PerspectiveChemistry

A Fresh Look at Electron Hydration

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Science  22 Oct 2004:
Vol. 306, Issue 5696, pp. 618-619
DOI: 10.1126/science.1104678

When an extra electron is added to water, a hydrated electron is formed. First discovered in 1962 (1), this fascinating species is of fundamental importance in radiation chemistry and in electron transfer processes in water, and has therefore been studied widely (2, 3). It remains unclear, however, how the hydrated electron moves through water and how the water molecules are arranged in its vicinity. Three reports in this issue describe experimental studies of negatively charged water clusters that shed light on these questions.

In bulk water, the hydrated electron is believed to be confined in a roughly spherical cavity with a radius of 0.22 to 0.24 nanometers (1 nm = 10−9 m), and to occupy an s-type ground electronic state. It is characterized by a broad electronic absorption near 1.7 eV, which can be thought of as a transition from the s state to an excited p state (13). Spectroscopic studies of hydrated electrons have revealed transient absorption on time scales of 50 femtoseconds (fs), (1 fs = 10−15 s) 200 to 300 fs, and 1 picosecond (ps) (1 ps = 10−3 fs) after excitation to the p state (4, 5). These time scales are typical for molecular motions. Some researchers have attributed the 50-fs process to hindered rotational motion of water molecules in the excited state and the 200- to 300-fs process to nonradiative decay of the p to the s state (4). Others have attributed the 50-fs process to ps decay and the 200- to 300-fs process to subsequent relaxation of the solvent in the s state (5). In these scenarios, the 1-picosecond time scale corresponds to long-time relaxation on the s state.

Bragg et al. (page 669) and Paik et al. (page 672) use pump-probe photoelectron spectroscopy to follow the dynamics of photoexcited clusters containing 15 to 50 water molecules and one excess electron (6, 7). Hammer et al. (page 675) use vibrational predissociation spectroscopy to elucidate the structures of smaller clusters with just four to six water molecules and one excess electron (8). All three studies are motivated by the fact that measurements of clusters can provide a level of detail that is difficult to achieve in studies of the bulk.

Bragg et al. (6) and Paik et al. (7) both provide evidence of fast (130 to 250 fs) dynamics associated with the decay of the p state to the s state of their clusters. Very short lifetimes have previously been reported for the excited states of such clusters (9). Bragg et al. find that the excited-state lifetimes decrease with increasing cluster size. They extrapolate to a value of 50 fs for bulk water and conclude that the 50-fs process observed in bulk water containing hydrated electrons is due to nonradiative conversion from the p to the s state, as suggested previously in (5).

Paik et al. investigate the fate of the s state after ps decay by selecting the energy of the electron that is ejected as a result of photoexcitation. Their experiments provide insights into the solvent dynamics in clusters of different sizes. The solvation dynamics are found to occur on a time scale of 300 to 450 fs, depending on cluster size. Because this time scale is similar to that of solvation dynamics in bulk water, Paik et al. conclude that the local solvent structure is critical for electron solvation. They also observe dynamics on a much longer time scale of 2 to 10 ps, which they ascribe to the breakage of hydrogen bonds followed by evaporation of a water monomer.

These measurements (6, 7) provide new insights into the dynamics of an excess electron interacting with hydrogen-bonded networks. However, the relevance of the new data for hydrated electron dynamics in the bulk depends on whether the excess electron is bound to the cluster surface or resides in its interior (10). Paik et al. do not reach a conclusion on this issue, whereas Bragg et al. argue that they are probing an interior-bound electron. If this is indeed the case, then Paik et al. probably also probe an interior-bound electron. Both studies would then be directly relevant to the dynamics of hydrated electrons.

Aside from the issue of interior versus surface binding, there is the question how the water molecules are arranged in the vicinity of the excess electron. This problem is addressed by Hammer et al. (8).

With the exception of the negatively charged water dimer, the geometrical structures of negatively charged water clusters have proven elusive. The report by Hammer et al. represents a major advance in establishing some of these structures (8). By clever use of mixed complexes of water and argon, the authors have been able to synthesize the elusive tetramer, either with normal or with deuterated water, as well as the pentamer and the hexamer. Their vibrational spectra show conclusively that in all three clusters, the excess electron binds in the vicinity of a water molecule that accepts two hydrogen bonds from adjacent molecules but does not itself donate any hydrogen bonds to the hydrogen-bonding network (see the figure). This arrangement is energetically unfavorable in neutral clusters. Its predominance in the negatively charged clusters shows how the excess electron disrupts the hydrogen-bonding network.

An excess electron binds to a cluster of five water molecules.

The water molecule closest to the diffuse excess electron (gray area) is in a double-acceptor hydrogen-bonding environment. Data from electronic structure calculations reported in (8).

The importance of the geometries with double-acceptor waters for the binding of excess electrons to water clusters was first proposed by Lee et al. (11) on the basis of electronic structure calculations for the negatively charged hexamer. Furthermore, Hammer et al. find that vibrational excitation of the OH stretch associated with the double-acceptor water molecule of the negatively charged tetramer and pentamer leads to rapid (50 to 300 fs) ejection of the excess electron.

In the small clusters studied by Hammer et al., the excess electron is surface-bound. In the interior of larger clusters and in bulk water, the hydrated electron may not be bound in the vicinity of double-acceptor water molecules. However, this type of arrangement could very well occur on ice surfaces or at the surface of large water clusters. The application of the vibrational predissociation technique of Hammer et al. to larger clusters may elucidate the location (surface or interior) of the excess electron.

These new experimental results for negatively charged water clusters are important benchmarks for theoretical studies of the structure and dynamics of excess electrons in aqueous environments. Recent theoretical studies have shown that dispersion interactions between the excess electron and the electrons of the water molecules make an important contribution to the binding energy of the former (12). Such interactions could play a role in determining whether the excess electron is surface- or interior-bound and could also affect its dynamics.

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