The water hexamer, shown, is the smallest assembly of water molecules that adopts a three-dimensional structure displaying hydrogen bonding patterns characteristic of liquid water and ice. The solid red spheres depict the positions for the oxygen atoms in the water hexamer structure from Pate's experiments. The full molecular structure, superimposed on the experimental positions, is obtained directly from quantum chemistry calculations. (Credit: University of Virginia)
Most understanding at that level is theoretical, requiring the use of supercomputers to make innumerable calculations over periods of weeks to make educated guesses as to the arrangements and structure of water clusters before they form into liquid water or ice.Water is the most abundant and one of the most frequently studied substances on Earth, yet its geometry at the molecular level — the simple two hydrogen atoms and one oxygen atom, and how they interact with other molecules, including other water — has remained somewhat of a mystery to chemists.
But a new study, using experimentation with a highly advanced spectrometer for molecular rotational spectroscopy, has removed some of the mystery and validates some very complex theory involving the way water molecules bond. It is published in the May 18 issue of the journal Science.
“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,” said Brooks Pate, a chemist in the University of Virginia’s College of Arts & Sciences who led the study. “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 properties of water, and how it interacts with itself and other molecules, is the basis for many processes in biology, and likely played a major role in the development of life on Earth. But understanding how those bonds form at the molecular level has been largely guesswork.
“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 his U.Va. team identified and imaged a three-dimensional geometry that a water molecule takes on that is the likely precursor structure for forming liquid water and ice. ”We found that the bonding strengths of liquid water actually begin to emerge even in a tiny cluster,” Pate said. “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.”
More here Chemists merge understanding of water molecule.
