PERSPECTIVE pubs.acs.org/JPCL
Unveiling Electron Promiscuity Dor Ben-Amotz* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States ABSTRACT: Although the wave-like proclivity of electrons for delocalization is familiar to every student of chemistry, it seems that electrons may have less respect for atomic and molecular boundaries than one might have considered proper. The boundaries in question include those between H-bonded dimers and within hydrated clusters, as well as those of aqueous cavities, colloidal suspensions, and macroscopic airwater and oilwater interfaces. Unveiling the promiscuous behavior of electrons at such frontiers may both raise eyebrows and demand acknowledgment.
D
espite our familiarity with the fascinating properties of electron particle-waves, recent experimental and theoretical results suggest that we may not yet sufficiently appreciate all of the interesting things that electrons are capable of doing. The penchant of electrons for occupying more than one place at a time is beautifully attested by electron diffraction experiments, as well as by the aesthetically pleasing shapes of atomic and molecular orbitals. Although we are accustomed to conceptually delineating boundaries between atoms and molecules, no such boundaries are clearly evident, and electrons do not seem to worry too much about crossing whatever boundaries we may choose to envision. Comparison of the results of various studies clearly implies that subtle differences in the shape of the electronwater pseudopotential can lead to remarkably different structure predictions. For example, whether hydrated electrons do or do not occupy empty cavities in water15 and whether electrons (as well as ions) do or do not have an affinity for airwater and oilwater interfaces are both questions whose answers are now greatly in flux.611 Moreover, it is becoming evident that electrons from one molecule or ion may often choose to distribute themselves promiscuously over neighboring molecules.1215 It is the broad aim of this Perspective to summarize these and other recent evolutions in our understanding of the interactions between electrons and molecules in the hopes of unveiling the threaded path that links these delocalized topics to each other, as well as suggesting future research directions and biological implications.
Comparison of the results of various studies clearly implies that subtle differences in the shape of the electronwater pseudopotential can lead to remarkably different structure predictions. r 2011 American Chemical Society
Hydrated Electrons. Hydrated electrons provide an interesting and practically important16 illustration of how hard it is to pin down the interactions between a single electron and molecules in a bulk liquid, or in a small fluid droplet, or in an anionic cluster.1 Early studies of solvated electrons produced when alkali metals were dissolved in liquid ammonia were attributed to electrons located in interstitial cavities, perhaps analogous to f-center defects in solids.17 Later observations of hydrated electrons produced when water was irradiated by high-energy electrons were attributed to electrons trapped in “a potential well formed by polarized water molecules”.18 Subsequent experimental and theoretical studies,1921 including recent ab initio molecular dynamics and hybrid quantumclassical simulations,22,23 have predominantly supported the notion that hydrated electrons occupy some sort of cavity in water. However, various alternative solvated electron structures, in which the electron is more closely associated with one or more solvent molecules, have also been proposed over the years.24,25 These include a recent hybrid quantumclassical simulation that suggests that hydrated electrons might more closely resemble an anionic cluster of water molecules whose density is higher, rather than lower, than the surrounding bulk water (see Figure 1B).2 The difficulty of pinning down the structure of electrons in water is underscored by comparing results obtained in the most recent quantumclassical simulations, which predict dramatically different hydrated electron structures.35 These studies all rely on ab initio quantum calculations to obtain a pseudopotential that describes the interaction between a quantum mechanical electron and the surrounding (classical) water molecules. Comparison of the results of these studies clearly implies that subtle differences in the shape of the electronwater pseudopotential can lead to remarkably different predictions in which the hydrated electron either resides primarily in a cavity or on top of water molecules. Purely ab Received: March 3, 2011 Accepted: April 21, 2011 Published: May 03, 2011 1216
dx.doi.org/10.1021/jz2002875 | J. Phys. Chem. Lett. 2011, 2, 1216–1222
The Journal of Physical Chemistry Letters
PERSPECTIVE
Figure 1. Alternative structures of the hydrated electron are exemplified in these two figures. (A) Hydrated electron residing in a cavity near the surface of a cluster of 32 water molecules, obtained from ab initio molecular dynamics simulations (the blue/gray contours correspond to hydrated electron densities of 0.003, 0.0005, and 0.0002 au). Copyright (2010) by the American Physical Society.22 (B) Anionic cluster structure of a hydrated electron in liquid water, obtained from quantumclassical simulations using an updated pseudopotential (the grid encloses 50% of the hydrated electron density). Reprinted with permission from AAAS.2
initio molecular dynamics simulations provide evidence that electrons prefer to reside in interstitial water cavities (see Figure 1A).22,26 However, such simulations have so far been limited to rather small water droplets containing only 32 water molecules and , therefore, may or may not accurately represent the structure of hydrated electrons in bulk water.
