Theoretical Characterization of Four Distinct Isomer Types in Hydrated-Electron Clusters, and Proposed Assignments for Photoelectron Spectra of Water Cluster Anions

2-in-1 Phase Space Sampling for Calculating the Absorption Spectrum of the Hydrated Electron

The investigation of vibrational effects on absorption spectrum calculations often employs Wigner sampling or thermal sampling. While Wigner sampling incorporates zero-point energy, it may not be suitable for flexible systems. Thermal sampling is applicable to anharmonic systems yet treats nuclei classically. The application of generalized smoothed trajectory analysis (GSTA) as a postprocessing method allows for the incorporation of nuclear quantum effects (NQEs), combining the advantages of both sampling methods. We demonstrate this approach in computing the absorption spectrum of a hydrated electron. Theoretical exploration of the hydrated electron and its embryonic forms, such as water cluster anions, poses a significant challenge due to the diffusivity of the excess electron and the continuous motion of water molecules. In many previous studies, the wave nature of atomic nuclei is often neglected, despite the substantial impact of NQEs on thermodynamic and spectroscopic properties, particularly for hydrogen atoms. In our studies, we examine these NQEs for the excess electrons in various water systems. We obtained structures from mixed classical-quantum simulations for water cluster anions and the hydrated electron by incorporating the quantum effects of atomic nuclei with the filtration of the classical trajectories. Absorption spectra were determined at different theoretical levels. Our results indicate significant NQEs, red shift, and broadening of the spectra for hydrated electron systems. This study demonstrates the applicability of GSTA to complex systems, providing insights into NQEs on energetic and structural properties.

Dynamics of the charge transfer to solvent process in aqueous iodide

Charge-transfer-to-solvent states in aqueous halides are ideal systems for studying the electron-transfer dynamics to the solvent involving a complex interplay between electronic excitation and solvent polarization. Despite extensive experimental investigations, a full picture of the charge-transfer-to-solvent dynamics has remained elusive. Here, we visualise the intricate interplay between the dynamics of the electron and the solvent polarization occurring in this process. Through the combined use of ab initio molecular dynamics and machine learning methods, we investigate the structure, dynamics and free energy as the excited electron evolves through the charge-transfer-to-solvent process, which we characterize as a sequence of states denoted charge-transfer-to-solvent, contact-pair, solvent-separated, and hydrated electron states, depending on the distance between the iodine and the excited electron. Our assignment of the charge-transfer-to-solvent states is supported by the good agreement between calculated and measured vertical binding energies. Our results reveal the charge transfer process in terms of the underlying atomic processes and mechanisms.

Spectroscopy and dynamics of the hydrated electron at the water/air interface

The hydrated electron, e-(aq), has attracted much attention as a central species in radiation chemistry. However, much less is known about e-(aq) at the water/air surface, despite its fundamental role in electron transfer processes at interfaces. Using time-resolved electronic sum-frequency generation spectroscopy, the electronic spectrum of e-(aq) at the water/air interface and its dynamics are measured here, following photo-oxidation of the phenoxide anion. The spectral maximum agrees with that for bulk e-(aq) and shows that the orbital density resides predominantly within the aqueous phase, in agreement with supporting calculations. In contrast, the chemistry of the interfacial hydrated electron differs from that in bulk water, with e-(aq) diffusing into the bulk and leaving the phenoxyl radical at the surface. Our work resolves long-standing questions about e-(aq) at the water/air interface and highlights its potential role in chemistry at the ubiquitous aqueous interface.

The Back Door to the Surface Hydrated Electron

We use a Mg+ metal to extend the size regime of aqueous clusters to extrapolate to the bulk limit of the vertical detachment energy (VDE) of the solvated electron to >3,200, a value between 1 to over 2 orders of magnitude larger than the one previously measured experimentally or computed theoretically. We relate the VDE to the energy difference between the Mg+(H2O)n and Mg2+(H2O)n systems and the metal's second ionization potential. The extrapolated bulk VDEs of the localized surface electron, which moves away from the metal as n increases, are 1.89 ± 0.01 eV for semiempirical (n ∼ 3,200; PM6-D3H4) and 1.73 ± 0.03 eV (n ∼ 150; HF) and 1.83 ± 0.02 eV (n ∼ 150; MP2) for ab initio, in excellent agreement with the 1.6-1.8 eV range of experimental results. The VDEs converge from above (larger values) to the bulk limit, in a manner that is qualitatively opposite from previous studies and experiments lacking a charged metal, a fact justifying the "back door" approach to the solvated electron.

