Water is essential for life, but the hydrogen bond—a critical interaction that brings H2O molecules together—remains complex and not fully understood.
Hydrogen bonds occur when hydrogen and oxygen atoms in water molecules interact, sharing an electronic charge. This charge-sharing creates a 3D network of hydrogen bonds that gives liquid water unique properties.
However, theoretical simulations have primarily explored the quantum phenomena underlying these networks.
Researchers led by Sylvie Roke at EPFL have developed a new method called correlated vibrational spectroscopy (CVS) that measures the behavior of water molecules within hydrogen bond networks.
CVS enables scientists to differentiate between interacting (H-bonded) molecules and randomly distributed, non-interacting molecules. This is significant because existing methods cannot distinguish these two types, so mixed measurements are provided instead.
Current spectroscopy methods detect the scattering of laser light caused by molecular vibrations, requiring assumptions about which interactions are being observed.
Each type of molecule with CVS has a distinct vibrational spectrum characterized by unique peaks. Notably, one peak corresponds to the movement of water molecules along hydrogen bonds, allowing direct measurement of properties like shared electronic charge and the strength of hydrogen bonds.
To differentiate between interacting and non-interacting molecules, researchers illuminated liquid water with femtosecond laser pulses in the near-infrared spectrum. These brief light bursts generated tiny charge oscillations and atomic displacements, leading to the emission of visible light.
The scattering pattern of this emitted light reveals information about the molecules’ spatial organization, while the photons’ color indicates atomic displacements within and between the molecules.
Roke said, “Typical experiments place the spectrographic detector at a 90-degree angle to the incoming laser beam, but we realized that we could probe interacting molecules simply by changing the detector position and recording spectra using certain combinations of polarized light. This way, we can create separate spectra for non-interacting and interacting molecules.”
The team performed additional CVS experiments to analyze hydrogen bond networks’ electronic and nuclear quantum effects. They varied the water pH by adding hydroxide ions to make it more basic or protons to increase acidity, allowing them to study the impact of these changes on the hydrogen bond dynamics.
PhD student Mischa Flór, the paper’s first author, said, “Hydroxide ions and protons participate in H-bonding, so changing the pH of water changes its reactivity. With CVS, we can now quantify exactly how much extra charge hydroxide ions donate to H-bond networks (8%) and how much charge protons accept from it (4%) – precise measurements that could never have been done experimentally before.”
The researchers highlight that their method, validated through theoretical calculations, can be applied to any material. Several new characterization experiments using this technique are already in progress.
Roke said, “The ability to quantify H-bonding strength directly is a powerful method that can be used to clarify molecular-level details of any solution, for example, containing electrolytes, sugars, amino acids, DNA, or proteins.”
“As CVS is not limited to water, it can also deliver information on other liquids, systems, and processes.”
Journal Reference:
- Mischa Flór et al., Dissecting the hydrogen bond network of water: Charge transfer and nuclear quantum effects. Science, DOI: 10.1126/science.ads4369