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How Does Water Freeze?

Research team from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences in collaboration with Tsinghua University, discover microstructures of the smallest ice cube.

Water freezing into ice might be common sense and could be of little importance, however, understanding the microstructure of ice and its hydrogen-bond network has proven to be challenging.

The low-energy structure of water octamer – ice – is predicted to be cubic in structure, with eight water molecules at the eight corners of the cube. The tri-coordination of water molecules to form this cubic structure has been identified at the surface of ice.

Few studies at the gas-phase have been successful in achieving experimental characterization of the water octamer structure together with two structure with almost the same energy levels were found.

A research team led by Professor Jiang Ling and Professor 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, revealed the coexistence of five cubic isomers in the smallest ice cube, including two with chirality. The team’s study was published in Nature Communications.

“We measured infrared spectra of size-selected neutral water octamer using the VUV-FEL-based infrared scheme,” said Professor Jiang.

“We observed the distinct features in the spectra, and identified additional cubic isomers with C2 and Ci symmetry, which coexisted with the global-minimum D2d and S4 isomers at finite temperature of the experiment,” said Professor Yang.

Prof. LI’s team conducted quantum chemical studies to understand the electronic structure of the water octamer. They found that the relative energies of these structures reflect topology-dependent, delocalized multi-centre hydrogen-bonding interactions.

The study demonstrated that even with a common structural motif, the degree of cooperativity among the hydrogen-bonding network created a hierarchy of distinct species. It provided crucial information for fundamental understanding of the formation processes of cloud, aerosol, and ice, especially under rapid cooling.

Their findings provide a benchmark for accurate description of the water intermolecular potentials to understand the macroscopic properties of water, and stimulate further study of intermediate-ice structures formed in the crystallization process of ice. [APBN]