Researchers have recently achieved a significant breakthrough by directly observing molecules engaged in hydrogen bonds in liquid water, assessing electronic and nuclear quantum effects that were previously only seen through theoretical simulations.
Water is essential for life, yet the complex interactions that bring H2O molecules together through hydrogen bonds are still not entirely understood. These bonds form when hydrogen and oxygen atoms from different water molecules interact, leading to the sharing of electronic charge. This charge-sharing is fundamental to the three-dimensional ‘H-bond’ network that gives liquid water its distinct properties; however, the quantum processes driving these networks have mostly been explored through theoretical models until now.
Now, under the leadership of Sylvie Roke, who heads the Laboratory for Fundamental BioPhotonics at EPFL’s School of Engineering, researchers have introduced a novel technique known as correlated vibrational spectroscopy (CVS). This method allows them to analyze the behavior of water molecules within hydrogen bond networks. Importantly, CVS distinguishes between the interacting molecules involved in the bonds and the randomly distributed, non-interacting molecules. In contrast, existing methods measure both types of molecules at once, making it challenging to differentiate between them.
“Existing spectroscopy techniques monitor the scattering of laser light from all molecules in a sample, which requires assumptions about the molecular interactions being observed,” Roke clarifies. “With CVS, each type of molecule has its own unique vibrational spectrum. Since each spectrum features distinct peaks linked to the movement of water molecules along the hydrogen bonds, we can directly assess their properties, including the extent of charge-sharing and the strength of the hydrogen bonds.”
The researchers claim that this innovative technique, published in Science, has transformative potential for characterizing interactions within any material.
A fresh perspective
To differentiate between interacting and non-interacting molecules, the scientists exposed liquid water to femtosecond (one quadrillionth of a second) laser pulses in the near-infrared range. These extremely brief flashes of light induce small charge oscillations and atomic movements in the water, resulting in the emission of visible light. This emitted light produces a scattering pattern that reveals critical data about how the molecules are organized spatially, while the color of the emitted photons provides insights into atomic movements occurring within and between the molecules.
“In typical experiments, the spectrographic detector is positioned at a 90-degree angle to the incoming laser. However, we discovered that by adjusting the detector’s position and using specific combinations of polarized light, we could analyze interacting molecules separately, thus generating distinct spectra for both non-interacting and interacting molecules,” Roke explains.
The research team conducted additional experiments with CVS to explore the electronic and nuclear quantum effects within hydrogen bond networks, experimenting with water pH levels by adding hydroxide ions (increasing basicity) or protons (increasing acidity).
PhD student Mischa Flór, the first author of the study, notes, “Hydroxide ions and protons take part in hydrogen bonding, so adjusting the water’s pH influences its reactivity.” Through CVS, the research team measured precisely how much additional charge hydroxide ions contribute to the hydrogen bond networks (8%) and how much charge protons receive from them (4%). These accurate measurements were supported by advanced simulations from collaborators in France, Italy, and the UK.
The researchers highlight that this method, which they also validated through theoretical calculations, can be applied to various materials, and multiple new characterization experiments are already in progress.
“The capability to directly quantify the strength of hydrogen bonds is a powerful tool for elucidating molecular-level details of any solution, such as those containing electrolytes, sugars, amino acids, DNA, or proteins,” Roke states. “Since CVS is not restricted to water, it promises a wealth of information about other liquids, systems, and processes as well.”
