In this work, experiment and theoretical analysis reveal the coexistence of five cubic isomers of water clusters (H2O)8, including two with chirality and the evidence for a noncyclic pentamer. The relative energies of these structures are found to reflect topology-dependent, delocalized multi-center hydrogen bonding interactions. These results demonstrate that even with a common structural motif, the degree of cooperativity among the hydrogen-bonding network creates a hierarchy of distinct species. The implications of these results on possible metastable forms of ice are speculated. Infrared spectroscopic study of neutral water clusters is crucial to understanding of the hydrogen-bonding networks in liquid water and ice. Here we report infrared spectra of size-selected neutral water clusters, (H2O)n (n = 3−6), in the OH stretching vibration region, based on threshold photoionization using a tunable vacuum ultraviolet free-electron laser. Distinct OH stretch vibrational fundamentals observed in the 3,500−3,600-cm−1 region of (H2O)5 provide unique spectral signatures for the formation of a noncyclic pentamer, which coexists with the global-minimum cyclic structure previously identified in the gas phase. The main features of infrared spectra of the pentamer and hexamer, (H2O)n (n = 5 and 6), span the entire OH stretching band of liquid water, suggesting that they start to exhibit the richness and diversity of hydrogen bonding networks in bulk water. As compared to previous spectroscopy of neutral clusters, the advanced VUV-FEL-based IR spectroscopic method has high sensitivity and size-selectivity.

Infrared spectroscopy of neutral water cluster and hydrogen bonding analysis: Smallest ice cube

ACS Spring 2022

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