Ab Initio Study of Lower Energy Phenol−Water1 ≤ n ≤ 4 Complexes:  Interpretation of Two Distinct Infrared Patterns in Spectra of Phenol−Water Tetramer

The lower energy portion of the potential energy surface of the interaction between phenol and four water molecules is thoroughly studied at a variety of computational levels including HF, MP2, and B3LYP in conjunction with 6-31G(d) basis set and MP2/6-31+G(d). The aim of the present work is 2-fold: first, to juxtapose this potential energy surface with those of the phenol−water n =1 - 3 complexes, whose bottoms are occupied by the structures exhibiting 2D ring-type arrangement of water molecules, and second, to offer a new sound theoretical interpretation of the experimental results obtained by the Stanley and Castleman and by the Mikami and Ebata groups via demonstrating a firm existence of the lower energy phenol−water4 isomer characterized by a three-dimensional arrangement of its water molecules. We show that it is precisely that isomer which is capable to provide a complete explanation of the puzzled “window” region in the IR stretching spectra of the phenol−water4 complex. Its three-dimensionality originates due that one of its water molecules form a π hydrogen bond with π cloud of the phenol ring. We explain a sort of “magic” of the number four of water molecules interacting with phenol in terms of that the ability of the phenolic OH-group to accept a hydrogen bond from water molecule becomes nearly exhausted when three water molecules form a two-dimensional ring and, therefore, competes with the ability of the π cloud of the phenol ring to form a π hydrogen bonding leading to a three-dimensional pattern.