The Spontaneous Electron-Mediated Redox Processes on Sprayed Water Microdroplets

Water is considered as an inert environment for the dispersion of many chemical systems. However, by simply spraying bulk water into microsized droplets, the water microdroplets have been shown to possess a large plethora of unique properties, including the ability to accelerate chemical reactions by several orders of magnitude compared to the same reactions in bulk water, and/or to trigger spontaneous reactions that cannot occur in bulk water. A high electric field (∼10⁹ V/m) at the air-water interface of microdroplets has been postulated to be the probable cause of the unique chemistries. This high field can even oxidize electrons out of hydroxide ions or other closed-shell molecules dissolved in water, forming radicals and electrons. Subsequently, the electrons can trigger further reduction processes. In this Perspective, by showing a large number of such electron-mediated redox reactions, and by studying the kinetics of these reactions, we opine that the redox reactions on sprayed water microdroplets are essentially processes using electrons as the charge carriers. The potential impacts of the redox capability of microdroplets are also discussed in a larger context of synthetic chemistry and atmospheric chemistry.

Surface or Internal Hydration - Does It Really Matter?

The precise location of an ion or electron, whether it is internally solvated or residing on the surface of a water cluster, remains an intriguing question. Subtle differences in the hydrogen bonding network may lead to a preference for one or the other. Here we discuss spectroscopic probes of the structure of gas-phase hydrated ions in combination with quantum chemistry, as well as H/D exchange as a means of structure elucidation. With the help of nanocalorimetry, we look for thermochemical signatures of surface vs internal solvation. Examples of strongly size-dependent reactivity are reviewed which illustrate the influence of surface vs internal solvation on unimolecular rearrangements of the cluster, as well as on the rate and product distribution of ion-molecule reactions.

Birth of the Hydrated Electron via Charge-Transfer-to-Solvent Excitation of Aqueous Iodide

A primary means to generate hydrated electrons in laboratory experiments is excitation to the charge-transfer-to-solvent (CTTS) state of a solute such as I^-(aq), but this initial step in the genesis of e^-(aq) has never been simulated directly using ab initio molecular dynamics. We report the first such simulations, combining ground- and excited-state simulations of I^-(aq) with a detailed analysis of fluctuations in the Coulomb potential experienced by the nascent solvated electron. What emerges is a two-step picture of the evolution of e^-(aq) starting from the CTTS state: I^-(aq) + hν → I^-*(aq) → I^•(aq) + e^-(aq). Notably, the equilibrated ground state of e^-(aq) evolves from I^-*(aq) without any nonadiabatic transitions, simply as a result of solvent reorganization. The methodology used here should be applicable to other photochemical electron transfer processes in solution, an important class of problems directly relevant to photocatalysis and energy transfer.

Experimental Observation of the Resonant Doorways to Anion Chemistry: Dynamic Role of Dipole-Bound Feshbach Resonances in Dissociative Electron Attachment

Anion chemical dynamics of autodetachment and fragmentation mediated by the dipole-bound state (DBS) have been thoroughly investigated in a state-specific way by employing the picosecond time-resolved or the nanosecond frequency-resolved spectroscopy combined with the cryogenically cooled ion trap and velocity-map imaging techniques. For the ortho-, meta-, or para-iodophenoxide anion (o-, m-, or p-IPhO^-), the C-I bond rupture occurs via the nonadiabatic transition from the DBS to the nearby valence-bound states (VBS) of the anion where the vibronic coupling into the S₁ (πσ*) state (repulsive along the C-I bond extension coordinate) should be largely responsible. Dynamic details are governed by the isomer-specific nature of the potential energy surfaces in the vicinity of the DBS-VBS curve crossings, as manifested in the huge different chemical reactivity of o-, m-, or p-IPhO^-. It is confirmed here that the C-I bond dissociation is mediated by DBS resonances, providing the foremost evidence that the metastable DBS plays the critical role as the doorway into the anion chemistry especially of the dissociative electron attachment (DEA). The fragmentation channel is dominant when it is mediated by the DBS resonances located below the electron-affinity (EA) threshold, whereas it is kinetically adjusted by the competitive autodetachment when the DBS resonances above EA convey the electron to the valence orbitals. The product yield of the C-I bond cleavage is strongly mode-dependent as the rate of the concomitant autodetachment is much influenced by the characteristics of the individual vibrational modes, paving a new way of the reaction control of the anion chemistry.

