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-(H₂O)_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-(H₂O)_n]⁺, the OH stretching vibrations of the water moieties strongly suggested that the n ≥ 3 cluster ions exhibit the proton-transferred form, [PhO-H₃O⁺(H₂O)_n-1], while the n = 1 and 2 ions are of the nontransferred form, [PhOH⁺-(H₂O)_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.