Results obtained in the most recent quantum-classical simulations predict dramatically different hydrated electron structures. Although a substantial number of experiments have been performed on hydrated electrons, it is not yet clear whether any of these are sufficient to provide a critical test of either the cavity or anionic cluster structures. For example, resonance Raman spectra of hydrated electrons reveal a red shift (decrease in frequency) of both the OH stretch and HOH bend vibrations of water.27 It has been suggested that these shifts are similar to those induced by increasing the density of water and thus support the anionic cluster structure of hydrated electrons,2 but this appears to conflict with the fact that the stretch and bend vibrations of ice shift in opposing directions at high pressure.28 On the other hand, Raman scattering calculations of neutral and anionic water clusters predict a red shift of both the bend and the stretch vibrations upon the addition of an excess electron to a neutral cluster.29 Although the latter predictions would seem to strongly support the anionic cluster model of hydrated electrons, previous calculations have shown that an electron in a water cavity might also produce red shifts in the surrounding water bend and stretch vibrations.30 Other experimental measurements may prove to provide more definitive tests of alternative hydrated electron models. For example, it remains to be seen whether the fast excited-state relaxation dynamics of hydrated electrons,2 as well as the oscillator strength31 and temperature/density dependence of the absorption spectrum,32 can be used to critically distinguish various hydrated electron structures.35 The experimental partial molar volume of a hydrated electron
should provide a key piece of evidence as its sign would presumably be negative for an anionic cluster structure and positive for a cavity structure. Experiments performed 40 years ago do in fact imply that a hydrated electron has an negative partial molar volume (and thus increases the density of water).33 However, even this is not yet sufficient to definitively distinguish the cavity and anionic cluster structures as a cavity-bound hydrated electron may also have a negative partial molar volume if the surrounding water molecules experience a sufficiently large electrostriction (electrostatically induced density increase). Electrons on the Surface of Water. Speculations regarding the structure of electrons dissolved in water also extend to electrons at aqueous interfaces. Both experimental and theoretical studies of anionic water clusters agree that electrons may occupy states that are either in the interior or on the surface of water clusters.11,34,35 Recent experiments provide tantalizing evidence that hydrated electrons may also be happy to reside at a macroscopic airwater interface.6,36 It is tempting to try to link these results to an even more provocative issue regarding the charge of the surface of pure water (in the absence of hydrated electrons). The charge of such an airwater interface, which remains a highly controversial issue,37,38 may result either from the preferential surface affinity of hydroxide (as opposed to hydronium) ions or perhaps from H-bond-induced electron transfer (as further described below). Various experiments provide evidence that both airwater and oilwater interfaces are negatively charged.9,39 For example, air bubbles in water, as well as colloidal suspensions of oil drops in water, are observed to electrophoretically migrate toward the anode in an externally applied electric field. Although such experiments do not precisely locate (or chemically identify) the excess surface charge, they indicate that there is about one excess negative charge for every 3 nm2 of oilwater surface area. On the other hand, photoelectron spectroscopic measurements have not found any evidence of excess hydroxide ions at the airwater interface.9 Moreover, although pH-dependent surface sum frequency measurements of water at a hydrophobic octadecyltrichlorosilane (OTS) interface have provided evidence that “...even the neat water/OTS interface is not neutral, but charged with OH ions”,38 the observed spectra failed to show an interfacial OH vibrational peak (which should appear as a narrow band at around 3630 ( 30 cm1),40,62 and the interpretations of such experimental results remain a subject of debate.41 1217
dx.doi.org/10.1021/jz2002875 |J. Phys. Chem. Lett. 2011, 2, 1216–1222
The Journal of Physical Chemistry Letters
PERSPECTIVE
Figure 2. Several theoretical approaches have been used to investigate the affinity of OH for an airwater interface. (A) Ab initio molecular dynamics simulations imply a small surface affinity (on the order of RT ≈ 0.5 kcal/mol). Copyright (2009), with permission from Elsevier.42 (B) Classical (polarizable) simulations predicts no surface affinity but imply that a large negative surface charge can nevertheless be produced as the results of correlations between the orientations and positions of Naþ and OH ions relative to the interface. Copyright (2010), with permission from the American Institute of Physics.43 (C) Dielectric continuum simulations imply that a very large surface affinity (of the order of 20RT) may result from the influence of OH on the local dielectric constant of water. Reproduced by permission of the PCCP Owner Societies.44
