The S∴π hemibond (two-center three-electron, 2c-3e, bond) is an attractive interaction between a sulfur atom and π electrons. The S∴π hemibond is of essential importance in understanding chemistry of sulfur radical cations, and its roles in biochemistry have recently attracted much interest. In the present study, we observe the S∴π hemibond in the simplest model system in the gas phase. Infrared spectroscopy is applied to the [benzene-(H2S)n]+ (n = 1-4) radical cation clusters. In n = 1, the CH stretch and SH stretch bands of the benzene and H2S moieties, respectively, are clearly different from those of the neutral molecules but similar to those of the ionic species. These vibrational features show that the positive charge is delocalized over the cluster due to the S∴π hemibond formation. In n = 2-4, the S∴S hemibond and S-π-S multicenter hemibond (three-center five-electron, 3c-5e, bond) can compete with the S∴π hemibond. The observed vibrational features clearly indicate that the S∴S hemibond formation is superior to the S∴π hemibond and S-π-S multicenter hemibond. Calculations of several dispersion-corrected density functionals are compared with the observations. While all the tested functionals qualitatively catch the feature of the S∴π hemibond, the energy order among the isomers of the different hemibond motifs strongly depends on the functionals. These results demonstrate that the [benzene-(H2S)n]+ clusters can be a benchmark of density functionals to evaluate the sulfur hemibonds.
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