Ultrafast Optical Kerr Effect Spectroscopy Reveals the Vibrational Fingerprint of Acetate–Water Hydrogen Bonds

Time-resolved optical Kerr effect (OKE) spectroscopy was employed to investigate the low-frequency vibrational dynamics of aqueous acetate solutions. While the isotropic OKE spectrum of neat water is broad and featureless, acetate solutions display a distinct band near 200 cm^-1. This feature increases systematically with acetate concentration, is absent in methyl acetate, and shows negligible dependence on the countercation, establishing it as the vibrational fingerprint of acetate-water hydrogen bonds. Comparison with hydroxide solutions demonstrates that the band is spectrally distinct from other anion-water vibrations. Quantum-chemical calculations further support the assignment, reproducing polarized vibrational modes in the same frequency region. Together, these results resolve long-standing ambiguities in the interpretation of acetate hydration and highlight the power of ultrafast OKE spectroscopy to isolate solute-specific hydrogen-bond vibrations in aqueous solutions. Beyond spectroscopy, these findings have implications for understanding electrolyte behavior in energy storage systems (e.g., lithium-ion batteries) and biological buffering processes.