The results shown in Figure 2 illustrate the widely differing theoretically predicted affinities of OH for an airwater interface, obtained using three different theoretical methods. Ab initio molecular dynamics simulations42 predict a small surface affinity of OH (Figure 2A) but do not include the influence of the counterion. Molecular dynamics simulations performed using multistate empirical valence bond potentials (Figure 2B)43 indicate that OH should have no affinity for an airwater interface (in agreement with previous simulations)9 but predict that a negative surface charge may nevertheless be generated as the result of correlations between the positions and orientations of OH and its counterion relative to the airwater interface. None of these molecular dynamics simulations predict a sufficient interfacial accumulation of OH to account for the experimentally observed negative surface charge (as described above). On the other hand, a recently proposed dielectric continuum argument, which relies on the reduced dielectric constant of the hydration shell around an hydrated ion (Figure 2C), implies that OH may have a sufficiently high affinity for a macroscopic airwater interface to explain the electrophoretic migration of oil drops and air bubbles.44 Some of these apparently contradictory experimental and theoretical results may perhaps be reconciled by considering the different length scales probed by various methods. In other words, it may be that the differential affinities of hydroxide and hydronium ions are not sharply localized at the surface but rather extend significantly into the bulk. Another way in which charge could be generated at the surface of pure water, without invoking selective-ion adsorption (or counterion correlations), is by the dipolar (or induced dipolar) alignment of interfacial water molecules. However, such an alignment could only produce a transient response to a static applied electric field45 and
therefore is incompatible with the observed electrophoretic mobility (and negative zeta-potantial) of colloidal oil drops and air bubbles. Thus, explaining the observed electrophoretic mobility appears to require that hydroxide ions, or some other negatively charged particles, do in fact accumulate at the surface of pure water. The electrophoretic experiments have been reproduced sufficiently carefully that it does not seem reasonable to attribute the negative surface charge to impurities, and therefore, it is hard to avoid the conclusion that it is due to a surface excess of hydroxide ions. However, recent discussions have raised the possibility that electrons themselves might perhaps be capable of producing the observed negative surface charge (private communication with Steve Rick, Paul Cremer, Tom Beck, Pavel Jungwirth, and Sylvie Roke).63,64
Explaining the observed electrophoretic mobility appears to require that hydroxide ions, or some other negatively charged particles, do in fact accumulate at the surface of pure water. The notion that electrons may be the particles that charge the surface of pure water is linked to theoretical results which indicate that the formation of a H-bond between two water molecules is accompanied by the transfer of electron density from the H-bond 1218
dx.doi.org/10.1021/jz2002875 |J. Phys. Chem. Lett. 2011, 2, 1216–1222
The Journal of Physical Chemistry Letters
PERSPECTIVE
Figure 3. Charge transfer from the H-bond acceptor to the H-bond donor (A), which is predicted to take place in a H-bonded water dimer, (B) may also give rise to a net charge on water molecules near an airwater interface (blue, positive; red, negative). The electron density contours in figure in (A) were obtained using an MP2/aug-cc-pVTZ calculation, from which a Bader atoms-in-molecules (AIM) analysis indicates an electron-transfer probability of ∼1.9% from the H-bond acceptor to the H-bond donor (private communication from Tom Beck). The results in (B) were obtained using a recently developed classical water model that includes intermolecular charge transfer (private communication from Steve Rick63,64) and predict a net negative surface charge of about 0.1% of an electron per nm2, when integrated over all of the water molecules within the first 0.5 nm from the interface (Gibbs dividing surface).
acceptor to the donor (as illustrated in Figure 3A and further discussed below).46 Such a charge-transfer mechanism could not significantly perturb the neutrality of bulk water because the latter water molecules necessarily have a statistically equivalent number of donor and acceptor H-bonds. On the other hand, the asymmetry of an airwater interface may lead to an imbalance in the number of H-bond donors and acceptors and thus could perhaps produce a net charge at the interface (as shown in Figure 3B). Preliminary estimates, based on the calculated charge transfer in a water dimer and the number of excess H-bond donors at an airwater interface, suggest that this mechanism may be capable of producing a surface charge of the right order of magnitude (private communication from Steve Rick).64 However, the key question is whether such a surface charging mechanism could or could not contribute to the observed electrophoretic mobility of colloidal oil drops and air bubbles.