Photoelectron Spectroscopy of Large Water Cluster Anions

Photoelectron spectra of large size selected water cluster anions (H₂O)_n^- (n = 100-1100) have been measured at a low cluster temperature (80 K). An extensive peak analysis has been conducted in order to determine average and isomer-resolved vertical detachment energies (VDE) of the hydrated electron. This allows us, in combination with the reevaluated data of the previously reported results on small- and medium-sized water cluster anions ( J. Chem. Phys. 2009, 131, 144303), to draw a comprehensive picture of the size-dependent development of the VDEs of water clusters. This allows for an improved extrapolation of the cluster VDEs to the bulk, which yields a value of 3.60 ± 0.03 eV. The general size dependence of the VDEs is in very good agreement with a standard dielectric model.

Ab Initio Simulations of Poorly and Well Equilibrated (CH3CN)n- Cluster Anions: Assigning Experimental Photoelectron Peaks to Surface-Bound Electrons and Solvated Monomer and Dimer Anions

Excess electrons in liquid acetonitrile are of particular interest because they exist in two different forms in equilibrium: they can be present as traditional solvated electrons in a cavity, and they can form some type of solvated molecular anion. Studies of small acetonitrile cluster anions in the gas phase show two isomers with distinct vertical detachment energies, and it is tempting to presume that the two gas-phase cluster anion isomers are precursors of the two excess electron species present in bulk solution. In this paper, we perform DFT-based ab initio molecular dynamics simulations of acetonitrile cluster anions to understand the electronic species that are present and why they have different binding energies. Using a long-range-corrected density functional that was optimally tuned to describe acetonitrile cluster anion structures, we have theoretically explored the chemistry of (CH₃CN)_n^- cluster anions with sizes n = 5, 7, and 10. Because the temperature of the experimental cluster anions is not known, we performed two sets of simulations that investigated how the way in which the cluster anions are prepared affects the excess electron binding motif: one set of simulations simply attached excess electrons to neutral (CH₃CN)_n clusters, providing little opportunity for the clusters to relax in the presence of the excess electron, while the other set allowed the cluster anions to thermally equilibrate near room temperature. We find that both sets of simulations show three distinct electron binding motifs: electrons can attach to the surface of the cluster (dipole-bound) or be present either as solvated monomer anions, CH₃CN^-, or as solvated molecular dimer anions, (CH₃CN)₂^-. All three species have higher binding energies at larger cluster sizes. Thermal equilibration strongly favors the formation of the valence-bound molecular anions relative to surface-bound excess electrons, and the dimer anion becomes more stable than the monomer anion and surface-bound species as the cluster size increases. The calculated photoelectron spectra from our simulations in which there was poor thermal equilibration are in good agreement with experiment, suggesting assignment of the two experimental cluster anion isomers as the surface-bound electron and the solvated molecular dimer anion. The simulations also suggest that the shoulder seen experimentally on the low-energy isomer's detachment peak is not part of a vibronic progression but instead results from molecular monomer anions. Nowhere in the size range that we explore do we see evidence for a nonvalence, cavity-bound interior-solvated electron, indicating that this species is likely only accessible at larger sizes with good thermal equilibration.

Benzene Radical Anion Microsolvated in Ammonia Clusters: Modeling the Transition from an Unbound Resonance to a Bound Species

The benzene radical anion, well-known in organic chemistry as the first intermediate in the Birch reduction of benzene in liquid ammonia, exhibits intriguing properties from the point of view of quantum chemistry. Notably, it has the character of a metastable shape resonance in the gas phase, while measurements in solution find it to be experimentally detectable and stable. In this light, our previous calculations performed in bulk liquid ammonia explicitly reveal that solvation leads to stabilization. Here, we focus on the transition of the benzene radical anion from an unstable gas-phase ion to a fully solvated bound species by explicit ionization calculations of the radical anion solvated in molecular clusters of increasing size. The computational cost of the largest systems is mitigated by combining density functional theory with auxiliary methods including effective fragment potentials or approximating the bulk by polarizable continuum models. Using this methodology, we obtain the cluster size dependence of the vertical binding energy of the benzene radical anion converging to the value of -2.3 eV at a modest computational cost.

