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

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 an extremely large low-frequency shift upon ionization, reflecting the significant enhancement of the π-hydrogen bond strength. The π-hydrogen bond energies in the cations were estimated on the basis of both the experiments and the theoretical calculations.