The three-electron two-center (3e–2c) bond plays an important role in structures and electron communication in biological systems involving cationic sulfur compounds. Although the nature of 3e–2c bonds and their theoretical formalism have attracted great interest, direct spectral identifications of 3e–2c-bound molecules are scarce. We observed the infrared spectra of the weakly 3e–2c-bound CH₃S∴S(H)CH₃ and the strongly 3e–2c-bound (CH₃SH)₂⁺ in a supersonic jet using infrared (IR) dissociation with vacuum-ultraviolet photoionization and time-of-flight detection. Protonation of CH₃S∴S(H)CH₃ to form [CH₃(H)S∴S(H)CH₃]⁺ significantly enhances the 3e–2c bond, characterized by a large red shift of the SH-stretching band with enhanced IR intensity, shortening of the calculated S–S distance from 3.00 to 2.86 Å, and a dissociation energy increased from 〜23 to 162 kJ mol^-1.

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