H<sub>2</sub> Formation on Interstellar Grains and the Fate of Reaction Energy

Pantaleone, Stefano; Enrique-Romero, Joan; Ceccarelli, C.; Ferrero, Stefano; Balucani, Nadia; Rimola, Albert; Ugliengo, Piero

doi:10.3847/1538-4357/ac0142

Cited by 44 publications

(35 citation statements)

References 38 publications

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“…It is important to note that in order to apply the RRKM theory, we implicitly assume that the intra-molecular energy redistribution of the reaction energy is faster than the reaction itself. This assumption is supported by recent ab initio molecular dynamics (AIMD) computations that show that a large fraction (≥50%) of the reaction energy is absorbed by the water ice in less than 1 ps (Pantaleone et al 2020(Pantaleone et al , 2021. We have checked a posteriori that the timescale of the reactions studied here is indeed longer that 1 ps.…”

Section: A Novel Treatment Of Surface Radical-radical Reactions Rate ...supporting

confidence: 73%

Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains

Enrique-Romero

Ceccarelli

Rimola

et al. 2021

A&A

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Context. Interstellar grains are known to be important actors in the formation of interstellar molecules such as H2, water, ammonia, and methanol. It has been suggested that the so-called interstellar complex organic molecules (iCOMs) are also formed on the interstellar grain icy surfaces by the combination of radicals via reactions assumed to have an efficiency equal to unity. Aims. In this work, we aim to investigate the robustness or weakness of this assumption. In particular, we consider the case of acetaldehyde (CH3CHO), one of the most abundant and commonly identified iCOMs, as a starting study case. In the literature, it has been postulated that acetaldehyde is formed on the icy surfaces via the combination of HCO and CH3. Here we report new theoretical computations on the efficiency of its formation. Methods. To this end, we coupled quantum chemical calculations of the energetics and kinetics of the reaction CH3 + HCO, which can lead to the formation of CH3CHO or CO + CH4. Specifically, we combined reaction kinetics computed with the Rice-Ramsperger–Kassel–Marcus theory (tunneling included) method with diffusion and desorption competitive channels. We provide the results of our computations in the format used by astrochemical models to facilitate their exploitation. Results. Our new computations indicate that the efficiency of acetaldehyde formation on the icy surfaces is a complex function of the temperature and, more importantly, of the assumed diffusion over binding energy ratio f of the CH3 radical. If the ratio f is ≥0.4, the efficiency is equal to unity in the range where the reaction can occur, namely between 12 and 30 K. However, if f is smaller, the efficiency dramatically crashes: with f = 0.3, it is at most 0.01. In addition, the formation of acetaldehyde is always in competition with that of CO + CH4. Conclusions. Given the poor understanding of the diffusion over binding energy ratio f and the dramatic effect it has on the formation, or not, of acetaldehyde via the combination of HCO and CH3 on icy surfaces, model predictions based on the formation efficiency equal to one should to be taken with precaution. The latest measurements of f suggest f = 0.3 and, if confirmed for CH3, this would rule out the formation of acetaldehyde on the interstellar icy surfaces. We recall the alternative possibility, which was recently reviewed, that acetaldehyde could be synthesized in the gas phase starting from ethanol. Finally, our computations show the paramount importance played by the micro-physics involved in the interstellar surface chemistry and call for extensive similar studies on different systems believed to form iCOMs on the interstellar icy surfaces.

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Section: A Novel Treatment Of Surface Radical-radical Reactions Rate ...supporting

confidence: 73%

Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains

Enrique-Romero

Ceccarelli

Rimola

et al. 2021

A&A

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“…Moreover, the energy released by the first reaction step will no longer be available for the second step, as water ices tend to efficiently dissipate chemical energy fast. 113,114 Therefore, R1 will lead to the formation of a highly stabilized acety-lene, which will hardly react with OH remaining stuck on the ice. However, this reaction can be an effective channel towards the formation of OH radicals on the ice surfaces without the need of a direct energy processing.…”

Section: Discussion and Astrophysical Implicationsmentioning

confidence: 99%

“…The very favorable reaction energies shown by the reactions make that the energy barriers of the isomerization processes lay below the pre-reactive asymptotic states and therefore, they can be overcome by making use of the nascent reaction energies. However, one should bear in mind that water ice surfaces are extraordinary third bodies 113,114 and accordingly, the direct transfer of the previous reaction energies to surmount the isomerization energy barriers is doubtful. To shed some light onto this aspect, dedicated ab initio molecular dynamics simulations are compulsory, which is out of the scope of the present work.…”

