unveiling electron promiscuity

Published: May 03, 2011

r 2011 American Chemical Society 1216 dx.doi.org/10.1021/jz2002875 | J. Phys. Chem. Lett. 2011, 2, 1216–1222

PERSPECTIVE

pubs.acs.org/JPCL

Unveiling Electron PromiscuityDor Ben-Amotz*Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States

Despite our familiarity with the fascinating properties ofelectron particle-waves, recent experimental and theoretical

results suggest that we may not yet sufficiently appreciate all of theinteresting things that electrons are capable of doing. The penchantof electrons for occupyingmore than one place at a time is beautifullyattested by electron diffraction experiments, as well as by the aesth-etically pleasing shapes of atomic and molecular orbitals. Althoughwe are accustomed to conceptually delineating boundaries betweenatoms and molecules, no such boundaries are clearly evident, andelectrons do not seem to worry too much about crossing whateverboundaries we may choose to envision.

Comparison of the results of various studies clearly impliesthat subtle differences in the shape of the electron�waterpseudopotential can lead to remarkably different structure predictions.For example, whether hydrated electrons do or do not occupy emptycavities in water1�5 and whether electrons (as well as ions) do or donot have an affinity for air�water and oil�water interfaces are bothquestions whose answers are now greatly in flux.6�11 Moreover, it isbecoming evident that electrons from one molecule or ion may oftenchoose to distribute themselves promiscuously over neighboringmolecules.12�15 It is the broad aim of this Perspective to summarizethese and other recent evolutions in our understanding of the inter-actions between electrons andmolecules in the hopes of unveiling thethreaded path that links these delocalized topics to each other, as wellas suggesting future research directions and biological implications.

Hydrated Electrons. Hydrated electrons provide an interestingand practically important16 illustration of how hard it is to pindown the interactions between a single electron and moleculesin a bulk liquid, or in a small fluid droplet, or in an anioniccluster.1 Early studies of solvated electrons produced when alkalimetals were dissolved in liquid ammonia were attributed toelectrons located in interstitial cavities, perhaps analogous tof-center defects in solids.17 Later observations of hydratedelectrons produced when water was irradiated by high-energyelectrons were attributed to electrons trapped in “a potentialwell formed by polarized water molecules”.18 Subsequent ex-perimental and theoretical studies,19�21 including recent abinitio molecular dynamics and hybrid quantum�classicalsimulations,22,23 have predominantly supported the notion thathydrated electrons occupy some sort of cavity in water. How-ever, various alternative solvated electron structures, in whichthe electron is more closely associated with one or more solventmolecules, have also been proposed over the years.24,25 Theseinclude a recent hybrid quantum�classical simulation thatsuggests that hydrated electrons might more closely resemblean anionic cluster of water molecules whose density is higher,rather than lower, than the surrounding bulk water (seeFigure 1B).2

The difficulty of pinning down the structure of electrons inwateris underscored by comparing results obtained in the most recentquantum�classical simulations, which predict dramatically differ-ent hydrated electron structures.3�5 These studies all rely on abinitio quantum calculations to obtain a pseudopotential thatdescribes the interaction between a quantum mechanical electronand the surrounding (classical) water molecules. Comparison of theresults of these studies clearly implies that subtle differences in theshape of the electron�water pseudopotential can lead to remark-ably different predictions � in which the hydrated electron eitherresides primarily in a cavity or on top of water molecules. Purely ab

Comparison of the results of variousstudies clearly implies that subtledifferences in the shape of the

electron�water pseudopotential canlead to remarkably different structure

predictions. Received: March 3, 2011Accepted: April 21, 2011

ABSTRACT: Although the wave-like proclivity of electrons for delocalization is familiar toevery student of chemistry, it seems that electrons may have less respect for atomic andmolecular boundaries than one might have considered proper. The boundaries in questioninclude those between H-bonded dimers and within hydrated clusters, as well as those ofaqueous cavities, colloidal suspensions, and macroscopic air�water and oil�water interfaces.Unveiling the promiscuous behavior of electrons at such frontiers may both raise eyebrows anddemand acknowledgment.

1217 dx.doi.org/10.1021/jz2002875 |J. Phys. Chem. Lett. 2011, 2, 1216–1222

The Journal of Physical Chemistry Letters PERSPECTIVE

initio molecular dynamics simulations provide evidence that elec-tronsprefer to reside in interstitialwater cavities (seeFigure 1A).22,26

However, such simulations have so far been limited to rather smallwater droplets containing only 32 water molecules and , therefore,may or may not accurately represent the structure of hydratedelectrons in bulk water.

