Spectroscopic observation of two-center three-electron bonded (hemi-bonded) structures of (H2S)n + clusters in the gas phase† †Electronic supplementary information (ESI) available: Calculated relative energies of stable isomers of (H2S)n + (n = 3–6), calculated harmonic frequencies of (H2S)4 + at the different calculation levels, comparison between the observed and simulated spectra of (H2S)n + (n = 3–6). See DOI: 10.1039/c6sc05361k Click here for additional data file.

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 hemibonded structures have been very scarce. In the present study, the presence of a stable hemi-bonded core (H2SrSH2) + in (H2S)n + (n 1⁄4 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 H3S –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 H2S molecules in the cluster for n 1⁄4 3 to 6. This also indicates the hemi-bond nature of the (H2SrSH2) + ion core and the small impact of the formation of a solvation shell on the ion core.