Water Radical Cations in the Gas Phase: Methods and Mechanisms of Formation, Structure and Chemical Properties

Review Water Radical Cations in the Gas Phase: Methods and Mechanisms of Formation, Structure and Chemical Properties

Dongbo Mi and Konstantin Chingin *

Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China University of Technology, Nanchang 330013, China; [email protected] * Correspondence: [email protected]

Received: 11 July 2020; Accepted: 29 July 2020; Published: 31 July 2020

+ Abstract: Water radical cations, (H2O)n •, are of great research interest in both fundamental and applied sciences. Fundamental studies of water radical reactions are important to better understand the mechanisms of natural processes, such as proton transfer in aqueous solutions, the formation of hydrogen bonds and DNA damage, as well as for the discovery of new gas-phase reactions and products. In applied science, the interest in water radicals is prompted by their potential in radiobiology and as a source of primary ions for selective and sensitive chemical ionization. However, in contrast to + protonated water clusters, (H2O)nH , which are relatively easy to generate and isolate in experiments, + the generation and isolation of radical water clusters, (H2O)n •, is tremendously diﬃcult due to their + ultra-high reactivity. This review focuses on the current knowledge and unknowns regarding (H2O)n • species, including the methods and mechanisms of their formation, structure and chemical properties.

Keywords: water radical cations; water radiolysis; ab initio dynamics; DFT calculations; ultrafast chemistry

1. Introduction Water is crucial for our existence on this planet and is involved in almost all biological and chemical processes [1]. The ionization of liquid water by photons, fast electrons, X-rays, heavy ions, etc., is increasingly employed in diverse fields such as photon science, radiotherapy, nuclear reactors, radiation chemistry, nuclear waste management, and so on [2–10]. Following the discovery of X-rays and natural radioactive phenomena, the chemistry of water radiolysis has been extensively studied for more than a century [11–13]. The interaction of highly energetic photons or charged particles with water results, in general, in the ejection of a quasi-free + electron from the valence shell, leaving behind a positively charged radical cation, H2O •, which then + becomes stabilized as a cluster, (H2O)n •. Gas-phase water radical cations can also be produced in air plasma under atmospheric pressure. Experimental data suggest that the proton transfer dynamics occur on a similar timescale as electron autoionization, with the proton transfer forming a Zundel-type + intermediate [HO* ... H ... H2O] •, which further ionizes to form a so-far undetected type of dicationic charge-separated species with high internal energy [14]. Characterization of generated water radical cations is usually done using mass spectrometry, + which can be combined with optical spectroscopy [15,16]. Two series of cluster ions ((H2O)nH and + (H2O)n •) in water ice have been detected simultaneously in experiments involving secondary ion + + mass spectrometry (SIMS), with Au , Au3 , and C60+ as primary ions [17]. Typically, protonated + (H2O)nH cations have been observed as the predominant products of water ionization [18–20]. Liu et al. recently employed a microfluidic chip combined with time-of-flight secondary ion mass + spectrometry (ToF-SIMS) using keV-energy ion irradiation of Bi3 as primary ions and only detected

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+ the protonated water and heavy water clusters [21]. The observation of radical (H2O)n • cations is highly uncommon. + 18 H2O • is estimated to form within a timescale of attoseconds (10− s) or subfemtoseconds at the most, based on the uncertainty relationship ∆E∆t h [22]. In pure water, the hot electrons generated ≈ after the ionization of the water relax into solvent molecules and become trapped as hydrated electron (e ) species, while H O+ rapidly forms oxidizing OH radicals via proton transfer, as shown in the hyd− 2 • • following equation [23], which is fully accomplished in less than 1 ps.

+ + H O • + H O OH + H O (1) 2 2 → • 3 In the past few decades, owing to the advent of femtosecond laser technology, several attempts + using various methods have been made to identify experimentally this H2O • cationic species [23–26]. + Nevertheless, a direct measurement of the H2O • decay has not yet been successfully done in pure water. It is of great interest to provide insight into the electronic signature of this radical cation by using a more sophisticated time-resolved experimental or theoretical method. + Recently, Mizuse reported the infrared spectra of water cluster radical cations (H2O)n • (n = 3–11) in the gas phase to understand the structural evolution of ionized water networks at the molecular level [15]. Further investigation has led us to have a deeper understanding of the precise structure of + gaseous water radical cations (H2O)n •. However, as has been pointed out, theoretical calculations often suﬀer from symmetry breaking, spin contamination and/or self-interaction errors in such open-shell doublet systems [27,28]. The reactivity of water exposed to ionizing radiation would be expected to depend on H-bond network structures around the created OH radical, which often reacts with a substance via one-electron • + oxidation, addition and H abstraction. However, direct investigation of (H2O)n • chemistry is still extremely challenging. Except for some dynamic simulations, only a few works on the oxidation of + small molecules, such as O2,C2H4 or CH2O by H2O • [29,30], and on other proton transfer and charge + migration processes of H2O •, have been reported. The great interest in water radical cations and their solvated clusters has prompted a large number of studies which have greatly enhanced our understanding of this highly unstable transient state of water over recent years. This review focuses on recent studies aimed at uncovering the mechanism of formation, structural characterization and chemical properties of the water radical cation and its clusters.

+ 2. Methods and Mechanisms of Formation of (H2O)n •

2.1. Electron Bombardment

+ The mechanism of the generation of H (H2O)n was first proposed by Good et al. in 1970 and was based on the results of experiments involving the electron bombardment of various species [18]. + + First, pure nitrogen was considered. The reaction for the formation of N3 from N has been + studied very little. The N3 bond dissociation energy has been shown to be much greater than that of + + N4 • when investigated under the same conditions [31]. Good et al. found the sum of the ions N2 • + and N4 • to constitute roughly 85% of the total ion intensity at all times during the experiments, and + + an almost complete conversion of N2 • to N4 • at long reaction times (t > 100 µs) was observed in the pressure range 0.5–4.5 torr [18]. Later, a series of experiments was conducted at 300 K with nitrogen containing traces of water. + + + + + + As shown in Figure1, the major ions observed were N 2 •,N4 •,H2O •,H3O ,H (H2O)2,H (H2O)3 + and H (H2O)4. These ions clearly form a first-order reaction sequence. These experiments indicated a + + + rapid decay of N2 • while N4 • is forming, followed by N4 • reaching a maximum and then decaying + + itself, and then H2O • forming, reaching a maximum and falling off, etc. The final ions H (H2O)2, + + H (H2O)3 and H (H2O)4 apparently reached constant concentrations after some 500–600 µs of reaction + time. The authors argued that the formation of H2O • here could not have proceeded by a direct + + charge transfer from N2 •, since the disappearance of N2 • was independent of the H2O concentration Molecules 2020, 25, x FOR PEER REVIEW 3 of 38 Molecules 2020, 25, 3490 3 of 37 from N2+•, since the disappearance of N2+• was independent of the H2O concentration ([H2O] < 8 × ([H10−20O] [N<2]28) and,10 due20 [N to] the2) and, rate due constant, to the ratedetermined constant, to determined be 1.9 × 10− to9 cc be2 molecule 1.9 10 −92·scc−12, beingmolecule a normal2 s 1, 2 × − 2 × − − · − beingvalue afor normal a charge-transfer-type value for a charge-transfer-type reaction. reaction.

Figure 1. Normalized ion-intensity curves for ions in moist nitrogen after being subjected to a 10 µs Figure 1. Normalized ion-intensity curves for ions in moist nitrogen after being subjected to a 10 μs electron pulse. Dashed lines represent theoretical curves calculated from integrated rate equations for electron pulse. Dashed lines represent theoretical curves calculated from integrated rate equations for consecutive reactions including reversible steps using average rate constants. Figure adapted from [18] consecutive reactions including reversible steps using average rate constants. Figure adapted from with permission. Copyright 1970 American Institute of Physics. [18] with permission. Copyright 1970 American Institute of Physics. + + H2O • was clearly indicated from these experiments to be the precursor of the H (H2O)n hydrates. +• + H2O was clearly indicated from these experiments to+ be the precursor+ of the H (H2O)n hydrates. Similarly, the rate constant of the reaction forming H3O from H2O • and H2O was determined to + +• Similarly, the9 rate2 constant 2of the1 reaction forming H3O from H2O9 and2 H2O was2 determined1 to be be 1.8 10− cc molecule− s− , very close to a value of 1.6 10− cc molecule− s− that had been 1.8 × 10×−9 cc2·molecule· −2·s−1, very· close to a value of 1.6 × 10−9 cc×2 molecule−2·s−1 that had· been previously previously determined by Thynne et al. and Gupta et al. [32,33]. determined by Thynne et al. and Gupta et al. [32,33]. + Perhaps most importantly, Good et al. suggested that an electron could transfer from H2O to N4 • Perhaps most importantly, Good et al. suggested that an+ electron could transfer+ from H2O to and result in the production of the primary water radical H2O • and then form H (H2O)n according N4+• and result in the production of the primary water radical H2O+• and then form H+(H2O)n to the following equations [18]: according to the following equations [18]: + N + e N • (2) 2 → 2 +• + N2 + e → N2+ (2) N2 • + 2N2 N4 • + N2 (3) +• → +• +N2 + 2N2 → N4 + + N2 (3) N •+ H O H O • + 2N (4) 4 2 → 2 2 N4+•+ H2O → H2O+• + 2N2 (4) In the case of moist oxygen and air, in addition to the reaction mechanism mentioned above, + P. KebarleIn theand case colleagues of moist oxygen proposed and a air, proper in addition alternative to the path reaction by which mechanism H (H2O) nmentionedcould be generated above, P. followingKebarle and the colleagues formation proposed of ions with a proper a mass alternative of 36 amu path [19], forby which theH+(H notation2O)n could H Obe+ generatedOH was 3 •• suggestedfollowing bythe Fehsenfeld formation andof ions Ferguson with a [34 mass]. These of 36 ions amu with [19], a mass for which of 36 amu the werenotation of very H3O low+•• intensityOH was insuggested all runs by and Fehsenfeld could be detectedand Ferguson only under[34]. These conditions ions with of steady a mass electron of 36 amu irradiation, were of indicating very low theseintensity ions in to all be runs reaction and intermediates could be detected and to only play un ader rather conditions central roleof steady in the reactionelectron mechanism.irradiation, Furthermore,indicating these P. Kebarleions to be and reaction colleagues intermediates proposed aand symmetrical to play a rather hybrid central structure role indicated in the reaction by the resonancemechanism. structures, Furthermore, as shown P. Kebarle in Figure and2, withcolleagues this structure proposed no a longer symmetrical showing hybrid any distinction structure + betweenindicated the by Hthe3O resonanceand •OH structures, parts of the as complex. shown in Figure 2, with this structure no longer showing any distinction between the H3O+ and •OH parts of the complex.

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Figure 2. Proposed resonance structures for the (a) (H4O2)+• and (b) (H5O2)+ ions. Figure adapted from [19] with permission. Copyright 1970 American Institute of Physics. + + Figure 2. Proposed resonance structures for the (a) (H4O2) • and (b) (H5O2) ions. Figure adapted Figure 2. Proposed resonance structures for the (a) (H4O2)+• and (b) (H5O2)+ ions. Figure adapted from 2.2. Coronafrom [Discharge19] with permission. at Atmospheric Copyright Pressure 1970 American Institute of Physics. [19] with permission. Copyright 1970 American Institute of Physics. 2.2.Traces Corona of Discharge H2O+• were at Atmospheric detected using Pressure a corona discharge source of the design shown in Figure 3 and with a distance of 0.5 mm from the corona discharge point to the sampling aperture. Here, the 2.2. Corona Discharge+ at Atmospheric Pressure samplingTraces procedure of H2O removed• were detected ions from using the a source corona chamber discharge in source about of 10 the−5 sdesign or less shownand the in ions Figure present3 and Traces of H2O+• were detected using a corona discharge source of the design shown in Figure 3 inwith the source a distance chamber of 0.5 mmunder from this the condition corona dischargeof operation point are to shown the sampling in Figure aperture. 4 [35]. In Here, this experiment, the sampling procedureand with a removeddistance ionsof 0.5 from mm thefrom source the corona chamber discharge in about point 10 5 tos or the less sampling and the aperture. ions present Here, in the the ion intensity of H2O+• was extremely low and other water− radicals, i.e., (H2O)n+•, were not sampling procedure removed ions from the source chamber in about 10−5 s or less and the ions present observed.source chamber under this condition of operation are shown in Figure4[ 35]. In this experiment, the ion + + intensityin the source of H chamber2O • was under extremely this condition low and of other operation water radicals,are shown i.e., in (HFigure2O)n 4• [35]., were In notthis observed. experiment, the ion intensity of H2O+• was extremely low and other water radicals, i.e., (H2O)n+•, were not observed.

