The Hydrated Excess Proton in the Zundel Cation H₅O₂⁺: The Role of Ultrafast Solvent Fluctuations

The nature of the excess proton in liquid water has remained elusive after decades of extensive research. In view of ultrafast structural fluctuations of bulk water scrambling the structural motifs of excess protons in water, we selectively probe prototypical protonated water solvates in acetonitrile on the femtosecond time scale. Focusing on the Zundel cation H₅O₂⁺prepared in room-temperature acetonitrile, we unravel the distinct character of its vibrational absorption continuum and separate it from OH stretching and bending excitations in transient pump-probe spectra. The infrared absorption continuum originates from a strong ultrafast frequency modulation of the H⁺transfer vibration and its combination and overtones. Vibrational lifetimes of H₅O₂⁺are found to be in the sub-100 fs range, much shorter than those of unprotonated water. Theoretical results support a picture of proton hydration where fluctuating electrical interactions with the solvent and stochastic thermal excitations of low-frequency modes continuously modify the proton binding site while affecting its motions.