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

Fabian Dahms, Rene Costard, Ehud Pines, Benjamin P. Fingerhut, Erik T.J. Nibbering, Thomas Elsaesser

Research output: Contribution to journalArticlepeer-review

55 Scopus citations

Abstract

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.

Original languageEnglish
Pages (from-to)10600-10605
Number of pages6
JournalAngewandte Chemie - International Edition
Volume55
Issue number36
DOIs
StatePublished - 26 Aug 2016

Keywords

  • Zundel cation
  • Zundel continuum
  • femtochemistry
  • hydrated protons
  • solvation dynamics

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