Infrared spectroscopy of phenol-(H₂O)_n>10: Structural strains in hydrogen bond networks of neutral water clusters

To investigate hydrogen bond network structures of tens of water molecules, we report infrared spectra of moderately size (n)-selected phenol-(H ₂O)_n (∼10 ≤ n ≤ ∼50), which have essentially the same network structures as (H₂O)_n+1. The phenyl group in phenol-(H₂O)_n allows us to apply photoionization-based size selection and infrared-ultraviolet double resonance spectroscopy. The spectra show a clear low-frequency shift of the free OH stretching band with increasing n. Detailed analyses with density functional theory calculations indicate that this shift is accounted for by the hydrogen bond network development from highly strained ones in the small (n < ∼10) clusters to more relaxed ones in the larger clusters, in addition to the cooperativity of hydrogen bonds.