Context. The isomerism of molecules in the interstellar medium and the mechanisms behind it are essential questions in the chemistry of organic molecules in space. In particular, for the simple formic and thioformic acids, the low temperatures found in molecular clouds indicate that cis-trans isomerization in the gas-phase must be impeded. Reactions happening on top of interstellar dust grains may explain the isomer interconversion at low temperatures. Aims. We studied the isomerization processes of formic and thioformic acid susceptible to happen on the surface of interstellar dust grains and initiated by H abstraction reactions. Similarly, deuterium enrichment of the acids can occur by the same mechanism. Our objective is to shed light on both topics to increase our understanding of key precursors of organic molecules in space. Methods. We determined the rate constants for the H abstraction reactions as well as the binding energies for the acids on water ice using ab-initio calculations and the instanton method for calculating the rate constants, including quantum tunneling. In addition, we tested the viability of the deuteration of formic acid with tailored experiments and looked for it on the L1544 source. Results. For formic acid, there is a clear dependence of the H abstraction reactions on the isomer of the reactant, with rate constants at ∼50 K that differ by 5 orders of magnitude. Correspondingly, we did not observe the trans-cis reaction in our experiments. In the case of thioformic acid, a very similar cis-trans reactivity is found for abstraction reactions at the thiol (-SH) group in contrast to a preferential reactivity found when abstractions take place at the -CH moiety. We found comparable binding energies for both isomers with average binding energies of around −6200 K and −3100 K for formic and thioformic acid, respectively. Our binding energy calculations show that the reactions are precluded for specific orientations, affecting the overall isomerization rate. For H abstractions initiated by deuterium atoms, we found very similar trends, with kinetic isotope effects varying in most cases between 13 and 20. Conclusions. Our results support the cis-trans interconversion of cis-formic acid on dust grains, suggesting that such an acid should not withstand the conditions found on these objects. On the other hand, the trans isomer is very resilient. Both isomers of thioformic acid are much more reactive. Non-trivial chemistry operates behind the apparent excess of its trans isomer found in cold molecular clouds and star-forming regions due to a subtle combination of preferential reactivity and binding with the surface. In the light of our results, all the deuterated counterparts of thioformic acid are viable molecules to be present on the ISM. In contrast, only the trans isomer of deuterated formic acid is expected, for which we provide upper bounds of detection. With the mechanisms presented in this paper, other mechanisms must be at play to explain the tiny fraction of cis-formic acid observed in i...

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Hydrogenation of small aromatic heterocycles at low temperatures

Cited by 19 publications

References 57 publications

Radical Addition and H Abstraction Reactions in C2H2, C2H4 and C2H6: A Gateway for Ethyl and Vinyl Bearing Molecules in the Interstellar Medium

Radical Addition and H Abstraction Reactions in C2H2, C2H4 and C2H6: A Gateway for Ethyl and Vinyl Bearing Molecules in the Interstellar Medium

Hydrogen abstraction reactions in formic and thioformic acid isomers by hydrogen and deuterium atoms

Hydrogen abstraction reactions in formic and thioformic acid isomers by hydrogen and deuterium atoms. Insights on isomerism and deuteration

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