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

TL;DR

This work investigates how HNSO forms on interstellar grain ices by combining quantum chemical calculations on ASW with three-phase astrochemical modeling that includes a multibinding treatment. It identifies NSO as a key intermediate formed primarily through NS+O and SO+N channels, with hydrogenation of NSO yielding cis-HNSO as the favored product; cis-HNSO is thermodynamically preferred and rapidly tunnels if isomerization occurs. On icy grains, HNSO abundance can rival that of OCS, making it a major sulfur reservoir in ices, while gas-phase HNSO remains much less abundant. The results underscore the importance of O and N diffusion on grains for sulfur chemistry in dense clouds and motivate targeted searches for HNSO and related H–N–O–S species in other astronomical environments, aided by a multibinding modeling approach that improves early-time abundance predictions.

Abstract

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.

Figures (4)

• Figure 1: Examples of the three binding sites on the cluster model of amorphous solid water (ASW) considered in this work. Red dotted lines indicate the hydrogen-bond network. The SO molecule is shown as a representative adsorbate.
• Figure 2: Spin density of the NSO radical obtained with an isovalue of 0.02 a.u.
• Figure 3: Eckart corrected isomerization rate constants for the gas phase cis-trans isomerization.
• Figure 4: Astrochemical model results for Models A--C see Table \ref{['tab:diff_models']}. In all models, the abundance of gas trans-HNSO is below 10$^{-19}$ due to spontaneous tunneling conversion.jiang_deciphering_2025. The black horizontal bar represents the observed abundance of HNSO while the red one represents the gaseous OCS abundance, both in in G+0.693.sanz-novo_discovery_2024. We note that a good agreement with observations of G+0.693 does not imply that the physical description of the cloud is accurate, see text.