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Accumulating evidence for the Zundel motif
In recent years, vibrational spectroscopy has been homing in on how water accommodates dissolved protons in acidic solution. Most such studies have examined adjacent stretching or bending modes. Dahms et al. pinpoint the vibrational dynamics of the acidic proton itself, which is sandwiched between two water molecules in a so-called Zundel motif. Comparing spectra in bulk water and in acetonitrile (a known Zundel host) revealed the persistence of this motif in aqueous acid on a subpicosecond time scale. Persistence is sustained by the hydrogen-bonding network among the surrounding water molecules.
Science, this issue p. 491
Abstract
Solvation and transport of excess protons in aqueous systems play a fundamental role in acid-base chemistry and biochemical processes. We mapped ultrafast proton excursions along the proton transfer coordinate by means of two-dimensional infrared spectroscopy, both in bulk water and in a Zundel cation (H5O2)+ motif selectively prepared in acetonitrile. Electric fields from the environment and stochastic hydrogen bond motions induce fluctuations of the proton double-minimum potential. Within the lifetime of a particular hydration geometry, the proton explores a multitude of positions on a sub-100-femtosecond time scale. The proton transfer vibration is strongly damped by its 20- to 40-femtosecond population decay. Our results suggest a central role of Zundel-like geometries in aqueous proton solvation and transport.
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