Electron Solvation in Liquid Ammonia: Lithium, Sodium, Magnesium, and Calcium As Electron Sources † ‡ Vitaly V

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Electron Solvation in Liquid Ammonia: Lithium, Sodium, Magnesium, and Calcium as Electron Sources † ‡ Vitaly V. Chaban*, and Oleg V. Prezhdo*, † Instituto de Cienciâ e Tecnologia, Universidade Federal de Saõ Paulo, 12231-280 Saõ Josédos Campos, SP Brazil ‡ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

ABSTRACT: 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 ff ffi fi the di usion coe cients of NH3 molecules of the rst 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.

■ INTRODUCTION properties of solvated electrons are still being actively fi The solvated electron constitutes an intriguing phenomenon investigated. Modern studies address ngerprints of solvated − that has continued to draw attention since its discovery.1 17 electrons in water, ammonia, acetonitrile, biphenyl in 11,13,22,23 Understanding the trends and peculiarities of electron solvation tetrahydrofuran, and other systems. The existence of in different solvents is helpful to a variety of fields, including the solvated electron can be hypothesized theoretically in some radiation chemistry, energy storage, and organic synthesis. polar solvents, but plausible experimental evidence has not yet Solvated electrons are particularly interesting in the context of been obtained. Downloaded via UNIV OF SOUTHERN CALIFORNIA on November 8, 2019 at 00:16:22 (UTC). 24 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. electron-transfer phenomena. Solvated electrons occupy spaces Schiller and Horvath considered a model consisting of a between solvent molecules and solute particles. Although the Rydberg atom interacting with thermodynamic fluctuations of solvated electron does not covalently bind to any of these the medium. Applied to supercritical water and ammonia, the entities, it interacts with them electrostatically. It can be said model provided good agreement with the experimental data. that both the solute and the solvent exhibit comparable Yazami and co-workers20 reported conductivity measurements affinities to the electron. The valence electron, therefore, fi and Fourier-transform infrared (FTIR) studies on solvated- obtains enough potential energy to exceed the rst ionization electron solutions obtained in solutions of lithium in potential of a metal. It is agreed in the research community that tetrahydrofuran with biphenyl as an electron acceptor. They lithium and sodium, in combination with a few types of polar fi −1 solvents, can act as donors of solvated electrons. These solvents achieved signi cant conductivity, 12.0 mS cm , using the include ammonia, water, tetrahydrofuran containing organic species ratio n(Li)/n(biphenyl)/n(solvent) = 1:1:8.2. The − radicals, and polyaromatic hydrocarbons.18 21 solutions exhibited metallic behavior. Fingerprint peaks were Lithium in liquid ammonia gives rise to the most well-known found in the FTIR spectra. example of the solvated electron. Humphry Davy was the first to describe blue-colored solutions of alkali metals in liquid Received: January 13, 2016 ammonia two centuries ago. A theoretical identification of the Revised: February 16, 2016 phenomenon of the solvated electron arrived much later. The Published: February 17, 2016