Recent discussions have raised the possibility that electrons themselves might perhaps be capable of producing the observed negative surface charge If the H-bond-induced charging of a water surface is analogous to an intermolecular polarizibility or dipolar alignment, in the sense that the induced charge separation is pinned to the surface, then it could not contribute to the observed electrophoretic mobility. On the other hand, H-bond-induced charge transfer might differ fundamentally from a dipolar alignment if the transferred electrons behave like other charged particles (such as hydroxide ions), which can be separated from their counterion in an external electric field, and thus could give rise to electrophoretic mobility. In other words, H-bondinduced charge transfer implies that electrons have an higher probability of residing on the H-bond donor and a lower probability of residing on the H-bond acceptor; therefore, why shouldn’t the resulting anionic and cationic water molecules (or clusters) be capable of separating from each other under the influence of an applied electric field? Moreover, the transfer of a single hydrogen atom
(proton plus electron) would convert such anionic and cationic waters to hydroxide and hydronium ions, which certainly could be electrophoretically separated. Such a hydrogen-atom-transfer process would also provide a direct link between H-bond-induced electron transfer and an interface-induced change in water’s ion product.47,48 Although the probability of electron transfer upon the formation of an H-bonded water dimer is rather small (near 1%), it is predicted to contribute substantially (∼20%) to the overall intermolecular binding energy.46 Perhaps even more surprisingly, electron transfer is also predicted to account for ∼30% of the intermolecular binding energy of the H2H2O dimer (although, in this case, the electron-transfer probability is only ∼0.1%).49 Water on Ions and Ions on Water. A much more substantial amount of intermolecular charge transfer is predicted to take place upon the hydration of anions,12 resulting, for example, in the delocalization of ∼20% of a chloride ion’s negative charge over the water molecules in its first hydration shell.13 Moreover, experimental X-ray absorption spectra imply that a significant degree of electron transfer occurs from water to divalent cations, such as Mg2þ (although much less charge is transferred to cations with lower charge density).14 The solvation of atomic ions in water is also predicted to induce a dipole moment on the ion, whose magnitude rivals that of dipolar molecules such as HCl (which has a dipole moment of ∼1 D). For example, some calculations suggest that the dipole moment of Cl(aq) may be as high as 1.6 D, although the true dipole moment of Cl(aq) is likely to be significantly smaller (∼0.6 D).13 The reason for the latter discrepancy may be traced to subtleties associated with the way in which classical polarizable potentials interact with the point charges on the surrounding water molecules (as further discussed below). This same issue also turns out to strongly influence the predicted affinity of ions for airwater and oilwater interfaces. Experiments, including mass spectrometry,50 X-ray photoelectron and fluorescence spectroscopies,51,52 and nonlinear optical surface spectroscopies,53 all point to the enhanced affinity of large anions for airwater interfaces, although the distribution of excess anions with respect to the interface again remains a subject of debate. Simulations suggest that the surface affinity of anions in liquid water and hydrate clusters are quite sensitive to both the polarizibility37 and size8,54 of the ions (see Figure 4). On the other hard, the experimental affinity of large anions for water droplet surfaces appears to be highly correlated with anion size but is not as well correlated with anion polarizability.50 1219
dx.doi.org/10.1021/jz2002875 |J. Phys. Chem. Lett. 2011, 2, 1216–1222
The Journal of Physical Chemistry Letters
PERSPECTIVE
Figure 4. The degree of affinity of large negative anions, such as I, for an airwater interface is quite sensitive to molecular polarizibility. (A) Comparison of nonpolarizable and polarizable molecular dynamics results for the reversible work (mean force potential) associated with moving an I ion relative to an airwater interface.61 (B) Snapshot and distribution function of I and Naþ ions near an airwater interface, obtained using molecular dynamics with polarizable potentials.37 Dielectric continuum models are also capable of predicting an interfacial affinity of I,8 as are nonpolarizable molecular models, when ion sizes are slightly adjusted.54 The true distribution may lie somewhere between the polarizable and nonpolarizable predictions (private communication with Pavel Jungwirth).
Recent studies have raised questions as to whether some classical (polarizable) potentials may be “over-polarized” with respect to ab initio calculations.7,15,55 Such an over-polarization can arise from short-range interactions between polarizabilities and point charges, resulting in unphysical singularities, which can be rectified by delocalizing the point charges using various “polarization damping” strategies.7,5557 Comparisons with ab initio calculations imply that the degree of polarization damping, which is employed in some commonly used potential functions, may not be sufficient and thus may lead to spurious predictions, despite the fact that the same potentials accurately predict ion hydration free energies. When a more realistic level of polarization dampling is introduced, the excess surface affinities of large anions, as well as the large induced dipoles on hydrated ions, are substantially decreased.7 Recent ab initio molecular dynamics simulations of iodide in water predict a substantially smaller induced dipole moment, relative to that obtained using some polarizable potentials, and provide a better fit to the local solvation structure of iodide as determined from extended X-ray absorption fine structure (EXAFS) experiments.15 What is still not clear is why over-polarized potentials are capable of accurately predicting hydration free energies. One possibility is that the over-polarization is compensating for ion water charge transfer (private communication with Tom Beck). If this is the case, then both ionwater charge transfer and realistic polarization damping may be required in order to self-consistently predict ion hydration thermodynamics, induced dipole moments, and surface affinities. Quasi-chemical theory,55 perhaps combined with ideas emerging from local molecular field theory,58 as well as the intermolecular extensions of molecular polarizabilities (private communication from Steve Rick63), may provide appealing strategies for accomplishing such an incorporation of electron transfer into liquid molecular dynamics simulations. Final Thoughts. Although this Perspective has encompassed recent speculations and debates regarding a broad range of phenomena, all of these are tied together by a common thread linking electrons and molecular boundaries; this is the key issue which determines the structure of hydrated electrons and drives the exchange of electrons between hydrated ions and water molecules, as well as between H-bonded water molecules. The latter electron transfer may also contribute, at least in part, to the observed negative charge of airwater and oilwater interfaces. Moreover, problems
associated with the over-polarization of classical potentials also hinge on charge delocalization, as over-polarization arises directly from the unphysical introduction of idealized point charges in classical intermolecular potentials. Intermolecular and intramolecular polarizability are closely related phenomena as both arise from the susceptibility of electron wave functions to applied electric fields. Although intramolecular electron delocalization is a much more familiar concept, it too may lead to consequences that are at odds with our preconceptions regarding atomic identities. For example, the positive charge on a tetraalkyl amine cation is not nearly as localized on the central nitrogen atom as one might have expected, but is rather widely distributed over the surrounding hydrocarbon chains, to an extent that the central nitrogen may be nearly neutral or perhaps even slightly negative.59 This charge delocalization may well contribute to the unusual reactivity of the associated alkane chains with hydroxide ions60 and might also be expected to significantly influence the hydrophobicity of the alkane chains.