Figure 3. Schematic diagram of a corona discharge source with an adjustable distance between the Figure 3. Schematic diagram of a corona discharge source with an adjustable distance between the corona discharge point and sampling aperture. Figure adapted from [35] with permission. Copyright corona discharge point and sampling aperture. Figure adapted from [35] with permission. Copyright 1976 American Chemical Society. 1976 American Chemical Society. Figure 3. Schematic diagram of a corona discharge source with an adjustable distance between the corona discharge point and sampling aperture. Figure adapted from [35] with permission. Copyright 1976 American Chemical Society.

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FigureFigure 4. Mass 4. Mass spectrum spectrum of of nitrogen nitrogen obtained with with a corona a corona discharge discharge point located point at located a distance at aof distance of 0.5 mm0.5 mm from from the the sampling sampling aperture. aperture. Figure Figure adapte adaptedd from from [35][ 35with] with permission. permission. Copyright Copyright 1976 1976 AmericanAmerican Chemical Chemical Society. Society.

KamabaraKamabara et al. et identified al. identified many many clusters clusters produced produced in anin an ambient ambient pressure pressure ionization ionization (API)(API) process throughprocess a collisional through dissociationa collisional dissociation method [36 method]. In their [36]. study,In their nitrogenstudy, nitrogen including including trace trace water water (including (including several ppm of water) was employed as the sample gas. The nitrogen flow rate was 1 severalL/min. ppm ofA corona water) discharge was employed from a needle as the electrode sample was gas. employed The nitrogen for ionization flow rate and wasproduced 1 L/min. a total A corona dischargedischarge from acurrent needle of electrode 1 μA. The was needle employed electrode for was ionization placed 4 andmm producedin front of athe total first discharge aperture current of 1 µA.electrode The needle and a electrodedischarge voltage was placed of 2.8 kV 4 mm was insupplied front ofbetween the first these aperture electrodes. electrode and a discharge voltage of 2.8Ions kV with was masses supplied of 74, between 56 and 46 these amu electrodes.were observed as the major ions in the dry nitrogen Ionsstreams with (with masses less ofthan 74, 1 56ppm and water). 46 amu These were ions observedas well as N as3+ have the major been reported ions in in the API dry experiments nitrogen streams with a β-ray ionizer [37]. The types of ions present in the dry nitrogen stream were clarified by (with less than 1 ppm water). These ions as well as N + have been reported in API experiments with acquiring spectra over a wide range of drift voltages, as3 shown in Figure 5. The ions observed were a β-ray ionizer [37]. The types of ions present in the dry nitrogen stream were clarified by acquiring identified to be N4+• for the 56 amu peak, H2O+• (not NH4+) for the 18 amu peak, H2O+••N2 (not NO2+•) spectrafor over the a46 wide amu rangepeak and of H drift2O+••N voltages,2•N2 for as the shown 74 amu in peak. Figure The5 dissociation. The ions observedpattern also were suggested identified to + + + +• + +• + be N4 that• for an the ion 56 corresponding amu peak, Hto2 theO 74• (not amu NHpeak4 had) for the the form 18 H amu2O •N peak,2•N2 H and2O not• N42 (not•H2O. NO In 2the• ) for the API spectra for the+ moist nitrogen (including several ppm of water), 32, 36, 37, 42,• 46, 47, 50, 55, 60, 46 amu peak and H2O • N2 N2 for the 74 amu peak. The dissociation pattern also suggested that an 72 and 74 amu peaks• were• the major ones observed. + + ion corresponding to the 74 amu peak had the form H2O • N2 N2 and not N4 • H2O. In the API Water clusters H+(H2O)n have been reported to be the major• components• in this concentration• spectra for the moist nitrogen (including several ppm of water), 32, 36, 37, 42, 46, 47, 50, 55, 60, 72 and range, consistent with those previously investigated using a β-ray ion source [38]. This difference was 74 amudueMolecules peaks to the 2020 were complete, 25, thex FOR majordifference PEER REVIEW ones between observed. the ion resident times and effective temperature in the corona6 of 38 discharge ion source and those in the β-ray ion source. The resident times of ions in corona discharge ion sources have been estimated to be only tens of microseconds. Therefore, the ion–molecule reactions, such as clusters forming, cannot reach an equilibrium determined by partial pressures of the components. In addition to this, the strong electric field in the ion source would raise the effective temperature of the ions. These issues apparently caused the differences between the ionization efficiency and mass patterns of the two ion sources. The 36 amu peak has been suggested to be probably due to two ions, one being H3O+••OH and the other NH4+•H2O. These two species apparently dissociated in two different ways at the high drift voltages, with the clusters of H3O+••OH dissociating to H3O+ and •OH (giving 19 amu ions) and clusters of NH4+•H2O dissociating to NH4+ and H2O (giving 18 amu ions). These dissociation results suggested an ion structure of H3O+••OH for this species, in good agreement with previous investigations mentioned above [19,34,39].

Figure 5. Drift voltage dependence of mass spectra for dry nitrogen. (a) 5 V, (b) 10 V, (c) 18 V, (d) 22 V, Figure 5. Drift voltage dependence of mass spectra for dry nitrogen. (a) 5 V, (b) 10 V, (c) 18 V, (d) 22 (e) 30 V,V, ( f()e) 40 30 V. V, Figure (f) 40 V. adapted Figure adapted from [36 from] with [36] permission. with permission. Copyright Copyright 1979 1979 American American ChemicalChemical Society. Society.

2.3. Photoionization of a Water Vapor Beam for the Formation of (H2O)n+• (n ≤ 3)

The detection of (H2O)2+• was reported first by Ng et al. [40], who measured the appearance potentials of (H2O)2+• and its dissociated product H3O+ using molecular beam photoionization mass spectrometry. The H2O molecular beam was produced by seeding water vapor at 89 °C at a pressure of 150 torr of Ar and then having it expand through a 0.15-mm-diameter Pyrex nozzle. Photoion yield curves of (H2O)2+•, H2O+• and H3O+ in the spectral range 950–1100 Å were acquired with a photo bandwidth of 2.5 Å full width at half maximum (FWHM) and are shown in Figure 6. Due to the weak signal of (H2O)2+•, no distinct threshold was observed. The ionization potential of 11.21 ± 0.09 eV corresponded to the point where the signal fell below 0.1 count/s and thus was only an upper bound; they found that the ionization potential of the dimer shifted 1.40 ± 0.09 eV from that of the monomer [40,41]. The efficiency of the photoionization producing the (H2O)2+• ion increased gradually above the threshold (11.21 ± 0.09 eV) indicative of a change in nuclear geometry. When the internal excitation of the dimer ion was increased to 0.52 eV, the H3O+ fragment ion was observed. However, no (H2O)n+• cluster ions were observed. Ng et al. argued that the inability to detect larger unprotonated ions (H2O)n+• (n ≥ 3) was most probably due to changes in geometry upon ionization, so that the potential minima of (H2O)n+• were far from the Franck–Condon region of the shallow potential minima of the neutral (H2O)n clusters [40]. Later, Shinohara et al. carried out a photoionization of supersonic cluster beams of water–argon mixtures (P ≥ 2 atm), with the vacuum-UV resonance lines at 11.83 and 11.62 eV [10]. As shown in Figure 7, with a stagnation pressure of 1.5 atm, all of the prominent peaks in the spectrum corresponded to protonated cluster ions with the general formula (H2O)nH+, and none corresponded to (H2O)2+•.

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+ Water clusters H (H2O)n have been reported to be the major components in this concentration range, consistent with those previously investigated using a β-ray ion source [38]. This difference was due to the complete difference between the ion resident times and effective temperature in the corona discharge ion source and those in the β-ray ion source. The resident times of ions in corona discharge ion sources have been estimated to be only tens of microseconds. Therefore, the ion–molecule reactions, such as clusters forming, cannot reach an equilibrium determined by partial pressures of the components. In addition to this, the strong electric field in the ion source would raise the effective temperature of the ions. These issues apparently caused the differences between the ionization efficiency and mass patterns of the two ion sources. The 36 amu peak has been suggested to be probably due to two ions, one being H O+ OH and 3 •• the other NH + H O. These two species apparently dissociated in two different ways at the high 4 • 2 drift voltages, with the clusters of H O+ OH dissociating to H O+ and OH (giving 19 amu ions) 3 •• 3 • and clusters of NH + H O dissociating to NH + and H O (giving 18 amu ions). These dissociation 4 • 2 4 2 results suggested an ion structure of H O+ OH for this species, in good agreement with previous 3 •• investigations mentioned above [19,34,39].

+ 2.3. Photoionization of a Water Vapor Beam for the Formation of (H O)n (n 3) 2 • ≤ + The detection of (H2O)2 • was reported ﬁrst by Ng et al. [40], who measured the appearance + + potentials of (H2O)2 • and its dissociated product H3O using molecular beam photoionization mass spectrometry. The H2O molecular beam was produced by seeding water vapor at 89 ◦C at a pressure of 150 torr of Ar and then having it expand through a 0.15-mm-diameter Pyrex nozzle. Photoion yield + + + curves of (H2O)2 •,H2O • and H3O in the spectral range 950–1100 Å were acquired with a photo bandwidthMolecules of 2.5 2020 Å,full 25, x FOR width PEERat REVIEW half maximum (FWHM) and are shown in Figure6. 7 of 38

Figure 6. Photoion yield curves of (H O) + ,H O+ and H O+ in the spectral range 95–1120 Å. Figure Figure 6. Photoion yield curves of2 (H22O)•2+•, H3 3O+ and H2O2+• in •the spectral range 95–1120 Å. Figure adapted fromadapted [40 ]from with [40] permission. with permission. Copyright Copyright 1977 1977 American Institute Institute of Physics. of Physics.

+ Due to the weak signal of (H2O)2 •, no distinct threshold was observed. The ionization potential of 11.21 0.09 eV corresponded to the point where the signal fell below 0.1 count/s and thus was only ± an upper bound; they found that the ionization potential of the dimer shifted 1.40 0.09 eV from that ± + of the monomer [40,41]. The eﬃciency of the photoionization producing the (H2O)2 • ion increased gradually above the threshold (11.21 0.09 eV) indicative of a change in nuclear geometry. When the ± + internal excitation of the dimer ion was increased to 0.52 eV, the H3O fragment ion was observed.

Figure 7. Vacuum-UV photoionization mass spectrum of water clusters at 11.83 and 11.62 eV (Ar resonance lamp; LiF window) with unit mass resolution. Conditions: 1.5 atm total stagnation pressure (seeded in Ar). Figure adapted from [10] with permission. Copyright 1986 American Institute of Physics.

Deuterium substitution studies were carried out by Shinohara et al. [10]. As shown in Figure 8, the change in isotope had no effect on the ratio (D2O)2+•/(D2O)2D+, and Shinohara et al. argued that proton transfers within water clusters were not affected by the deuterium substitution [10].

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+ However, no (H2O)n • cluster ions were observed. Ng et al. argued that the inability to detect larger + unprotonated ions (H2O)n • (n 3) was most probably due to changes in geometry upon ionization, ≥ + so that the potential minima of (H2O)n • were far from the Franck–Condon region of the shallow potential minima of the neutral (H2O)n clusters [40]. Later, Shinohara et al. carried out a photoionization of supersonic cluster beams of water–argon mixtures (P 2 atm), with the vacuum-UV resonance lines at 11.83 and 11.62 eV [10]. As shown in ≥ Figure7, with a stagnation pressure of 1.5 atm, all of the prominent peaks in the spectrum corresponded +• + +• to protonated clusterFigure 6. ionsPhotoion with yield the curves general of (H2O) formula2 , H3O and (H HO)2O Hin +the, andspectral none range corresponded 95–1120 Å. Figure to (H O) + . adapted from [40] with permission. Copyright 1977 American2 n Institute of Physics. 2 2 •

Figure 7. FigureVacuum-UV 7. Vacuum-UV photoionization photoionization mass mass spectrum spectrum of water of water clusters clusters at 11.83 and at 11.8311.62 eV and (Ar 11.62 eV (Ar resonanceresonance lamp; lamp; LiF LiF window window)) with with un unitit mass mass resolution. resolution. Conditions: Conditions: 1.5 atm total stagnation 1.5 atm totalpressure stagnation pressure (seeded(seeded in inAr). Ar). Figure adapted adapted from from [10] [ 10with] with permission. permission. Copyright Copyright 1986 American 1986 AmericanInstitute of Institute of Physics. Physics.