An interesting series of studies concerning excess electrons in one to conduct relevant simulations and to characterize the liquid acetonitrile were reported by Doan and Schwartz.11,22 experimental results. Most ab initio simulations to date have The excess electron exists in two forms in liquid acetonitrile: been performed using relatively small and typically finite − the traditional solvated electron absorbing in the near-IR region systems,1,9,36 41 and more systematic investigations are and a solvated molecular dimer anion. The latter absorbs desirable. In the present article, we report a study of the weakly in the visible spectral region. The solvated electron is solvated electron in liquid ammonia, where the source of the localized right after being produced, but it tends to form a electron is an atom of lithium, sodium, magnesium, or calcium dimer anion later. Yoshida and co-workers25 used pulse (Figure 1). The calculations were implemented using ab initio radiolysis to study the solvated electron in alkylammonium ionic liquids. A number of different cations and anions were combined to reveal the effects of the ions. The absorption peak at 1100 nm in all studied ionic liquids was ascribed to the solvated electron. The reaction rate constant of the identified electron with pyrene was found to exceed viscosity-based diffusion-controlled limits by 1 order of magnitude. The authors made an interesting conclusion that the macroscale viscosity of the alkylammonium ionic liquids appeared systematically higher than the effective viscosity on the molecular scale. Vertical electron binding energies were directly measured by Suzuki and co-workers for the solvated electron in methanol and ethanol.23 Time-resolved photoelectron spectroscopy at ultralow kinetic energy was applied to liquid beams of sodium iodide solutions. The solvated electron was formed Figure 1. Electron density distribution in the equilibrated system from the iodide anions by reactions involving charge transfer to containing a metal atom (Li, Na, Mg, Ca) and 32 NH3 molecules. The the solvent. The authors concluded that the cavity radii in water location of the electron donor atom is highlighted in red. Note that all and low alcohols were very similar. simulated systems were neutral, because the metals were supplied as − Rossky and collaborators12,26 28 pioneered time-domain atoms, rather than as cations. modeling of solvated electrons, motivated by ultrafast pump− − probe experiments.29 32 They showed that the shape of the molecular dynamics (MD) simulations of the neutral periodic cavity created by the solvated electron depends strongly on the systems powered by plane-wave DFT. We investigate the quantum state of the electron. The cavity is spherical in the effects of the solvated electron on the structures and dynamics ground state, whereas it is elongated in the excited state. Their of these solutions. simulations demonstrated and characterized a complex inter- play between electronic and nuclear degrees of freedom, ■ METHODOLOGY involving solvation dynamics, charge transfer, and nonradiative Electronic structure calculations and adiabatic molecular electronic transitions. dynamics simulations were computed by means of the Vienna Having studied solvated electrons in water clusters, Turi, ab Initio Simulation Package (VASP).42 VASP uses pure DFT Sheu, and Rossky12 identified distinct spectral signatures of the with a converged plane-wave basis set, which allows for the electron’s surface and interior states and concluded, based on efficient simulation of periodic (infinite) systems. A metal atom an analysis of experimental data, that the electron in small water (Li, Na, Mg, or Ca) was surrounded by 32 ammonia molecules clusters is stabilized by surface-bound states. Jacobson and maintaining the experimental density (ca. 730 kg m−3). The Herbert33 investigated the temperature dependence of solvated four resulting systems were placed into periodic cubic cells; electrons in water clusters. Having characterized four types of additionally, a larger system, comprising one lithium atom and states, namely, dipole-bound, surface-bound, partially embed- 72 ammonia molecules, was simulated (Table 1). Metals were ded, and cavity states, they showed by extrapolation to large added to the simulated systems as atoms, rather than ions, so cluster sizes that electrons in very cold clusters prefer the cavity that the neutrality of the periodic cells was preserved. state whereas warm clusters create surface-bound electron The generalized gradient approximation for the exchange- states. As the cluster size decreases, the surface-bound state correlation functional proposed by Perdew, Burke, and transforms into the partially embedded state. Ernzerhof was employed,43 as was also done in prior studies In addition to previously known surface and cavity states, on similar systems.40,44 The projector-augmented wave method Sommerfeld and Jordan identified a new binding motif in which to substitute ultrasoft pseudopotentials was used for all atoms.45 an excess electron permeates the hydrogen-bonding network.34 Electrostatic binding of an excess electron dominates only in Table 1. Simulated Systems and Their Parameters the isomers with large dipole moments, whereas polarization and correlation effects prevail in all other water cluster isomers. no. of box side system no. of NH3 no. of explicit volume length − − 34 a 3 Clusters from (H2O)12 to (H2O)24 were considered. no. metal molecules electrons electrons (Å ) (Å) Shkrob used density functional theory (DFT) calculations on 1 Li 32 323 257 1253 10.78 singly negatively charged water clusters (comprising 2, 8, 20, 2 Na 32 331 257 1289 10.88 and 24 molecules) to interpret solution-phase electron 3 Mg 32 332 258 1292 10.89 paramagnetic resonance (EPR)/electron spin−echo envelope 4 Ca 32 340 264 1328 10.99 35 modulation (ESEEM) experiments on an aqueous electron. 5 Li 72 723 577 2803 14.10 The majority of solvated-electron studies have been experimental. The current state of the ab initio methods allows aIn each case, there was one metal atom.

