The aqueous excess proton is a ubiquitous species, playing a prominent role in redox, environmental, and biophysical chemistry. However, due to fast dynamics occurring on femtosecond to picosecond timescales, a molecular-level understanding of the structures of the excess proton and the detailed dynamics of proton transfer in water has remained elusive. We have taken advantage of the femtosecond time resolution and the sensitivity towards hydrogen-bonding configurations provided by ultrafast IR spectroscopy to study the structural dynamics of the aqueous proton. With broadband two-dimensional IR spectroscopy, we find that the excited state absorption of the hydrated proton stretching vibration lies at higher frequency than the ground state bleach, and both display substantial inhomogeneous broadening. This information provides evidence for a persistent asymmetric “Zundel-like” configuration of the excess proton in between the idealized H3O+ and shared H5O2+ species. With polarization-sensitive transient absorption spectroscopy, we find that the picosecond kinetics of proton transfer are encoded in the polarization anisotropy decay of the hydrated proton bending mode. The temperature dependence of the timescale of the bend anisotropy relaxation follows Arrhenius behavior, revealing an energy barrier of 2.5 kcal/mol for proton transfer, comparable to but smaller than the barrier of 3.7 kcal/mol for hydrogen bond reorganization in liquid water. Together with the viscosity and counterion dependence of the anisotropy decay timescales, this provides strong evidence that the proton transfer process is driven primarily by collective solvent motion, with the barrier to this process being affected by the presence of the structural defects in the hydrogen bond network induced by the excess protons.

Uncovering the structure and dynamics of aqueous proton transfer with ultrafast infrared spectroscopy

2019 Spring National Meeting

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