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The Dynamics of Protons in Liquid Water Viewed through Ultrafast IR Spectroscopy

Event Details

Event Dates: 

Friday, November 13, 2015 - 4:00pm

Seminar Location: 

  • JILA Auditorium

Speaker Name(s): 

Andrei Tokmakoff

Speaker Affiliation(s): 

University of Chicago
Seminar Type/Subject

Scientific Seminar Type: 

  • Phys Chem/Chem Phys Seminar

Event Details & Abstract: 

Aqueous proton transport is generally accepted to occur by the sequential transfer of an excess proton between hydrogen bonded water molecules rather than diffusion of a particular proton. Because the aqueous proton can adopt a variety of configurations that evolve on femtosecond time-scales, the dynamics of this process have proven difficult to study experimentally. As a result, this process has been almost solely studied with molecular dynamics simulations, which describe the process in terms of interchange between two limiting structures: the Eigen (H3O+(H2O)3) and Zundel (H+(H2O)2) complexes. We investigated aqueous protons in hydrochloric acid solutions using ultrafast broadband 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 (1,3) = (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 indicate that the Zundel complex is a significant―if not major―percentag​e of all aqueous proton species. These results provide important new constraints on the mechanism of aqueous proton transfer, indicating for instance that the Zundel species is not merely a transition state between Eigen configurations.