Infrared and Raman perspectives on vibrational coupling in liquid water

Vibrational coupling in liquid water underpins fundamental chemical and physical processes, including energy dissipation and chemical reactivity. Here, we investigate the impact of the vibrational coupling on the O-H stretching band by simulating the IR, isotropic Raman, and anisotropic Raman spectra for a range of isotopic mixtures. We show that vibrational coupling can be consistently decomposed for IR and anisotropic Raman using the standard additive separation into coupled, uncoupled, and inter-/intramolecular contributions. In contrast, isotropic Raman does not generally permit this decomposition because its selection rules correlate intra- and intermolecular contributions, making the inferred intramolecular term dependent on the chosen coupling definition. This finding also cautions against analyses based solely on parallel Raman spectra, which mix isotropic and anisotropic responses. Finally, by comparing classical dynamics with quantum-nuclei dynamics from quasi-centroid molecular dynamics, we find that, aside from the expected frequency redshift, vibrational coupling in the fundamental stretching band is largely classical. Together, these results guide the interpretation of Raman and IR measurements and inform simulation protocols aimed at quantitatively resolving vibrational coupling in liquid water.