Quantum mechanical atomistic investigation of the hydrogen addition to carbon monosulfide on interstellar water ice

The successive hydrogenation of carbon monosulfide (CS) on the surfaces of interstellar icy grains is regarded as a highly efficient mechanism to generate thioformaldehyde H_2CS and methanethiol or methyl mercaptan CH_3SH in the interstellar medium (ISM). Over recent years, numerous experimental studies have investigated these reactions, offering valuable insights at the macroscopic scale. Nevertheless, several concerns remain, such as atomistic insights into quantitative energetic data and the actual role of water ice in these processes. These aspects are especially critical given that water constitutes the primary component of the ice mantles of dust grain cores. Here, we present quantum chemical simulations combined with kinetic calculations on the successive H addition to CS on water ice surfaces, giving rise to H_2CS and CH_3SH. Our aim is to determine their energetic and kinetic features in the ISM. The potential energy surfaces of the hydrogenation steps were characterized by means of methods based on density functional theory (DFT). The unimolecular rate constants were obtained by applying the Rice-Ramsperger-Kassel-Marcus theory using the calculated energy barriers. The reactions were simulated on structural cluster models, consisting of 3, 18, and 33 water molecules. Our main finding is that the interaction of CS with the water ice surfaces happens through a hydrogen bond that is exclusive to the C atom, leading to a shortening (and strengthening) of the CS bond. Consequently, the C atom becomes less prone to its hydrogenation towards H_2CS formation, thereby increasing the energy barrier compared to the gas-phase reaction. In contrast, the H-bond interaction of H_2CS with the water ice is through the S atom, leading to an enlargement (and weakening) of the C-S bond. Consequently, the C atom is more prone to receiving H atoms to form CH_3SH, thereby reducing the energy barriers, compared to its gas-phase analogue. Comparisons with H additions to CO to form H_2CO (formaldehyde) and CH_3OH (methanol) indicates that irrespective of the activation and inactivation of the C atoms, the formation of H_2CS and CH_3SH presents lower energy barriers than the formation of H_2CO and CH_3OH. Despite these differences, rate-constant calculations and the kinetic analysis of all these processes (i.e., H additions to CS and CO) indicate that all of them are largely favorable at very low interstellar temperatures due to tunneling effects.