A new spectroscopy method reveals water's quantum secrets

For the first time, researchers at EPFL have directly observed molecules involved in hydrogen bonds in liquid water. This groundbreaking work measures electronic and nuclear quantum effects previously accessible only through theoretical simulations.

Water is vital for life, yet the intricate interactions that link H2O molecules through hydrogen bonds remain complex. These bonds form when hydrogen and oxygen atoms from different water molecules share electronic charge, creating a three-dimensional H-bond network that gives water its unique properties.

Led by Sylvie Roke, the team developed a new technique called correlated vibrational spectroscopy (CVS). This method allows scientists to distinguish between interacting molecules in the H-bond network and non-interacting molecules, overcoming limitations of traditional spectroscopy methods that report mixed results.

Roke explains that current techniques require assumptions about molecular interactions, but CVS provides distinct vibrational spectra for each type of molecule. This enables precise measurements of properties like electronic charge sharing and H-bond strength.

To achieve this, the researchers used femtosecond laser pulses to illuminate liquid water, inducing tiny charge oscillations and atomic movements that trigger the emission of visible light. This light reveals important information about molecular arrangement and interactions.

Further experiments aimed to analyze the quantum effects of H-bond networks by adjusting the water's pH with hydroxide ions or protons. Ph.D. student Mischa Flór noted that these changes affect water's reactivity and allow for quantifying how much charge hydroxide ions (8%) and protons (4%) contribute to the H-bond network.

The researchers emphasize that CVS can be applied to various materials, with several new characterization experiments underway. Roke states that this method can directly quantify H-bonding strength, providing valuable insights into molecular interactions in solutions containing electrolytes, sugars, amino acids, DNA, and proteins.