Infrared spectroscopy of size-selected benzene-water cluster cations [C₆H₆-(H₂O)_n]⁺ (n = 1-23): Hydrogen bond network evolution and microscopic hydrophobicity

Infrared (IR) spectra of size-selected benzene-water cluster cations ([C₆H₆-(H₂O)_n]^-, n = 1-23) were measured in the OH and CH stretch regions to investigate the cluster structures, especially at large sizes. In the size range of n = 4-23, the IR spectra show features almost identical to those of protonated water cluster cations H⁺(H₂O)_m consistent with the occurrence of the intracluster proton-transfer reaction from the benzene moiety to the water moiety, as suggested in the previous IR and electronic spectroscopic studies of the small-sized clusters. The structure of the protonated water moiety was found to be almost the same as that of H ⁺(H₂O)_n up to n = 23, including the characteristic three-dimensional cage structure at n = 21. On the basis of the IR spectra, it was demonstrated that the phenyl radical participates in the direct solvation of the H₃O⁺ core in the clusters of n ≤ 10. In the larger sized clusters, while the protonated water moiety forms the exclusive hydrogen bond network, the phenyl radical is finally pushed out to the exterior of the network, reflecting the microscopic hydrophobicity of the aromatic ring.