Direct Observation of Weak Hydrogen Bonds in Microsolvated Phenol: Infrared Spectroscopy of OH Stretching Vibrations of Phenol−CO and −CO₂ in S₀ and D₀

Infrared spectroscopy was applied to phenol−CO and phenol−CO 2 clusters in a molecular beam, and the OH stretching vibrations of these clusters were observed both in their neutral (S 0 ) and in their cationic (D 0 ) ground states. Their OH frequency in S 0 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 CO 2 molecules. The OH stretching vibrations in D 0 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.