Although a substantial number of experiments have beenperformed on hydrated electrons, it is not yet clear whetherany of these are sufficient to provide a critical test of either thecavity or anionic cluster structures. For example, resonanceRaman spectra of hydrated electrons reveal a red shift(decrease in frequency) of both the OH stretch and HOH bendvibrations of water.27 It has been suggested that these shifts aresimilar to those induced by increasing the density of water and thussupport the anionic cluster structure of hydrated electrons,2 but thisappears to conflict with the fact that the stretch and bend vibrations ofice shift in opposing directions at high pressure.28 On the other hand,Raman scattering calculations of neutral and anionic water clusterspredict a red shift of both the bend and the stretch vibrations uponthe addition of an excess electron to a neutral cluster.29 Although thelatter predictions would seem to strongly support the anionic clustermodel of hydrated electrons, previous calculations have shown thatan electron in a water cavity might also produce red shifts in thesurrounding water bend and stretch vibrations.30

Other experimental measurements may prove to provide moredefinitive tests of alternative hydrated electronmodels. For example, itremains to be seen whether the fast excited-state relaxation dynamicsof hydrated electrons,2 as well as the oscillator strength31 andtemperature/density dependence of the absorption spectrum,32 canbeused to critically distinguish various hydrated electron structures.3�5

The experimental partial molar volume of a hydrated electron

should provide a key piece of evidence as its sign wouldpresumably be negative for an anionic cluster structure andpositive for a cavity structure. Experiments performed 40 yearsago do in fact imply that a hydrated electron has an negativepartial molar volume (and thus increases the density of water).33

However, even this is not yet sufficient to definitively distinguishthe cavity and anionic cluster structures as a cavity-boundhydrated electron may also have a negative partial molar volumeif the surrounding water molecules experience a sufficiently largeelectrostriction (electrostatically induced density increase).

Electrons on the Surface of Water. Speculations regarding thestructure of electrons dissolved in water also extend to electrons ataqueous interfaces. Both experimental and theoretical studies ofanionic water clusters agree that electronsmay occupy states that areeither in the interior or on the surface ofwater clusters.11,34,35 Recentexperiments provide tantalizing evidence that hydrated electronsmay also be happy to reside at amacroscopic air�water interface.6,36

It is tempting to try to link these results to an evenmore provocativeissue regarding the charge of the surface of pure water (in theabsence of hydrated electrons). The charge of such an air�waterinterface, which remains a highly controversial issue,37,38 may resulteither from the preferential surface affinity of hydroxide (as opposedto hydronium) ions or perhaps from H-bond-induced electrontransfer (as further described below).

Various experiments provide evidence that both air�waterand oil�water interfaces are negatively charged.9,39 For example, airbubbles in water, as well as colloidal suspensions of oil drops inwater, are observed to electrophoretically migrate toward the anodein an externally applied electric field. Although such experiments donot precisely locate (or chemically identify) the excess surfacecharge, they indicate that there is about one excess negative chargefor every 3 nm2 of oil�water surface area. On the other hand,photoelectron spectroscopic measurements have not found anyevidence of excess hydroxide ions at the air�water interface.9

Moreover, although pH-dependent surface sum frequency mea-surements of water at a hydrophobic octadecyltrichlorosilane(OTS) interface have provided evidence that “...even the neatwater/OTS interface is not neutral, but charged with OH� ions”,38

the observed spectra failed to show an interfacial OH� vibrationalpeak (which should appear as a narrow band at around 3630 (30 cm�1),40,62 and the interpretations of such experimentalresults remain a subject of debate.41

Figure 1. Alternative structures of thehydrated electron are exemplified in these twofigures. (A)Hydrated electron residing in a cavitynear the surface of a cluster of32watermolecules, obtained from ab initiomolecular dynamics simulations (the blue/gray contours correspond to hydrated electron densities of 0.003, 0.0005, and0.0002 au). Copyright (2010) by the American Physical Society.22 (B) Anionic cluster structure of a hydrated electron in liquid water, obtained fromquantum�classical simulations using an updated pseudopotential (the grid encloses 50% of the hydrated electron density). Reprintedwith permission fromAAAS.2

Results obtained in the most recentquantum-classical simulations pre-dict dramatically different hydrated

electron structures.



