Theoretical studies of geometric structures of phenol-water clusters and their infrared absorption spectra in the O–H stretching region

The structures of the phenol-(H2O)n clusters (n≤4) are determined with ab initio molecular orbital methods, and their infrared spectra are calculated to analyze the experimental spectra reported in the preceding paper. The experimental infrared spectra of phenol-(H2O)n clusters for n≤4 have ‘‘window regions,’’ which are intervals of two types of OH stretching modes of the water molecules. The calculated IR spectra of isomers with a ring structure will reproduce these window regions. The ring is formed by a network of the hydration bonds of the −OH group of the phenol and water molecules. For n=4, two kinds of spectra are reported in the experiments. One spectrum has a window region similar to that of n≤3. It is, therefore, identified to the isomers of a ring structure. The other one has several bands in the window region. The calculations for several isomers and large clusters suggest that this spectra might be attributed either (i) to the mixture of several branched isomers, (ii) to the products of evaporation of large clusters, or (iii) to the product of the proton transfer reaction in phenol-(H2O)4 cluster.