Spectral Characterization of Three-Electron Two-Center (3e-2c) Bonds of Gaseous CH₃S-S(H)CH₃ and (CH₃SH)₂⁺ and Enhancement of the 3e-2c Bond upon Protonation

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.