The results shown in Figure 2 illustrate the widely differingtheoretically predicted affinities of OH� for an air�water interface,obtained using three different theoretical methods. Ab initio molec-ular 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 empiri-cal valence bond potentials (Figure 2B)43 indicate that OH�

should have no affinity for an air�water interface (in agreementwith previous simulations)9 but predict that a negative surfacechargemay nevertheless be generated as the result of correlationsbetween the positions and orientations of OH� and its counter-ion relative to the air�water interface. None of these moleculardynamics simulations predict a sufficient interfacial accumulationof OH� to account for the experimentally observed negativesurface charge (as described above). On the other hand, arecently proposed dielectric continuum argument, which relieson the reduced dielectric constant of the hydration shell aroundan hydrated ion (Figure 2C), implies that OH� may have asufficiently high affinity for a macroscopic air�water interface toexplain the electrophoretic migration of oil drops and airbubbles.44 Some of these apparently contradictory experimentaland theoretical results may perhaps be reconciled by consideringthe different length scales probed by various methods. In otherwords, it may be that the differential affinities of hydroxide andhydronium ions are not sharply localized at the surface but ratherextend significantly into the bulk.

Another way in which charge could be generated at the surfaceof purewater, without invoking selective-ion adsorption (or counter-ion correlations), is by the dipolar (or induced dipolar) alignment ofinterfacial water molecules. However, such an alignment could onlyproduce 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 torequire that hydroxide ions, or some other negatively chargedparticles, do in fact accumulate at the surface of pure water. Theelectrophoretic experiments have been reproduced sufficiently care-fully that it does not seem reasonable to attribute the negative surfacecharge to impurities, and therefore, it is hard to avoid the conclusionthat it is due to a surface excess of hydroxide ions. However, recentdiscussions have raised the possibility that electrons themselvesmight perhaps be capable of producing the observed negative surfacecharge (private communication with Steve Rick, Paul Cremer, TomBeck, Pavel Jungwirth, and Sylvie Roke).63,64

The notion that electrons may be the particles that charge thesurface of pure water is linked to theoretical results which indicatethat the formation of a H-bond between two water molecules isaccompanied by the transfer of electron density from the H-bond

Explaining the observed electro-phoreticmobility appears to requirethat hydroxide ions, or some othernegatively charged particles, do infact accumulate at the surface of

pure water.

Figure 2. Several theoretical approaches have been used to investigate the affinity of OH� for an air�water interface. (A) Ab initio molecular dynamicssimulations 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 ofcorrelations between the orientations and positions of Naþ and OH� ions relative to the interface. Copyright (2010), with permission from theAmerican Institute of Physics.43 (C) Dielectric continuum simulations imply that a very large surface affinity (of the order of 20RT) may result from theinfluence of OH� on the local dielectric constant of water. Reproduced by permission of the PCCP Owner Societies.44



acceptor to the donor (as illustrated in Figure 3A and furtherdiscussed below).46 Such a charge-transfer mechanism could notsignificantly perturb the neutrality of bulk water because the latterwater molecules necessarily have a statistically equivalent number ofdonor and acceptor H-bonds. On the other hand, the asymmetry ofan air�water interface may lead to an imbalance in the number ofH-bond donors and acceptors and thus could perhaps produce a netcharge at the interface (as shown in Figure 3B). Preliminaryestimates, based on the calculated charge transfer in a water dimerand the number of excess H-bond donors at an air�water interface,suggest that this mechanism may be capable of producing a surfacecharge of the right order of magnitude (private communication fromSteve Rick).64 However, the key question is whether such a surfacecharging mechanism could or could not contribute to the observedelectrophoretic mobility of colloidal oil drops and air bubbles.