Deuterium substitution studies were carried out by Shinohara et al. [10]. As shown in Figure 8, Deuterium substitution studies were carried out by Shinohara et al. [10]. As shown in Figure8, the change in isotope had no effect on the ratio (D2O)2+•/(D2O)2D+, and Shinohara et al. argued that + + the changeproton in isotope transfers had within no e waterﬀect onclusters the were ratio no (Dt affected2O)2 • by/(D the2O) deuterium2D , and substitution Shinohara [10]. et al. argued that proton transfers within water clusters were not aﬀected by the deuterium substitution [10]. Molecules 2020, 25, x FOR PEER REVIEW 8 of 38

Figure 8. ArFigure lamp 8.(LiF Ar lamp window; (LiF window; 11.83 11.83 and and 11.62 11.62 eV) eV) photoionization photoionization mass mass spectrum spectrum of heavy of water heavy water clusters (D2O)nclusters with (D2 unitO)n with mass unit resolution. mass resolution. Conditions: Conditions: 1.5 1. atm5 atm total total stagnationstagnation pressure pressure (seeded (seeded in in Ar). Figure adaptedAr). Figure from adapted [10] with from permission. [10] with permission. Copyright Copyright 1986 1986 American American Institute Institute of of Physics. Physics.

Tunable vacuumTunable ultraviolet vacuum ultraviolet (VUV) (VUV) photoionization photoionization studies studies of of water water clusters clusters were performed performed using using 10–14 eV synchrotron radiation and analyzed using reflectron time-of-flight (TOF) mass 10–14 eV synchrotronspectrometry, radiation devised by and Belau analyzed et al. [42]. using Thre reﬂectrone series of peaks time-of-ﬂight comprising (TOF) unprotonated mass spectrometry, and devised byprotonated Belau et al.water [42 ].clusters Three and series their ofmetastable peaks comprisingfragments were unprotonated distinguished in and the protonated spectrum. water However, only two unprotonated water species were indicated here, namely H2O+• and (H2O)2+•, and they were observed at extremely low ion intensity. In all of the experiments reported so far, the protonated cluster ions were by far the predominant ions indicated by the mass spectra, whereas the unprotonated cluster ions were either not detectable or yielded peaks that were orders of magnitude weaker than those corresponding to protonated ions. According to an acquired water dimer photoelectron spectrum [43], the 11.83 eV photon can effectively produce the first vertical ionization, which has been interpreted as the ejection of an electron from the out-of-plane nonbonding molecular orbital (MO) (2a’’) localized on the proton donor oxygen atom. Tomoda and Kimura have suggested the potential minimum for (H2O)2+• to be far from the wide Franck–Condon region expected from the shallow potential minimum of (H2O)2 [44]. They have also suggested the presence of some reaction paths with no activation energy barriers between the vertically ionized points [(H2O)2+•]*vip and the dissociation limiting point yielding H3O+ and •OH.

2.4. Photoionization of a Molecular Beam for the Formation of (H2O)n+• (n > 3)

Shinohara et al. [10] expected the failure to observe (H2O)n+• to imply the occurrence of geometrical changes upon ionization. The geometries of (H2O)n+• species differ quite considerably from those of (H2O)n due to the shorter O-O distances of the ionized form and thus the Franck– Condon transitions from the potential curves of the neutral (H2O)n clusters do not cover the potential minima of (H2O)n+• along the proton transfer coordinates [44,45]. They concluded that in water clusters, except for (H2O)2+•, the Franck–Condon excitations from the neutral clusters (H2O)n to the corresponding ionic states cannot produce the parent cluster ions (H2O)n+•, even in the near-threshold ionization, as illustrated in Figure 9.

Molecules 2020, 25, 3490 8 of 37 clusters and their metastable fragments were distinguished in the spectrum. However, only two + + unprotonated water species were indicated here, namely H2O • and (H2O)2 •, and they were observed at extremely low ion intensity. In all of the experiments reported so far, the protonated cluster ions were by far the predominant ions indicated by the mass spectra, whereas the unprotonated cluster ions were either not detectable or yielded peaks that were orders of magnitude weaker than those corresponding to protonated ions. According to an acquired water dimer photoelectron spectrum [43], the 11.83 eV photon can eﬀectively produce the ﬁrst vertical ionization, which has been interpreted as the ejection of an electron from the out-of-plane nonbonding molecular orbital (MO) (2a”) localized on the proton donor oxygen atom. + Tomoda and Kimura have suggested the potential minimum for (H2O)2 • to be far from the wide Franck–Condon region expected from the shallow potential minimum of (H2O)2 [44]. They have also suggested the presence of some reaction paths with no activation energy barriers between the vertically + * + ionized points [(H2O)2 •] vip and the dissociation limiting point yielding H3O and •OH.

+ 2.4. Photoionization of a Molecular Beam for the Formation of (H2O)n • (n > 3) + Shinohara et al. [10] expected the failure to observe (H2O)n • to imply the occurrence of geometrical + changes upon ionization. The geometries of (H2O)n • species diﬀer quite considerably from those of (H2O)n due to the shorter O-O distances of the ionized form and thus the Franck–Condon transitions + from the potential curves of the neutral (H2O)n clusters do not cover the potential minima of (H2O)n • along the proton transfer coordinates [44,45]. They concluded that in water clusters, except for + (H2O)2 •, the Franck–Condon excitations from the neutral clusters (H2O)n to the corresponding + ionic states cannot produce the parent cluster ions (H2O)n •, even in the near-threshold ionization, as illustrated in Figure9. Molecules 2020, 25, x FOR PEER REVIEW 9 of 38

Figure 9. FigureSchematic 9. Schematic energy energy level level diagram diagram of of water water clusters clusters (H2O)n (H along2O) then along reaction the channels. reaction The channels. The label “vip”label “vip” stands stands for for vertically vertically ionizedionized points points (sic). (sic). Figure Figure adapted adapted from [10] from with [10 permission.] with permission. CopyrightCopyright 1986 American 1986 American Institute Institute of Physics. of Physics.

+• +• However, (H+ 2O)n (2 ≤ n ≤ 10) and (Ar)m•(H2O)n (2 ≤+ m ≤ 3; 2 ≤ n ≤ 7) were detected in mass However, (H O)n (2 n 10) and (Ar)m (H O)n (2 m 3; 2 n 7) were detected in spectra, exhibited2 • in Figure≤ ≤ 10, for the first time• when2 supersonic• ≤ cluster≤ beams≤ of≤ water–argon mass spectra,mixtures exhibited were photoionized, in Figure 10 with, for the the vacuum-UV ﬁrst time re whensonance supersonic lines in the near cluster threshold beams at ofhigher water–argon mixtures werestagnation photoionized, pressures (≥3 atm). with Further the vacuum-UV investigation demonstrated resonance linesthat, at in a much the near higher threshold stagnation at higher stagnationpressure pressures of 5 atm, ( 3 the atm). quantity Further of unprotonated investigation water demonstrated clusters increased that, markedly, at a muchto such higheran extent stagnation that the signal≥ intensities of (H2O)n+• exceeded those of the corresponding protonated cluster ions pressure of 5 atm, the quantity of unprotonated water clusters increased markedly, to such an extent (H2O)nH+. Shinohara et al. [10] also compared the mass spectra of the supersonic H2O beam seeded + that the signalin Ar by intensities electron impact of (H with2O) thosen • exceededsubjected to 11.83 those eV of photons, the corresponding but in the latter case, protonated (H2O)n+• was cluster ions + (H2O)nH .not Shinohara observed, suggesting et al. [10] a also lack of compared production the of unprotonated mass spectra ions of at the high supersonic energies. H2O beam seeded in + Ar by electron impact with those subjected to 11.83 eV photons, but in the latter case, (H2O)n • was not observed, suggesting a lack of production of unprotonated ions at high energies.

Figure 10. Vacuum-UV photoionization mass spectrum of water clusters at 11.83 and 11.62 eV (Ar resonance lamp; LiF window) with unit mass resolution. Condition: 3.0 atm stagnation pressure and 200 accumulations. Figure adapted from [10] with permission. Copyright 1986 American Institute of Physics.

Molecules 2020, 25, x FOR PEER REVIEW 9 of 38

Figure 9. Schematic energy level diagram of water clusters (H2O)n along the reaction channels. The label “vip” stands for vertically ionized points (sic). Figure adapted from [10] with permission. Copyright 1986 American Institute of Physics.

However, (H2O)n+• (2 ≤ n ≤ 10) and (Ar)m•(H2O)n+• (2 ≤ m ≤ 3; 2 ≤ n ≤ 7) were detected in mass spectra, exhibited in Figure 10, for the first time when supersonic cluster beams of water–argon mixtures were photoionized, with the vacuum-UV resonance lines in the near threshold at higher stagnation pressures (≥3 atm). Further investigation demonstrated that, at a much higher stagnation pressure of 5 atm, the quantity of unprotonated water clusters increased markedly, to such an extent that the signal intensities of (H2O)n+• exceeded those of the corresponding protonated cluster ions (H2O)nH+. Shinohara et al. [10] also compared the mass spectra of the supersonic H2O beam seeded Molecules 2020, 25, 3490in Ar by electron impact with those subjected to 11.83 eV photons, but in the latter case, (H2O)n+• was 9 of 37 not observed, suggesting a lack of production of unprotonated ions at high energies.

Figure 10. Vacuum-UV photoionization mass spectrum of water clusters at 11.83 and 11.62 eV (Ar resonance lampFigure; LiF10. Vacuum-UV window) photoionization with unit massmass spectrum resolution. of water Condition: clusters at 11.83 3.0 and atm 11.62 stagnation eV (Ar pressure and 200 accumulations.resonance lamp; Figure LiF window) adapted with from unit mass [10] resolu withtion. permission. Condition: 3.0 Copyrightatm stagnation 1986pressure American and Institute 200 accumulations. Figure adapted from [10] with permission. Copyright 1986 American Institute of of Physics. Physics.

Finally,based on systematic studies, Shinohara et al. [10] concluded that carrying out near-threshold + photoionization with Ar as a “cooling gas” is necessary for the generation of the (H2O)n • ions and they + proposed a formation mechanism involving (H2O)n • species being generated via the photoionization + of water–argon binary clusters, (Ar)m (H2O)n + hγ (H2O)n • + mAr + e−. Here, the excess energies • → + * produced during ionization can be randomized within the [(Ar)m (H O)n ] clusters (so-called • 2 • vip intra-cluster excess energy dissipation) and finally be used to release argon atoms, giving rise to the + + stable (H O)n ions and various (Ar) (H O)n ions produced by the near-threshold photoionization, 2 • k• 2 • suggesting the effective Franck–Condon excitations of the neutral (Ar)m (H2O)n clusters to the related + * + • [(Ar)m (H O)n ] . They further claimed that (H O)n ions could also be observed via ionization • 2 • vip 2 • of the (CO2)m (H2O)n [46] binary system as well as (N2O)m (H2O)n clusters. • • + + + Shiromaru et al. [47] determined the appearance potentials for (H2O)2 •, (H2O)3 •, (H2O)2H and + (H2O)3H using synchrotron radiation. The results suggested that water clusters in their experiment were + directly ionized since the photoionization efficiency curve of H2O • showed no distinct autoionization + + structure. Furthermore, in contrast to the (H2O)2 • ion, (H2O)3 • was not observed to have been generated from the water trimer, a result attributed to the Franck–Condon region covering a small part of the potential minimum for the water trimer ion as compared to that for the dimer ion. As for the threshold ionization produced using synchrotron radiation, the Franck–Condon assumption having restricted direct vertical ionization involving the ladder climbing mechanism was in agreement with the results obtained by Shinohara et al. [10] to rationalize the lack of unprotonated water clusters. Hydrogen-bonded water clusters were formed with inert gases adsorbed to them in a strong molecular beam expansion carried out by Jongma et al. [48]. A schematic overview of the molecular beam photoionization reflectron TOF mass spectrometer that they used is shown in Figure 11. A room-temperature mixture of water vapor and carrier gas (total backing pressure between 2 and 6 bar) was expanded through a pulsed (200 µs duration) solenoid valve (general valve). The standard orifice of the valve was replaced with a gradually sloped conical nozzle (smallest diameter: 0.8 mm) to enhance water cluster formation. The gas mixture was expanded into the source chamber (pumped by an 1100 L/s turbomolecular pump). Under these conditions, the cooling during the expansion allowed for the efficient production of clusters containing up to ~80 water monomers per cluster. After passing a 0.5 mm diameter skimmer, the molecular beam entered the ionization region (pumped by a 400 L/s turbomolecular pump) of the reflectron TOF mass spectrometer. Single-photon ionization of the water clusters was carried out with vacuum-ultraviolet radiation. The water cluster ions were extracted by the applied electric fields into the drift tube. A mass gate mounted in this tube was used to select part of the spectrum. Parent and daughter ions were separated in time in the reflectron and detected using an microchannel plate (MCP) detector after passing a field-free region. Molecules 2020, 25, x FOR PEER REVIEW 10 of 38

