By Sotiris S. Xantheas
The structure and properties of the proton in water are of fundamental importance in many areas of chemis- try 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 volt-age-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 Eigen-type 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
The challenge is that these bands are of-
ten quite broad in condensed-phase environ-
ments 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 neigh-
bor’s different response to the positive charge
(7). Isotopic substitution with deuterium in
neighboring water molecules enables identi-
fication 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 struc-
tural motifs and associated hopping process
in aqueous cluster networks and ultimately
in aqueous solution at room temperature.
The current “bottom-up” (cluster) ap-
proach is complementary to recent “top-
down” (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 concen-
trated (4M) aqueous hydrochloric acid solu-
tion (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.
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. j
REFERENCES AND NOTES
1. C. T. Wolke et al., Science 354, 1131(2016).
2. J. M. Mayer, Annu. Rev. Phys. Chem. 55, 363 (2004).
3. C. Costentin, M. Robert, J.-M. Savéant,
5. M. Eigen, Ange w. Chem. Int. Ed. 3, 1 (1964).
6. M. L. Huggins, J. Phys. Chem. 40, 723 (1936).
7. C. T. Wolke et al., J. Chem. Phys. 144,074305(2016).
8. M. Thämer, L. De Marco, K. Ramasesha, A. Mandal,
A. Tokmakoff, Science 350, 78 (2015).
9. S.S.Xantheas, Nature 457,673(2009).
10. O. Vendrell, F. Gatti, H.-D. Meyer, Angew. Chem. Int. Ed. 48,
11. Y. Wu, H.Chen,F. Wang, F.Paesani,G.A.Voth, J. Phys.
Chem. B 112, 467 (2008).
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 North west National Laboratory (PNNL) is a multipro-gram national laboratory operated for DOE by Battelle.
Spying on the neighbors’ pool
Spectral signatures are obtained for the movement
of protons in cold water clusters
Physical Sciences Division, Pacific Northwest National
Laboratory, 902 Battelle Boulevard, MS K1-83, Richland, WA
99352, USA. Email: email@example.com
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.