If the H-bond-induced charging of a water surface is analogousto an intermolecular polarizibility or dipolar alignment, in the sensethat the induced charge separation is pinned to the surface, then itcould not contribute to the observed electrophoreticmobility.On theother hand, H-bond-induced charge transfer might differ fundamen-tally from a dipolar alignment if the transferred electrons behave likeother charged particles (such as hydroxide ions), which can beseparated from their counterion in an external electric field, and thuscould give rise to electrophoretic mobility. In other words, H-bond-induced charge transfer implies that electrons have an higher prob-ability of residing on the H-bond donor and a lower probability ofresiding on the H-bond acceptor; therefore, why shouldn’t theresulting anionic and cationicwatermolecules (or clusters) be capableof separating from each other under the influence of an appliedelectric field? Moreover, the transfer of a single hydrogen atom

(proton plus electron) would convert such anionic and cationicwaters to hydroxide and hydronium ions, which certainly could beelectrophoretically separated. Such a hydrogen-atom-transfer processwould also provide a direct link between H-bond-induced electrontransfer and an interface-induced change in water’s ion product.47,48

Although the probability of electron transfer upon the forma-tion of an H-bonded water dimer is rather small (near 1%), it ispredicted to contribute substantially (∼20%) to the overall inter-molecular binding energy.46 Perhaps even more surprisingly, elec-tron transfer is also predicted to account for ∼30% of theintermolecular binding energy of the H2�H2O dimer (although,in this case, the electron-transfer probability is only ∼0.1%).49

Water on Ions and Ions on Water. A much more substantialamount of intermolecular charge transfer is predicted to take placeupon the hydration of anions,12 resulting, for example, in thedelocalization of ∼20% of a chloride ion’s negative charge over thewater molecules in its first hydration shell.13 Moreover, experimentalX-ray absorption spectra imply that a significant degree of electrontransfer occurs fromwater to divalent cations, such asMg2þ (althoughmuch less charge is transferred to cationswith lower charge density).14

The solvation of atomic ions in water is also predicted to induce adipole moment on the ion, whose magnitude rivals that of dipolarmolecules such as HCl (which has a dipole moment of∼1 D). Forexample, some calculations suggest that the dipole moment ofCl�(aq) may be as high as 1.6 D, although the true dipole momentofCl�(aq) is likely to be significantly smaller (∼0.6D).13The reasonfor the latter discrepancy may be traced to subtleties associated withtheway inwhich classical polarizable potentials interact with the pointcharges on the surrounding water molecules (as further discussedbelow). This same issue also turns out to strongly influence thepredicted affinity of ions for air�water and oil�water interfaces.

Experiments, including mass spectrometry,50 X-ray photo-electron and fluorescence spectroscopies,51,52 and nonlinearoptical surface spectroscopies,53 all point to the enhanced affinityof large anions for air�water interfaces, although the distributionof excess anions with respect to the interface again remains asubject of debate. Simulations suggest that the surface affinity ofanions in liquid water and hydrate clusters are quite sensitive toboth the polarizibility37 and size8,54 of the ions (see Figure 4). Onthe other hard, the experimental affinity of large anions for waterdroplet surfaces appears to be highly correlated with anion sizebut is not as well correlated with anion polarizability.50

Recent discussions have raised thepossibility that electrons them-

selves might perhaps be capable ofproducing the observed negative

surface charge

Figure 3. Charge transfer from theH-bond acceptor to theH-bond donor (A), which is predicted to take place in aH-bonded water dimer, (B) may also giverise to a net charge on water molecules near an air�water interface (blue, positive; red, negative). The electron density contours in figure in (A) were obtainedusing an MP2/aug-cc-pVTZ calculation, from which a Bader atoms-in-molecules (AIM) analysis indicates an electron-transfer probability of∼1.9% from theH-bond acceptor to the H-bond donor (private communication from Tom Beck). The results in (B) were obtained using a recently developed classical watermodel that includes intermolecular charge transfer (private communication from Steve Rick63,64) and predict a net negative surface charge of about 0.1% of anelectron per nm2, when integrated over all of the water molecules within the first 0.5 nm from the interface (Gibbs dividing surface).



Recent studies have raised questions as to whether some classical(polarizable) potentials may be “over-polarized” with respect to abinitio calculations.7,15,55 Such an over-polarization can arise fromshort-range interactions between polarizabilities and point charges,resulting in unphysical singularities, which can be rectified bydelocalizing the point charges using various “polarization damping”strategies.7,55�57 Comparisons with ab initio calculations imply thatthe degree of polarization damping, which is employed in somecommonly used potential functions, may not be sufficient and thusmay lead to spurious predictions, despite the fact that the samepotentials accurately predict ion hydration free energies. When amore realistic level of polarization dampling is introduced, the excesssurface affinities of large anions, as well as the large induced dipoleson hydrated ions, are substantially decreased.7 Recent ab initiomolecular dynamics simulations of iodide in water predict asubstantially smaller induced dipole moment, relative to thatobtained using some polarizable potentials, and provide a betterfit to the local solvation structure of iodide as determined fromextended X-ray absorption fine structure (EXAFS) experiments.15

