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

Abstract

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 therefore 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 angstrom, and a dissociation energy increased from similar to 23 to 162 kJ mol(-1).