Bottom-Up Formation of the Simplest Geminal Thiol─Methanedithiol (CH ₂ (SH) ₂ )─and the Methyl Hydrodisulfide (H ₃ CSSH) Isomer in Interstellar Analogue Ices

Geminal dithiols─organic molecules bearing two thiol groups on the same carbon atom─are versatile synthons in organic and atmospheric chemistry, exhibiting significantly greater stability than their oxygen analogues. Here, we report the first formation of the smallest geminal dithiol, methanedithiol (CH 2 (SH) 2 ), and its structural isomer, methyl hydrodisulfide (CH 3 SSH), in low-temperature model interstellar ices composed of methane and hydrogen sulfide via energetic electron irradiation, simulating secondary electrons generated by galactic cosmic rays. Both isomers were identified in the gas phase using isomer-selective vacuum ultraviolet (VUV) photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS), guided by quantum chemically computed adiabatic ionization energies, and confirmed through isotopic labeling and ultraviolet photolysis studies. These results not only demonstrate that methanedithiol and methyl hydrodisulfide can form on ice-coated interstellar nanoparticles and represent promising candidates for future astronomical detection, but they also provide fundamental insights into the nonequilibrium synthesis of geminal dithiols in extraterrestrial environments.