2501 DOI: 10.1021/acs.jpcb.6b00412 J. Phys. Chem. B 2016, 120, 2500−2506 The Journal of Physical Chemistry B Article

The plane-wave energy cutoff was set to 400 eV for charge is a pyramid, with the nitrogen atom constituting one of the computation and to 250 eV for MD simulations. The systems vertices. Because nitrogen is more electronegative than were gradually heated from 0 to 250 K by the conventional hydrogen, ammonia coordinates the cations through the velocity rescaling procedure. The production MD runs were nitrogen atom. Figures 3 and 4 report the average numbers performed with an integration time step of 0.5 fs. Every system of valence electrons on the individual ammonia molecules. was simulated for 10.0 ps after equilibration to record molecular trajectories for further processing. Partial electronic charges were computed following the Bader partitioning scheme46,47 that is part of the quantum theory of atoms in molecules. The definition of an atom is drawn purely from the charge density distribution. In typical molecular systems, charge density reaches a minimum between atoms. This minimum is considered to be a natural place to separate atoms from each other. The radial distribution function (RDF) shows the extent to which the local density at a given interatomic distance exceeds the average density with respect to a certain atom type. The cumulative coordination number Figure 3. Excess negative charge localized on each ammonia molecule (CCN) indicates how many solvent molecules are located in the alkali metal systems: Li, red solid line; Na, green dashed line. fi within a certain radius of the solute particle. The CCN is Some strongly charged NH3 molecules belonging to the rst solvation proportional to the integral of the RDF taken from zero to the shell (FSS) of the metal atom are denoted by FSS. The results are − × −19 given distance. The mean-squared displacement (MSD) given in electron charges, qe = 1.602 10 C. characterizes the mobility of particles in the infinite system. The slope of the MSD with respect to the time axis provides the diffusion coefficient, D, numerically. The Visual Molecular Dynamics (VMD) package48 was used for the preparation of molecular images. ■ RESULTS AND DISCUSSION Figure 2 shows partial electron charges (deficient electrons) on each metal atom. Because of their smaller sizes and, hence,

Figure 4. Excess negative charge localized on each ammonia molecule in the alkaline earth metal systems: Mg, red solid line; Ca, green dashed line. Some strongly charged NH3 molecules belonging to the ﬁrst solvation shell of the metal atom are denoted by FSS. The results − × −19 are given in electron charges, qe = 1.602 10 C.

The single solvated electron present in the alkali metal systems is distributed relatively uniformly among the surrounding NH3 molecules (two solvation shells). This result Figure 2. Deficient electrons on the metal atoms in the equilibrated is rather surprising, as the ammonia molecules of the first systems. The analysis was performed following the Bader algorithm. solvation shell (FSS) could be expected to obtain systematically more electron density. However, Figure 3 demonstrates that the numbers of electrons localized on all ammonia molecules higher electron densities, smaller atoms (Li, Mg) are stronger are quite similar, irrespective of the solvation shell. The electron donors. Larger atoms (Na, Ca) are somewhat weaker situation is different in the case of the alkaline earth metals electron donors, although the difference is not dramatic. The (Mg, Ca; see Figure 4). Some solvent molecules accommodate alkaline earth elements tend to form doubly charged cations. more electron density than others. Detailed analysis of the Therefore, they donate two electrons (1.58−1.65e), and the locations of these atoms revealed that many of them belong to solvated-electron concentration is higher in the cases of Mg and the FSS of the metal atoms. Therefore, larger numbers of Ca. The electron deficiencies per equivalent of donated electrons favor less uniform distributions. One can expect that electron, defined to be 1 for Li and Na, and 2 for Mg and the same impact would be achieved with a high concentration Ca, are very similar. Interestingly, the observed trend in of alkali atoms in the simulations, because more electrons electron deficiency down a column of the periodic table does would be solvated, leading to higher concentrations of excess not follow the corresponding trend in the ionization potential, electrons. as one might expect. Na and Ca are less electron deficient than Table 2 presents the standard deviations of the electron Li and Mg, respectively, even though they should ionize more charges on the solvent molecules surrounding the four metal easily. This effect arises because smaller ions are capable of atoms. The standard deviations characterize how evenly the interacting with solvent molecules more strongly. A stronger charge is spread within the solvent. The data support our donor−acceptor interaction facilitates greater electron transfer. conclusion that the solvated electron is spread more uniformly The solvated electron has to be shared between the solute in the alkali metal systems than in the alkaline earth systems. and the solvent to exist in equilibrium. The ammonia molecule For instance, the data in Table 2 show that the charge is

