Structures of size-selected hydrogen-bonded phenol-(H₂O)_n clusters in S₀, S₁ and ion

The structures of the size-selected hydrogen-bonded phenol-(H₂O)_n clusters were investigated by analysing the OH stretching vibrational spectra in S₀, S₁ and the ionic states. Several spectroscopic methods were applied to measure the vibrational spectra of the clusters; Ionization detected IR (IDIR) and fluorescence detected IR (FDIR) spectroscopies for the neutral clusters, and trapped ion infrared multiphoton dissociation spectroscopy for the ionic clusters. In S₀, characteristic red shifts of the OH stretch vibrations upon hydrogen bond (H-bond) formation were observed and it was confirmed that the most stable clusters for 2 ≤ n ≤ 4 have the ring-form structure. The observed OH stretching vibrations fit well with the calculated ones which were obtained by Watanabe and Iwata with ab initio molecular orbital calculations. In S₁, the reduction of the frequency of the OH stretch vibrations upon H-bond was found to be much larger compared to that in S₀. The result demonstrates the evidence that a proton of the phenolic OH group is more attracted to H₂O (proton acceptor), that is, an acidity of phenol increases in S₁. The increase of the acidity in S₁ leads a drastic distortion of the ring structure of the phenol-(H₂O)₂ cluster, which is the smallest ring-form cluster. In the ionic cluster, an indication of the proton transfer was obtained for the clusters with n ≥ 3 by the observation of the OH stretch vibration characteristic to the hydrated hydronium ions.