New spectroscopy method maps water’s hydrogen-bonded network | Research

A new spectroscopy technique allows researchers to map hydrogen-bonded networks and study how changes in conditions affect them. Derived from the long-known hyper-Raman scattering, the technique provides new information about bulk water and can provide direct experimental insight into a wide range of problems that have so far been solved only computationally.

Hydrogen bonds are responsible for the abnormal properties of water. The exact nature of hydrogen bonding remains somewhat of a mystery, mostly because hydrogen bonds are difficult to interrogate directly experimentally. Direct information about hydrogen bonds comes from their stretching mode. ‘This is the displacement of water molecules across hydrogen bonding, and so this is exactly what you need to investigate hydrogen bonds,’ he says Sylvie Roke at EPFL in Switzerland. However, excitation is extremely difficult to measure because the spectral region is difficult to access and is filled with numerous other low-energy excitations.

To measure the signal directly, Roke and colleagues at EPFL turned to hyper-Raman scattering, a nonlinear spectroscopic technique first developed in 1965 but not widely used. Roke says: ‘I always asked myself: “Why would anyone do this spectroscopy?” However, using symmetry considerations, the researchers found that if they recorded four hyper-Raman spectra of each sample, varying the position of the detector and the polarization of the light, they could measure separate spectra of interacting and non-interacting molecules. “It’s a matter of adding and subtracting spectra, providing information that was previously only accessible via computer,” says Roke.

Direct measurements

The researchers validated their technique, which they call correlative vibrational spectroscopy (CVS), by recording the spectra of both water and tetrachloromethane. Water with significant hydrogen bonding has a CVS spectrum that indicates a hydrogen bond stretching mode for interacting molecules. Tetrachloromethane, a liquid composed almost entirely of non-interacting molecules, produced a flat CVS spectrum. They then used deuterated water to study the effect of quantum delocalization of the smaller hydrogen nucleus in deuterium and how changes in pH affect the transfer of electronic charge to the hydrogen-bonded network. They found that both effects were mixed. We don’t see much nuclear quantum effect when we change ‘OH’^– to OD^–Roke says; ‘If we change H’₃HE⁺ to D₃HE⁺ ‘We’re seeing a big change and the effect of the hydrogens is to reduce the amount of charge transfer.’ Collaborators in France, Italy and the United Kingdom performed calculations to interpret the differences.

Researchers believe the technique has great potential. In particular, Roke points to the possibility of resolving long-standing debates about the role of water in protein denaturation. It also highlights the potential to study interactions in other liquids or amorphous materials. ‘There’s an almost infinite number of things you can do that before you would have to go to a computer; ‘What happens if I do this and that?’ ask. then take a measurement and hope the two might be linked somehow,’ says Roke.

chemical physicist Anders Nilsson Researchers from Stockholm University in Sweden believe they have already produced important findings using the new technique: ‘In particular, results regarding the asymmetry in the charge distribution in hydrogen bonds of solvated OH^– and protons; “It’s surprising that the first one is much more displaced, containing three hydration shells,” he says. ‘The importance of nuclear quantum effects – that they lead to a weakening of hydrogen bonds in the network – has been known for decades, but now we have the numbers from new data from the comparison between H and H.₂O and D₂O. This information will have implications for our understanding of water.’