In biochemical systems, the delocalization of charge between “ionic” and “hydrophobic” groups may prove to play an important role in the structure and reactivity of soluble proteins. In biochemical systems, the delocalization of charge between “ionic” and “hydrophobic” groups may prove to play an important role in the structure and reactivity of soluble proteins, whose surface areas are typically nearly half occupied by hydrophobic side chains, decorated by neighboring ionic and polar groups. Moreover, the negative surface charge of water at macroscopic (and colloidal) oilwater interfaces may also prove to be of biochemical relevance, if a similar charge accumulation occurs in the vicinity of hydrophobic patches on the surfaces of proteins and other biological assemblies. 1220
dx.doi.org/10.1021/jz2002875 |J. Phys. Chem. Lett. 2011, 2, 1216–1222
The Journal of Physical Chemistry Letters
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Website: http://www.chem. purdue.edu/bendor/.
’ BIOGRAPHY Dor Ben-Amotz is a professor of physical chemistry at Purdue University. His recent experimental and theoretical interests include hydration shell spectroscopy, nanoscale hydrophobicity, hyperspectral imaging, and new ways of teaching physical chemistry. ’ ACKNOWLEDGMENT This work was facilitated by the National Science Foundation (CHE-0847928), as well as by fruitful discussions with the following people (in alphabetical order): Heather Allen, Dave Bartels, James Beattie, Tom Beck, Max Berkowitz, Paul Cremer, Mark Johnson, Ken Jordan, Pavel Jungwirth, Chris Mundy, Sandeep Patel, Lawrence Pratt, Steve Rick, Sylvie Roke, Peter Rossky, Benjamin Schwartz, Doug Tobias, and Laszlo Turi. ’ REFERENCES (1) Jordan, K. D.; Johnson, M. A. Downsizing the Hydrated Electron’s Lair. Science 2010, 329, 42–43. (2) Larsen, R. E.; Glover, W. J.; Schwartz, B. J. Does the Hydrated Electron Occupy a Cavity? Science 2010, 329, 65–69. . Comment on “Does the Hydrated (3) Turi, L.; Madarasz, A Electron Occupy a Cavity?” Science 2011, 331, 1387-c. (4) Jacobson, L. D.; Herbert, J. M. Comment on “Does the Hydrated Electron Occupy a Cavity?” Science 2011, 331, 1387-d. (5) Larsen, R. E.; Glover, W. J.; Schwartz, B. J. Response to Comments on “Does the Hydrated Electron Occupy a Cavity?” Science 2011, 331, 1387-e. (6) Sagar, D. M.; Bain, C. D.; Verlet, J. R. R. Hydrated Electrons at the Water/Air Interface. J. Am. Chem. Soc. 2010, 132, 6917–6919. (7) Wick, C. D. Electrostatic Dampening Dampens the Anion Propensity for the AirWater Interface. J. Chem. Phys. 2009, 131, 084715/1–084715/6. (8) Levin, Y.; dos Santos, A. P.; Diehl, A. Ions at the AirWater Interface: An End to a Hundred-Year-Old Mystery? Phys. Rev. Lett. 2009, 103, 257802/1–257802/4. (9) Winter, B.; Faubel, M.; Vacha, R.; Jungwirth, P. Behavior of Hydroxide at the Water/Vapor Interface. Chem. Phys. Lett. 2009, 474, 241–247. (10) Tobias, D. J.; Hemminger, J. C. Chemistry — Getting Specific About Specific Ion Effects. Science 2008, 319, 1197–1198. (11) Turi, L.; Sheu, W. S.; Rossky, P. J. Characterization of Excess Electrons in Water-Cluster Anions by Quantum Simulations. Science 2005, 309, 914–917. (12) Combariza, J. E.; Kestner, N. R.; Jortner, J. EnergyStructure Relationships for Microscopic Solvation of Anions in Water Clusters. J. Chem. Phys. 1994, 100, 2851–2864. (13) Zhao, Z.; Rogers, D. M.; Beck, T. L. Polarization and Charge Transfer in the Hydration of Chloride Ions. J. Chem. Phys. 2010, 132, 014502/1–014502/10. (14) Cappa, C. D.; Smith, J. D.; Messer, B. M.; Cohen, R. C.; Saykally, R. J. Effects of Cations on the Hydrogen Bond Network of Liquid Water: New Results from X-ray Absorption Spectroscopy of Liquid Microjets. J. Phys. Chem. B 2006, 110, 5301–5309. (15) Fulton, J. L.; Schenter, G. K.; Baer, M. D.; Mundy, C. J.; Dang, L. X.; Balasubramanian, M. Probing the Hydration Structure of Polarizable Halides: A Multiedge XAFS and Molecular Dynamics Study of the Iodide Anion. J. Phys. Chem. B 2010, 114, 12926–12937.
