Easy Come, Easy Go

Whereas the diffusion of water molecules in the bulk liquid depends entirely on breaking hydrogen bonds, the diffusion of proton defects (i.e., an excess proton in acid or a proton deficit in base) is expedited by proton hopping across hydrogen bonds.  The details of this process are well understood in acid, and the process in base was believed to occur in analogous fashion. However, theoretical studies of hydroxide have given highly divergent predictions of solvation structures and diffusion rates, depending on the chosen recipe for such simulations: some predicted the traditionally expected solvation structures and some predicted the experimentally observed diffusion trends, but none do both. Now Seyit Kale, a graduate student in Prof. Judith Herzfeld’s group, has studied proton defects using the group’s recently developed LEWIS force field.[1] The LEWIS simulations obtain the correct relative diffusion rates with hydroxide solvation structures that are analogous to those of hydronium,[2] thereby supporting the traditional picture of the “proton hole”. The authors also catch and characterize proton transfer events, identifying similar “special pairs”[3] as the intermediates in both cases (see figure).

[1]       S. Kale, J. Herzfeld, J. Chem. Phys. 2012, 136, 084109.
[2]       S. Kale, J. Herzfeld, Angew. Chem. Int. Edit. 2012 in press. DOI: 10.1002/anie.201203568.
[3]       O. Markovitch, H. Chen, S. Izvekov, F. Paesani, G. A. Voth, N. Agmon, J. Phys. Chem. B. 2008, 112, 9456-9466.


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