2502 DOI: 10.1021/acs.jpcb.6b00412 J. Phys. Chem. B 2016, 120, 2500−2506 The Journal of Physical Chemistry B Article − Table 2. Standard Deviations in the Charges Localized on stronger Mg NH3 binding, as suggested by the larger height of a,b Ammonia Molecules in the Four Metal Systems this peak. Solvation of Na in NH3 is weakest, 7 units, according to the MD simulations, although the solvated electron is σ × −2 metal atom ( 10 e) classically known to exist in this system. It is easy to see that the Li 0.13 deficient electrons (Figure 2) do not directly correlate with the Na 0.10 RDFs. The second peaks are located within 0.4−0.5 nm, but are Mg 0.17 nevertheless quite modest, ca. 2 units. We suppose that these Ca 0.20 peaks are properly pronounced at somewhat lower temper- a − × −19 b Results are given in electron charges, qe = 1.602 10 C. The atures, such as 200−230 K. Our simulations were performed at threshold for a molecule to be considered charged was set to 0.03e in 250 K, which is slightly above the normal boiling point of pure the systems with Li and Na, which donate one electron, and to 0.06e ammonia. Note that metals decrease this boiling point, so the in the systems with Mg and Ca, which donate two electrons. These − fl simulations were likely done for pressures of metal ammonia values were chosen to be commensurate with thermal uctuations of systems below 1 bar. The simulations at relatively high this property. temperature were carried out to accelerate the dynamics in the investigated systems. delocalized most evenly in the case of Na and least evenly in the ff case of Ca, the difference being a factor of 2. The e ect of the metal atom on the structure of NH3 in its first and second solvation shells (Figure 6) is insignificant. The The behaviors of the solvated electrons in the studied fi systems can be rationalized further in terms of structural rst peaks located between 0.32 and 0.35 nm (in perfect properties, such as RDFs (Figures 5 and 6). The metal atoms agreement with the van der Waals diameter of nitrogen) are broadened. The second peaks are absent. This sort of RDF confirms that the MD simulations were performed with good accuracy, because the results are well expected both qualitatively and quantitatively. As is known classically, cations exhibit coordination numbers of either four or six depending on their size, charge, and solvent nature. Figure 7 shows that both ionized alkali atoms are

Figure 5. Metal−nitrogen radial distribution functions depending on the electron donor, as indicated in the legend.

Figure 7. Cumulative coordination numbers of the electron donors with respect to NH3 molecules.

coordinated by four NH3 molecules. In turn, Mg and Ca are coordinated by six NH3 molecules. Therefore, charge plays a major role in this case. This observation is also important to show that the behaviors of the chosen metal atoms in the ammonia solution are similar to the behaviors of the fi Figure 6. Nitrogen−nitrogen radial distribution functions in different corresponding cations. Note that the FSS is better de ned in systems, as indicated in the legend. the case of Li and Mg, because cumulative coordination numbers do not grow until the first minimum in the RDF (Figure 5). are strongly coordinated in the NH3 solutions, which is in The dynamics of the ions and molecules (Figure 8)in agreement with their ionization upon the liberation of the solution constitutes a fine tool that characterizes the structure solvated electrons. When the solvated electron is cleaved, the and solvation in general very well. Stronger solvation implies interaction between the metal and NH3 becomes predom- slow dynamics of the solvation shells. In addition, the shape and inantly electrostatic, especially in the case of the alkaline earth mass of the cation are important. The least mobile NH3 metals. The position of the first peak in the metal−nitrogen molecules are observed in the lithium solution. The fastest RDFs is in line with the ion charge and the empirical atomic NH3 molecules are in the sodium and magnesium solutions. covalent radii, as published by Slater:49 r(Li) < r(Mg) < r(Na) Ionized magnesium and its solvation shell are unexpectedly = r(Ca). In turn, the height of the first peak is largely influenced mobile, likely because of the low atomic mass of Mg. It is by the charge acquired by the metal (Figure 2). The highest noteworthy that Mg is more mobile than Ca, with diffusion peak is that of Mg, at 16 units, whereas Li and Ca exhibit similar coefficients of 3.2 × 10−9 versus 1.4 × 10−9 m2 s−1, respectively. heights of 11−12 units. Despite having equal atomic radii, the Although Mg is lighter than Ca, 24 versus 40 amu, Mg binds − RDF peak for Mg NH3 appears at somewhat smaller distances NH3 more strongly. Because the solvent molecules in the Mg − than that for Ca NH3. This should be understood as a result of shells are somewhat less mobile than those in the Ca shells, we