PERSPECTIVE
(16) Garrett, B. C.; Dixon, D. A.; Camaioni, D. M.; Chipman, D. M.; Johnson, M. A.; Jonah, C. D.; Kimmel, G. A.; Miller, J. H.; Rescigno, T. N.; Rossky, P. J.;Role of Water in Electron-Initiated Processes and Radical Chemistry: Issues and Scientific Advances. Chem. Rev. 2005, 105, 355–389. (17) Ogg, R. A. Physical Interaction of Electrons with Liquid Dielectric Media — The Properties of MetalAmmonia Solutions. Phys. Rev. 1946, 69, 668–669. (18) Hart, E. J.; Boag, J. W. Absorption Spectrum of Hydrated Electron in Water and in Aqueous Solutions. J. Am. Chem. Soc. 1962, 84, 4090–4095. (19) Kevan, L. Solvated Electron-Structure in Glassy Matrices. Acc. Chem. Res. 1981, 14, 138–145. (20) Schwartz, B. J.; Rossky, P. J. Aqueous Solvation Dynamics with a Quantum-Mechanical Solute — Computer-Simulation Studies of the Photoexcited Hydrated Electron. J. Chem. Phys. 1994, 101, 6902–6916. (21) Tang, Y.; Shen, H.; Sekiguchi, K.; Kurahashi, N.; Mizuno, T.; Suzuki, Y. I.; Suzuki, T. Direct Measurement of Vertical Binding Energy of a Hydrated Electron. Phys. Chem. Chem. Phys. 2010, 12, 3653–3655. (22) Marsalek, O.; Uhlig, F.; Frigato, T.; Schmidt, B.; Jungwirth, P. Dynamics of Electron Localization in Warm Versus Cold Water Clusters. Phys. Rev. Lett. 2010, 105, 043002/1–043002/4. (23) Jacobson, L. D.; Herbert, J. M. A One-Electron Model for the Aqueous Electron That Includes Many-Body Electron-Water Polarization: Bulk Equilibrium Structure, Vertical Electron Binding Energy, and Optical Absorption Spectrum. J. Chem. Phys. 2010, 133, 154506/ 1–154506/19. (24) Tuttle, T. R.; Golden, S. Solvated Electrons — What Is Solvated. J. Phys. Chem. 1991, 95, 5725–5736. (25) Sobolewski, A. L.; Domcke, W. Hydrated Hydronium: A Cluster Model of the Solvated Electron? Phys. Chem. Chem. Phys. 2002, 4, 4–10. (26) Boero, M.; Parrinello, M.; Terakura, K.; Ikeshoji, T.; Liew, C. C. First-Principles Molecular-Dynamics Simulations of a Hydrated Electron in Normal and Supercritical Water. Phys. Rev. Lett. 2003, 90, 226403. (27) Tauber, M. J.; Mathies, R. A. Structure of the Aqueous Solvated Electron from Resonance Raman Spectroscopy: Lessons from Isotopic Mixtures. J. Am. Chem. Soc. 2003, 125, 1394–1402. (28) Aoki, K.; Yamawaki, H.; Sakashita, M. Pressure-Tuned Fermi Resonance in Ice-VII. Science 1995, 268, 1322–1324. (29) Wu, D. Y.; Duani, S.; Liu, X. M.; Xu, Y. C.; Jiang, Y. X.; Ren, B.; Xu, X.; Lin, S. H.; Tian, Z. Q. Theoretical Study of Binding Interactions and Vibrational Raman Spectra of Water in Hydrogen-Bonded Anionic Complexes: (H2O)N (N = 2 and 3), H2O 3 3 3 X (X = F, Cl, Br, and I), and H2O 3 3 3 M (M = Cu, Ag, and Au). J. Phys. Chem. A 2008, 112, 1313–1321. (30) Shkrob, I. A.; Glover, W. J.; Larsen, R. E.; Schwartz, B. J. The Structure of the Hydrated Electron. Part 2. A Mixed Quantum/Classical Molecular Dynamics Embedded Cluster Density Functional Theory: Single-Excitation Configuration Interaction Study. J. Phys. Chem. A 2007, 111, 5232–5243. (31) Hare, P. M.; Price, E. A.; Stanisky, C. M.; Janik, I.; Bartels, D. M. Solvated Electron Extinction Coefficient and Oscillator Strength in High Temperature Water. J. Phys. Chem. A 2010, 114, 1766–1775. (32) Bartels, D. M.; Takahashi, K.; Cline, J. A.; Marin, T. W.; Jonah, C. D. Pulse Radiolysis of Supercritical Water. 