Spectroscopic and photometric observations show that the interstellar medium contains ordinary matter, consisting of a mixture of gas and submicron sized dust grains. This matter is not evenly distributed but condensed regions forming the so-called interstellar clouds. The ca. 240 detected gas-phase species indicate a great chemical diversity and complexity in space, which are inversely proportional: while there is a wide chemical diversity for species with less than 5 atoms (most of them of inorganic character), more reduced is the list of species with more than 5 atoms (most of them of organic character). Dust grains consist of refractory materials (e.g., silicates) that are covered by ice mantles dominated by water. The presence of dust grains is especially important for the synthesis of molecules that otherwise would be impossible to form via simple gas-phase reactions (according to the observed abundances) because they provide active surfaces where important chemical reactions can occur. However, accurate atomistic details of the physico-chemical features (i.e., structure, energetics, and dynamics) of chemical processes taking place on the surface of grains are very scarce. This kind of information can be gently obtained by means of state-of-the-art quantum chemical simulations, by providing molecular pictures of the simulated processes, the chemical steps and related energetics, and allow us to unveil the role of the grains on them. In the present contribution, results obtained using quantum chemical calculations for the formation of H2, H2O and NH3 on silicates and water ice surfaces are presented. The reactions have been studied by using periodic models of the surfaces (crystalline silicate and amorphous water ice). For all of them, the potential energy surfaces have been characterized, obtaining the energy features of the reaction mechanisms. These data have been served as the basis to i) analyze if the presence of Fe in the silicate surface is of importance (H2 formation case), ii) do a kinetic treatment and assess how important the tunneling effects are (H2O formation case), and iii) unravel if the surfaces can exert a third body effect that stabilizes the newly formed product (NH3 formation case).

Fundamental chemical reactions occurring on surfaces of interstellar silicates and water ices: A quantum chemical approach

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