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

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.

Are the Accompanying Cations of Doping Anions Influential in Conducting Organic Polymers? The Case of the Popular PEDOT

“…This is in good agreement with the solvation energies following the trend Cs + < Na + < Li + . [21] In the case of [H 5 O 2 (ACN) 4 ] + , the center of the cation is not shielded by the solvent molecules, [22] and therefore it can easily interactw ith COSANE. Althoughg as-phase calculations (even with solventm odels) do not necessarily account accurately for cation-anioni nteractions within the bulk of al iquid, the results of these calculations indicatet hat the simple model system described above, containing the solvated cation and the COSANE anion, provide an explanation of the impact of accompanying cationso nt he PEDOT/doping-anion stoichiometry that is consistent with the experimental observations.…”

Moving Water Droplets: The Role of Atmospheric CO2 and Incident Radiant Energy in Charge Separation at the Air–Water Interface

“…Those unbalanced protons residing in the droplet and stripped of their counter-ions (left on the solid) should be highly mobile. When they are transferred along hydrogen-bonded water molecules, fluctuations of dipole moments are induced, giving rise to enhanced infrared emission . Yet, as shown by both experiments and theory, − , upon evaporation, protons preferentially escape from the air–water interface, leaving the latter negatively charged.…”

“…Water evaporation (which can support development of negative charge) has been shown to depend primarily on the librational motions of water molecules resulting from intermolecular interactions and particularly water rotations within the hydrogen-bonded network. − Librational motions can be induced by infrared radiation at wavelengths centered around 4.65 μm (weak broad absorption) and 15 μm (strong absorption) . In the same spectral regions lies the absorption of different configurations of hydrated protons that are constantly being transferred along chains of hydrogen-bonded water molecules. ,, Apparently, human skin (with the human finger being very effective in inducing droplet movement) emits IR mainly in the wavelength range from 2 to 20 μm with 90% of radiation at 8–15 μm . Cellulose-based materials, such as paper tissue and cork, emit radiation mostly in the long wavelength infrared from 7 to 20 μm and above .…”

Estimations of FH···X hydrogen bond energies from IR intensities: Iogansen's rule revisited

“…The intensity of proton donor stretching band A increases by about one or two orders of magnitude upon formation of a medium strong hydrogen‐bonded complex. It should be noted that although the overall structure of a vibrational band sometimes can be inextricable being a manifold of hot and combination bands (sometimes referred to as Zundel continuum for water molecules in clusters), 33,35,36 the relative increase of its intensity upon formation of a hydrogen bond can be reliably determined 37,38 . To calibrate the band intensities and associate their increase with the strength of a complex, both intensities of the bands of monomers or characteristic bands of other groups (which are not involved in complexation) can be used.…”