ScienceWeek

CHEMICAL PHYSICS: ON THE SOLVATED ELECTRON

The following points are made by D.H. Paik et al (Science 2004 306:672):

1) The nature of the solvated electron, which was first observed in liquid ammonia in 1864, continues to pose several fundamental problems. When the solvent medium is water, the hydrated electron becomes essential to a myriad of physical, chemical, and biological processes. In a simple picture of an electron in a cavity, the description of the hydrated electron state structure is analogous to that of a hydrogen atom, with a ground state of s-type and an excited state of p-type character. However, the hydrated electron is far more complex because of the ultrafast dynamics of structural change, solvation, and recombination.

2) After postulation of the existence of the hydrated electron and the discovery of its absorption, experimental and theoretical efforts have focused on studies in bulk water in which the "cavity" is surrounded by a continuum of other water molecules. A key issue for understanding electron hydration is knowledge of the time scales involved: the motion of water molecules toward the equilibrium structure, and the lifetime of the electron in the different states it occupies. In bulk water, early femtosecond transient absorption studies [1,2] resolved electron hydration dynamics using excitation by two ultraviolet photons to eject bound electrons from water molecules or solute anions.

3) During the succeeding decade, different research groups have provided a vast amount of experimental data on the time scales of relaxation and the theoretical underpinnings of the hydrated electron system [3-5]. Among these was the first three-pulse experiment [3], in which a population of ground-state hydrated electrons created by an initial laser pulse was subsequently studied using two additional pulses, the first of which excited the electrons from the s- to the p-state, and the second of which probed either state. More recently, studies have been made with pulses as short as 5 fs in order to elucidate the different relaxation pathways [5]. In these bulk studies, there remain unanswered questions, especially regarding the microscopic molecular structure and dynamics of hydration.

4) In summary: The authors directly observed the hydration dynamics of an excess electron in the finite-sized water clusters of (H2O)(sub-n)(-) with n = 15, 20, 25, 30, and 35. The authors initiated the solvent motion by exciting the hydrated electron in the cluster. By resolving the binding energy of the excess electron in real time with femtosecond resolution, the authors captured the ultrafast dynamics of the electron in the presolvated ("wet") and hydrated states and obtained, as a function of cluster size, the subsequent relaxation times. The solvation time (300 femtoseconds) after the internal conversion [140 femtoseconds for (H2O)(sub-35)(-)] was similar to that of bulk water, indicating the dominant role of the local water structure in the dynamics of hydration. In contrast, the relaxation in other nuclear coordinates was on a much longer time scale (2 to 10 picoseconds) and depended critically on cluster size.

References (abridged):

1. A. Migus, Y. Gauduel, J. L. Martin, A. Antonetti, Phys. Rev. Lett. 58, 1559 (1987)

2. F. H. Long, H. Lu, K. B. Eisenthal, Phys. Rev. Lett. 64, 1469 (1990)

3. J. C. Alfano, P. K. Walhout, Y. Kimura, P. F. Barbara, J. Chem. Phys. 98, 5996 (1993)

4. K. Yokoyama, C. Silva, D. H. Son, P. K. Walhout, P. F. Barbara, J. Phys. Chem. A 102, 6957 (1998)

5. A. Baltuka, M. F. Emde, M. S. Pshenichnikov, D. A. Wiersma, J. Phys. Chem. A 103, 10065 (1999)

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ON SOLVATED ELECTRONS IN CLUSTERS OF POLAR MOLECULES

Notes by ScienceWeek:

In general, the term "solvation" refers to the association or combination of a solute unit (e.g., ionic, molecular, or particulate) with solvent molecules. This association may involve chemical or physical interactions or both, and may vary in degree from a loose and indefinite complex to the formation of a distinct chemical compound, with such an entity containing a definite number of solvent molecules per solute molecule. Solvation occurring in aqueous solutions is referred to as "hydration".

In general, solvation is often the key process in the phenomenon of solubility, since the interaction energy of solute with solvent must be greater than the interaction energy of solute molecules with themselves in order for the solute to dissolve in the solvent.

Some textbooks consider solvation to signify hydration, but this is an error: when a piece of paraffin, for example, dissolves in benzene, the dissolution of the paraffin results from the solvation of paraffin molecules by benzene molecules, the solvation in this case involving only van der Waals interactions. Hydration is merely a specific type of solvation, one that involves water molecules as solvent.

The following points are made by M. Gutowski et al (Phys. Rev. Lett. 2002 88:143001):

1) A cluster of polar molecules can host an excess electron in at least two ways. First, the excess electron can be tethered to the cluster by its interaction with the cluster's dipole moment. Second, the electron can localize inside the cluster, bulk analogs being the hydrated and ammoniated electrons.[1] While the structural reorganization of the cluster, due to attachment of an excess electron, is typically small for dipole-bound electrons, it is usually quite significant for "solvated electrons", since the solvation occurs at the expense of breaking of preexisting hydrogen bonds. The solvated electron structures, however, provide more contact interactions between the polar molecules and the excess electron. For these reasons, it is often assumed that dipole-bound electrons dominate for small polar clusters, whereas large clusters form solvated electrons.

