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

Abstract

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.

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

“…For instance, in acetaldehyde formation by the coupling of CH 3 and HCO, the coupling temperature window was found to be between the temperatures at which CH 3 becomes mobile on the surface (between 9 and 15 K, depending on the adopted diffusion barrier) and the temperature at which the methyl radical would desorb (30 K). 55 Another drawback is that radical–radical couplings are often assumed to be barrierless because they are driven by the coupling of the opposite electronic spins of the radicals. However, the reactions can exhibit energy barriers because the radicals, to proceed with the coupling, need to break the interactions with the icy surfaces.…”

Desorption of organic molecules from interstellar ices, combining experiments and computer simulations: Acetaldehyde as a case study

“…Besides its ubiquity and comparatively high gas phase abundance mentioned in the introduction, it has been tentatively suggested as a possible carrier of the weak 7.41 µ m band recorded in observations of the Infrared Space Observatory (ISO-SW) towards the young stellar object W 33A (Schutte et al 1999). As demonstrated by theoretical studies, acetaldehyde can be formed from the recombination of CH 3 and CHO on dust grains (Lamberts et al 2019;Enrique-Romero et al 2021). Such recombination is a highly exothermic process that, in combination with the relatively low binding energy of the molecule with ASW, may suggest a return to the gas phase of the nascent molecule via non-thermal effects.…”

“…To this aim, theoretical calculations and experiments can provide helpful information. For example, concerning acetaldehyde, two theoretical works were recently published that investigate the formation of CH 3 CHO on CO or water-ice surfaces (Lamberts et al 2019;Enrique-Romero et al 2021). These latter studies found a route for CH 3 HO formation on dust grains via recombination of CH 3 and CHO radicals.…”

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

“…3,[51][52][53] And, additionally, they have also been assumed to study the formation of iCOMs by means of quantum chemical simulations. [54][55][56][57] By compiling all the available works, it seems that both paradigms are necessary to explain the presence, distribution and abundance of the wide and rich organic chemistry in the ISM. Thus, knowing whether the formation of an iCOM is dominated by surface or gas-phase chemistry is a case-by-case problem.…”