Revealing that the Gas-phase Reaction of CN with H2S Can Be a Source of HSCN in Interstellar Molecular Clouds

Abstract

Sulfur-bearing molecules are key constituents of the interstellar medium (ISM). Particularly, hydrogen sulfide (H2S) and cyano (CN) radicals are key precursors of prebiotic molecules in the ISM. However, the ultralow-temperature gas-phase reactivity remains poorly characterized yet. We report the first experimental and theoretical investigation of the CN + H2S reaction under conditions relevant to cold molecular clouds. Rate coefficients were determined between 11.7 and 45.5 K using the Cinétique de Réaction en Ecoulement Supersonique Uniforme technique coupled with pulsed laser photolysis–laser-induced fluorescence, yielding negligible temperature dependence values around 4.0 × 10−10 cm3 s−1 in excellent agreement with complementary rate coefficients calculations. AutoMeKin and coupled-cluster theory reveal that the dominant channel involves CN addition to H2S, followed by H elimination, forming HSCN. This pathway is energetically more favorable than the previously assumed HCN + SH channel and exhibits submerged transition states, suggesting efficient reactivity at ultracold temperatures. Astrochemical modeling indicates that inclusion of this reaction in chemical networks enhances HSCN abundances in dark clouds, with contributions comparable to those from dissociative recombination routes. Although the CN + H2S reaction is absent from current astrochemical databases, our results demonstrate its potential role in sulfur–nitrogen coupling and the formation of prebiotic molecules in the ISM. These findings underscore the need to update chemical models to account for this process and improve predictions of sulfur chemistry in star-forming regions.