2) Low energy electrons are of paramount importance in radiation-induced chemical processes, and negatively charged clusters of polar molecules have been extensively studied, both experimentally and theoretically. The isolated ten-electron molecules, HF, H(sub2)O, and NH(sub3) , are closed-shell species, which are not known to form stable associations with an excess electron, i.e., stable anions. However, their hydrogen-bonded assemblies, even as small as dimers, are known to trap excess electrons and to form stable anions in the gas phase]. Early studies of small anionic clusters composed of intact polar molecules concentrated on dipole-bound electrons, also known as surface electron states, in which the cluster's dipole moment is fortified by coalignment of the dipoles of the monomers forming the cluster. On the other hand, solvated electrons (sometimes referred to as internal electron states) were considered inherent to large clusters of polar molecules. Recently, however, theoretical studies have contradicted these classical views.

3) The authors demonstrate that dipole-bound electrons and solvated electrons coexist in as small a cluster as (HF)(sub-3)(--), and they suggest that the stability of these anions with respect to the neutral cluster results not only from the excess electron binding energy, but also from favorable entropic effects, which reflect the greater "floppiness" of the anionic structures.

References (abridged):

1. E.J. Hart and M. Anbar, The Hydrated Electron (Wiley-Interscience, New York, 1970)

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CHEMISTRY: ON ULTRAFAST PROBES OF WATER STRUCTURE

The following points are made by Y. Zubavicus and M. Grunze (Science 2004 304:974):

1) Despite the apparent simplicity of the water molecule, liquid water is one of the most mysterious substances in our world. Because of its vital importance for virtually any area of human activity, ranging from geophysics through cell biology, there has been a continuous attempt to explain the behavior of liquid water by referring to the properties of the water molecule. Bulk water is truly multifaceted (1). This pertains to a common beverage and solvent as well as to one of the 13 known crystalline modifications of ice or one of the three, or probably infinite number, of its amorphous modifications (2,3). Not to mention overcooled, supercritical, interfacial, low-dimensional cluster, confined, and many other forms of water.

2) The interaction of a water molecule with its neighbors -- the so-called water "structure" -- is of key importance for understanding the unique properties of aqueous systems. Since the first two-state-mixture model (capable of explaining the anomalous dependencies of some macroscopic properties of liquid water on temperature and external pressure) was suggested by Roentgen (4), tremendous progress has been made with sophisticated models augmented with extended theoretical formalisms (such as flickering cluster, percolation, fluctuating charges, random network, continuum models, and so forth) (5).

3) Nevertheless, many structural aspects of liquid water remain a subject of debate. The traditional techniques for structural studies of ice and liquid water, such as neutron and x-ray diffraction, are yielding diminishing returns, owing to the collective efforts of many generations of researchers. New results should instead be expected with new techniques unrelated to these mainstream research efforts. For instance, studies of water-based systems under confinement or extreme pressure and temperature conditions could extend the existing knowledge base. Alternatively, one could focus on the structural flexibility of water as reflected in the structural relaxation following an external perturbation or the fluctuations within a network of hydrogen bonds.

4) The question arises whether recent results with ultrafast probes can be explained within the commonly envisioned "static picture" of water, taking into account the dynamical behavior of H2O molecules in the liquid phase, together with the near-edge x-ray absorption fine structure, which probes instantaneous positions of the oxygen and hydrogen atoms in an ensemble of water molecules, integrated over time. Whether -- or to what extent -- the time-averaged instantaneous positions reflect the equilibrium position of the oxygen atoms in the hydrogen bond network of water will be a subject for debate; that is, if the results obtained with the attosecond "snapshot" approach are comparable to the time- and ensemble-averaged results obtained by x-ray and neutron diffraction. What is disturbing, and probably of major consequence for all molecular dynamics simulations of water performed with established and widely used computer program packages, is their disagreement with the water structure "snapshots" measured experimentally by Wernet et al (Science 2004 304:995).

References (abridged):

1. P. Ball, H2O: A Biography of Water (Weidenfeld & Nicolson, London, 1999)

2. C. A. Tulk et al., Science 297, 1320 (2002)

3. M. Guthrie, J. Urquidi, C. A. Tulk, D. KLug, J. Neuefeind, Phys. Rev. B 68, 184110 (2003)

4. W. C. Roentgen, Ann. Phys. Chem. 45, 91 (1892)

5. I. Ohmine, H. Tanaka, Chem. Rev. 93, 2545 (1993)

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