Effect of ice charging on the astrochemistry of interstellar sulfur-bearing species on amorphous solid water

Abstract

Understanding where up to 99% of the expected sulfur is hidden in dense molecular clouds remains one of the long-standing unresolved problems in astrochemistry. Binding energies (BEs) control desorption, diffusion, and residence times of molecules on amorphous-solid-water (ASW) mantles and hence also determine these input parameters in gas–grain models. The effect of excess negative charge in ASW on sulfur adsorption remains entirely unexplored. We aim to derive statistically robust and physically interpretable BE distributions for atomic S and the sulfur-bearing molecules H_2S, SO_2, and OCS on neutral and negatively charged ASW and assess how excess negative charge alters their retention and potential role in the sulfur reservoir of cold, dense molecular clouds. The basis set superposition error (BSSE) and zero-point energy (ZPE) corrected BEs of S, SO_2, OCS, and H_2S were calculated via density functional theory (DFT) using the ORCA software. Molecule-specific DFT levels of theory were first selected from benchmark calculations on neutral and charged small water complexes against coupled-cluster reference energies. The selected protocols were then applied to study adsorption on neutral and charged ASW clusters. A range of adsorption sites were sampled on five independent amorphous ice clusters, yielding BE distributions that account for the site heterogeneity of ASW. Neutral ASW yields broad, site-dependent BE distributions consistent with previous water-ice estimates. On charged ASW, three general cases are identified: the BE always increases due to electron transfer (S-atom, SO_2), increases slightly without any electron transfer (H_2S), or remains the same unless an electron transfer occurs under specific conditions (OCS). These findings are inherently linked to the molecular properties of these molecules. Excess negative charge does not uniformly increase sulfur binding on ASW. Instead, it produces molecule-specific adsorption regimes, showing that charged ASW must be treated explicitly in gas–grain models of sulfur chemistry.