Infrared spectroscopic signature of the structural diversity of the water heptamer

As a key species in understanding the hydrogen-bonding network transitions between liquid water and ice, the neutral water heptamer is a challenging experimental target, owing to the richness of low-lying isomers. Here, we report the size-specific infrared spectra of confinement-free, neutral water heptamer (H2O)(7) based on threshold photoionization using a tunable vacuum ultraviolet free electron laser. The complexity of the observed spectra indicates that many nearly isoenergetic isomers are present at finite temperatures. Two classes of prism- and cage-like structures are identified in a high-pressure pulsed supersonic expansion of water heptamer, in which the former is favored energetically at low temperatures and serves as a major contributor to the experimental spectrum. These findings provide key information for filling the gap between well-studied water hexamer and octamer.

Automated summary

This study reports the infrared spectra of neutral water heptamer and identifies its structures. The results demonstrate the presence of multiple nearly isoenergetic isomers at finite temperatures. These findings contribute to bridging the gap between the understanding of water hexamer and octamer.