New model suggests source of spectral broadening in liquid water

Liquid water is awash in molecular chaos. Individual water molecules constantly rotate and rearrange as they forge and break hydrogen bonds with their neighbors. This bustle blurs the intermolecular modes that recent spectroscopic measurements have struggled to resolve, and microscopic theories based on molecular dynamics simulations have failed to predict the precise source of this spectroscopic smearing.

A new paper by Sidler and Hamm presents a model that suggests an origin for the observed mode broadening, arguing that it is the inhomogeneities in the stretching of intermolecular hydrogen bonds that are most likely responsible.

At the molecular scale, rapid bond reconfiguration tends to make each bond look the same on average, while heterogeneous structural changes to the hydrogen-bond network are only apparent on short time scales. Prior models could not distinguish between the two—both of which can broaden the THz and Raman spectra probed by two-dimensional spectroscopy.

The researchers developed a simple model that treats each bond as an anharmonic oscillator and builds in both sources of broadening from the beginning. Using just six independent parameters, the model showed good quantitative agreement with the spectroscopic response of water measured in experiments.

Hamm says it’s surprising that the spectrum seems to arise in the way it does, since it implies that the heterogeneous network of hydrogen bonds lives long enough to be resolved. The authors suggest that their model could help refine the understanding of the information that can be extracted in future spectroscopy experiments.

Source: “Feynman diagram description of 2D-Raman-THz spectroscopy applied to water,” by David Sidler and Peter Hamm, The Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5079497 .