Perspectives

Spying on the neighbors' pool

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Science  02 Dec 2016:
Vol. 354, Issue 6316, pp. 1101
DOI: 10.1126/science.aal1413

The structure and properties of the proton in water are of fundamental importance in many areas of chemistry and biology. The high mobility of the proton in an aqueous solution is understood in terms of its “hopping” between neighboring water molecules, as suggested by the two-century-old Grotthuss mechanism. The barrier for this process intimately depends on the proton's surrounding environment, which is manifested by the connectivity of the immediate hydrogen-bonding network as well as its dynamics caused by thermal fluctuations. On page 1131 of this issue, Wolke et al. (1) shed new light on the role that the proton's water neighbors play toward facilitating positive charge translocation within a hydrogen-bonded network in a cold water cluster.

Understanding the speciation and reactivity of the proton in an aqueous environment begins with acids and bases, which can transfer (either donate or accept) a proton, according to Brønsted and Lowry. This process was further explained by Lewis in terms of changes in acids' and bases' electronic structure in an attempt to offer a generalization of the Arrhenius theory. Simple proton transfers or the ones coupled to an electron transfer determine speciation, valence, and reactivity in aqueous media (2) and explain electrochemical processes (3), whereas voltage-gated proton channels play an essential role in the function of many cells (4).

The water environment plays a role in the molecular-level description of the proton, the two limiting cases being the Eigentype H3O+(H2O)m (5) and the Zundel-type H5O2+(H2O)n (6) cations in water clusters of varying size (see the figure). Infrared (IR) vibrational spectroscopy is a powerful experimental tool for identifying the spectral signatures associated with the underlying water network structure. With the aid of theoretical calculations, spectral bands can be decoded and assigned to the causal molecular vibrations.

The challenge is that these bands are often quite broad in condensed-phase environments at room temperature and smear out the fundamental vibrations occurring at the molecular level. By selectively tagging the proton's water neighbors, placed at known distinct positions within a cluster, Wolke et al. showed that they could isolate each neighbor's different response to the positive charge (7). Isotopic substitution with deuterium in neighboring water molecules enables identification of spectral patterns arising from their interaction with the proton. Thus, how each neighbor was altered by its local environment could be accounted for quantitatively. The present study shows that it is now possible to identify the spectroscopic signatures along the proton transfer pathway and quantify the correlation between the hydrogen-bonded OH stretching frequency and its surrounding environment in a cold aqueous cluster.

This study represents an important step toward understanding the proton's structural motifs and associated hopping process in aqueous cluster networks and ultimately in aqueous solution at room temperature. The current “bottom-up” (cluster) approach is complementary to recent “topdown” (solution) experimental ultrafast two-dimensional IR (2D-IR) spectroscopic measurements used to probe the spectral correlations between the stretching and bending vibrations in the constituent water molecules of the Zundel cation in a concentrated (4M) aqueous hydrochloric acid solution (8). The analysis of the 2D-IR spectra obtained during that study suggested an unexpected large concentration of Zundel-type cations, further reinforcing their role in the proton transfer mechanism.

Proton pathways

The two limiting protonated water structures, the Eigen- and Zundel-type cations, are shown in water cluster minima of different sizes. In each structure, a proton shuttles to an adjacent water molecule's oxygen atom along a hydrogen bond.

GRAPHIC: C. BICKEL/SCIENCE

Theory can help bridge the gap between these “bottom-up” and “top-down” approaches that aim at understanding proton speciation and dynamical properties in aqueous environments of varying size, composition, and external conditions (e.g., temperature and pressure). The interpretation of the measured spectral features can be enhanced by theory, even if existing theoretical approaches are currently challenged (9) when called on to accurately describe the vibrations of even the fundamental units of those cations in cold aqueous clusters (10). However, more approximate methods that are currently available to treat the collective motions in extended systems [such as density functional theory models, multistate valence bond (11) models, or both] cannot yet offer a first principles–based approach to the problem of accurately describing both network structure, its fluctuations, and the corresponding spectral signatures. New theoretical methodologies are needed that accurately account for the network's collective interactions and fluctuations, as well as approaches for decoding the spectral patterns associated with the underlying molecular motions in liquids.

References and Notes

Acknowledgments: This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

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