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Water and Aqueous Solutions

Hydrogen bond dynamics in water

Water has many important and unusual properties, such as acid–base chemistry, solubility and hydrophobicity, transport of electrical, chemical and thermal energy, and phase transitions. Despite their importance, we do not have a predictive molecular understanding of these properties. The unique properties of water originate from water's hydrogen bond network. This network features strong intermolecular interactions that result in collective motions spanning many water molecules, yet it remains fluid enough to reorganize on ultrafast time scales. We have used 2D IR experiments of isotope-diluted water to monitor the fluctuations of this network and build a mechanistic picture of how waters structure evolves with time. Hydrogen bonds in water are broken only momentarily during a switching of hydrogen-bonding partners involving the concerted motion of multiple water molecules. We have used similar experiments on H2O in order to describe the extended communication of water molecules through this network (vibrational excitons). Our current efforts extend these methods to more accurately describe water's influence on molecular solutes varying from ions to proteins.



Structure and transport of the aqueous proton

Excess protons in water and hydroxide ions display anomalously high diffusion rates, which is explained by the Grotthuss mechanism, the sequential translocation of an ion by proton transfer along a network of hydrogen bonds. Proton transport of an excess proton is commonly described in terms of shifting between two idealized structures: the Eigen complex, a triply solvated hydronium core, and the Zundel complex, a proton shared between two waters. The shifting of this proton involves a significant reorganization of the liquid structure. Therefore, the structure and transport of protons are intimately connected with the hydrogen-bonding dynamics of water.

We are using 2D IR to visualize the time-dependent molecular structure of protons in water to understand how these dynamics influence aqueous charge transport. Our studies of the aqueous proton are directed at the infrared continuum band, which is observed upon adding strong acids to water. This continuum is thought to arise from variations in the proton potential as it varies continuously with the strength of the hydrogen bond it participates in. In recent experiments we identified a crosspeak between the proton-related bend vibration (1760 cm−1) and strongly H-bonded O—H stretches at 3200 cm−1, which are frequencies in good agreement with the predictions for vibrational frequencies of the Zundel species. Additional studies are targeting other regions of the acid continuum band to identify the exchange dynamics between different solvated proton complexes.


Computational spectroscopy of aqueous proton transfer

In collaboration with Greg Voth's group at UChicago, we develop new theoretical methods to more accurately model and predict the spectroscopic behavior of aqueous proton complexes and transport in water. These methods draw clusters from simulations of Voth's reactive multistate empirical valence bond (MS-EVB) models, and use DFT calculations to study how microscopic structure and dynamics are manifested in IR spectroscopy. We have developed a semi-empirical single oscillator spectroscopic map that predicts the O—H stretch frequency in isotopically dilute acid solutions, and can be used to describe how it's participation in different proton complexes and proton transfer events influence IR frequency trajectories. In order to understand the coupled anharmonic vibrations of transiently solvated proton–water clusters in isotopically pure solution, we have used normal mode analysis on instantaneous proton–water clusters taken from simulations to reveal vibrational frequency patterns associated with aqueous proton clusters that vary between Eigen and Zundel type configurations.


Proton transfer in aqueous hydroxide solutions

A similar proton transfer process is thought to occur in aqueous hydroxide solutions, in which the aqueous hydroxide ion accepts a proton from a solvating water molecule, leading to the translocation of the ion. We performed 2D IR experiments to assign the spectral features to the OH ion and its solvation shell, and to characterize the kinetics of exchange between these species. Using isotope dilution experiments in D2O we identified a spectral feature that decays on a ~110 fs time scale that we assign to the relaxation of a Zundel-like DO..H..OD species, and use chemical exchange measurements set a 3 ps lower limit on the exchange time between water and ions. Experiments on hydroxide in H2O reveals that the ion exists in a three- or four-coordinate solvent bound complex in which the ion and solvation shell are strongly coordinated between transfer events.