A new spin on water research: Bonding

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  • Published: Jun 1, 2012
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: A new spin on water research: Bonding

Escaping the theoretical cage

Molecular rotational spectroscopy has been used to remove some of the mystery regarding the anomalous properties of water due to its unique bonding mode.

Modelling water
(Credit: Pate et al).

Molecular rotational spectroscopy has been used to remove some of the mystery regarding the anomalous properties of water due to its unique bonding mode.

Water is an endlessly fascinating molecule, not least because it is essential to life. The narrow range of temperatures in which it can exist in all three main states of matter, its specific heat capacity, its expansion on freezing, its remarkable solvating capacity, are all targets for the curious investigator. We have many clues regarding its geometry, its molecular shape, even the hydrogen bonds that hook together, however transiently, individual H2O molecules, are understood to some degree at the molecular level. And yet, the superficially simple configuration of an oxygen atom flanked at an angle by two hydrogen atoms remains an enigma.

Much of what we know about water is based on our everyday experience, underpinned by sophisticated theoretical models that usually require supercomputing prowess to crunch countless numbers over weeks and months. These nevertheless offer us nothing more than an educated guess as to the arrangement of water molecules as they form liquid clusters liquid prior to freezing into ice, for instance. Thankfully, a new experimental study based on molecular rotational spectroscopy has now provided some much needed physical validation of the theoretical.

"We set out to determine quantitatively the structure that small assemblies of water adopt, and then compare them to theory to see how well current quantum chemistry predicts the properties of molecules," explains chemist Brooks Pate of the University of Virginia's College of Arts & Sciences in Charlottesville, Virginia. "We found experimentally that modern quantum chemistry has reached the point where its theories are proving out in the lab regarding the unusual directional bonding properties of water clusters."

The study not only gets to the heart of why icebergs float and why lakes freeze from the top down, but also helps explain some of the innumerable biological interactions involving water and in some cases deliberately excluding water. How water interacts with itself and either hydrophobic or hydrophilic molecules plays no small part in life on Earth. If life exists elsewhere in the universe, it would not be too much of a surprise to find that it also relies on water as its vital solvent.

Solidifying experiment and theory

"For the first time, now we have an actual physical picture of what water's molecules put together look like, and it turns out they adopt three different geometries," Pate said. "This is in agreement with theory." Pate and colleagues have identified and imaged a three-dimensional geometry adopted by a water molecule and suggest that it is the precursor structure for formation of liquid water and ice. "We found that the bonding strengths of liquid water actually begin to emerge even in a tiny cluster," Pate explains. "The challenge is figuring out how it interacts with other molecules and how the forces between two molecules of water can be described quantitatively, because the orientation of how the waters come at each other makes a big difference in the binding."

Pate's experimental work meshes well with the quantum chemistry of George Shields' team at Bucknell University in Lewisburg, Pennsylvania, who have been working on the theoretical details for many years. The researchers point out that theoretically water should form hexamers, these would be the smallest possible cluster having a three-dimensional hydrogen-bonding network at minimum energy. Calculations have shown several possible low-energy configurations depending on the specifics of the computations. Earlier experimental work has even demonstrated the existence of cage, book, and cyclic isomers, but none have been seen together.

The difficulty has been in developing techniques that are sensitive enough to image the water molecules and their orientation. Pate's team developed broadband rotational spectroscopy in a pulsed supersonic expansion to unambiguously identify all three isomers using oxygen-18 water. They were also able to determine the relative isomer populations using this technique under different expansion conditions and so show that the cage isomer is the minimum energy structure. The team also identified predicted heptamer and nonamer structures.

"This will allow chemists to transfer what we've learned to larger systems," Pate explains. "We are checking to see if theory can get right the structures of the arrangements of water molecules so that that information can be used to see how water interacts in larger systems." Larger systems might include biopolymers, such as proteins and DNA and how surrounding water molecules might interact with those molecules through hydrogen bonding. "It is very satisfying to see that the experimental work we did, completely independently of theory, came together so perfectly with the theory," Pate says.

"A key feature of the theoretical work is that it includes the effects of 'zero-point vibrational energy' on the cluster structure and makes this an 'apples-to-apples' comparison for theory and experiment," Pate told SpectroscopyNOW. The Virginia and Pennsylvania teams worked with colleagues from the Polish Academy of Sciences, in Warsaw, Poland.

"Structures of Cage, Prism, and Book Isomers of Water Hexamer from Broadband Rotational Spectroscopy", Science, 2012, 336, 897-901; DOI: 10.1126/science.1220574

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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