2503 DOI: 10.1021/acs.jpcb.6b00412 J. Phys. Chem. B 2016, 120, 2500−2506 The Journal of Physical Chemistry B Article

size of the simulation box. By increasing the box size, one both creates an opportunity for electron localization due to the presence of a larger number of solvation shells and increased solvent ﬂuctuations and heterogeneity and decreases the concentration of solvated species. Because the simulation cell is periodically replicated in plane-wave DFT, the systems under investigation represent rather concentrated solutions of metal atoms. We repeated the calculation for the Li system by increasing the amount of solvent by more than a factor of 2. Figure 10 depicts charges on solvent molecules for the

Figure 8. Dynamics of NH3 and metal atoms: (left) mean-squared displacements (MSDs) of nitrogen atoms constituting the first two solvation shells of the respective metal atom; (right) diffusion coefficients, D, of the metal atoms (red solid line) and ammonia molecules constituting the first two solvation shells of the respective metal atom (green dashed line). The simulations were performed at 250 K. assume that NH3 becomes slower as a result of strong binding to the cation. It should be noted that pure density functionals of the type used in the present work tend to delocalize electrons. Figure 10. Excess negative charge localized on ammonia molecules in the Li + 72NH3 system. The results are given in electron charges, qe = Compared with an extra electron in a pure solvent, for −1.602 × 10−19 C. instance, the problem is not particularly strong in the present systems, because the solvated electrons are localized by interaction with the metal cation. To test this known pitfall simulation comprising 72 ammonia molecules. Dilution fosters of pure DFT, we computed the electron distribution within the further electron delocalization and, therefore, increases the − NH3 molecules (Figure 9) using Møller Plesset perturbation electron volume. Note that the charges on the ammonia molecules are generally smaller in the larger system (cf. Figure 10 and Figure 3). The electron remains delocalized among the solvent molecules, supporting our original conclusion. ■ CONCLUSIONS In this article, we have reported ab initio MD simulations of Li, Na, Mg, and Ca in NH3 solutions. To our knowledge, this is the first systematic plane-wave DFT investigation comparing the alkali and alkaline earth metals in liquid ammonia. The metal atoms act as an electron donor, sharing electrons with the solvent, giving rise to the so-called solvated electron. Even though solvated electrons were first observed in ammonia, electrons solvated by bulk water and water clusters have fi Figure 9. Excessive/de cient electrons localized on the NH3 received much greater attention, as discussed in the molecules in the doubly negatively charged complex of six NH3 Introduction. Studies of the hydrated electron have revealed a molecules (left), in the neutral Mg(NH3)6 complex (center), and in broad spectrum of solvation structures, showing dependences 2+ the Mg solvation shell (right). on temperature, cluster size, and interaction potential model, suggesting that further investigations into ammoniated theory of the second order and the 6-311++G** split-valence electrons are needed. Whereas the cases of alkali atoms as triple-ζ basis set. Figure 9 demonstrates that the solvated electron donors were considered before, information regarding electron is shared by six NH3 molecules. The partial charges in Mg and Ca is scarce, irrespective of the solvent. Having studied the case of the positively charged magnesium cation are the electronic properties, we correlated them with the structure completely uniform. This result agrees with the corresponding and dynamics of the solution. This work provides new insights data from Figure 4. Figure 4 shows that the ammonia molecules regarding the structure and dynamics of the solvated electron in the first solvation shell are more electron-rich than other donated by alkali and alkaline earth elements in periodic ammonia molecules. It also shows that the NH3 molecules in systems. the FSS have comparable charges (see points denoted FSS in The single solvated electron present in the alkali metal the plot). The central panel of Figure 9 shows a similar systems is distributed more or less uniformly among the distribution of charges. The left panel of Figure 9 demonstrates surrounding solvent molecules. Quite unexpectedly, addition of that NH3 molecules are much more electron-rich in the absence the second electron in the case of alkaline earth metals favors a of the metal cation and that the variation in the charges on the less uniform distribution of the electron density. Lighter atoms, individual NH3 molecules is greater. namely, Li versus Na and Mg versus Ca, are somewhat stronger Because the excess electron contributed by the metal atoms electron donors, which is also rather surprising, because heavier is significantly delocalized, especially for the alkali metals, it is atoms in the same column of the periodic table have smaller appropriate to investigate the dependence of the results on the ionization potentials. The explanation for this finding resides in

