In a groundbreaking discovery by RIKEN chemists, an elusive structure involving two water molecules, which had long been predicted but never observed, has finally been isolated. This discovery has the potential to have wide-ranging implications in various fields such as astrochemistry and the corrosion of metals. The research paper detailing this discovery has been published in The Journal of Physical Chemistry Letters.

When an energetic particle or photon knocks out an electron from a water molecule, it creates a positive ion (cation; H2O+) and an electron, a process known as ionization of water. This ionization event can trigger a cascade of subsequent reactions with nearby molecules. Understanding the ionization of water is crucial for a variety of applications, including biological processes, radiation chemistry, and the promotion of corrosion at water-metal interfaces.

Calculations have long predicted that following the ionization of a water molecule, two isomers of a positively charged ion of a water dimer would form rapidly. One isomer, known as H3O+·OH, has been previously observed and occurs when a proton is transferred from one water molecule to another. The other isomer, with a half-bond or hemibonded structure (H2O·OH2)+, has never been isolated or confirmed through spectroscopic measurements due to its higher energy compared to the proton-transfer dimer.

RIKEN’s Innovative Approach

Susumu Kuma and his team at the RIKEN Atomic, Molecular, and Optical Physics Laboratory managed to isolate both water dimer ions by trapping them in tiny droplets of cold helium. By using infrared spectroscopy, they were able to determine the structures of these elusive ions. The researchers created an ultracold environment where water molecules cooled rapidly as helium atoms evaporated from the surface of the droplets, facilitating the formation of the metastable hemibonded isomer due to its quick stabilization within the cold droplets.

Advancements in Structural Analysis

The spectroscopic signatures of the molecular ions isolated by Kuma and his team closely resembled those of bare ions without the presence of helium. This enabled the researchers to directly compare the measurements of the bare ions with results from quantum-chemical calculations, greatly enhancing the structural analysis of the dimers. The discovery of these hemibonded water cations is expected to catalyze further studies in the field, particularly in understanding the radiation chemistry of water.

Future Directions

Looking ahead, Kuma’s team plans to expand their research by searching for other structures that have not yet been observed. They aim to extend the size of the water complex cations in helium droplets to uncover additional insights into the behavior and properties of these unique molecular ions. The discovery of the water dimer ions represents a significant step forward in our understanding of physical chemistry and opens up new avenues for exploration in related fields.


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