Hydrogen-bond Network Reorganization in Liquid Water under Vibrational Strong Coupling

Water mediates chemical transformation, and energy exchanges through a flexible hydrogen-bond network, while how collective responses emerge from local hydrogen-bond structures remains a fundamental question. Here we show that the molecular ensemble of liquid water is collectively reorganized under vibrational ultra-strong coupling (V-USC) in a Fabry–Perot cavity. Distinct infrared spectroscopic responses of the OH-stretching bands associated with network, intermediate and isolated water reveal an unusual retardation of thermal disordering within the hydrogen-bond network. Significant increases in the population fractions of intermediate and isolated water at elevated temperature indicate a non-conventional redistribution of hydrogen-bonded water configurations under V-USC. Precise tuning of the coupling strength leads to selective narrowing of the upper polariton linewidth towards values comparable to the homogeneous limit of water molecules. Correlation analysis on the independent behavior of population redistribution and linewidth narrowing suggests extended intermolecular correlations in V-USC water beyond those of bulk water. The present findings suggest that vacuum-field-mediated collective interactions can reorganize hydrogen-bond networks even under dark conditions, providing novel insight into the equilibrium and dynamical responses of aqueous molecular ensembles.