Characterization of the Hydrogen-Bonded Cluster Ions [Phenol−(H₂O)n]⁺ (n = 1−4), (Phenol)₂⁺, and (Phenol−Methanol)⁺ As Studied by Trapped Ion Infrared Multiphoton Dissociation Spectroscopy of Their OH Stretching Vibrations

OH stretching vibrations of hydrogen-bonded cluster ions of phenol (PhOH), [PhOH−(H 2 O) n ] + ( n = 1−4), (PhOH) 2 +, and (PhOH−methanol) + have been observed with infrared photodissociation spectroscopy in combination with an ion-trapping technique. Cluster ions were efficiently generated by ionization of phenol followed by a jet expansion and were mass-selectively stored by the radio frequency ion trap method, which allows us to observe infrared multiphoton dissociation yield spectra of size-selected cluster ions. For [PhOH−(H 2 O) n ] +, the OH stretching vibrations of the water moieties strongly suggested that the n ≥ 3 cluster ions exhibit the proton-transferred form, [PhO−H 3 O + (H 2 O) n - 1 ], while the n = 1 and 2 ions are of the nontransferred form, [PhOH + −(H 2 O) n ]. As for (PhOH) 2 +, the infrared spectra indicate that the dimer ion is characterized as the open form, in which the phenol ion acts as a proton donor and the neutral phenol as an acceptor through their single hydrogen bond. The similar open form is also found for (PhOH−methanol) +, in which the phenol ion acts as a proton donor.