An Infrared Study of π-Hydrogen Bonds in Micro-solvated Phenol: OH Stretching Vibrations of Phenol−X (X = C6H6, C2H4, and C2H2) Clusters in the Neutral and Cationic Ground States

Abstract

Infrared spectra of phenol−X (X = C6H6, C2H4, and C2H2) clusters in the neutral and cationic ground states were observed in the OH stretching vibrational region. For the neutral ground state, infrared−ultraviolet double resonance spectroscopy was utilized to observe the infrared spectra. A small low-frequency shift of the OH vibration of the phenol site in all the clusters represented the characteristic feature for their π-hydrogen-bonded structures, which were also confirmed by density functional theoretical calculations. The OH frequency shifts did not remarkably depend on the type of the π-electrons. The correlation between the proton affinity of X and the OH frequency shift, which has been known for conventional σ-hydrogen-bonded phenol clusters, was held in phenol−C2H4 and −C2H2, while phenol−C6H6 showed a clear deviation from the correlation. For the cationic ground state, infrared photodissociation spectroscopy was used to observe the infrared spectra. The OH frequency of these clusters exhibited a...