Looking Deeper into Water

Water and its unique properties are considered to be an essential condition for life – and yet there is much that scientists still do not understand about it. The key to unlocking water’s mysteries lies in understanding the interactions that form its hydrogen-bond network. Despite their apparent simplicity, the physics and chemistry associated with these interactions have proven to be very challenging.

One of the ways scientists have gone about investigating liquid water’s internal structure is through a technique called vibrational spectroscopy, which is measures which wavelengths of light are absorbed by different vibrations. This technique, however, has some limitations. “Current spectroscopy methods measure the scattering of light caused by the vibrations of all molecules in a system, so you have to guess or assume that what you are seeing is due to the molecular interaction you’re interested in,” explains Sylvie Roke of the Ecole Polytechnique Fédérale de Lausanne, Switzerland, a world leading expert in the spectroscopy of water. Roke leads the international team of experimentalists and theoreticians that developed a new experimental method called “correlated vibrational spectroscopy” (CVS), which overcomes some of the drawbacks of current techniques, and which has recently been published in Science.

“With CVS, the vibrational mode of each different type of molecule has its own vibrational spectrum. And because each spectrum has a unique peak corresponding to water molecules moving back and forth along the H-bonds, we can measure directly their properties, such as the extent of electronic polarization along the bond, and subsequently, how H-bond strength is impacted,” Roke adds.

Ali Hassanali of the ICTP’s Condensed Matter and Statistical Physics section is among the theoreticians from Italy, France and Ireland who participated in the collaboration. “With their technique, Roke and her group are able to separate the vibrational spectrum of water into contributions from single molecules and contributions that emerge from their interactions with other water molecules,” he explains.

Hassanali, who performed some of the electronic structure simulations that contributed to the breakthrough, also explains that by combining experimental measurements with results from simulations, the team was able to propose how hydrogen-bonds in different environments are modulated by a coupling of electronic and nuclear degrees of freedom. “This gives us important insights into the nature of hydrogen bonds, which are at the heart of water’s many complexities,” he adds.

The experimental results from Roke’s group are consistent with numerical simulations, that until now had been the only window into the electronic and nuclear quantum properties of hydrogen bonds. “What is even more powerful about this new technique is that CVS can potentially be applied to any liquid, and therefore it opens up the way to a better understanding of many other complex materials,” Hassanali concludes.

The article: “Dissecting the hydrogen bond network of water: Charge transfer and nuclear quantum effects”, Science DOI: 10.1126/science.ads4369 (2024), https://www.science.org/doi/10.1126/science.ads4369