Water is synonymous with life, but the dynamic, multifaceted interaction that brings H2O molecules together – the hydrogen bond – remains mysterious. Hydrogen bonds result when hydrogen and oxygen atoms between water molecules interact, sharing electronic charge in the process. This charge-sharing is a key feature of the three-dimensional 'H-bond' network that gives liquid water its unique properties, but quantum phenomena at the heart of such networks have thus far been understood only through theoretical simulations.
Now, researchers led by Sylvie Roke, head of the Laboratory for Fundamental BioPhotonics in EPFL's School of Engineering, have published a new method – correlated vibrational spectroscopy (CVS) – that enables them to measure how water molecules behave when they participate in H-bond networks. Crucially, CVS allows scientists to distinguish between such participating (interacting) molecules, and randomly distributed, non-H-bonded (non-interacting) molecules. By contrast, any other method reports measurements on both molecule types simultaneously, making it impossible to distinguish between them.
"Current spectroscopy methods measure the scattering of laser 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," Roke explains. "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 how much electronic charge is shared, and how H-bond strength is impacted."
The method, which the team says has "transformative" potential to characterize interactions in any material, has been published in Science .
Looking at things from a new angle
To distinguish between interacting and non-interacting molecules, the scientists illuminated liquid water with femtosecond (one quadrillionth of a second) laser pulses in the near-infrared spectrum. These ultra-short bursts of light create tiny charge oscillations and atomic displacements in the water, which trigger the emission of visible light. This emitted light appears in a scattering pattern that contains key information about the spatial organization of the molecules, while the color of the photons contains information about atomic displacements within and between molecules.
"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. In this way, we can create separate spectra for non-interacting and interacting molecules," Roke says.
The team conducted more experiments aimed at using CVS to tease apart the electronic and nuclear quantum effects of H-bond networks, for example by changing the pH of water through the addition of hydroxide ions (making it more basic), or protons (more acidic).
"Hydroxide ions and protons participate in H-bonding, so changing the pH of water changes its reactivity," says PhD student Mischa Flór, the paper's first author. "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." These values were explained with the aid of advanced simulations conducted by collaborators in France, Italy, and the UK.
The researchers emphasize that the method, which they also corroborated via theoretical calculations, can be applied to any material, and indeed several new characterization experiments are already underway.
"The ability to quantify directly H-bonding strength 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," Roke says. "As CVS is not limited to water, it can also deliver a wealth of information on other liquids, systems, and processes."
