The hydrated excess proton plays a crucial role in proton transport through bulk water, acid-base chemistry, membrane protein proton pumps, and energy storage in hydrogen fuel cells. Much activity has been pointed at the elucidation of the microscopic mechanisms for proton transport but the molecular structures prevailing under thermal equilibrium conditions have remained elusive. Such structures undergo changes on ultrafast time scales, a fact that has strongly hampered their separation and analysis. Here, we apply femtosecond infrared spectroscopy to probe the ultrafast vibrational response of hydrated protons. Focusing on the Zundel cation H5O2+ [1], selectively prepared in acetonitrile solution, we unravel the distinct character of the broadband absorption continuum, also known as Zundel continuum, from OH stretching and bending excitations. Vibrational lifetimes of all modes are in the (sub-) 100 fs time range, much shorter than for bulk water or water monomers. Supported by theoretical calculations, we provide a new picture of proton hydration where the infrared absorption continuum is a hallmark of the ultrafast frequency modulation of the proton transfer vibration and of coupled combination and overtone transitions. The strong broadening of the infrared lineshape is mainly caused by fluctuating electrical interactions with the polar solvent.

Hydrated excess proton-ultrafast vibrational dynamics of the Zundel cation H5O2+

253rd National Meeting (2017)

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