3. Spectrum and Thermodynamics of the Hydrated Electron. J. Phys. Chem. A 2005, 109, 1299–1307. (33) Hentz, R. R.; Brazier, D. W. Gamma Radiolysis of Liquids at High Pressures 0.10. Reaction HþOH and Partial Molal Volume of Hydrated Electron. J. Chem. Phys. 1971, 54, 2777–2780. (34) Hammer, N. I.; Shin, J. W.; Headrick, J. M.; Diken, E. G.; Roscioli, J. R.; Weddle, G. H.; Johnson, M. A. How Do Small Water Clusters Bind an Excess Electron? Science 2004, 306, 675–679. (35) Roscioli, J. R.; Hammer, N. I.; Johnson, M. A.; Diri, K.; Jordan, K. D. Exploring the Correlation between Network Structure and Electron Binding Energy in the (H2O)7 Cluster through Isomer-Photoselected 1221
dx.doi.org/10.1021/jz2002875 |J. Phys. Chem. Lett. 2011, 2, 1216–1222
The Journal of Physical Chemistry Letters Vibrational Predissociation Spectroscopy and Ab Initio Calculations: Addressing Complexity Beyond Types IIII. J. Chem. Phys. 2008, 128, 104314/1–104314/11. (36) Siefermann, K. R.; Liu, Y. X.; Lugovoy, E.; Link, O.; Faubel, M.; Buck, U.; Winter, B.; Abel, B. Binding Energies, Lifetimes and Implications of Bulk and Interface Solvated Electrons in Water. Nat. Chem. 2010, 2, 274–279. (37) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/Water Interface. Chem. Rev. 2006, 106, 1259–1281. (38) Tian, C. S.; Shen, Y. R. Structure and Charging of Hydrophobic Material/Water Interfaces Studied by Phase-Sensitive Sum-Frequency Vibrational Spectroscopy. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15148–15153. (39) Beattie, J. K.; Djerdjev, A. N.; Warr, G. G. The Surface of Neat Water Is Basic. Faraday Discuss. 2009, 141, 31–39. (40) (a) Walrafen, G. E.; Douglas, R. T. W. Raman Spectra from Very Concentrated Aqueous Naoh and from Wet and Dry, Solid, and Anhydrous Molten, LiOH, NaOH, and KOH. J. Chem. Phys. 2006, 124, 114504/1–114504/14. (b) Robertson, W. H.; Diken, E. G.; Price, E. A.; Shin, J. W.; Johnson, M. A. Spectroscopic Determination of the OH Solvation Shell in the OH 3 (H2O)(N) Clusters. Science 2003, 299, 1367–1372. (41) Ye, S.; Nihonyanagi, S.; Uosaki, K. Sum Frequency Generation (SFG) Study of the pH-Dependent Water Structure on a Fused Quartz Surface Modified by an Octadecyltrichlorosilane (OTS) Monolayer. Phys. Chem. Chem. Phys. 2001, 3, 3463–3469. (42) Mundy, C. J.; Kuo, I. F. W.; Tuckerman, M. E.; Lee, H. S.; Tobias, D. J. Hydroxide Anion at the AirWater Interface. Chem. Phys. Lett. 2009, 481, 2–8. (43) Wick, C. D.; Dang, L. X. The Behavior of NaOH at the AirWater Interface: A Computational Study. J. Chem. Phys. 2010, 133, 024705/1–024705/8. (44) Gray-Weale, A.; Beattie, J. K. An Explanation for the Charge on Water’s Surface. Phys. Chem. Chem. Phys. 2009, 11, 10994–11005. (45) Bonthuis, D. J.; Horinek, D.; Bocquet, L.; Netz, R. R. Electrokinetics at Aqueous Interfaces without Mobile Charges. Langmuir 2010, 26, 12614–12625. (46) Khaliullin, R. Z.; Bell, A. T.; Head-Gordon, M. Electron Donation in the WaterWater Hydrogen Bond. Chem.