Gas-Phase\nand Model Ice-Surface Reactions of S(<sup>1</sup>D) with Water and\nMethanol: A Computational Investigation\nand Implications for Cosmochemistry/Astrochemistry

Gas-phase reactions of atomic sulfur in its first electronically\nexcited metastable state, S(<sup>1</sup>D), with water and methanol\nhave been theoretically investigated to characterize their potential\nenergy surfaces, the reaction mechanisms, and the product branching\nfractions. According to our results, both reactions proceed with the\nformation of bound intermediates that, for the isolated systems, decompose\ninto products because of the large energy content with which they\nare formed. The SO(a <sup>1</sup>Δ) + H<sub>2</sub> channel\nis the only open one for the S(<sup>1</sup>D) + H<sub>2</sub>O reaction,\nwhile many channels are open for the S(<sup>1</sup>D) + CH<sub>3</sub>OH reaction. For the latter case, statistical estimates of the product\nbranching fractions indicate that the main channels are those leading\nto CH<sub>2</sub>OH + SH, H<sub>2</sub>CO + H<sub>2</sub>S, H<sub>2</sub>CS + H<sub>2</sub>O, and CH<sub>3</sub> + HSO. The mechanism\nof the related O(<sup>1</sup>D) + CH<sub>3</sub>SH reaction has also\nbeen unveiled. Since the reaction intermediates can be stabilized\nby energy loss to surrounding species on ice or in liquid water, to\ngain some insights into the possible effects of water molecules, we\nhave also analyzed how the two reactions behave when four additional\nwater molecules are added. The conclusion is that the initial intermediates\nformed by the insertion or addition mechanism, namely, HOSH (hydrogen\nthioperoxide) and H<sub>2</sub>OS for S(<sup>1</sup>D) + H<sub>2</sub>O and CH<sub>2</sub>OHSH (mercaptomethanol), CH<sub>4</sub>OS and\nCH<sub>3</sub>OSH (methyl thioperoxide) for S(<sup>1</sup>D) + CH<sub>3</sub>OH, as well as CH<sub>3</sub>SOH (methyl sulfenic acid) for\nO(<sup>1</sup>D) + CH<sub>3</sub>SH, will probably be stabilized by\nthe interaction with the additional water molecules. Our results can\nhelp in understanding sulfur chemistry in space, especially in the\ncase of comets. On the one hand, the S(<sup>1</sup>D) + H<sub>2</sub>O gas-phase reaction could account for the additional SO source necessary\nto explain the observed distribution of this species obtained by using\nthe Plateau de Bure interferometer of Institut de Radioastronomie\nMillimétrique (IRAM) for the Hale Bopp comet. On the other\nhand, some of the S/O-containing molecules identified by ROSINA (Rosetta\nOrbiter Spectrometer for Ion and Neutral Analysis) during the enhanced\ndust emission events of the 67/P comet (e.g., species with the gross\nformula HSO, H<sub>2</sub>SO, and CH<sub>4</sub>OS) could be the results\nof the chemistry occurring on ice that we have exposed in this work.