The water heptamer, (H₂O)₇, is significant in the context of so-called amorphous precursor to the "crystal-like" cubic structures. The neutral (H₂O)₇ ice-like cluster is a key species in understanding the hydrogen-bonding network transitions between liquid water and ice.

Recently, a research team led by Prof. JIANG Ling and Prof. YANG Xueming from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences, in collaboration with Prof. LI Jun from Tsinghua University, has revealed the structural complexity of ice-like clusters.

The study was published in Cell Reports Physical Science on Feb. 1.

Prof. JIANG's group and Prof. YANG's group developed a method of infrared spectroscopy of neutral clusters based on a tunable vacuum ultraviolet free electron laser (VUV-FEL). This method provides a new opportunity for the study of vibrational spectra of a wide variety of neutral clusters that could not be studied before.

They measured infrared (IR) spectra of size-selected neutral (H₂O)₇ using the VUV-FEL-based IR scheme. A variety of diverse bands observed in the IR spectrum of the water heptamer provide evidence for the presence of various low-lying structures at finite temperatures.

Prof. LI's team carried out quantum chemical studies to identify the structure of the low-lying isomers and to assign the observed IR spectral features.

The consistency of the experimental and calculated infrared spectra established general trends in the structural motifs. They found that the major contribution for the experimental spectrum originated from the prism-like isomers, although the minor contribution of the higher-energy cage-like isomers was non-negligible.

This study improved the understanding of structural diversity and complexity of water heptamer in the context of the formation and growth of ordered structures in larger systems.