Spectroscopic observation of two-center three-electron bonded (hemi-bonded) structures of (HS) clusters in the gas phase.

A two-center three-electron 2c-3e bond (hemi-bond) is a non-classical chemical bond, and its existence has been supposed in radical cation clusters with lone pairs. Though the nature of the hemi-bond and its role in the reactivity of radical cations have attracted great interest, spectroscopic observations of hemi-bonded structures have been very scarce. In the present study, the presence of a stable hemi-bonded core (HS∴SH) in (HS) ( = 3-6) in the gas phase is demonstrated by infrared spectroscopy combined with quantum chemical calculations. The spectral features of the free SH stretch of the ion core show that the hemi-bond motif of the ion core is maintained up to the completion of the first H-bonded solvation shell. All of the observed spectra are well reproduced by the minimum energy hemi-bonded isomers, and no sign of the proton-transferred ion core type HS-SH, which is estimated to have a much higher energy, is found. Spin density calculations show that the excess charge is almost equally delocalized over the two HS molecules in the cluster for = 3 to 6. This also indicates the hemi-bond nature of the (HS∴SH) ion core and the small impact of the formation of a solvation shell on the ion core.