Molecular Dynamics Characterization of Dielectron Hydration in Liquid Water with Unique Double Proton Transfers

Radiation chemistry of water and aqueous solutions has always been an interesting scientific issue owing to involving electronic excitations, ionization of solvated species, and formation of radiolytic species and many elementary reactions, but the underlying mechanisms are still poorly understood. Here, we for the first time molecular dynamics characterize the hydration dynamics of two correlated electrons and their triggered unique phenomena in liquid water associated with radiolysis of water using the combined hybrid functional and nonlocal dispersion functional. Hydration of two electrons may experience two distinctly different mechanisms, one forming a spin-paired closed-shell unicaged dielectron hydrate (e₂^2-_aq) and the other forming a spin-paired metastable open-shell bicaged hydrated electron pair (e^-_aq···e^-_aq) which exhibits intriguing antiferromagnetic spin coupling dynamics (in a range of -40 cm^-1 to -500 cm^-1). e^-_aq···e^-_aq can recombine to e₂^2-_aq through a unique solvent fluctuation-controlled gradual-flowing mechanism, and enlarging fluctuation can promote the conversion. Interestingly, we directly observe that e₂^2-_aq as the precursor can trigger hydrogen evolution via unique continuous spontaneous double proton transfer to the dielectron with a short-lived H^-_aq intermediate, but e^-_aq···e^-_aq does not directly. This is the first direct observation for the connection between e₂^2-_aq and spontaneous hydrogen evolution including participation of H^-_aq in aqueous solution, bridging relevant experimental phenomena. This work also evidences an unnoticed process, the double proton transfer mediated charge separation, and presents the first detailed analysis regarding the evolution dynamics of e₂^2-_aq for the understanding of the radiolysis reactions in aqueous solutions.

Ab Initio Molecular Dynamics Simulations of Solvated Electrons in Ammonia Clusters

We investigated excess electron solvation dynamics in (NH₃)_n^- ammonia clusters in the n = 8-32 size range by performing finite temperature molecular dynamics simulations. In particular, we focused on three possible scenarios. The first case is designed to model electron attachment to small neutral ammonia clusters (n ≤ ∼10) that form hydrogen-bonded chains. The excess electron is bound to the clusters via dipole bound states, and persists with a VDE of ∼160 meV at 100 K for the n = 8 cluster. The coupled nuclear and electronic relaxation is fast (within ∼100 fs) and takes place predominantly by intermolecular librational motions and the intramolecular umbrella mode. The second scenario illustrates the mechanism of excess electron attachment to cold compact neutral clusters of medium size (8 ≤ n ≤ 32). The neutral clusters show increasing tendency with size to bind the excess electron on the surface of the clusters in weakly bound, diffuse, and highly delocalized states. Anionic relaxation trajectories launched from these initial states provide no indication for excess electron stabilization for sizes n < 24. Excess electrons are likely to autodetach from these clusters. The two largest investigated clusters (n = 24 and 32) can accommodate the excess electron in electronic states that are mainly localized on the surface, but they may be partly embedded in the cluster. In the last 500 fs of the simulated trajectories, the VDE of the solvated electron fluctuates around ∼200 meV for n = 24 and ∼500 meV for n = 32, consistent with the values extrapolated from the experimentally observed linear VDE-n^-1/3 trend. In the third case, we prepared neutral ammonia cluster configurations, including an n = 48 cluster, that contain possible electron localization sites within the interior of the cluster. Excess electrons added to these clusters localize in cavities with high VDEs up to 1.9 eV for n = 48. The computed VDE values for larger clusters are considerably higher than the experimentally observed photoelectric threshold energy for the solvated electron in bulk ammonia (∼1.4 eV). Molecular dynamics simulations launched from these initial cavity states strongly indicate, however, that these cavity structures exist only for ∼200 fs. During the relaxation the electron leaves the cavity and becomes delocalized, while the cluster loses its initial compactness.

Importance of self-interaction-error removal in density functional calculations on water cluster anions

Removing self-interaction errors in density functional approximations results in significantly improved vertical detachment energies of water anions and is essential for obtaining orbital energies consistent with electron binding energies.

Role of hemibonding in the structure and ultraviolet spectroscopy of the aqueous hydroxyl radical

The presence of a two-center, three-electron hemibond in the solvation structure of the aqueous hydroxl radical has long been debated, as its appearance can be sensitive to self-interaction error in density functional theory.

