Computational Characterization of CH ₄ S ₂ Isomers as Key Candidates in Interstellar Sulfur Chemistry

The sulfur depletion problem in the interstellar medium motivates a detailed characterization of sulfur-bearing molecules such as CH4S2, whose isomers may act as potential reservoirs of interstellar sulfur. Using high-level ab initio methods, we identified eight structural isomers of CH4S2, with 1-methylhydrodisulfide (CH3SSH) as the global minimum and methanedithiol lying 3.1 kcal·mol–1 higher in energy. These low-energy isomers show both kinetic and thermodynamic stability against decomposition into H2CS and H2S, even in the presence of water. In contrast, hypervalent sulfur isomers are more than 60 kcal·mol–1 higher in energy. Rotational constants obtained with the jun-Cheap Scheme (jun-ChS), including anharmonic corrections reproduce experimental values within 0.17%, ensuring reliable spectral predictions. The stability of CH4S2 isomers under interstellar conditions and their dipole moments (up to 1.9 D) suggest that they are promising targets for radioastronomical detection and may help clarify the chemical pathways of sulfur in cold molecular clouds.