Exploring chemical pathways for the interstellar molecule HOCS⁺: Preferential formation of the O-protonated carbonyl sulfide isomer

¹ Computational Chemistry Group, Departamento de Química Física y Química Inorgánica, Facultad de Ciencias, Universidad de Valladolid, 47011 Valladolid, Spain
² Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain

^★ Corresponding author: pilar.redondo@uva.es

Received: 12 September 2025
Accepted: 24 October 2025

Abstract

Context. The recent interstellar detection of the high-energy O-protonated carbonyl sulfide isomer (HOCS⁺) toward the molecular cloud G+0.693-0.027 contrasts with the non-detection of its lower-energy S-protonated counterpart, HSCO⁺, the global minimum in energy. This raises questions regarding the occurrence of selective formation pathways of these [H,C,S,O]⁺ isomers in space.

Aims. In this work, we aim to explore the most likely gas-phase formation routes for both HOCS⁺ and HSCO⁺ beyond the direct protonation of OCS (i.e., HCS⁺ + OH, HCO⁺ + SH, HOC⁺ + SH, and HCO + SH⁺) to help rationalize previous observational results. Methods. We first explored the thermodynamic feasibility of the aforementioned reactions using high-level double-hybrid B2PLYPD3∕aug-cc-pVTZ and CCSD(T)-F12∕cc-pVTZ-F12 computations. For the reaction HCS⁺ + OH, found to be the most thermodynamically favorable, we characterized the stationary points on its corresponding potential energy surface (PES). In addition, we also used a composite approach to refine relative energies and employed the statistical rate theory and master equation simulations to estimate rate constants and branching ratios.

Results. We show that HOCS⁺ is preferentially formed through the reaction of HCS⁺ with OH, providing a plausible chemical explanation for its interstellar presence and the non-detection of the low energy isomer. Nevertheless, while the branching ratio computed at a T ~T_kin(G+0.693) = 70-140 K is qualitatively consistent with the observations, its value is two orders of magnitude larger than the derived HOCS⁺/HSCO⁺ lower limit observational ratio (of ≥2.3). This suggests that if the upper limit of HSCO⁺ is close to the real abundance, additional formation pathways may also play a significant role in shaping the isomeric ratio.

Conclusions. These results highlight that including all isomers in a given family, along with their isomer-preferential formation pathways, in astrochemical models, which are in many cases isomer-insensitive, is essential to understand their formation routes.