Probing the Structural Evolution of the Hydrated Electron in Water Cluster Anions (H2O)n-, n ≤ 200, by Electronic Absorption Spectroscopy

Electronic absorption spectra of water cluster anions (H₂O)_n^–, n ≤ 200, at T = 80 K are obtained by photodissociation spectroscopy and compared with simulations from literature and experimental data for bulk hydrated electrons. Two almost isoenergetic electron binding motifs are seen for cluster sizes 20 ≤ n ≤ 40, which are assigned to surface and partially embedded isomers. With increasing cluster size, the surface isomer becomes less populated, and for n ≥ 50, the partially embedded isomer prevails. The absorption shifts to the blue, reaching a plateau at n ≈ 100. In this size range, the absorption spectrum is similar to that of the bulk hydrated electron but is slightly red-shifted; spectral moment analysis indicates that these clusters are reasonable model systems for hydrated electrons near the liquid–vacuum interface.

Picture of the wet electron: a localized transient state in liquid water

A transient state of the excess electron in liquid water preceding the development of the solvation shell, the so-called wet electron, has been invoked to explain spectroscopic observations, but its binding energy and atomic structure have remained highly elusive. Here, we carry out hybrid functional molecular dynamics to unveil the ultrafast solvation mechanism leading to the hydrated electron. In the pre-hydrated regime, the electron is found to repeatedly switch between a quasi-free electron state in the conduction band and a localized state with a binding energy of 0.26 eV, which we assign to the wet electron. This transient state self-traps in a region of the liquid which extends up to ∼4.5 Å and involves a severe disruption of the hydrogen-bond network. Our picture provides an unprecedented view on the nature of the wet electron, which is instrumental to understanding the properties of this fundamental species in liquid water.

Selectivity in Electron Attachment to Water Clusters

Electron attachment onto water clusters to form water cluster anions is studied by varying the point of electron attachment along a molecular beam axis and probing the produced cluster anions using photoelectron spectroscopy. The results show that the point of electron attachment has a clear effect on the final distribution of isomers for a cluster containing 78 water molecules, with isomer I formed preferentially near the start of the expansion and isomer II formed preferentially once the molecular beam has progressed for several millimeters. These changes can be accounted for by the cluster growth rate along the beam. Near the start of the expansion, cluster growth is proceeding rapidly with condensing water molecules solvating the electron, while further along the expansion, the growth has terminated and electrons are attached to large and cold preformed clusters, leading to the isomer associated with a loosely bound surface state.

Structure of the aqueous electron

A cavity or excluded-volume structure best explains the experimental properties of the aqueous or “hydrated” electron.

Thermal Equilibration Controls H-Bonding and the Vertical Detachment Energy of Water Cluster Anions

One of the outstanding puzzles in the photoelectron spectroscopy of water anion clusters, which serve as precursors to the hydrated electron, is that the excess electron has multiple vertical detachment energies (VDEs), with different groups seeing different distributions of VDEs. We have studied the photoelectron spectroscopy of water cluster anions using simulation techniques designed to mimic the different ways that water cluster anions are produced experimentally. Our simulations take advantage of density functional theory-based Born-Oppenheimer molecular dynamics with an optimally tuned range-separated hybrid functional that is shown to give outstanding accuracy for calculating electron binding energies for this system. We find that our simulations are able to accurately reproduce the experimentally observed VDEs for cluster anions of different sizes, with different VDE distributions observed depending on how the water cluster anions are prepared. For cluster anion sizes up to 20 water molecules, we see that the excess electron always resides on the surface of the cluster and that the different discrete VDEs result from the discrete number of hydrogen bonds made to the electron by water molecules on the surface. Clusters that are less thermally equilibrated have surface waters that tend to make single H-bonds to the electron, resulting in lower VDEs, while clusters that are more thermally equilibrated have surface waters that prefer to make two H-bonds to the electron, resulting in higher VDEs.

Tuning Solvated Electrons by Polar-Nonpolar Oxide Heterostructure

Solvated electron states at the oxide/aqueous interface represent the lowest energy charge-transfer pathways, thereby playing an important role in photocatalysis and electronic device applications. However, their energies are usually higher than the conduction band minimum (CBM), which makes the solvated electrons difficult to utilize in charge-transfer processes. Thus it is essential to stabilize the energy of the solvated electron states. Taking LaAlO₃/SrTiO₃ (LAO/STO) oxide heterostructure with H₂O-adsorbed monolayer as a prototypical system, we show using DFT and ab initio time-dependent nonadiabatic molecular dynamics simulation that the energy and dynamics of solvated electrons can be tuned by the electric field in the polar-nonpolar oxide heterostructure. In particular, for LAO/STO with p-type interface, the CBM is contributed by the solvated electron state when LAO is thicker than four unit cells. Furthermore, the solvated electron band minimum can be partially occupied when LAO is thicker than eight unit cells. We propose that the tunability of solvated electron states can be achieved on polar-nonpolar oxide heterostructure surfaces as well as on ferroelectric oxides, which is important for charge and proton transfer at oxide/aqueous interfaces.

