Atom Addition Formation of Thionylimide (HNSO) on Interstellar Dust Grains: Chemical routes requiring oxygen and nitrogen atom surface diffusion

We investigate the formation of the recently detected HNSO molecule using quantum chemical calculations on ices and astrochemical models in tandem. Our results indicate that HNSO is efficiently produced on grain surfaces through reactions involving atomic oxygen and nitrogen atoms with the radicals NS and SO, forming NSO as a key intermediate. Subsequent hydrogenation of NSO leads to HNSO, with a clear preference for the lowest energy cis conformer, while the trans form is metastable and may be short-lived under typical interstellar conditions. The models predict that solid HNSO can reach abundances comparable to icy OCS, placing it among the major sulfur-bearing species in interstellar ices. Gas-phase abundances, in contrast, remain lower than those of OCS. The implementation of a multibinding scheme in our models clarifies the role of diffusive chemistry in the production of HNSO at early times, improving agreement with observations. These findings suggest that reactions involving diffusing O and N atoms on icy grains contribute significantly to sulfur chemistry and beyond in dense clouds and motivate further searches for molecules containing simultaneously H, N, O and S in other astronomical environments.

💡 Research Summary

The paper investigates the origin of thionylimide (HNSO), a newly detected interstellar molecule containing hydrogen, nitrogen, sulfur and oxygen, by combining quantum‑chemical calculations on amorphous solid water (ASW) ice surfaces with astrochemical kinetic modeling. The authors first identify plausible grain‑surface routes to the NSO radical, the immediate precursor of HNSO. Three elementary atom‑addition reactions are considered: NO + S, SO + N, and NS + O. Using a dual‑level DFT approach (geometry optimizations at ωB97M‑D4‑gCP/def2‑SVP and single‑point energies at ωB97M‑D4/def2‑TZVPPD) on a 33‑water‑molecule cluster, they evaluate binding energies of the reactants (SO, NS) on three representative sites (Pocket, dH, Pentamer) and compute reaction energetics for twelve possible addition pathways. Both SO + N → NSO and NS + O → NSO are found to be highly exothermic (ΔH ≈ ‑70 to ‑80 kcal mol⁻¹) and essentially barrierless, whereas NO + S is disfavored due to a large rearrangement barrier. The calculated binding energies (≈ 4–5 kcal mol⁻¹) indicate that atomic O and N can diffuse on ASW at 10 K more readily than the heavier radicals, supporting the feasibility of the low‑energy routes.

Subsequent hydrogenation of NSO (H + NSO → HNSO) preferentially yields the cis conformer, which is the only isomer observed astronomically. High‑level CCSD(T)-F12 calculations show that the trans‑to‑cis isomerization barrier exceeds 30 kcal mol⁻¹, making tunneling rates negligible on interstellar timescales; thus trans‑HNSO is expected to be short‑lived.

To translate these microscopic results into observable abundances, the authors implement a “multibinding” scheme in the NAUTILUS astrochemical code. This scheme samples the distribution of binding energies for O and N on the ice surface, thereby providing a realistic diffusion rate rather than a single averaged value. Model runs for a dense cloud representative of the Galactic Center cloud G+0.693–0.027 show that, once O and N diffusion is enabled, NSO and subsequently HNSO build up rapidly on grains. The solid‑phase abundance of HNSO can reach ≈10⁻⁹–10⁻⁸ relative to H₂, comparable to that of OCS, a well‑known sulfur carrier. In the gas phase, HNSO remains 1–2 dex lower than OCS, consistent with the observed gas‑phase upper limits. The model also highlights the importance of the radicals NS and SO as reservoirs that feed the O/N addition pathways; their release during shock‑induced ice sputtering in G+0.693 explains the high observed column density.

The study concludes that (1) diffusion of atomic O and N on icy grains is efficient enough to drive non‑hydrogenation chemistry, (2) NSO is a key intermediate formed via barrierless O/N addition to NS or SO, and (3) the multibinding approach markedly improves the agreement between modeled and observed HNSO abundances. These findings broaden our understanding of sulfur chemistry beyond simple hydrogenation networks, suggest that other NSO‑related species may be detectable, and provide a framework for incorporating heavy‑atom diffusion into astrochemical models of complex interstellar molecules.