Finally, based on systematic studies, Shinohara et al. [10] concluded that carrying out near- threshold photoionization with Ar as a “cooling gas” is necessary for the generation of the (H2O)n+• ions and they proposed a formation mechanism involving (H2O)n+• species being generated via the photoionization of water–argon binary clusters, (Ar)m•(H2O)n + hγ → (H2O)n+• + mAr + e−. Here, the excess energies produced during ionization can be randomized within the [(Ar)m•(H2O)n+•]*vip clusters (so-called intra-cluster excess energy dissipation) and finally be used to release argon atoms, giving rise to the stable (H2O)n+• ions and various (Ar)k•(H2O)n+• ions produced by the near-threshold photoionization, suggesting the effective Franck–Condon excitations of the neutral (Ar)m•(H2O)n clusters to the related [(Ar)m•(H2O)n+•]*vip. They further claimed that (H2O)n+• ions could also be observed via ionization of the (CO2)m•(H2O)n [46] binary system as well as (N2O)m•(H2O)n clusters. Shiromaru et al. [47] determined the appearance potentials for (H2O)2+•, (H2O)3+•, (H2O)2H+ and (H2O)3H+ using synchrotron radiation. The results suggested that water clusters in their experiment were directly ionized since the photoionization efficiency curve of H2O+• showed no distinct autoionization structure. Furthermore, in contrast to the (H2O)2+• ion, (H2O)3+• was not observed to have been generated from the water trimer, a result attributed to the Franck–Condon region covering a small part of the potential minimum for the water trimer ion as compared to that for the dimer ion. As for the threshold ionization produced using synchrotron radiation, the Franck–Condon assumption having restricted direct vertical ionization involving the ladder climbing mechanism was in agreement with the results obtained by Shinohara et al. [10] to rationalize the lack of unprotonated water clusters. Hydrogen-bonded water clusters were formed with inert gases adsorbed to them in a strong molecular beam expansion carried out by Jongma et al. [48]. A schematic overview of the molecular beam photoionization reflectron TOF mass spectrometer that they used is shown in Figure 11. A room-temperature mixture of water vapor and carrier gas (total backing pressure between 2 and 6 bar) was expanded through a pulsed (200 μs duration) solenoid valve (general valve). The standard orifice of the valve was replaced with a gradually sloped conical nozzle (smallest diameter: 0.8 mm) to enhance water cluster formation. The gas mixture was expanded into the source chamber (pumped by an 1100 L/s turbomolecular pump). Under these conditions, the cooling during the expansion allowed for the efficient production of clusters containing up to ~80 water monomers per cluster. After passing a 0.5 mm diameter skimmer, the molecular beam entered the ionization region (pumped by a 400 L/s turbomolecular pump) of the reflectron TOF mass spectrometer. Single-photon ionization of the water clusters was carried out with vacuum-ultraviolet radiation. The water cluster ions were extracted by the applied electric fields into the drift tube. A mass gate mounted in this tube Molecules 2020was, 25 used, 3490 to select part of the spectrum. Parent and daughter ions were separated in time in the 10 of 37 reflectron and detected using an microchannel plate (MCP) detector after passing a field-free region.

Figure 11.FigureSchematic 11. Schematic overview overview of the of molecular the molecular beam be photoionizationam photoionization reflectron reflectron TOFTOF massmass spectrometer used by Jongmaspectrometer et al. Figureused by adaptedJongma et from al. Figure [48] with adapted permission. from [48] Copyrightwith permission. 1998 AmericanCopyright 1998 Chemical Society. American Chemical Society. Figure 12a shows a typical mass spectrum obtained by Jongma et al. [48], as recorded under modest Figure 12a shows a typical mass spectrum obtained by Jongma et al. [48], as recorded under expansionmodest conditions expansion using conditions Ar as using a carrier Ar as gasa carrier (backing gas (backing pressure: pressure: 2 bar). 2 bar). Cluster Cluster ions,ions, each each containing up to about Molecules 75 water 2020, 25, molecules x FOR PEER REVIEW were readily observed. Evidence of in source decay11 of 38 (ISD), as an asymmetric peak shape with a tail to longer flight times (higher masses when converted to the mass scale), wascontaining clearly observed, up to about 75 as water shown molecules in Figure were read 12ilyb observed. [48]. Asymmetric Evidence of in peaksource shapesdecay (ISD), were clearly as an asymmetric peak shape with a tail to longer flight times n(higher masses when converted31 to the observed formass the scale), protonated was clearly water observed, clusters as shown (abbreviated in Figure 12b as [48]. P Asymmetricin the figure, peak suchshapes aswere P clearly). Furthermore, a mass spectrumobserved composed for the protonated of a series water ofclusters strong (abbreviated quartets as with Pn in the additional figure, such weak as P31). mass Furthermore, features between the quartetsa wasmass spectrum obtained composed after the of a backingseries of strong pressure quartets and with driving additional voltage weak mass of features the pulsed between source were the quartets was obtained after the backing pressure and driving voltage of the pulsed source were increased, as shown in Figure 12c. The low-mass members of the quartets appeared 1 amu lower increased, as shown in Figure 12c. The low-mass members of the quartets appeared 1 amu lower than n + than the P thefamily. Pn family. This This family family of of mass mass features features nominally nominally has has the theformula formula (H2O)n (H+• and2O) isn formally• and is formally designateddesignated as Un (shown as Un (shown in the in figure, the figure, such such as as U U3131).

Figure 12. MolecularFigure 12. Molecular beam beam photoionization photoionization reﬂectronreflectron TO TOFF mass mass spectrometer spectrometer detected by detected Jongma et by Jongma et al. (a) Massal. (a spectrum) Mass spectrum as recorded as recorded using using aa weakweak H H2O/Ar2O/ Arexpansion. expansion. (b) Small (b )part Small of the part same of the same spectrum asspectrum displayed as displayed in (a), startingin (a), starting at p at31 p.31 A. A progression progression of of“triplets” “triplets” separated separated by 18 amu by is 18 clearly amu is clearly n seen. The first peak of each group is the parent pn ion for n = 31–36. The second and third peaks are seen. The ﬁrstdaughter peak ion of peaks, each produced group isby thethe loss parent of one p andion two for water n =monomers31–36. from The the second protonated and parent third peaks are daughter ionion, peaks, respectively. produced (c) Mass by spectrum the loss recorded of one un andder twostrong water expansion monomers conditions. from An extra the peak protonated with parent ion, respectively.a mass that (c) Massis 1 amu spectrum less than that recorded of the pn under parent is strong observed expansion and assigned conditions. as the “unprotonated” An extra peak with a mass thatwater is 1 amucluster. less The thansmall thatpeaks of between the p ntheparent quartets is are observed the same andunprotonated assigned ions as with the varying “unprotonated” numbers of Ar atoms attached to them. Figure adapted from [48] with permission. Copyright 1998 water cluster.American The smallChemical peaks Society. between the quartets are the same unprotonated ions with varying numbers of Ar atoms attached to them. Figure adapted from [48] with permission. Copyright 1998 American ChemicalUnambiguous Society. evidence for H3O+•(H2O)n-1••OH as the structure of Un was provided by the observation of a loss of •OH from Un, as shown in a TOF spectrum between 37 and 38 microseconds (Figure 13a) [48]. Even clearer evidence for this conclusion was provided by the TOF spectrum between 51 and 52 microseconds (Figure 13b), i.e., at around the arrival time of U20 and P20 (solid trace). Here, evidence for the occurrence of U21 → P20 + •OD was provided by inspection of the solid trace of this spectrum, with a 20 ns difference between the two peaks (the solid line peak and the dotted line peak around 51.4 μs) clearly visible. Further analysis of the heavy water cluster ions by

Molecules 2020, 25, 3490 11 of 37

+ n Unambiguous evidence for H3O (H2O)n 1 •OH as the structure of U was provided by the • − • observation of a loss of OH from Un, as shown in a TOF spectrum between 37 and 38 microseconds • (Figure 13a) [48]. Even clearer evidence for this conclusion was provided by the TOF spectrum between 51 and 52 microseconds (Figure 13b), i.e., at around the arrival time of U20 and P20 (solid trace). Here, evidence for the occurrence of U21 P20 + OD was provided by inspection of the solid → • trace of this spectrum, with a 20 ns diﬀerence between the two peaks (the solid line peak and the Molecules 2020, 25, x FOR PEER REVIEW 12 of 38 dotted line peak around 51.4 µs) clearly visible. Further analysis of the heavy water cluster ions by n JongmaJongma et al. et [al.48 ][48] revealed revealed the the loss loss of of •ODOD fromfrom Un toto be be much much more more probab probablele than than the loss the of loss D2O. of D O. • 2 The dominationThe domination of theof the metastable metastable decay decay byby •ODOD lo lossss was was consistent consistent with with an •OD an OD molecule molecule having having • • beenbeen formed formed in the in the proton proton transfer transfer reaction reaction andand then having having diffused diﬀused away away from from the charge the charge center center of of the cluster,the cluster, as discussed as discussed in in detail detail below. below.

FigureFigure 13. Molecular13. Molecular beam beam photoionization photoionization reflectron reflectron TOFTOF mass spectrometer spectrometer detected detected byby Jongma Jongma et et al. (a) Smallal. (a) part Small of part the TOFof the spectrum TOF spectrum obtained obtained using using a D 2aO D/2NO/N2 expansion.2 expansion. Spectra Spectra withwith (solid line) line) and withoutand (dashedwithout line)(dashed unprotonated line) unprotonated cluster cluster signals sign presentals present are superimposed are superimposed and scaledand scaled to the to intensitythe of theintensity p10 peak. of the The p10 double peak. peakThe double at 37.45 peakµs at (solid 37.45 line) μs (solid is due line) to lossis due of ODto loss from of theOD unprotonatedfrom the 10 10 11 11 clusterunprotonated (u ) and due cluster to the (u loss) and of Ddue2O fromto the the loss protonated of D2O from cluster the (p protonated). This assignment cluster (p is). confirmedThis by theassignment dotted spectrum, is confirmed where by th onlye dotted p11 spectrum,p10 + D whereO is only possible. p11 → (pb10) + Similar D2O is topossible. (a) for ( largerb) Similar clusters. → 2 to (a) for larger clusters. The presence of the peak due to loss of D2O from the protonated cluster is The presence of the peak due to loss of D2O from the protonated cluster is hardly detectable under conditionshardly wheredetectable the under unprotonated conditions species where the is present. unprotonated The small species peak is present. to the rightThe small of this peak (solid to the line) is right of this (solid line) is due to the daughter ion produced by the loss of OD followed by the loss of due to the daughter ion produced by the loss of OD followed by the loss of D2O from the unprotonated D2O from the unprotonated cluster. The peak position of daughter ions due to the loss of two D2O cluster. The peak position of daughter ions due to the loss of two D O monomers from protonated monomers from protonated clusters is at a slightly different position, as2 is obvious from the dashed clustersspectrum, is at a slightlyand is not di observedfferent position, in spectra as where is obvious the unprotonated from the dashed cluste spectrum,rs appear (solid and is line). not observedFigure in spectraadapted where from the [48] unprotonated with permission. clusters Copyright appear 1998 (solid American line). Figure Chemical adapted Society. from [48] with permission. Copyright 1998 American Chemical Society. Jongma et al. [48] also confirmed that the formation of unprotonated ions would be possible only duringJongma expansion et al. [48 conditions] also confirmed strong enough that theto produce formation neutral of unprotonatedwater clusters with ions adsorbed would beAr. possibleTo onlymore during fully expansion explore this conditions phenomenon, strong the enough authors toacquired produce photoionization neutral water mass clusters spectra with foradsorbed several Ar. To morecarrier fully gases, explore including thisphenomenon, Kr, O2, N2, CO2 the and authors even CO. acquired All of these photoionization carrier gases massexhibited spectra behaviors for several qualitatively similar to that of Ar. In all cases, the Un family was always detected simultaneously with carrier gases, including Kr, O2,N2, CO2 and even CO. All of these carrier gases exhibited behaviors n qualitativelyU Mm families similar (M to =that neutral of Ar. carrier In all gas cases, molecules, the Un Kr,family O2, N was2, CO always2 and even detected CO). simultaneouslyThis result was with n consistent with the idea that mixed neutral clusters must first form in the molecular beam expansion U Mm families (M = neutral carrier gas molecules, Kr, O2,N2, CO2 and even CO). This result was before unprotonated water clusters can be observed. However, Jongma et al. [48] disagreed with the point of view regarding the “active parent ion cooling mechanism” proposed by Shinohara et al., [10] at least for the large clusters. Jongma et al. attributed this phenomenon to rapid evaporative cooling. Once the energy in the cluster ion becomes low enough, the mobility of the •OH radical in the cluster would be sufficiently reduced so that the •OH radical would become trapped in the cluster, leading