What is still not clear is why over-polarized potentials arecapable of accurately predicting hydration free energies. One possi-bility is that the over-polarization is compensating for ion�water charge transfer (private communication with Tom Beck). Ifthis is the case, then both ion�water charge transfer and realisticpolarization damping may be required in order to self-consistentlypredict ion hydration thermodynamics, induced dipole moments,and surface affinities. Quasi-chemical theory,55 perhaps combinedwith ideas emerging from local molecular field theory,58 as well as theintermolecular extensions of molecular polarizabilities (private com-munication from Steve Rick63), may provide appealing strategies foraccomplishing such an incorporation of electron transfer into liquidmolecular dynamics simulations.

Final Thoughts. Although this Perspective has encompassedrecent speculations and debates regarding a broad range of phe-nomena, all of these are tied together by a common thread linkingelectrons and molecular boundaries; this is the key issue whichdetermines the structure of hydrated electrons and drives theexchange of electrons between hydrated ions and water molecules,as well as between H-bonded water molecules. The latter electrontransfermay also contribute, at least in part, to the observed negativecharge of air�water and oil�water interfaces. Moreover, problems

associatedwith the over-polarization of classical potentials also hingeon charge delocalization, as over-polarization arises directly from theunphysical introduction of idealized point charges in classicalintermolecular potentials.

Intermolecular and intramolecular polarizability are closely re-lated phenomena as both arise from the susceptibility of electronwave functions to applied electric fields. Although intramolecularelectron delocalization is a much more familiar concept, it too maylead to consequences that are at odds with our preconceptionsregarding atomic identities. For example, the positive charge on atetraalkyl amine cation is not nearly as localized on the centralnitrogen atom as one might have expected, but is rather widelydistributed over the surrounding hydrocarbon chains, to an extentthat the central nitrogen may be nearly neutral or perhaps evenslightly negative.59 This charge delocalizationmaywell contribute tothe unusual reactivity of the associated alkane chains with hydroxideions60 and might also be expected to significantly influence thehydrophobicity of the alkane chains.

In biochemical systems, the delocalization of charge between“ionic” and “hydrophobic” groups may prove to play an importantrole in the structure and reactivity of soluble proteins, whose surfaceareas are typically nearly half occupied by hydrophobic side chains,decorated by neighboring ionic and polar groups. Moreover, thenegative surface charge of water at macroscopic (and colloidal)oil�water interfaces may also prove to be of biochemical relevance,if a similar charge accumulation occurs in the vicinity of hydrophobicpatches on the surfaces of proteins and other biological assemblies.

In biochemical systems, the delo-calization of charge between

“ionic” and “hydrophobic” groupsmay prove to play an important rolein the structure and reactivity of

soluble proteins.

Figure 4. The degree of affinity of large negative anions, such as I�, for an air�water 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 air�water interface.61 (B) Snapshot and distribution function of I� andNaþ ions near an air�water interface, obtained usingmoleculardynamics with polarizable potentials.37 Dielectric continuum models are also capable of predicting an interfacial affinity of I�,8 as are nonpolarizablemolecular models, when ion sizes are slightly adjusted.54 The true distribution may lie somewhere between the polarizable and nonpolarizablepredictions (private communication with Pavel Jungwirth).



’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 PurdueUniversity. His recent experimental and theoretical interestsinclude hydration shell spectroscopy, nanoscale hydrophobicity,hyperspectral imaging, and new ways of teaching physicalchemistry.

’ACKNOWLEDGMENT

This work was facilitated by the National Science Foundation(CHE-0847928), as well as by fruitful discussions with thefollowing people (in alphabetical order): Heather Allen, DaveBartels, James Beattie, Tom Beck, Max Berkowitz, Paul Cremer,Mark Johnson, Ken Jordan, Pavel Jungwirth, Chris Mundy,Sandeep Patel, Lawrence Pratt, Steve Rick, Sylvie Roke, PeterRossky, Benjamin Schwartz, Doug Tobias, and Laszlo Turi.

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unveiling electron promiscuity

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