A systematic computational study of radical—radical reactions on interstellar ice
surfaces
J. Enrique-Romero,1,2 A. Rimola,2 and C. Ceccarelli1
1Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble(IPAG), 38000 Grenoble, France
2Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, 08193,
Catalonia, Spain
Understanding how insterstellar complex organic molecules (iCOMs) are formed anddestroyed is of high importance to predict the ultimate organic complexity reached in theinterstellar medium (ISM) ([1], [2]). Two paradigms are invoked in the literature. Both argue
that simple molecules and atoms are hydrogenated on the interstellar grain surfaces during thecold prestellar phase. Following this first step, one paradigm assumes that iCOMs appear as aresult of gas-phase chemical processes, whereas the other predicts that radical-radical
reactivity on the grain surfaces is the major responsible for the observed chemical complexity.
The latter has profited of much popularity among astrochemical models in the last years, eventhough some basic assumptions of the paradigm are still a topic of debate. Among them, theradical-radical reactivity assumption, which is extremely difficult to quantify experimentally.
We propose a complementary and, sometimes, alternative method: theoretical quantum
chemical calculations, which can provide a precious atomistic perspective from which tostudy such processes (e.g. [3], [4], [5]).
In this contribution, we present our recent systematic quantum chemical study on the surface
reactivity of several radical pairs. Some of the studied systems are CH3 + X and HCO + X (X= CH3, HCO, NH2, CH2OH and CH3O). According to observational evidences, the icy
mantles that cover interstellar dust grains are dominated by water ([6]). We, therefore, use twocluster-like ice models consisting on 18 and 33 water molecules to simulate the grain surface
where the radical-radical reaction occurs. We then study the reactivity of the above biradicalsystems by means of static quantum chemical calculations to obtain the potential energy
surfaces of several complex species and the approximate efficiency of the different reaction
product channels. We found that many reactions leading to iCOMs have non negligible
activation barriers and that the transfer of a hydrogen atom from a radical acting as a donnor
to its partner, acting as an acceptor are important competitive processes. The occurrence ofone process or the other could entirely depend on the relative orientation of the radicals upon
encounter, namely on the water ice structure and interaction with the two radicals. These
results indicate that the fraction of iCOMs generated in the current astrochemical models iscertainly overestimated since (i) it is assumed that reactions always occur and (ii) thecompetitive reactions are not included.
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C. Ceccarelli, S. Viti, N. Balucani, et al., 2018, MNRAS 476(1), p. 1371-1383.
A. Rimola, D. Skouteris, N. Balucani, et al., 2018, ACS Earth and Space Chemistry 2(7), 720-734.
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