Size-Dependent Hydrogen Bonds of Cluster Ions between Phenol Cation Radicals and Water Molecules: A Molecular Orbital Study

The molecular structures and vibrational frequencies of [C 6 H 5 OH−(H 2 O) n ] •+ ( n = 1−4) are studied by employing ab initio molecular orbital methods. Since the hydrogen bond between phenol cation radicals and water molecules is much stronger than that of the neutral phenol−water system, the position of the proton of phenol cation radicals depends on the number of water molecules in the clusters [C 6 H 5 OH−(H 2 O) n ] •+ . Although the stable structure of [C 6 H 5 OH−(H 2 O) n ] •+ varies depending on the method used in the calculation, the result obtained with the B3LYP density functional method gives good agreement with the experimental IR spectra. The proton-nontransferred form is found to be most stable for n = 1 and n = 2 clusters. In the cases of n ≥ 3, the most stable structures are the proton-transferred form. There are two types of structures obtained for n = 3 clusters, where the branched form is more stable than the chained form. The optimized structures for n = 3 and n = 4 clusters show that the H 3 O + moiety prefers to interact with the phenoxy radical and two water molecules.