Molecules 2020, 25, 3490 12 of 37 consistent with the idea that mixed neutral clusters must first form in the molecular beam expansion before unprotonated water clusters can be observed. However, Jongma et al. [48] disagreed with the point of view regarding the “active parent ion cooling mechanism” proposed by Shinohara et al., [10] at least for the large clusters. Jongma et al. attributed this phenomenon to rapid evaporative cooling. Once the energy in the cluster ion becomes low enough, the mobility of the OH radical in the cluster • would be sufficiently reduced so that the OH radical would become trapped in the cluster, leading to • the formation of “unprotonated” clusters. Such a conclusion could actually be proven under favorable conditions in a post source decay (PSD) experiment. Cluster ion distributions of water in a molecular beam were investigated by Radi et al. using femtosecondMolecules 2020, 25 ionization, x FOR PEER REVIEW at 780 nm and reflectron time-of-flight mass spectrometry13 of 38 since + * vertical ionizationto the formation from of the “unprotonated” ground state clusters. to the Such potential a conclusion energy couldsurface actually be of proven the [(H under2O) n •] ion could be achievedfavorable [conditions49]. Furthermore, in a post source since decay intramolecular (PSD) experiment. energy transfer in intermediate states is not expected duringCluster ionization, ion distributions cluster heatingof water in a femtosecond molecular beam photoionization were investigated isby likely Radi et to al. be using insignificant. femtosecond ionization at 780 nm and reflectron+ time-of-flight mass spectrometry since vertical However, unprotonated species of the form (H O)n have not been observed, with the exception of ionization from the ground state to the potential2 •energy surface of the [(H2O)n+•]* ion could be + the dimer ionachieved (H2O) [49].2 •Furthermore,, which was since attributed intramolecular by Radiet energy al. transfer to the in Franck–Condon-restricted intermediate states is not direct vertical ionization.expected during ionization, cluster heating in femtosecond photoionization is likely to be insignificant. However, unprotonated species of the form+ (H2O)n+• have not been observed, with the Mizuse et al. [50] reported IR spectra of (H2O)n • (n = 3–11) water cluster cations; specifically, exception of the dimer ion (H2O)2+•, which was attributed by Radiet al. to the Franck–Condon- they acquiredrestricted photodissociation direct vertical ionization. spectra by using a tandem quadrupole mass spectrometer and a + coherent IR source.Mizuse IR et spectra al. [50] reported of (H2O) IRn spectra• Ar of (n (H=2O)3–7)n+• (n were = 3–11) also water acquired cluster usingcations; essentially specifically, the same + • + technique.they As shownacquired inphotodissociation Figure 14,H spectra(H2O) byn andusing H a tandem(H2O) nquadrupoleAr yielded mass much spectrometer stronger and peaks a than coherent+ IR source.+ IR spectra of (H2O)n+••Ar (n = 3–7) were also• acquired using essentially+ the same + did (H2O)n • and (H2O)n • Ar. In this study, the water cluster cations (H2O)n • and (H2O)n • Ar technique. As shown in• Figure 14, H+(H2O)n and H+(H2O)n•Ar yielded much stronger peaks than did • were generated in a supersonic jet expansion. A gaseous mixture of H O (trace; 17,18O-depleted (H2O)n+• and (H2O)n+••Ar. In this study, the water cluster cations (H2O)n+• and2 (H2O)n+••Ar were 16 16 H2 O, 99.99%generatedO, in ISOTEC)a supersonic and jet expansion. Ar (5 MPa) A gaseous was expanded mixture of H into2O (trace; a vacuum 17,18O-depleted chamber H216O, through a high-pressure99.99% pulsed 16O, ISOTEC) valve and (Even–Lavie Ar (5 MPa) was valve) expanded [51]. in Theto a vacuum gas pulse chamber was through crossed a high-pressure by a 200 eV electron beam frompulsed an electron valve (Even–Lavie gun (Omegatron) valve) [51]. in The the gas collisional pulse was regioncrossed ofby thea 200 jet. eV Clusterelectron beam ions from formed an became electron gun (Omegatron) in the collisional region of the jet. Cluster ions formed became larger and larger andwere were cooled cooled following following the collisions. the collisions. The cluster The ion cluster of interest ion ofwas interest selected wasby the selected first mass by the first mass spectrometerspectrometer (with (with aa mass resolution resolution (Δm) (of∆ m)~ 1) and of ~was 1) irradiated and was with irradiated coherent IR with light. coherentIR spectra IR light. IR spectra werewere acquired by by monitoring monitoring the photofragment the photofragment intensity intensity as a function as aof function the IR wavelength. of the IR The wavelength. The secondsecond mass spectrometermass spectrometer was was tuned tunedto to selectselect the the fragments fragments of both of both the H the2O-loss H O-lossand the and•OH-loss the OH-loss +• 2 channels (Δm ~ 3) for spectroscopy of bare (H2O)n+ . • channels (∆m ~ 3) for spectroscopy of bare (H2O)n •.

Figure 14. Mass spectrum of ions produced using the present ion source. Figure adapted from [15]

with permission. Copyright 2011 The Royal Society of Chemistry. Figure 14. Mass spectrum of ions produced using the present ion source. Figure adapted from [15] with permission. Copyright 2011 The Royal Society of Chemistry. In addition to using the “colder” cluster source, Mizuse et al. further tried a “warmer” ion source, i.e., withIn aaddition lower to stagnation using the “colder” pressure cluster that source should, Mizuse reduce et theal. further degree tried of collisionala “warmer” coolingion [50]. source, i.e., with a lower stagnation pressure that should reduce the degree of collisional cooling [50]. + They argued that cooling eﬃciency and the choice of the carrier gas are critical to the yields of (H2O)n •. They argued that cooling efficiency and the choice of the carrier gas are critical to the yields of Figure 15 shows a comparison of typical mass distributions resulting from using the “colder” and (H2O)n+•. + “warmer” ion sources.Figure 15 Forshows the a “warmer”comparison of ion typical source, mass only distributions H (H2O) resultingn was from observed, using the consistent “colder” with the previous massand “warmer” spectrometry ion sources. studies For the by “warmer” Shinohara ion etsource, al. and only Jongma H+(H2O)n etwas al. observed, discussed consistent above [10,48]. As for thewith “colder” the previous ion source, mass spectrometry carrier gas studies dependencies by Shinohara et were al. and also Jongma checked et al. discussed by Mizuse above et al. [50]. [10,48]. As for the “colder” ion source, carrier gas dependencies were also checked by Mizuse et al. [50]. When Ne was used as a carrier gas instead of Ar, the relative yields of (H2O)n+• were lower. In

Molecules 2020, 25, 3490 13 of 37

Molecules 2020, 25, x FOR PEER REVIEW + 14 of 38 When Ne was used as a carrier gas instead of Ar, the relative yields of (H2O)n • were lower. In the case + +• of He, onlytheMolecules acase very of 2020 He, small, 25 only, x FOR amounta very PEER small REVIEW of amount (H 2O) nof (H• was2O)n produced, was produced, also also in in agreementagreement with with previous previous14 of 38 mass spectrometrymass studiesspectrometry [48 ].studies [48]. the case of He, only a very small amount of (H2O)n+• was produced, also in agreement with previous mass spectrometry studies [48].

Figure 15. Cluster ion distributions obtained with (a) a “colder” cluster ion source and (b) “warmer” Figure 15. Cluster ion distributions obtained with (a) a “colder” cluster ion source and (b) “warmer”

cluster ioncluster source. ion source. Figure Figure adapted adapted from from [15] [15 ]wi withth permission. permission. Copyright Copyright 2011 The Royal 2011 Society The Royal of Society of Chemistry.Chemistry.Figure 15. Cluster ion distributions obtained with (a) a “colder” cluster ion source and (b) “warmer” cluster ion source. Figure adapted from [15] with permission. Copyright 2011 The Royal Society of FigureChemistry. 16 shows an IR photodissociation mass spectrum of (H2O)5+•. Mizuse+ et al. [15] selected Figure 16 shows an IR photodissociation mass spectrum of (H2O)5 •. Mizuse et al. [15] selected +• −1 this ion using the first mass filter and then (H2O)5 + was irradiated by an IR light pulse of 3734 cm . 1 this ion using the first mass filter and then (H O) was irradiated by+• an IR light pulse of 3734 cm . The secondFigure mass 16 shows filter then an IR analyzed photodissociation the m/z2 ratios mass5 of• spectrumthe resulting of (H ions,2O) 5and. Mizuse then the et spectrumal. [15] selected was − The secondobtained.this mass ion usingThe filter IR the thenlight first analyzedof mass this filterphoton theand energy thenm/z (Hratios was2O) 5+•resonant of was the irradiated resultingwith the by antisymmetric an ions, IR light and pulse then stretch of the3734 of spectrum thecm −1. was obtained.terminalThe second IR water light mass moiety. of filter this With then photon theanalyzed measurement, energy the m/z ratios was only resonantofthe the •OH-loss resulting with fragment ions, the and antisymmetricwas then found the spectrum actually stretch wasto of the obtained. The IR light of this photon energy was resonant+• with the antisymmetric stretch of the terminal watercontribute moiety. to the observed With the IR measurement,spectra, at least for only (H2O) the3–5 OH-loss. This result fragment implied the was presence found of actually to +terminal •water moiety. With the measurement, only the •OH-loss fragment was found actually to H (H2O)n−1 OH-type structures. • + contributecontribute to the observed to the observed IR spectra, IR spectra, at leastat least for for (H (H22O)O)33–5–5+•. •This. This result result implied implied the presence the presence of of + H (H2O)n H1+•(HOH-type2O)n−1•OH-type structures. structures. −

+ 2.5. Stabilization of (H2O)n • Produced by Electron Impact in Helium Nanodroplets + Rapidly removing excess energy is required for (H2O)n • to survive the ionization process. An alternative means of preventing the dissociation of the parent cluster ion is to carry out the ionization in a helium nanodroplet. Helium nanodroplets provide a low equilibrium temperature Molecules 2020, 25, x FOR PEER REVIEW 15 of 38

