Although the freezing of water is a more common phenomenon, it has been difficult to gain insights into the microstructure of ice and its hydrogen-bonding networks.
It is proposed that the water octamer’s low-energy structure is nominally cubic, including eight tri-coordinated water molecules arranged at the eight corners of the cube. Tri-coordinated water molecules such as these have been found at the surface of the ice.
The experimental characterization of water octamer has been performed only by a few gas-phase studies, and the result is two almost isoenergetic structures with D2d and S4 symmetry.
At present, there is a change to this insight. A team of researchers under the guidance of 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, unraveled that five cubic isomers coexist in the tiniest ice cube, of which two exhibit chirality.
The study was reported in the Nature Communications journal on October 28th, 2020.
Prof. JIANG and Professor YANG devised an infrared spectroscopy technique for neutral clusters that relies on a tunable vacuum ultraviolet free-electron laser (VUV-FEL).
This technique paved a new path for the analysis of vibrational spectra of an extensive range of neutral clusters that could not be studied earlier.
Professor JIANG stated, “We measured infrared spectra of size-selected neutral water octamer using the VUV-FEL-based infrared scheme.”
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.
YANG Xueming, Professor, Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Prof. LI’s group performed quantum chemical studies to comprehend the water octamer’s electronic structure. They identified that the relative energies of such structures reflect topology-dependent, delocalized multi-center hydrogen-bonding interactions.
The study showed that even with a general structural motif, the extent of cooperativity among the hydrogen-bonding network led to a hierarchy of unique species. It offered essential data for a basic understanding of the processes involved in the formation of ice, aerosol, and cloud, specifically under rapid cooling.
The study results offer a stepping-stone for the precise description of the intermolecular potentials of water to gain insights into the macroscopic properties of water and inspire further study of intermediate-ice structures that form during the crystallization process of ice.
Li, G., et al. (2020) Infrared spectroscopic study of hydrogen bonding topologies in the smallest ice cube. Nature Communications. doi.org/10.1038/s41467-020-19226-6.