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−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 S0 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 D0 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.