2.5. Stabilization of (H2O)n+• Produced by Electron Impact in Helium Nanodroplets

Rapidly removing excess energy is required for (H2O)n+• to survive the ionization process. An alternative means of preventing the dissociation of the parent cluster ion is to carry out the ionization inMolecules a helium2020, 25nanodroplet., 3490 Helium nanodroplets provide a low equilibrium temperature (0.414 ofK), 37 possess an exceptionally high thermal conductivity and can dissipate energy rapidly as a result of evaporative(0.4 K), possess loss an of exceptionally weakly bound high helium thermal atoms conductivity [52]. The andrelease can dissipateof gas phase energy (H2 rapidlyO)n+• cluster as a result ions from this environment has previously been reported by Lewerenz et al. [53] and Fröchtenicht+ et al. of evaporative loss of weakly bound helium atoms [52]. The release of gas phase (H2O)n • cluster ions [54].from The this investigation environment by has Fröchtenicht previously been et al. reported was concerned by Lewerenz primarily et al. with [53] recording and Fröchtenicht infrared et spectra al. [54]. ofThe neutral investigation water clusters by Fröchtenicht and recording et al. waselectron concerned impact primarily mass spectra, with in recording which only infrared much spectra weaker of signalsneutral due water to clusters unprotonated and recording water electroncluster ions impact were mass reported. spectra, The in which study only by Lewerenz much weaker et al. signals also involveddue to unprotonated electron impact water ionization cluster ionsmass were spectromet reported.ry. Relatively The study low-resolution by Lewerenz et mass al.also spectra involved were obtained,electron impact yet signals ionization corresponding mass spectrometry. to unprotonat Relativelyed water low-resolution cluster ions were mass spectrareported were as shoulders obtained, onyet the signals much corresponding stronger H to+(H unprotonated2O)n peaks. As water shown cluster in ionsFigure were 17, reported Yang aset shouldersal. [55] more on the closely much investigated+ the electron impact mass spectrometry of water clusters in helium nanodroplets. Not stronger H (H2O)n peaks. As shown in Figure 17, Yang et al. [55] more closely investigated the electron onlyimpact did mass they spectrometry report seeing of watersignals clusters corresponding in helium nanodroplets.to both H+(H2 NotO)n onlyand did(H2O) theyn+•, reportas reported seeing previously, but they also reported+ the observation of+ He(H2O)n+• cluster ions for the first time. Based signals corresponding to both H (H2O)n and (H2O)n •, as reported previously, but they also reported on ab initio calculations, Yang+ et al. suggested that, as was the case for H+(H2O)n ions, the preferential the observation of He(H2O)n • cluster ions for the ﬁrst time. Based on ab initio calculations, Yang et al. location for a positive charge in large +(H2O)n+• clusters is at the surface of the cluster, specifically on suggested that, as was the case for H (H2O)n ions, the preferential location for a positive charge in a dangling O-H+ bond to which a single helium atom can attach via a charge-induced dipole large (H2O)n • clusters is at the surface of the cluster, speciﬁcally on a dangling O-H bond to which a interaction.single helium atom can attach via a charge-induced dipole interaction.

Figure 17. The n = 14, 20 and 24 regions of a mass spectrum recorded from water clusters (H2O)n in Figure 17. The n = 14, 20 and 24 regions of a mass spectrum recorded from water clusters (H2O)n in helium nanodroplets. Figure adapted from [55] with permission. Copyright 2007 American Institute helium nanodroplets. Figure adapted from [55] with permission. Copyright 2007 American Institute of Physics. of Physics. + 2.6. Formation of (H2O)n • Using High-Energy Photons or Particles 2.6. Formation of (H2O)n+• Using High-Energy Photons or Particles A tabletop soft X-ray laser was applied for the ﬁrst time by Dong et al. as a high-energy photon sourceA tabletop for chemical soft dynamicsX-ray laser experiments was applied in for the the study first oftime water by [Dong56]. Speciﬁcally, et al. as a high-energy a 26.5 eV soft photon X-ray sourcelaser (pulse for chemical duration dynamics of ~ 1 ns) wasexperiments employed. in Asthe shownstudy inof Figurewater [56].18a, whenSpecifically, pure He a was26.5 usedeV soft as theX- + + raycarrier laser gas, (pulse a weak duration signal of for ~ (H1 ns)2O) was2 • was employed. observed As on shown the low-mass in Figure side 18a, of when (H2O) pure2H ,He the was daughter used + +• + + asion the of (Hcarrier2O)3 gas,•. When a weak 5% Arsignal was for mixed (H2O) into2 thewas He observed expansion on gas, the thelow-mass (H2O)2 side• signal of (H strengthened2O)2H , the + +• +•+ daughterand the H ion3O ofsignal (H2O) weakened3 . When (see5% FigureAr was 18 mixedb). For into a 20%Ar the He/80% expansion He expansion, gas, the the (H (H22O)O)22 •signalmass + + strengthenedfeature was much and the larger H3O than signal that weakened for H3O (Figure(see Figure 18c). 18b). The intensityFor a 20%Ar/80% of the signal He expansion, corresponding the +• + + (Hto2 theO)2 protonatedmass feature cluster was ion much (H2 O)larger2H thanalso decreasedthat for H as3O the (Figure concentration 18c). The of intensity Ar in the of binding the signal gas correspondingwas increased. to Single the protonated photon ionization cluster ion by (H a 26.52O)2 eVH+ also photon decreased has much as the more concentration energy (ca. of 15 Ar eV) in than the that required for ionization of all the water clusters. If this energy were to remain in the clusters, + + only H2O • and its fragments could be observed. Nonetheless, (H2O)2 • was observed even with no Ar present in the expansion. This observation suggests that almost all of the excess energy in these clusters (ca. 15 eV) was removed by the exiting electron, consistent with the arguments proposed by Shiromaru et al. [47]. Molecules 2020, 25, x FOR PEER REVIEW 16 of 38 binding gas was increased. Single photon ionization by a 26.5 eV photon has much more energy (ca. 15 eV) than that required for ionization of all the water clusters. If this energy were to remain in the clusters, only H2O+• and its fragments could be observed. Nonetheless, (H2O)2+• was observed even with no Ar present in the expansion. This observation suggests that almost all of the excess energy in these clusters (ca. 15 eV) was removed by the exiting electron, consistent with the arguments Molecules 2020, 25, 3490 15 of 37 proposed by Shiromaru et al. [47].

Figure 18. TOF mass spectra of unprotonated water dimer ion formed in diﬀerent carrier gases: (a) pure Figure 18. TOF mass spectra of unprotonated water dimer ion formed in different carrier gases: (a) He gas, (b) 5% Ar mixed in He gas and (c) 20% Ar mixed in He gas, respectively. Figure adapted pure He gas, (b) 5% Ar mixed in He gas and (c) 20% Ar mixed in He gas, respectively. Figure adapted from [56] with permission. Copyright 2006 American Institute of Physics. from [56] with permission. Copyright 2006 American Institute of Physics. At even higher energies, new reaction channels would open; e.g., ionizing the inner valence electronsAt even in water higher would energies, lead to new the subsequentreaction channels autoionization would open; process e.g., identiﬁed ionizing asthe intermolecular inner valence coulombelectrons decayin water (ICD) would [57 ,lead58]. to The the unequivocalsubsequent au signaturetoionization of thisprocess process identified has been as intermolecular observed in simultaneouscoulomb decay measurements (ICD) [57,58]. of low-energy The unequivocal electrons sign andature photoelectrons of this process generated has frombeen inner-valence observed in shellssimultaneous using vacuum-ultraviolet measurements of low-energy light. ICD electrons occurs whenand photoelectrons the excited particle generated is only from loosely inner- attachedvalence shells to neighboring using vacuum-ultraviolet particles by, for light. example, ICD occurs van der when Waals the excited forces orparticle hydrogen is only bonding loosely (asattachedMolecules shown 2020 to in ,neighboring Figure25, x FOR 19 PEER). particles REVIEW by, for example, van der Waals forces or hydrogen bonding17 of(as 38 shown in Figure 19).

Figure 19. Schematic of intermolecular coulombic decay (ICD) of an inner-valence vacancy in a Figure 19. Schematic of intermolecular coulombic decay (ICD) of an inner-valence vacancy in a hydrogen-bonded network of water molecules. This interatomic autoionization process takes place hydrogen-bonded network of water molecules. This interatomic autoionization process takes place when an inner-valence vacancy created by (photo)ionization on molecule M (a) is ﬁlled by an electron when an inner-valence vacancy created by (photo)ionization on molecule M (a) is filled by an electron from an outer-valence orbital of the same molecule, while another outer-valence electron is emitted from an outer-valence orbital of the same molecule, while another outer-valence electron is emitted from the nearest-neighbor molecule M’ (see b). Figure adapted from [57] with permission. Copyright from the nearest-neighbor molecule M’ (see b). Figure adapted from [57] with permission. Copyright 2010 Springer Nature. 2010 Springer Nature.

Jahnke et al. reported the direct observation of an ultrafast transfer of energy across the hydrogen bridge in (H2O)2, illustrated in Figure 20 [58]. They argued that this decay is faster than the proton transfer that is usually a prominent pathway in the case of electronic excitation of small water clusters and leads to dissociation of the water dimer into two H2O+• ions.

Molecules 2020, 25, x FOR PEER REVIEW 17 of 38

Figure 19. Schematic of intermolecular coulombic decay (ICD) of an inner-valence vacancy in a hydrogen-bonded network of water molecules. This interatomic autoionization process takes place when an inner-valence vacancy created by (photo)ionization on molecule M (a) is filled by an electron from an outer-valence orbital of the same molecule, while another outer-valence electron is emitted from the nearest-neighbor molecule M’ (see b). Figure adapted from [57] with permission. Copyright Molecules 2020, 25, 3490 16 of 37 2010 Springer Nature.

JahnkeJahnke etet al.al. reported reported the the direct direct observation observation of an of ultrafast an ultrafast transfer transfer of energy of acrossenergy the across hydrogen the 2 2 hydrogenbridge in (Hbridge2O)2 in, illustrated (H O) , illustrated in Figure in 20 Figure[ 58]. They20 [58]. argued They thatargu thised that decay this is decay faster is than faster the than proton the protontransfer transfer that is usuallythat is usually a prominent a prominent pathway pathway in the case in the of electroniccase of electronic excitation excitation of small of water small clusters water + 2 +• clustersand leads and to leads dissociation to dissociation of the water of the dimer water into dimer two into H2O two• ions. H O ions.

Figure 20. (a) Geometry of the water dimer. (b,c) An electron from the inner valence shell of one of the molecules of the dimer is ejected by absorption of a photon (b) and then the energy released by de-excitation at this site is transferred to the neighboring site, where a second, low-energy electron is emitted. Figure adapted from [58] with permission. Copyright 2010 Springer Nature.

Electron impact ionization measurements with 70 eV electrons [55] have shown the formation of + + H2O • radical cations with a yield of 37.5% (with the rest indicated to be H3O ). As a result of the impacts of these high-energy electrons, the electrons from all valence orbitals were indicated to be ejected. In the experiment carried out by Jahnke et al. [58], 100% of the autoionization events resulted from the ICD process, but the efficiency of the ICD process in the water dimer was not determined. As Svoboda et al. have noted [59], it is difficult to estimate the fraction of the water dimers with the ionized 2a1 electrons that are deactivated via the internal conversion process without becoming further ionized. The results of the chemical calculation simulations done by Svoboda et al., together with the electron impact measurements of Buck and Winter [60], could provide a hint regarding the efficiency of the ICD process. Svoboda et al. made a rough estimate based on the assumption that all of the valence electrons would be equally likely to become ionized by a 70 eV electron. The efficiency of the ICD process was estimated in this way to be between 20% and 40%. Since fast nuclear dynamics could take place even upon 1a1 ionization [14,59] as well as upon 2a1 electron ionization, the system can then efficiently lose potential energy, thus closing the autoionization channel and hence reducing the ICD process efficiency to much lower than 100%.

+ 3. Structural Properties of (H2O)n •: Simulations and Experimental Studies Studies of clusters have provided detailed insights into structural trends and dynamics of hydrogen-bonded water networks, as shown in Figure 21[61–72]. Molecules 2020, 25, x FOR PEER REVIEW 18 of 38

Electron impact ionization measurements with 70 eV electrons [55] have shown the formation of H2O+• radical cations with a yield of 37.5% (with the rest indicated to be H3O+). As a result of the impacts of these high-energy electrons, the electrons from all valence orbitals were indicated to be ejected. In the experiment carried out by Jahnke et al. [58], 100% of the autoionization events resulted from the ICD process, but the efficiency of the ICD process in the water dimer was not determined. As Svoboda et al. have noted [59], it is difficult to estimate the fraction of the water dimers with the ionized 2a1 electrons that are deactivated via the internal conversion process without becoming further ionized. The results of the chemical calculation simulations done by Svoboda et al., together with the electron impact measurements of Buck and Winter [60], could provide a hint regarding the efficiency of the ICD process. Svoboda et al. made a rough estimate based on the assumption that all of the valence electrons would be equally likely to become ionized by a 70 eV electron. The efficiency of the ICD process was estimated in this way to be between 20% and 40%. Since fast nuclear dynamics could take place even upon 1a1 ionization [14,59] as well as upon 2a1 electron ionization, the system can then efficiently lose potential energy, thus closing the autoionization channel and hence reducing the ICD process efficiency to much lower than 100%.