—Eur. J. 2009, 15, 851–855. (47) Knecht, V.; Levine, Z. A.; Vernier, P. T. Electrophoresis of Neutral Oil in Water. J. Colloid Interface Sci. 2010, 352, 223–231. (48) Vacha, R.; Buch, V.; Milet, A.; Devlin, P.; Jungwirth, P. Autoionization at the Surface of Neat Water: Is the Top Layer pH Neutral, Basic, or Acidic? Phys. Chem. Chem. Phys. 2007, 9, 4736–4747. (49) Belpassi, L.; Reca, M. L.; Tarantelli, F.; Roncaratti, L. F.; Pirani, F.; Cappelletti, D.; Faure, A.; Scribano, Y. Charge-Transfer Energy in the WaterHydrogen Molecular Aggregate Revealed by Molecular-Beam Scattering Experiments, Charge Displacement Analysis, and Ab Initio Calculations. J. Am. Chem. Soc. 2010, 132, 13046–13058. (50) Cheng, J.; Hoffmann, M. R.; Colussi, A. J. Anion Fractionation and Reactivity at Air/Water: Methanol Interfaces. Implications for the Origin of Hofmeister Effects. J. Phys. Chem. B 2008, 112, 7157–7161. (51) Ghosal, S.; Hemminger, J. C.; Bluhm, H.; Mun, B. S.; Hebenstreit, E. L. D.; Ketteler, G.; Ogletree, D. F.; Requejo, F. G.; Salmeron, M. Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides. Science 2005, 307, 563–566. (52) Padmanabhan, V.; Daillant, J.; Belloni, L.; Mora, S.; Alba, M.; Konovalov, O. Specific Ion Adsorption and Short-Range Interactions at the Air Aqueous Solution Interface. Phys. Rev. Lett. 2007, 99, 086105/ 1–086105/4. (53) Allen, H. C.; Casillas-Ituarte, N. N.; Sierra-Hernandez, M. R.; Chen, X. K.; Tang, C. Y. Shedding Light on Water Structure at AirAqueous Interfaces: Ions, Lipids, and Hydration. Phys. Chem. Chem. Phys. 2009, 11, 5538–5549. (54) Horinek, D.; Herz, A.; Vrbka, L.; Sedlmeier, F.; Mamatkulov, S. I.; Netz, R. R. Specific Ion Adsorption at the Air/Water Interface: The Role of Hydrophobic Solvation. Chem. Phys. Lett. 2009, 479, 173–183.
PERSPECTIVE
(55) Rogers, D. M.; Beck, T. L. Quasichemical and Structural Analysis of Polarizable Anion Hydration. J. Chem. Phys. 2010, 132, 014505/1–014505/12. (56) Masia, M. Ab Initio Based Polarizable Force Field Parametrization. J. Chem. Phys. 2008, 128, 184107/1–184107/4. (57) Slipchenko, L. V.; Gordon, M. S. Damping Functions in the Effective Fragment Potential Method. Mol. Phys. 2009, 107, 999–1016. (58) Hu, Z. H.; Weeks, J. D. Efficient Solutions of Self-Consistent Mean Field Equations for Dewetting and Electrostatics in Nonuniform Liquids. Phys. Rev. Lett. 2010, 105, 140602/1–140602/4. (59) Luzhkov, V. B.; Osterberg, F.; Acharya, P.; Chattopadhyaya, J.; Aqvist, J. Computational and Nmr Study of Quaternary Ammonium Ion Conformations in Solution. Phys. Chem. Chem. Phys. 2002, 4, 4640–4647. (60) Chempath, S.; Boncella, J. M.; Pratt, L. R.; Henson, N.; Pivovar, B. S. Density Functional Theory Study of Degradation of Tetraalkylammonium Hydroxides. J. Phys. Chem. C 2010, 114, 11977–11983. (61) Dang, L. X.; Chang, T. M. Molecular Mechanism of Ion Binding to the Liquid/Vapor Interface of Water. J. Phys. Chem. B 2002, 106, 235–238. (62) Robertson, W. H.; Diken, E. G.; Price, E. A.; Shin, J. W.; Johnson, M. A. Spectroscopic Determination of the OH Solvation Shell in the OH 3 (H2O)(N) Clusters. Science 2003, 299, 1367–1372. (63) Lee, A. J.; Rick, S. W.; The effects of charge transfer on the properties of liquid water J. Chem. Phys., in press. (64) Vacha, R.; Rick, S. W.; Jungwirth, P.; de Aguiar, H. B.; de Beer, A. G. F.; Samson, J.-S.; Roke, S.; The orientation and charge of water at the hydrophobic oil droplet-water interface. J. Am. Chem. Soc., in press.
1222
dx.doi.org/10.1021/jz2002875 |J. Phys. Chem. Lett. 2011, 2, 1216–1222