Ab initio molecular dynamics study of solvated electrons in methanol clusters

Stable surface excess electronic states in small methanol cluster anions were identified and characterized in ab initio molecular dynamics simulations.

Electronic Excited State Lifetimes of Anionic Water Clusters: Dependence on Charge Solvation Motif

An ongoing controversy about water cluster anions concerns the electron-binding motif, whether the charge center is localized at the surface or within the cluster interior. Here, mixed quantum-classical dynamics simulations have been carried out for a wide range of cluster sizes (n ≤ 1000) for (H₂O)_n^- and (D₂O)_n^-, based on a nonequilibrium first-order rate constant approach. The computed data are in good general agreement with time-resolved photoelectron imaging results (n ≤ 200). The analysis reveals that, for surface state electrons, the cluster size dependence of the excited state electronic energy gap and the magnitude of the nonadiabatic couplings have compensating influences on the excited state lifetimes: the excited state lifetime for surface states reaches a minimum for n ∼ 150 and then increases for larger clusters. It is concluded that the electron resides in a surface-localized motif in all of these measured clusters, dominating at least up to n = 200.

Hydrated Electrons in Water Clusters: Inside or Outside, Cavity or Noncavity?

In this work, we compare the applicability of three electron–water molecule pseudopotentials in modeling the physical properties of hydrated electrons. Quantum model calculations illustrate that the recently suggested Larsen–Glover–Schwartz (LGS) model and its modified m-LGS version have a too-attractive potential in the vicinity of the oxygen. As a result, LGS models predict a noncavity hydrated electron structure in clusters at room temperature, as seen from mixed one-electron quantum–classical molecular dynamics simulations of water cluster anions, with the electron localizing exclusively in the interior of the clusters. Comparative calculations using the cavity-preferring Turi–Borgis (TB) model predict interior-state and surface-state cluster isomers. The computed associated physical properties are also analyzed and compared to available experimental data. We find that the LGS and m-LGS potentials provide results that appear to be inconsistent with the size dependence of the experimental data. The simulated TB tendencies are qualitatively correct. Furthermore, ab initio calculations on static LGS noncavity structures indicate weak stabilization of the excess electron in regions where the LGS potential preferably and strongly binds the electron. TB calculations give stabilization energies that are in line with the ab initio results. In conclusion, we observe that the cavity-preferring pseudopotential model predicts cluster physical properties in better agreement with experimental data and ab initio calculations than the models predicting noncavity structures for the hydrated electron.

Electron Solvation in Liquid Ammonia: Lithium, Sodium, Magnesium, and Calcium as Electron Sources

A free electron in solution, known as a solvated electron, is the smallest possible anion. Alkali and alkaline earth atoms serve as electron donors in solvents that mediate outer-sphere electron transfer. We report herein ab initio molecular dynamics simulations of lithium, sodium, magnesium, and calcium in liquid ammonia at 250 K. By analyzing the electronic properties and the ionic and solvation structures and dynamics, we systematically characterize these metals as electron donors and ammonia molecules as electron acceptors. We show that the solvated metal strongly modifies the properties of its solvation shells and that the observed effect is metal-specific. Specifically, the radius and charge exhibit major impacts. The single solvated electron present in the alkali metal systems is distributed more uniformly among the solvent molecules of each metal's two solvation shells. In contrast, alkaline earth metals favor a less uniform distribution of the electron density. Alkali and alkaline earth atoms are coordinated by four and six NH3 molecules, respectively. The smaller atoms, Li and Mg, are stronger electron donors than Na and Ca. This result is surprising, as smaller atoms in a column of the periodic table have higher ionization potentials. However, it can be explained by stronger electron donor-acceptor interactions between the smaller atoms and the solvent molecules. The structure of the first solvation shell is sharpest for Mg, which has a large charge and a small radius. Solvation is weakest for Na, which has a small charge and a large radius. Weak solvation leads to rapid dynamics, as reflected in the diffusion coefficients of NH3 molecules of the first two solvation shells and the Na atom. The properties of the solvated electrons established in the present study are important for radiation chemistry, synthetic chemistry, condensed-matter charge transfer, and energy sources.

