Spectral signatures of four-coordinated sites in water clusters: Infrared spectroscopy of phenol-(H₂O)_n (∼20 ≤ n ≤ ∼50)

We report infrared spectra of phenol-(H₂O)_n (∼20 ≤ n ≤ ∼50) in the OH stretching vibrational region. Phenol-(H ₂O)_n forms essentially the same hydrogen bond (H-bond) network as that of the neat water cluster, (H₂O)_n+1. The phenyl group enables us to apply the scheme of infrared-ultraviolet double resonance spectroscopy combined with mass spectrometry, achieving the moderate size selectivity (0 ≤ Δn ≤ ∼6). The observed spectra show clear decrease of the free OH stretch band intensity relative to that of the H-bonded OH band with increasing cluster size n. This indicates increase of the relative weight of four-coordinated water sites, which have no free OH. Corresponding to the suppression of the free OH band, the absorption peak of the H-bonded OH stretch band rises at ∼3350 cm^-1. This spectral change is interpreted in terms of a signature of four-coordinated water sites in the clusters.