The chemistry leading to interstellar sulphur-bearing organic molecules is still in need to be fully characterized. Sulphur is present in space with an abundance of 16 ppm and it is an interesting element for prebiotic chemistry since it is present in amino acids and several other biomolecules. Simple S-containing species, such as OCS, CS, CH₃SH, H₂CS, as well as C₂S and C₃S, have been detected in a variety of objects. More complex sulphur species have been detected in the coma of the comet 67P/Churyumov-Grasimenko, such as CH₄OS, C₂H₆OS, C₃H₆OS and CH₄S₂.
In this contribution, we present a theoretical investigation of two reactions involving atomic sulphur in its first electronically excited state, ¹D, with two molecules, water and methanol, which are abundant in interstellar/cometary ice. S(¹D) can be produced on the ice surface by UV-induced photodissociation of precursor molecules, such as OCS, that is, one the few molecules for which a secure identification in interstellar ice has been provided. The ¹D state is metastable with a long radiative lifetime and is very reactive, differently from the case of the ground ³P state of atomic sulphur.
The role of electronically excited metastable atoms in the ice chemistry has been recently considered in the research groups of Oberg and Herbst for the similar case of atomic oxygen. According to those studies, the formation of O(¹D) opens up new reaction pathways which are not amenable in the case of the O(³P) reactions. According to our results, the SO(a ¹Δ_g) + H₂ channel is the only open channel for the S(¹D) + H₂O reaction, while many channels are open for the S(¹D) + CH₃OH reaction. For the latter case, statistical estimates of the product branching fractions indicate that the main channels are those leading to CH₂OH + SH, H₂CO + H₂S, H₂CS + H₂O, and CH₃ + HSO.
To gain some insight into the possible effects of additional water molecules, as in ice or liquid water matrices, we have also analyzed how the two reactions behave when four additional water molecules are added. The conclusion is that the initial intermediates formed by the insertion or addition mechanism − namely HOSH (hydrogen thioperoxide) and H₂OS for S(¹D) + H₂O and CH₂OHSH (mercaptomethanol), CH₄OS and CH₃OSH (methyl thioperoxide) for S(¹D) + CH₃OH, as well as CH₃SOH (methyl sulfenic acid) for O(¹D) + CH₃SH − will probably be stabilized by the interaction with the additional water molecules.

Cecilia Ceccarelli

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