To explore the microhydration structures of protonated phenol, size-selective infrared spectroscopy of protonated phenol-(water)_n clusters (n = 1–5) was performed. The protonation of phenol can occur either at the phenyl ring or at the hydroxy group. The coexistence of the two isomer types separated by the high isomerization barrier was reconfirmed for bare protonated phenol. Preferential hydration of the hydroxy group initially occurs in both the two isomer types of protonated phenol. Development of the water hydrogen-bond network is localized around the hydroxy group up to n = 2. Intracluster proton transfer from the phenol moiety to the water moiety was observed in n ≥ 3–4. The water moiety with the H₃O⁺ ion core resides on the phenyl ring, and the water moiety is bound to the phenyl ring with a π-hydrogen bond. Such a structure is in striking contrast to those of phenol⁺-(water)_n radical cation clusters, in which the water moiety is located away from the phenyl ring even when intracluster proton transfer occurs.

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