Hydrogen Bonding in Aromatic Alcohol-Water Clusters: A Brief Review

Recent experimental and theoretical advances on the aromatic alcohol-water clusters are reviewed, focusing on the structure of the hydrogen bonding between the alcoholic OH group and the binding water molecules. The interplay of experimental observations and theoretical calculations for the elucidation of the structure is demonstrated for phenol-water, benzyl alcohol-water, substituted phenol-water, naphthol-water and tropolone -water clusters. Discussion is made on assigning the role (either proton-donating or -accepting) of the hydroxyl group by measuring the shifts of infrared frequency of the OH stretching mode in the cluster from that of bare aromatic alcohol for the experimental determination of the cluster structure. and biology, profoundly affecting the properties of the molecule. It is well known, for example, that the three dimensional structures of protein are mostly the results of the hydrogen bonding between the constituting amino acids. In the aqueous solution, the interactions of the amino acids with the solvent molecules, 3-8 another example of hydrogen bonding, may also play a fundamental role of the structure and reaction of protein. Therefore, systematic study on the hydrogen bond can reveal invaluable information for the structure and biochemical activity of protein. Since there are extremely many solvent molecules in the solution phase, however, it is more useful to study the hydrogen bonding in the clusters 9-23 consisting of a solute and a few solvent molecules. Hydrogen bonding between the hydroxyl (OH) group and the water molecule(s) in organic alcohol-water clusters has been studied intensively as a prototypical model for more complex system. The organic alcohol-water clusters are of moderate size for quantum chemical calculations, allowing the comparison with the experimental observations by the molecular beam and the UV-IR double resonance techniques. Elucidation of the structures of their conformers lying closely in energy may also give a lot of useful information for the interactions between the alcohol molecule and the water molecules in the solution phase 24 on the molecular level. In the present review we discuss recent advances on the understanding of hydrogen bonding in aromatic alcohol- water clusters. Experimental measurements and theoretical calculations of the structure, binding energy, 25,26 and infrared frequency of phenol-water, benzyl alcohol-water, substituted phenol-water, β-naphthol-water and tropolone-water clusters are reviewed. Elucidation of the nature of hydrogen bonding in these clusters by the analysis of the shifts in the vibronic bands observed by spectroscopic techniques is exemplified. Electrostatic and steric effects of substitution at the phenyl ring on the structure of hydrogen bonding in the substituted phenol-water clusters are discussed. The OH stretching frequency is described in relation to the structure and the strength of the hydrogen bond in the clusters. Phenol-water, benzyl alcohol-water and phenylpropyl alcohol-water clusters. The phenol-(H2O)n clusters have been studied most intensively by many investigators 15-22 as the prototypical system for organic alcohol-water cluster. As depicted in Figure 1, the role of the OH group in the phenol- (H2O)n cluster can be either proton-donating or -accepting. The conformer (P11) with the proton-donating OH group is calculated to be of lower energy than the proton-accepting one (P12). Experimentally, only the conformer with the proton -donating OH has been observed so far, and its measured binding energy is 5.47(±0.09) kcal/mol. 27 The kinetic stability of the conformer with the proton-accepting OH group is not known, however, if the latter conformer may be observed experimentally, the different behavior of the OH stretching frequencies in the complexes P11 and P12 (significant red shift for P11 and slight blue shift for P12; see Table 1) from that of the bare phenol would be key observable for distinguishing the two conformers. Figure 2 presents the benzyl alcohol-H 2O clusters corre- sponding to the phenol-H2O clusters in Figure 1. In the