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Quantumness of water molecules might explain unexpected behaviors

Water is vital to life as we know it, but there is still a great deal unknown when it comes to correctly modeling its properties. Now researchers have discovered room-temperature water may be even more bizarre than once suspected — quantum physics suggest its hydrogen atoms can travel surprisingly farther than before thought, report findings detailed in the Proceedings of the National Academy of Sciences.

Water is just made of two hydrogen atoms and an oxygen atom, but despite its apparent simplicity, liquid water displays a remarkable number of unusual properties, such as how it decreases in density upon freezing, and the existence of some 19 different forms of ice. Scientists traditionally ascribe water’s peculiar behavior to the hydrogen bond. Water is polar — partial electric charges separate within the molecule, leading to slightly positively charged hydrogen ends and a negatively charged oxygen middle. As such, the hydrogens in one water molecule can get attracted to the oxygen in another, a hydrogen bond that can help explain why water has such a high boiling point, for example.

All of water’s anomalies, together with its unquestionably vital role in climate and life on Earth, have led to intense research around the globe, but still much remains unknown about it. To shed light on water’s behavior, materials scientist Michele Ceriotti at the University of Oxford in England and his colleagues modeled how the atomic nuclei of water’s hydrogen might behave in a quantum way — that is, not like points as the above explanation of hydrogen bonding from classical physics would suggest, but as more delocalized, cloud-like objects.

Unexpectedly, the researchers discovered that after accounting for these nuclear quantum effects, water’s protons can drive hydrogen atoms in water molecules far closer to oxygen atoms in other water molecules at room temperature than previously thought.

“We see that there is about one chance in 1,000 to see one proton closer to the acceptor atom than to the atom it is covalently bound to,” Ceriotti says. Instead of seeing two H2O molecules, this pair would spontaneously ionize, more closely resembling an OH- and a H3O+.

The chance of such closeness occurring when one does take nuclear quantum effects into account is roughly 10,000 times greater than in previous simulations where one did not consider such effects.

“At first we were a bit skeptical of our results, because of the unavoidable approximations that underlie the description of the electronic structure for a system with hundreds of atoms: we could not be sure of how much our findings were affected by these approximations,” Ceriotti says. “To lift these doubts, we performed some checks with a more accurate and much more computationally demanding technique, and observed almost no difference.”

The researchers believe this effect is closely related to the one that different isotopes of hydrogen have on the pH of water. While normal water has a neutral pH of 7, deuterated water possessing deuterium, a hydrogen isotope whose nuclei each possess a neutron, has a slightly alkaline pH of 7.43, while tritiated water possessing tritium, a hydrogen isotope whose nuclei each possess two neutrons, has a more alkaline pH of 7.61. The pH of a solution is linked to how much free hydrogen ion activity it has, and heavier isotopes of hydrogen would find it more difficult to move farther and free themselves from the oxygen atoms they were bonded to than lighter isotopes.

The researchers suggest that any perturbations that might influence hydrogen bonds, such as pressure, interactions with interfaces, and the presence of dissolved ions, could further radically alter water’s behavior. In addition, on a more speculative level, Ceriotti noted there are somewhat controversial experimental observations that water confined in spaces only nanometers or billionths of a meter wide — say, within a carbon nanotube — “exhibits very different behavior than bulk water. If quantum fluctuations can already induce wide proton excursions in bulk water under ambient conditions, one can imagine that the combined effect of quantum fluctuations and the perturbation to the hydrogen bond network could help explain the observed anomalies.”

Ceriotti cautions that at times, “claims of ‘quantumness’ of water are used to justify homeopathy or similar pseudoscientific theories.” In contrast, the effects they witness are very limited, and do not support the existence of any more far-reaching effect.

Categories: Chemistry
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