Reactivity of HCO with CH3 and NH2 on Water Ice Surfaces. A Comprehensive Accurate Quantum Chemistry Study

Séminaire doctorant de Joan Enrique Romero (IPAG), jeudi 4 juillet 2019 à 11h00, salle Manuel Forestini

Interstellar complex organic molecules (iCOMs) have a great importance for the astronomical community since they can be used as tracers for the physical conditions of the interstellar medium (ISM).

Moreover, in the last years they have gained a lot of attention since they could have played a role in the origin of life on Earth as they could have been inherited by small bodies of the Solar System, e.g. carbonaceous chondrites and comets [Caselli & Ceccarelli 2012].

Thus, understanding how iCOMs are formed and destroyed is of high importance to predict the ultimate organic complexity reached in the interstellar medium (ISM). Two paradigms are invoked : one assumes that complex species appear as a result of several gas-phase chemical processes ; the other that radical-radical reactivity occurring on the surface of interstellar grains are the responsible for the observed chemical complexity. The latter is somewhat more popular among astrochemical models, even though their relative importance is still a topic of debate. Checking the radical-radical reactivity assumption experimentally is an unprecedented difficult task to perform. Quantum chemistry methods, on the other hand, provide a suitable atomistic perspective from which to study such processes.

In this contribution we present our recent quantum chemical studies on the surface reactivity of radical couples.
According to observational evidences, the icy mantles that cover interstellar dust grains are dominated by water (Boogert et al. 2014), for this reason we use a cluster-like model made of 33 water molecules.
On top of them we study the reactivity of biradical systems, such as CH3 + HCO, by means of static quantum chemical calculations to study the formation of, in this example, acetaldehyde (CH3CHO). Besides the formation of the iCOM (CH3CHO), we also observe competitive processes leading back to simpler species, in this example : CH4 + CO. These results indicate that the fraction of iCOMs generated in our current astrochemical models is overestimated since the considered reactions do not include the competitive channel. All in all, the
occurrence of one process or the other could entirely depend on the relative orientation of the radicals upon encounter.