# Atom Addition Formation of Thionylimide (HNSO) on Interstellar Dust Grains: Chemical Routes Requiring Oxygen and Nitrogen Atom Surface Diffusion

## TL;DR

This paper explores how the molecule HNSO forms on interstellar dust grains through reactions involving oxygen and nitrogen atoms.

## Contribution

The study identifies a new chemical route for HNSO formation involving surface diffusion of oxygen and nitrogen atoms.

## Key findings

- HNSO is efficiently produced on grain surfaces via reactions with atomic oxygen and nitrogen.
- The cis conformer of HNSO is favored over the metastable trans form under interstellar conditions.
- Solid HNSO abundances can rival those of OCS in interstellar ices.

## 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.