Proton transport in liquid water has proven difficult to study experimentally as a result of the rapidly changing configurations of the solvated excess proton on femtosecond time-scales. As a result, this process has been almost solely studied with molecular dynamics simulations, which describe the process in terms of exchange between two limiting structures: the Eigen (H3O+(H2O)3) and Zundel (H+(H2O)2) complexes. We investigated aqueous hydrochloric acid using ultrafast two-dimensional infrared spectroscopy in which we excite the O-H stretching vibration of water species participating in strong hydrogen bonds and follow the response between 3600-1500 cm-1. These experiments reveal a crosspeak between stretching and bending vibrations characteristic of the flanking waters of the Zundel complex at (w1,w3) = (3200 cm-1, 1760 cm-1). From time-dependent shifts of the Zundel stretch-bend cross peak, we determined a lower limit on the lifetime of the Zundel complex of 480 femtoseconds. Our estimates suggest that the Zundel complex is a significant -- if not major -- percentage of all aqueous proton species, and not just a transition state between Eigen configurations.

Award Address (Ahmed Zewail Award in Ultrafast Science and Technology sponsored by the Ahmed Zewail Endowment Fund established by the Newport Corporation (Newport)). Dynamics of protons in liquid water viewed through ultrafast IR spectroscopy

251st National Meeting (2016)

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