2504 DOI: 10.1021/acs.jpcb.6b00412 J. Phys. Chem. B 2016, 120, 2500−2506 The Journal of Physical Chemistry B Article the ability of the smaller atoms to interact more strongly with (10) Hare, P. M.; Price, E. A.; Stanisky, C. M.; Janik, I.; Bartels, D. M. the solvent molecules, creating more opportunities for donor− Solvated Electron Extinction Coefficient and Oscillator Strength in acceptor interactions and charge transfer. High Temperature Water. J. Phys. Chem. A 2010, 114, 1766−1775. Both alkali atoms are coordinated by four NH molecules. In (11) Doan, S. C.; Schwartz, B. J. Nature of Excess Electrons in Polar 3 Fluids: Anion-Solvated Electron Equilibrium and Polarized Hole- turn, Mg and Ca are coordinated by six NH3 molecules. Charge − plays a major role in this case. The structure of the first Burning in Liquid Acetonitrile. J. Phys. Chem. Lett. 2013, 4, 1471 1476. solvation shell is sharpest for Mg, which has a large charge and (12) Turi, L.; Sheu, W. S.; Rossky, P. J. Characterization of Excess a small radius. Li and Ca show similar solvation-shell features, Electrons in Water-Cluster Anions by Quantum Simulations. Science whereas solvation of Na in NH3 is weakest among the 2005, 309, 914−917. considered metal atoms. Ionized Na exhibits a larger radius (13) Mones, L.; Turi, L. A New Electron-Methanol Molecule than Li but a smaller charge than Ca. Lighter atoms have a large Pseudopotential and Its Application for the Solvated Electron in admixture of covalence in the ionic bonds that they form with Methanol. J. Chem. Phys. 2010, 132, 154507. the solvent molecules, because of wave-function overlapping. (14) Renou, F.; Pernot, P.; Bonin, J.; Lampre, I.; Mostafavi, M. Weak solvation generally implies rapid solvation-shell dynam- Solvated Electron Pairing with Earth Alkaline Metals in THF 2 fi Reactivity of the (MgII,e−) Pair with Aromatic and Halogenated ics. Indeed, the NH3 molecules of the rst two solvation shells s and the metal atom itself exhibit fastest dynamics in the Na Hydrocarbon Compounds. J. Phys. Chem. A 2003, 107, 6587−6593. solution. The reported results advance the understanding of the (15) Stuart, C. M.; Tauber, M. J.; Mathies, R. A. Structure and behavior of the solvated electron in different systems, as Dynamics of the Solvated Electron in Alcohols from Resonance fi Raman Spectroscopy. J. Phys. Chem. A 2007, 111, 8390−8400. required in a variety of elds, including energy storage, (16) Šmídova,́ D.; Lengyel, J.; Pysanenko, A.; Med, J.; Slavícek,̌ P.; radiation chemistry, and organic synthesis. Farnik, M. Reactivity of Hydrated Electron in Finite Size System: − Sodium Pickup on Mixed N2O Water Nanoparticles. J. Phys. Chem. ■ AUTHOR INFORMATION Lett. 2015, 6, 2865−2869. (17) Savolainen, J.; Uhlig, F.; Ahmed, S.; Hamm, P.; Jungwirth, P. Corresponding Authors Direct Observation of the Collapse of the Delocalized Excess Electron *E-mail: [email protected] (V.V.C.). in Water. Nat. Chem. 2014, 6, 697−701. *E-mail: [email protected] (O.V.P.). (18) Buck, U.; Dauster, I.; Gao, B.; Liu, Z. F. Infrared Spectroscopy Notes of Small Sodium-Doped Water Clusters: Interaction with the Solvated − fi Electron. J. Phys. Chem. A 2007, 111, 12355 12362. The authors declare no competing nancial interest. (19) Hashimoto, K.; Daigoku, K. Ground and Low-Lying Excited States of Na(NH3)n and Na(H2O)n Clusters: Formation and ■ ACKNOWLEDGMENTS Localization of Solvated Electron. Chem. Phys. Lett. 2009, 469,62−67. (20) Tan, K. S.; Grimsdale, A. C.; Yazami, R. Synthesis and V.V.C. was funded through CAPES. O.V.P. acknowledges Characterization of Biphenyl-Based Lithium Solvated Electron support from the U.S. Department of Energy (Grant DE- Solutions. J. Phys. Chem. B 2012, 116, 9056−9060. SC0014429). 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2505 DOI: 10.1021/acs.jpcb.6b00412 J. Phys. Chem. B 2016, 120, 2500−2506 The Journal of Physical Chemistry B Article

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2506 DOI: 10.1021/acs.jpcb.6b00412 J. Phys. Chem. B 2016, 120, 2500−2506