The Dynamics of Hydroxide Transport in Water

Cherry Emerson Series

It is believed that the aqueous hydroxide ion possesses an anomalously fast diffusion constant because of its ability to accept a proton from a neighboring water molecule, leading to translocation of the ion. This proposal is similar to the Grotthus mechanism for transport of an excess proton in water. We use femtosecond 2D IR spectroscopy to investigate this process through the vibrational dynamics of the OH stretching vibration in aqueous NaOD solutions. Our experiments separate the dynamics of the hydroxide from those of its first solvation shell, characterize the kinetics for proton exchange and show signatures of the intermolecular proton transfer dynamics. The interpretation of these experiments is assisted with spectroscopic modeling based on empirical valence bond simulations of this process. Our results show that transfer proceeds transiently through a Zundel-like configuration that evolves as a result of the fluctuations of the water hydrogen bonding network.

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