The Bending Mode of Water: A Powerful Probe for Hydrogen Bond Structure of Aqueous Systems

[Image: see text] Insights into the microscopic structure and dynamics of the water’s hydrogen-bonded network are crucial to understand the role of water in biology, atmospheric and geochemical processes, and chemical reactions in aqueous systems. Vibrational spectroscopy of water has provided many such insights, in particular using the O–H stretch mode. In this Perspective, we summarize our recent studies that have revealed that the H–O–H bending mode can be an equally powerful reporter for the microscopic structure of water and provides more direct access to the hydrogen-bonded network than the conventionally studied O–H stretch mode. We discuss the fundamental vibrational properties of the water bending mode, such as the intermolecular vibrational coupling, and its effects on the spectral lineshapes and vibrational dynamics. Several examples of static and ultrafast bending mode spectroscopy illustrate how the water bending mode provides an excellent window on the microscopic structure of both bulk and interfacial water.