Section: Discussion and Astrophysical Implicationsmentioning

confidence: 99%

Non-Energetic Formation of Ethanol via CCH Reaction with Interstellar H2O Ices. A Computational Chemistry Study

Perrero¹,

Enrique-Romero²,

Bachs³

et al. 2022

Preprint

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Ethanol (CH 3 CH 2 OH) is a relatively common molecule, often found in star forming regions. Recent studies suggest that it could be a parent molecule of several so-called interstellar complex organic molecules (iCOMs). Yet, the formation route of this species remains debated. In the present work, we study the formation of ethanol through the reaction of CCH with one H 2 O molecule belonging to the ice, as a test case to investigate the viability of chemical reactions based on a "radical + ice component" scheme as an alternative mechanism for the synthesis of iCOMs, beyond the usual radical-radical coupling. This has been done by means of DFT calculations adopting two clusters of 18 and 33 water molecules as ice models. Results indicate that CH 3 CH 2 OH can potentially be formed by this proposed reaction mechanism. The reaction of CCH with H 2 O on the water ice clusters can be barrierless (thanks to the help of boundary icy water molecules acting as proton transfer assistants) leading to the formation of vinyl alcohol precursors (H 2 CCOH and CHCHOH). Subsequent hydrogenation of vinyl alcohol yielding ethanol is the only step presenting a low activation energy barrier. We finally discuss the astrophysical implications of these findings.

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“…At each new added water molecule the structure is first optimized at GFN-FF level. As discussed before, to simulate the conditions of the ISM in which a fraction of the heat of the water formation is transferred to the grain increasing its local temperature, 47 every 10 added water molecules a NVT molecular dynamics at 10 K is performed for 1 ps to simulate the water relaxation, directly followed by a GFN-FF optimization. We counted, on average, from a sample of 20 clusters of 200 water molecules, an increase of 6 hydrogen bonds as a result of the MD step followed by a GFN-FF optimization.…”

Section: Computational Detailsmentioning

confidence: 99%

Computer Generated Realistic Interstellar Icy Grain Models: Physicochemical Properties and Interaction with NH₃

Germain

Tinacci

Pantaleone

et al. 2022

ACS Earth Space Chem.

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Interstellar grains are composed by a rocky core (usually amorphous silicates) covered by an icy mantle, the most abundant molecule being H 2 O followed by CO, CO 2 , NH 3 , and also radicals in minor quantities. In dense molecular clouds, gas-phase chemical species freeze onto the grain surface, making it an important reservoir of molecular diversity/complexity whose evolution leads to interstellar complex organic molecules (iCOMs). Many different models of water clusters have appeared in the literature, but without a systematic study on the properties of the grain (such as the H-bonds features, the oxygen radial distribution function, the dangling species present on the mantle surface, the surface electrostatic potential, etc.). In this work, we present a computer procedure (ACO-FROST) grounded on the newly developed semiempirical GFN2 tight-binding quantum mechanical method and the GFN-FF force field method to build-up structures of amorphous ice of large size. These methods show a very favorable accuracy/cost ratio as they are ideally designed to take noncovalent interactions into account. ACO-FROST program can be tuned to build grains of different composition mimicking dirty icy grains. These icy grain models allow studying the adsorption features (structure, binding energy, vibrational frequencies, etc.) of relevant species on a large variety of adsorption sites so to obtain a statistically meaningful distribution of the physicochemical properties of interest to be transferred in numerical models. As a test case, we computed the binding energy of ammonia adsorbed at the different sites of the icy grain surface, showing a broad distribution not easily accounted for by other more size limited icy grain models. Our method is also the base for further refinements, adopting the present grain in a more rigorous QM:MM treatment, capable of giving binding energies within the chemical accuracy.

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H₂ Formation on Interstellar Grains and the Fate of Reaction Energy

Cited by 44 publications

References 38 publications

Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains

Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains

Non-Energetic Formation of Ethanol via CCH Reaction with Interstellar H2O Ices. A Computational Chemistry Study

Computer Generated Realistic Interstellar Icy Grain Models: Physicochemical Properties and Interaction with NH₃

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H2 Formation on Interstellar Grains and the Fate of Reaction Energy

Cited by 44 publications

References 38 publications

Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains

Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains

Non-Energetic Formation of Ethanol via CCH Reaction with Interstellar H2O Ices. A Computational Chemistry Study

Computer Generated Realistic Interstellar Icy Grain Models: Physicochemical Properties and Interaction with NH3

Contact Info

Product

Resources

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H₂ Formation on Interstellar Grains and the Fate of Reaction Energy

Computer Generated Realistic Interstellar Icy Grain Models: Physicochemical Properties and Interaction with NH₃