Effects of solvent molecules on hemi-bonded (CH3SH)2+: infrared absorption of [(CH3SH)2–X]+ with X = H2O, (CH3)2CO, or NH3 and (CH3SH)n+ (n = 3–6)

Abstract

Three-electron two-center (3e-2c) hemi-bonds play important roles in the oxidation and electron transport of proteins and are implicated to be involved in some neurodegenerative diseases. Our previous investigations on infrared (IR) spectra of (CH₃SH)₂⁺ using vacuum-ultraviolet photoionization, infrared dissociation, and time-of-flight detection have shown that (CH₃SH)₂⁺ is (3e-2c)-bonded. To investigate the influence of the solvent molecules on the (3e-2c)-bonded (CH₃SH)₂⁺ in a supersonic jet, we added H₂O or (CH₃)₂CO or NH₃ or (CH₃SH)_n (n = 1–4) to (CH₃SH)₂⁺ and investigated their IR action spectra. The (3e-2c)-bonded (CH₃SH)₂⁺ ion core was maintained when a molecule of H₂O or (CH₃)₂CO or CH₃SH binds, indicating that the ion core is more stable than the hydrogen bond, whereas the (3e-2c)-bond became broken by a NH₃ molecule because the proton transfer led to a more stable hydrogen-bonded structure. The spectral features of the SH-stretching modes of (CH₃SH)_n⁺ (n = 3–6) indicate that the (3e-2c)-bonded (CH₃SH)₂⁺ ion core is maintained and the first two additional CH₃SH are H-bonded to the free SH groups of the ion core. For larger clusters with n = 5 and 6, the additional solvent molecules likely bind to the first solvation shell. These results show also that the (3e-2c)-bonded S∴S structure is more stable than the S∴O and S∴N structures in [(CH₃SH)₂–X]⁺ with X = H₂O or (CH₃)₂CO or CH₃SH or NH₃.