Structures of size-selected hydrogen-bonded phenol-(H2O)(n) clusters in S-0, S-1 and ion

The structures of the size-selected hydrogen-bonded phenol-(H2O)(n) clusters were investigated by analysing the OH stretching vibrational spectra in S-0, S-1 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 So, 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 less than or equal to n less than or equal to 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 S-1, the reduction of the frequency of the OH stretch vibrations upon H-bond was found to be much larger compared to that in So. 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 S-1. The increase of the acidity in S-1 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 withn greater than or equal to 3 by the observation of the OH stretch vibration characteristic to the hydrated hydronium ions.