Molecular Interactions Found in Extreme Phases of Water Ice

Water is found everywhere even though it is not the same everywhere. When frozen under extreme temperatures and pressures, ice takes on a range of complex crystalline structures.

The molecular environment and network structures of different phases of water ice (CREDIT: C.G. Salzmann)

Many of the behaviors and properties of these exotic ices continue to be mysterious, however, recently, a team of Researchers provided new understanding. These Researchers examined how water molecules interact with one another in three types of ice and discovered the interactions depended firmly on the orientation of the molecules and the overall structure of the ice. The results have been described by the Researchers in The Journal of Chemical Physics, from AIP Publishing.

The new work has given us spectacular new insights on how water molecules behave when packed in dense and structurally complex environments. Ultimately, this knowledge will enable us to understand liquid water as well as water surrounding biomolecules in a much better fashion.

Christoph Salzmann, University College London

It is an established fact that water is indeed essential for life. However, it is also unique because of its bent molecular shape, with two hydrogen atoms hanging off an oxygen atom at an angle. The overall molecule comprises of an electrical polarity, with negatively and positively charged sides. Water molecules can fit together in different ways when solidifying into ice because of these properties.

The molecules gather into a lattice with structural units shaped like hexagons as water typically freezes on Earth. But at very high pressures and low temperatures, the molecules can assemble themselves in more complex ways, producing 17 different phases - some of which may be present on the icy moons of the outer planets.

While the structures themselves are known, Scientists do not yet completely understand how the molecules interact and affect one another, explained Peter Hamm of the University of Zurich. In these ice phases, the molecules are attached together, influencing one another as if they were all attached with springs.

In order to understand these interactions, Salzmann, Hamm and their team employed 2D infrared spectroscopy on three ice phases with diverse structures: ice II, ice V and ice XIII. In this method, a sequence of ultrashort infrared laser pulses was fired in order to excite the molecular bonds in the ice, making them vibrate.

The molecule emits light at infrared frequencies as the molecular vibrations settle back down to a lower energy state. By measuring how the intensity of the infrared emission relies on the frequencies of both the emitted radiation and the pulse - producing 2D spectra - the Researchers will be able to determine how the molecules interact with one another.

After gathering data on the ice, some of which had to be frozen at below -200 oC and at pressures several thousand times that of the atmosphere at sea level, the Researchers made use of computer simulations of molecular interactions in order to help interpret their results. While the simulations corresponded with the data for ice II, they did not for ice XIII and V, which speaks to the complexity of the system.

Still, insights from this kind of analysis can enable informing computer simulations employed for modeling the behavior of biological molecules such as proteins, which are generally surrounded by water.

Water ice is important, and we need to be able to understand it on a very microscopic level," Hamm said. "Then we can better understand how water interacts with other molecules, and particularly biomolecules.

Peter Hamm, The University of Zurich

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