Theoretical simulations of the reaction of excited atomic sulfur (¹D) with water on an 18-molecule ice cluster reveal that the ice environment stabilizes novel products (H₂OS and HOSH) by dissipating energy and altering reaction pathways, demonstrating that gas-phase reaction models are insufficient for interstellar ice chemistry.
In this contribution, we present a theoretical investigation of the reaction involving atomic sulfur in its first electronically excited state, 1D, and H2O on an ice-surface model. This study is motivated by the work of Giustini et al. (ACS Earth Space Chem., 2024, 8, 2318), which indicated a strong effect of the presence of four additional water molecules in the S(1D) + H2O reaction compared to the pure gas-phase case. Our simulation treats the long-range interactions (H-bonds and dispersion forces) with the ice water molecules in a much more realistic way being based on the use of a cluster of 18 water molecules, thus overcoming the limits of the small cluster used by Giustini et al. According to our results, S(1D) reacts via two possible reaction mechanisms: (1) addition to the O atom of a water molecule with the formation of H2OS or (2) insertion into one of the O–H bonds of a water molecule with the formation of HOSH. Both H2OS and HOSH are stabilized on ice by energy dissipation rather than isomerizing or dissociating into two products as seen in the gas-phase reaction. The interaction with surrounding water molecules affects the entire reaction pathway by stabilizing intermediate species, reducing some barriers, and impeding the only two-product open channel of the gas-phase reaction. S(1D) can be produced by UV-induced photodissociation of various precursor molecules on the surface of interstellar or cometary ice or by other high-energy processes induced by electrons or cosmic rays also in the ice bulk. Therefore, our results can be of help in elucidating the mysterious sulfur chemistry occurring in the icy mantles of interstellar grains or in cometary nuclei. Furthermore, this study demonstrates that the product branching ratios of gas-phase reactions should not be uncritically used in modeling interstellar ice chemistry.
This work has been published in ACS Earth and Space Chemistry.
Link to the article in ACS Earth Space Chem. In gold open access: https://pubs.acs.org/doi/10.1021/acsearthspacechem.4c00351