Water’s pyramid shape confirmed

Researchers at Johannes Gutenberg University Mainz (JGU) have confirmed the original model of the molecular structure of water. The findings have made it possible to resolve a long-standing scientific debate about liquid H₂O’s structure.

The tetrahedral model was first hypothesised nearly 100 years ago. It assumes that each water molecule forms a hydrogen bond with four adjacent molecules. Research conducted in 2004 almost overturned this concept, when it was suggested that water molecules form bonds with only two other molecules.

“The quality of the results was excellent but they merely represent a snapshot of the situation,” explained Professor Dr Thomas Kühne.

Using computer simulations, Kühne and his team have been able to refute this ‘double bonding’ theory based on new types of combinations of two computational methods developed recently by his research group.

Hydrogen bonds, which give water some of its unique properties such as its liquid aggregate state and high boiling point, are formed due to the different charges carried by the oxygen and hydrogen atoms.

It was traditionally suggested that water has a tetrahedral structure at room temperature so that on average, each water molecule would be linked with four adjacent ones via two donor and acceptor bonds.

“In our theoretical approach, the median result we observed over time was always for quadruple bonding,” Kühne confirmed.

He says that the reason for the surprising results observed in 2004 was not indicative of double bonding, “but of instantaneous asymmetrical fluctuation”.

That study used X-ray absorption spectroscopy and results were obtained using water molecules with high levels of momentary asymmetry, which is why essentially only two strong hydrogen bonds were observed in an otherwise tetrahedral structure.

“Our findings have important implications as they help reconcile the symmetric and asymmetric views on the structure of water,” the researchers write in the paper published in Nature Communications.