Infrared Spectroscopy of (Benzene-H2S-Xn)+, X = H2O (n = 1 and 2) and CH3OH (n = 1), Radical Cation Clusters: Microsolvation Effects on the S-π Hemibond.

An unconventional covalent bond in which three electrons are shared by two centers is called hemibond. Hemibond formation frequently competes with proton transfer (or ionic hydrogen bond formation), but there have been a few experimental reports on such competition. In the present study, we focus on the (benzene-H2S)+ radical cation cluster, which is a model system of the S-π hemibond. The stability of the S-π hemibond to the microsolvation by water and methanol is explored with infrared spectroscopy of (benzene-H2S-Xn)+, X = H2O (n = 1 and 2) and CH3OH (n = 1), clusters. We also perform energy-optimization and vibrational simulations of (benzene-H2S-Xn)+. By comparison among the observed and simulated spectra, we determine the intermolecular binding motifs in (benzene-H2S-Xn)+. While the S-π hemibonded isomer is exclusively populated in (benzene-H2S-H2O)+, both the hemibonded and proton-transferred isomers coexist in [benzene-H2S-(H2O)2]+ and (benzene-H2S-CH3OH)+. Breaking of the S-π hemibond by the microsolvation is observed, and its solvent and cluster size dependence is interpreted by the proton affinity and the coordination property of the solvent moiety.