Direct observation of weak hydrogen bonds in microsolvated phenol: Infrared of OH stretching vibrations of phenol-CO and -CO₂ in S₀ and D₀

Infrared spectroscopy was applied to phenol-CO and phenol-CO₂ clusters in a molecular beam, and the OH stretching vibrations of these clusters were observed both in their neutral (S₀) and in their cationic (D₀) ground states. Their OH frequency in So shows a small but definite low-frequency shift, indicating that the phenol moiety acts as a proton donor in their hydrogen-bonded structures. The magnitude of the frequency shifts well correlates with proton affinities of the proton-accepting CO and CO₂ molecules. The OH stretching vibrations in Do showed a substantial low-frequency shift, representing that the hydrogen-bond strength is remarkably enhanced upon ionization. Density functional theoretical calculations of the cluster structures were carried out to support discussion.