Glucose-Promoted Localization Dynamics of Excess Electrons in Aqueous Glucose Solution Revealed by Ab Initio Molecular Dynamics Simulation

Ab initio molecular dynamics simulations reveal that an excess electron (EE) can be more efficiently localized as a cavity-shaped state in aqueous glucose solution (AGS) than in water. Compared with that (∼1.5 ps) in water, the localization time is shortened by ∼0.7-1.2 ps in three AGSs (0.56, 1.12, and 2.87 M). Although the radii of gyration of the solvated EEs are all close to 2.6 Å in the four solutions, the solvated EE cavities in the AGSs become more compact and can localize ∼80% of an EE, which is considerably larger than that (∼40-60% and occasionally ∼80%) in water. These observations are attributed to a modification of the hydrogen-bonded network by the introduction of glucose molecules into water. The water acts as a promoter and stabilizer, by forming voids around glucose molecules and, in this fashion, favoring the localization of an EE with high efficiency. This study provides important information about EEs in physiological AGSs and suggests a new strategy to efficiently localize an EE in a stable cavity for further exploration of biological function.

Direct observation of the collapse of the delocalized excess electron in water

It is generally assumed that the hydrated electron occupies a quasi-spherical cavity surrounded by only a few water molecules in its equilibrated state. However, in the very moment of its generation, before water has had time to respond to the extra charge, it is expected to be significantly larger in size. According to a particle-in-a-box picture, the frequency of its absorption spectrum is a sensitive measure of the initial size of the electronic wavefunction. Here, using transient terahertz spectroscopy, we show that the excess electron initially absorbs in the far-infrared at a frequency for which accompanying ab initio molecular dynamics simulations estimate an initial delocalization length of ≈ 40 Å. The electron subsequently shrinks due to solvation and thereby leaves the terahertz observation window very quickly, within ≈ 200 fs.

Relaxation mechanism of the hydrated electron

The relaxation dynamics of the photoexcited hydrated electron have been subject to conflicting interpretations. Here, we report time-resolved photoelectron spectra of hydrated electrons in a liquid microjet with the aim of clarifying ambiguities from previous experiments. A sequence of three ultrashort laser pulses (~100 femtosecond duration) successively created hydrated electrons by charge-transfer-to-solvent excitation of dissolved anions, electronically excited these electrons via the s→p transition, and then ejected them into vacuum. Two distinct transient signals were observed. One was assigned to the initially excited p-state with a lifetime of ~75 femtoseconds, and the other, with a lifetime of ~400 femtoseconds, was attributed to s-state electrons just after internal conversion in a nonequilibrated solvent environment. These assignments support the nonadiabatic relaxation model.

Electron at the Surface of Water: Dehydrated or Not?

The hydrated electron is a crucial species in radiative processes, and it has been speculated that its behavior at the water surface could lead to specific interfacial chemical properties. Here, we address fundamental questions concerning the structure and energetics of an electron at the surface of water. We use the method of ab initio molecular dynamics, which was shown to provide a faithful description of solvated electrons in large water clusters and in bulk water. The present results clearly demonstrate that the surface electron is mostly buried in the interfacial water layer, with only about 10 % of its density protruding into the vapor phase. Consequently, it has a structure that is very similar to that of an electron solvated in the aqueous bulk. This points to a general feature of charges at the surface of water, namely, that they do not behave as half-dehydrated but rather as almost fully hydrated species.

Visualization of chemical reaction dynamics: toward understanding complex polyatomic reactions

Polyatomic molecules have several electronic states that have similar energies. Consequently, their chemical dynamics often involve nonadiabatic transitions between multiple potential energy surfaces. Elucidating the complex reactions of polyatomic molecules is one of the most important tasks of theoretical and experimental studies of chemical dynamics. This paper describes our recent experimental studies of the multidimensional multisurface dynamics of polyatomic molecules based on two-dimensional ion/electron imaging. It also discusses ultrafast photoelectron spectroscopy of liquids for elucidating nonadiabatic electronic dynamics in aqueous solutions. (Communicated by Hiroo INOKUCHI, M.J.A.)