3. Structural Properties of (H2O)n+•: Simulations and Experimental Studies Studies of clusters have provided detailed insights into structural trends and dynamics of hydrogen-bonded water networks, as shown in Figure 21 [61–72]. Another advance in studying hydrated clusters in the gas phase is to understand chemical reactions in aqueous solutions from microscopic observations, since there are circumstances in which condensed phase studies may be unable to adequately investigate such detailed reaction mechanisms. Reducing the number of molecules in a cluster system can provide deeper insights into Moleculessuch a complicated2020, 25, 3490 system and has been used to characterize the reaction mechanism of an important17 of 37 chemical reaction: ionizing radiation-induced reactions of water [73–83].

++ Figure 21.21. Experimentally characterized structuresstructures ofof HH (H2O)nn. .Figure Figure adapted adapted from from [15] [15] with permission. Copyright 2011 The RoyalRoyal Society of Chemistry.

Another advance in studying hydrated clusters in the gas phase is to understand chemical reactions in aqueous solutions from microscopic observations, since there are circumstances in which condensed phase studies may be unable to adequately investigate such detailed reaction mechanisms. Reducing the number of molecules in a cluster system can provide deeper insights into such a complicated system and has been used to characterize the reaction mechanism of an important chemical reaction: ionizing radiation-induced reactions of water [73–83]. + + Structures of (H2O)n • would be expected to reﬂect the inherent instability of H2O • and the reactivity of cationized water networks. Attaining such knowledge is quite important for understanding + ionizing-radiation-induced chemistry in water. However, although (H2O)n • has been produced in trace amounts in several experimental studies, structural information on these species is still limited. This section focuses on this problem.

+ 3.1. Investigation on the Structure of the Water Dimer Radical Cation (H2O)2 • + The structure of (H2O)2 • is very important in radiation chemistry [12,84,85], as is its role in the + deionization of O2 • in the ionosphere [39,86]. Two structural motifs have been suggested. In early theoretical works, the difference between the energy of the optimized dimer cation and that of the cation with the optimal geometry of the neutral dimer calculated at the M˝oller-Plesset second-order perturbation theory (MP2) level was found to be almost the same as that calculated at the Unrestricted Hartree-Fock theory (UHF) level, specifically 2.57 and 2.56 eV, respectively [87]. This result may indicate that the electron correlation is important for estimating the absolute stabilization energy of + the cation, but not so much with regards to geometry optimization [88], suggesting that (H2O)2 • + would adopt a charge-resonance-stabilized structure reminiscent of hydrazine [H2O ... OH2] •, i.e., a so-called dimer cation [89]. Recent fine theoretical studies were consistent with the presence of a structure resulting from 2 proton transfer. Gill and Radom characterized the A” state of the hydrogen-bonded (Cs symmetry) 2 isomer and the Bu state of the hemi-bonded (C2h symmetry) isomer as two minima at the MP4/6-311G (MC)**//MP2/6-31G* level of theory [90]. The energy difference between the isomers was found to be + 8.9 kcal/mol. Sodupe, Oliva and Bertran located five structures [91] of the ionized water dimer (H2O)2 • Molecules 2020, 25, 3490 18 of 37

and argued for a similarity between its Cs symmetry transition state and its C1 minimum structure, but with the OH rotated out of the plane. The energy diﬀerence between these two structures was • predicted to be 0.03 kcal/mol. Cheng et al. [92] determined fourteen stationary points for the water dimer radical cation on its doublet electronic state potential energy surface by carrying out harmonic vibrational frequency analyses using coupled cluster theory with single and double excitations (CCSD) and CCSD with perturbative triple excitations (CCSD (T)). Two stationary points were found to be local minima: isomer + 1 (C1 symmetry), with H3O -•OH character (hydrogen-bonded system), and isomer 7 (C2 symmetry), Molecules 2020, 25, x FOR +PEER REVIEW 20 of 38 with [H2O ... H2O] • character (dimer cation). Adiabatic energies of the ionization of (H O) to isomers 1 and 7 were determined to be 10.81 quantified the extent of proton delocalization 2by 2using wave functions that were obtained in the and 11.19 eV, respectively, with the former being in excellent agreement with the experimental value analysis of the spectrum [99] and found this extent to be much smaller than that of the binary of 10.8–10.9 eV. The critical dissociation energy of isomer 1 to H O+ and OH was predicted to ion/water complexes reported earlier, indicative of the initially localized3 bridging• proton exploring be 26.4 kcal/mol, while the dissociation energy of isomer 7 to H O+ and H O was determined an intra-cluster proton transfer configuration to enhance the covalent2 •nature of2 the H-bond to the to be 34.7 kcal/mol. As shown in Figure 22, at the aug-cc-pVQZ CCSD (T) level of theory, proton acceptor and meaning that this species would be best described as an H3O+-•OH ion-radical the hydrogen-bonded 1 and hemi-bonded 7 minima were determined to be separated by 8.8 kcal/mol, complex. with an interconversion barrier (1 10 7) of 15.1 kcal/mol. → →

Figure 22. Illustrations of the proton transfer and dissociation process for the dimer cations. Figure 22. Illustrations of the proton transfer and dissociation process for the dimer cations. (a) (a) Schematic of the potential energy surface (in kcal/mol, zero-point vibrational energy (ZPVE) Schematic of the potential energy surface (in kcal/mol, zero-point vibrational energy (ZPVE) corrected corrected values in parentheses) showing proton transfer and dissociation processes for the dimer values in parentheses) showing proton transfer and dissociation processes for the dimer cation and cation and hydrogen-bonded water dimer radical cation structures 1, 7 and 10 at the aug-cc-pVQZ hydrogen-bonded water dimer radical cation structures 1, 7 and 10 at the aug-cc-pVQZ+ CCSD (T) CCSD (T) level of theory. (b) Singly occupied molecular orbitals (SOMO) of the (H2O)2 • equilibria 1 level of theory. (b) Singly occupied molecular orbitals (SOMO) of the (H2O)2+• equilibria 1 and 7 [92]. and 7 [92]. Figure adapted from [92] with permission. Copyright 2009 American Chemical Society. Figure adapted from [92] with permission. Copyright 2009 American Chemical Society. + For several cases, particularly the binary ion/water complexes involving OH−,O−,F−, or H3O , +• GardenierCollisional et al. reportedstudies of binding the (H energies2O)2 ion considerably in the gas greaterphase by than Stace one and would co-workers expect for [100], a typical for + • ionexample,/water were complex more with consistent bonding with dominated an [H3O by- ionOH/ dipole]ion-radical or dispersion structure interactions than with [the93, 94dimer] as well cation as hydrogenstructure bonding.[61,99]. This These ion-radical large interaction structure energies (shown reﬂect in Figure an electronic 23) was structure also indicated in which theto sharedbe the protonminimum-energy is partially transferredarrangement from in thea theoretical water molecule paper to by the Pieniazek molecular et ion. al. The [101]. extent Very of thisrecently, eﬀect +• +• canGardenier be characterized et al. presented by values IR spectra of ρ, as of illustrated argon-tagged in the (H following2O)2 and equation: (H2O)2 and they only observed the proton-transferred [H3O+-•OH] type (shown in Figure 23) [99]. They found the resulting bands involving the displacement of the bridging proton to be broad and appear as a strong triplet centered around 2000 cm−1, with this observation completely ruling out a contribution from a hydrazine-like isomer. Other observed band positions are shown in Figure 24.

Molecules 2020, 25, 3490 19 of 37

ρ = [r r 0] [r r 0] (5) XH − XH − X’H − X’H where X represents the acceptor (or donor) molecule and X’ represents the donor (or acceptor) molecule. 0 The rXH is the XH bond length in the X-H-X’ complex, and rXH is the value in the isolated molecule or molecular ion, calculated at the same level of theory; this equation was first reported by Scheiner and used by Leopold and A. Johnson to characterize proton delocalization and vibration-induced proton transfer [95–98]. By this definition, larger values of |ρ| reflect greater localization of the shared proton, whereas values close to zero suggest equal sharing. A. Johnson and his co-workers quantified the extent of proton delocalization by using wave functions that were obtained in the analysis of the spectrum [99] and found this extent to be much smaller than that of the binary ion/water complexes reported earlier, indicative of the initially localized bridging proton exploring an intra-cluster proton transfer configuration to enhance the covalent nature of the H-bond to the proton acceptor and meaning + that this species would be best described as an H3O -•OH ion-radical complex. + Collisional studies of the (H2O)2 • ion in the gas phase by Stace and co-workers [100], + for example, were more consistent with an [H3O -•OH]ion-radical structure than with the dimer cation structure [61,99]. This ion-radical structure (shown in Figure 23) was also indicated to be the minimum-energy arrangement in a theoretical paper by Pieniazek et al. [101]. Very recently, + + Gardenier et al. presented IR spectra of argon-tagged (H2O)2 • and (H2O)2 • and they only observed + the proton-transferred [H3O -•OH] type (shown in Figure 23)[99]. They found the resulting bands involving the displacement of the bridging proton to be broad and appear as a strong triplet centered 1 around 2000 cm− , with this observation completely ruling out a contribution from a hydrazine-like isomer.Molecules 2020 Other, 25, observed x FOR PEER band REVIEW positions are shown in Figure 24. 21 of 38

+•+ Figure 23. (a,b) TheThe equilibriumequilibrium structuresstructures ofof (H(H22O)O)22 • reportedreported by by Pieniazek Pieniazek et et al. [[101].101]. ( c,d) The +•+ structures of (H (H2O)O)2 •ArArn withn with (c) ( cn) = n 1= and1 and (d) (nd =) n2 reported= 2 reported by Gardenier by Gardenier et al. et[99]. al. Selected [99]. Selected bond 2 2 •• bondlengths lengths are reported are reported in Ångstroms. in Ångstroms. Figure Figure adap adaptedted from from [99] [99 ]with with permission. permission. Copyright Copyright 2009 American Chemical Society.

Figure 24. Vibrational predissociation spectra of (a) (H2O)2+••Ar, monitoring the loss of Ar and (b)

(H2O)2+••Ar2, monitoring the loss of two Ar species per molecule for excitations above 2400 cm−1 and the loss of one Ar in the low-energy region. Schematic displacement vectors are included to illustrate qualitative assignments of observed features. The subscript sp (ll) refers to the parallel displacement of the bridging proton along the heavy-atom (O-O) axis. The superscripts sym and asym denote,

Molecules 2020, 25, x FOR PEER REVIEW 21 of 38

Figure 23. (a,b) The equilibrium structures of (H2O)2+• reported by Pieniazek et al. [101]. (c,d) The

structures of (H2O)2+••Arn with (c) n = 1 and (d) n = 2 reported by Gardenier et al. [99]. Selected bond Moleculeslengths2020, 25are, 3490 reported in Ångstroms. Figure adapted from [99] with permission. Copyright 200920 of 37 American Chemical Society.

Figure 24.24. Vibrational predissociationpredissociation spectra ofof ((aa)) (H(H2O)2+•+•Ar,Ar, monitoring monitoring the the loss loss of of Ar and ((b)b) 2 2 •• (H(H2O)2+•+•ArAr2, ,monitoring monitoring the the loss loss of of two two Ar Ar species species per per molecule forfor excitationsexcitations aboveabove 24002400 cm cm−11 and 2 2 •• 2 − the loss of one Ar in the low-energy region. Schemati Schematicc displacement vectors are included to illustrate qualitative assignments of observed features. The subscriptsubscript sp (ll) refers to the parallel displacement of the bridgingbridging protonproton alongalong thethe heavy-atomheavy-atom (O-O)(O-O) axis.axis. The superscripts sym and asym denote, + respectively, the symmetric and asymmetric stretches of dangling OH groups of the H3O moiety,

each of which is bound to an Ar atom in the n = 2 system. The subscript •OH refers to the OH stretch on the hydroxyl moiety. Calculated anharmonic values for υsp (ll) and 2υsp (ll) are shown with dashed red lines. Figure adapted from [99] with permission. Copyright 2009 American Chemical Society.

