Water molecules interact strongly with each other through hydrogen bonds. This efficient intermolecular coupling causes strong delocalization of molecular vibrations in bulk water. We study intermolecular coupling at the air/water interface and find intermolecular coupling 1) to be significantly reduced and 2) to vary strongly for different water molecules at the interface-whereas in bulk water the coupling is homogeneous. For strongly hydrogen-bonded OH groups, coupling is roughly half of that of bulk water, due to the lower density in the near-surface region. For weakly hydrogen-bonded OH groups that absorb around 3500 cm(-1) , which are assigned to the outermost, yet hydrogen-bonded OH groups pointing towards the liquid, coupling is further reduced by an additional factor of 2. Remarkably, despite the reduced structural constraints imposed by the interfacial hydrogen-bond environment, the structural relaxation is slow and the intermolecular coupling of these water molecules is weak.

Weinheim: Wiley
Angew. Chem., Int. Ed.

Hsieh, C.-S., Okuno, M., Hunger, J., Backus, E., Nagata, Y., & Bonn, M. (2014). Aqueous Heterogeneity at the Air/Water Interface Revealed by 2D-HD-SFG Spectroscopy. Angew. Chem. Int. Ed., 53(31), 8146–8149. doi:10.1002/anie.201402566