Sulphur reactions on interstellar water ice grains 

Introduction:  Astrochemistry is a multidisciplinary subject that allows us to investigate formation and destruction routes of molecules found in extraterrestrial environments, such as planetary atmospheres, comets or the interstellar medium (ISM) [1]. To this day, there is still a lack of knowledge concerning the chemistry of minor species, for example, there are important issues related to the presence of sulphur in the ISM, like the sulphur depletion problem [2]. Many sulphur species (H2S, OCS, SO, S2, SO2 and CS2) have been identified in the coma of the comet 67P/Churyumov-Grasimenko [3] and some have been proposed as the main carriers of S, such as H2S and OCS, condensed in the icy mantles of interstellar dust grains. In this contribution, we present a theoretical investigation of the reactions involving atomic sulphur, in the first electronically excited state 1D, with water. S(1D) is produced by UV-induced photodissociation of precursor molecules, such as OCS [4] and CS2 [5], which are relatively abundant in extraterrestrial environments. In. particular, OCS is among the few molecules for which a secure identification in interstellar ice has been provided [6-7]. S(1D) + H2O: The presence S(1D) precursors, like OCS, on interstellar ice is documented. Therefore, if formed on the ice surface, S(1D) will be able to react with water. According to our calculations, the reaction proceeds either by S(1D) addition to one of the lone pairs of O or via insertion into one of the two O-H bonds. Two different intermediates can be formed: H2OS and HOSH. Theoretical method:  We first characterized the gas-phase potential energy surface (PES) of the reaction S(1D) + H2O. We then optimized the geometry of an 18-H2O cluster ice surface at DFT level of theory. We identified several surface binding sites on the grain, based on the amount of available hydrogen bonds that engage the sulfur atom with the surface. For each identified site, we computed the binding energies of the sulfur atom in order to select the best configuration with which to investigate the catalytic effect of water molecules. Conclusions: In this contribution we show the catalytic effect of the water ice: water molecules actively participate to the H-transfer process, reducing the energy barriers compared to the analogous gas-phase steps. References: [1] Caselli P. and Ceccarelli C. (2012) Astron. Astro- phys. Rev., 56, 20 [2] Vidal T. H. G. and Wakelam V. (2018) Mon. Not. R. Astron. Soc., 5575–5587, 474 [3] Hänni N. et al. (2022) Nat. Commun., 3639, 13 [4] Kim, M. H. et al. (2004) Can. J. Chem. 880-884, 82 [5] Black, G. and Jusinski, L.E. (1986) Chem. Phys. Lett., 90–92, 124 [6] Gibb E. L., et al. (2000) Astrophys. J., 347, 536 [7] Gibb E. L., et al. (2004) Astrophys. J. S. S., 35, 15