+ 3.2. Structural Trends for Water Radical Cations (H O)n (3 n 10 and n > 10) 2 • ≤ ≤ Though conﬁrmation of the formation of the OH radical in the dimer cation is an indication of • important progress, the ion-core structure in larger clusters was at this stage still ambiguous because the ion-core structure often changes with solvation. Further experimental studies were thus required to elucidate the general trends of the network structures and the interplay between the protonated site and the OH radical under the hydration environment. • Based on MP2/UHF/4-31++G** data, Novakovskaya et al. arranged the H O+ and OH fragments 3 • and divided them into two groups: either the OH fragment acting exclusively as a proton acceptor • + in all of its hydrogen bonds or its being directly bonded to H3O and acting also as a proton donor in an H-bond with a water molecule [88]. They concluded the structures of the ﬁrst group to be + slightly more stable, with the H3O fragment tightly bonded to water molecules. For larger clusters, more of the positive charge would be shared by water molecules, but the charge of each molecule + would decrease with increasing n and with increasing distance between the molecule and H3O fragment. In contrast, the H-bond between OH and neighbor fragments would gradually weaken • with increasing cluster size. Molecules 2020, 25, x FOR PEER REVIEW 22 of 38

respectively, the symmetric and asymmetric stretches of dangling OH groups of the H3O+ moiety, each of which is bound to an Ar atom in the n = 2 system. The subscript •OH refers to the OH stretch

3.2. Structural Trends for Water Radical Cations (H2O)n+• (3 ≤ n ≤ 10 and n > 10) Though confirmation of the formation of the •OH radical in the dimer cation is an indication of important progress, the ion-core structure in larger clusters was at this stage still ambiguous because the ion-core structure often changes with solvation. Further experimental studies were thus required to elucidate the general trends of the network structures and the interplay between the protonated site and the •OH radical under the hydration environment. Based on MP2/UHF/4-31++G** data, Novakovskaya et al. arranged the H3O+ and •OH fragments and divided them into two groups: either the •OH fragment acting exclusively as a proton acceptor in all of its hydrogen bonds or its being directly bonded to H3O+ and acting also as a proton donor in an H-bond with a water molecule [88]. They concluded the structures of the first group to be slightly more stable, with the H3O+ fragment tightly bonded to water molecules. For larger clusters, more of the positive charge would be shared by water molecules, but the charge of each molecule would decrease with increasing n and with increasing distance between the molecule and H3O+ fragment. Molecules 2020, 25, 3490 21 of 37 In contrast, the H-bond between •OH and neighbor fragments would gradually weaken with increasing cluster size. + BesidesBesides IR IR spectra, spectra, the the structure evolutionevolution ofof ionizedionized waterwater radicalradical cationscations (H (H22O)O)nn+•• withwith n n == 5–85–8 waswas studied studied using using ab ab initio initio methods methods by by M.-K. M.-K. Tsai, Tsai, J.-L. J.-L. Kuo Kuo and and their their co-workers. co-workers. Both Both the the size size and and + temperaturetemperature dependence dependence of the the structure of (H2O)O)n+• andand solvation solvation of of the the •OHOH radical radical were were analyzed analyzed 2 • • systematicallysystematically by by using using a a structure structure searching searching meth methodod based based on on the the previous previous understanding understanding of of the the hydrogenhydrogen bond bond (H-bond) (H-bond) networks networks in in neutra neutrall and and protonated protonated water clusters (Figure 25).25).

Figure 25. Infrared spectra of neat and Ar-tagged (H O) + compared with simulated spectra of 2 7+• • Figure 25.+ Infrared spectra of neat and Ar-tagged (H2O)7 compared with simulated spectra of (H2O)7 • at 200 K and 75 K [102]. Figure adapted from [102] with permission. Copyright 2014 The (H2O)7+• at 200 K and 75 K [102]. Figure adapted from [102] with permission. Copyright 2014 The Royal Society of Chemistry. Royal Society of Chemistry. The agreement between the calculated and experimentally determined IR spectra in the free OH • stretchThe region agreement conﬁrmed between the preferencethe calculated of the andOH experi radicalmentally to stay determined on the terminal IR spectra site of thein the H-bond free • •OHnetwork stretch for region n = 5 andconfirmed n = 6. the Furthermore, preference of M.-K. the •O Tsai,H radical J.-L. Kuo to stay and on their the co-workersterminal site found of the that H- bondthe OHnetwork radical for begann = 5 and to form n = 6. H-bonds Furthermore, with waterM.-K. Tsai, molecules J.-L. Kuo as an and H-bond their co-workers donor for nfound= 7 and that 8 the •OH• radical began to form H-bonds with water molecules as an H-bond donor for n = 7 and 8 (as (asMolecules shown 2020 in, 25 Figure, x FOR 26 PEER). REVIEW 23 of 38 shown in Figure 26).

+ Figure 26. The most stable structures ofof (H(H22O)nn+•• inin different diﬀerent topological topological groups, groups, as determined using + ab initioinitio methods.methods. To guideguide thethe eyes,eyes, thethe OO atomsatoms ofof HH33OO+,, the •OHOH radicalradical andand HH22O are shown inin • yellow,yellow, blueblue andand red.red. The relativerelative energiesenergies withwith zero-point energy (ZPE) correction (in kcalkcal/mol)/mol) are shownshown inin parentheses.parentheses. Figure adapted from [[102]102] withwith permission.permission. Copyright 2014 The Royal Society of Chemistry.

Vibrational signatures of fully solvated •OH were found to be located at 3200–3400 cm−1, coinciding with the additional peaks found in previous experimental data obtained by Mizuse et al. [15]. Besides the theoretical calculation, three groups have reported structural motifs (for n ≥ 3) from their mass spectrometric analyses, but with these three motifs being different from each other [48,100,103]. Figure 27 shows the structural motifs constructed based on their results. In Figure 27, structure (a) shows a solvated “dimer cation core” type, as suggested by Yamaguchi et al. No •OH radical exists in this type [103]. Both structures (b) and (c) are of the proton-transferred type, but with these two differing in the position of the •OH radical. In structure (b), suggested by Angel and Stace, the H3O+ ion and •OH radical are in contact with each other and fully hydrated via three hydrogen bonds [100]. However, in structure (c), suggested by Jongma et al., the ion-radical pair is dissociated, with the •OH radical located at the terminus of the network [48]. Structure (a) is expected to display a reactivity very different from the reactivities of structures (b) and (c) due to the absence of the •OH radical. Furthermore, if the •OH radical is formed as in structures (b) and (c), its location relative to the protonated site may be a crucial factor for the reactivity of ionized water networks because the mobility of the •OH radical may depend on whether or not the radical is tightly bound to the protonated (charged) site.

Figure 27. Three different structural motifs of (H2O)n+• (n ≥ 3). These structures were constructed from previous results [48,100,103]. The protonated site is indicated by a border. The •OH radical moiety is circled. (a) “Dimer cation core”-type structure. (b) Structure with the H3O+ ion in contact with the •OH

Molecules 2020, 25, x FOR PEER REVIEW 23 of 38

Figure 26. The most stable structures of (H2O)n+• in different topological groups, as determined using

ab initio methods. To guide the eyes, the O atoms of H3O+, the •OH radical and H2O are shown in yellow, blue and red. The relative energies with zero-point energy (ZPE) correction (in kcal/mol) are shown in parentheses. Figure adapted from [102] with permission. Copyright 2014 The Royal Society Moleculesof Chemistry.2020, 25, 3490 22 of 37

Vibrational signatures of fully solvated •OH were found to be located at 3200–3400 cm−1, Vibrational signatures of fully solvated OH were found to be located at 3200–3400 cm 1, coinciding coinciding with the additional peaks found• in previous experimental data obtained by− Mizuse et al. with[15]. the additional peaks found in previous experimental data obtained by Mizuse et al. [15]. Besides the theoretical calculation, three groups have reported structural motifs (for n 3) from their Besides the theoretical calculation, three groups have reported structural motifs (for≥ n ≥ 3) from masstheir spectrometricmass spectrometric analyses, analyses, but with but these with three these motifs three being motifs diff erentbeing from different each otherfrom [48each,100 other,103]. Figure[48,100,103]. 27 shows Figure the 27 structural shows the motifs structural constructed motifs basedconstructed on their based results. on their In Figure results. 27, In structure Figure 27, (a) shows a solvated “dimer cation core” type, as suggested by Yamaguchi et al. No OH radical exists structure (a) shows a solvated “dimer cation core” type, as suggested by Yamaguchi• et al. No •OH inradical this typeexists [103 in this]. Both type structures [103]. Both (b) structures and (c) are (b) of and the (c) proton-transferred are of the proton-transferred type, but with type, these but with two differing in the position of the OH radical. In structure (b), suggested by Angel and Stace, the H O+ these two differing in the position• of the •OH radical. In structure (b), suggested by Angel and Stace,3 ion and + OH radical are in contact with each other and fully hydrated via three hydrogen bonds [100]. the H3O• ion and •OH radical are in contact with each other and fully hydrated via three hydrogen However, in structure (c), suggested by Jongma et al., the ion-radical pair is dissociated, with the OH bonds [100]. However, in structure (c), suggested by Jongma et al., the ion-radical pair is dissociated,• radicalwith the located •OH radical at the terminuslocated at ofthe the terminus network of [th48e]. network Structure [48]. (a) Structure is expected (a) tois displayexpected a to reactivity display verya reactivity different very from different the reactivities from the reactivities of structures of structures (b) and (c) (b) due and to (c) the due absence to the ofabsence the OH of the radical. •OH • Furthermore,radical. Furthermore, if the OHif the radical •OH radical is formed is formed as instructures as in structures (b) and (b) (c), and its (c), location its location relative relative to the to • protonatedthe protonated site maysite bemay a crucialbe a crucial factor factor for the for reactivity the reactivity of ionized of ionized water networks water networks because thebecause mobility the ofmobility the OH of radicalthe •OH may radical depend may on depend whether on or whether not the radicalor not the is tightly radical bound is tightly to the bound protonated to the • (charged)protonated site. (charged) site.

+ Figure 27. Three didifferentﬀerent structuralstructural motifs motifs of of (H (HO)2O)nn+• (n(n ≥ 3).3). These These structures structures were were constructed constructed from 2 • ≥ previousprevious results [48,100,103]. [48,100,103]. The The protonated protonated site site is is indicated indicated by by a aborder. border. The The •OHOH radical radical moiety moiety is +• iscircled. circled. (a) (“Dimera) “Dimer cation cation core”-type core”-type structure. structure. (b) Structure (b) Structure with the with H3 theO+ ion H3 Oin contaction in with contact the with•OH the OH radical. (c) Structure with the ion and radical separated from each other by solvent. Figure • adapted from [15] with permission. Copyright 2011 The Royal Society of Chemistry.

+ To reveal the structures of (H2O)n • at the molecular level, such as network shapes, ion core motifs and the location of the OH radical, Mizuse carried out a systematic investigation combining • IR spectroscopy, especially in the OH stretch region, and the inert gas (such as Ar) attachment + technique [50]. In contrast to the case for the H (H2O)n system, free OH stretch band patterns of + + (H2O)n • were found to be quite similar to those of protonated water clusters (H2O)nH (shown in Figure 28b) reported by several groups so far [16,69,71,104–109]. + + However, the spectra of (H2O)n • were noted to not be exactly identical to those of (H2O)nH in 1 + the 3500–3600 cm− region: an additional band was observed for (H2O)3–6 • and it is indicated by the arrow in Figure 28. The frequency of this band was too low to be attributed to any free OH stretch of the H2O moieties, as the gas-phase frequency of the stretch vibration of the OH radical has been 1 • observed to be 3570 cm− . Mizuse et al. assigned this band to the free stretch of an OH radical moiety • + in the clusters. This assignment was consistent with the OH radical stretch frequencies in (H2O)2 • 1 • • Ar 1,2 of 3511 and 3499 cm− , respectively [99]. Molecules 2020, 25, x FOR PEER REVIEW 24 of 38

To reveal the structures of (H2O)n+• at the molecular level, such as network shapes, ion core motifs and the location of the •OH radical, Mizuse carried out a systematic investigation combining IR spectroscopy, especially in the OH stretch region, and the inert gas (such as Ar) attachment technique [50]. In contrast to the case for the H+(H2O)n system, free OH stretch band patterns of Molecules(H22020O)n+•, 25 were, 3490 found to be quite similar to those of protonated water clusters (H2O)nH+ (shown in23 of 37 Figure 28b) reported by several groups so far [16,69,71,104–109].

++• FigureFigure 28. (28.a) IR(a) spectraIR spectra of of (H (H2O)2O)n n• (n(n == 3–11) in in the the free free OH OH stretch stretch region. region. (b) IR (b )spectra IR spectra and and representative structures of (H2O)nH++. (Dotted curves mark the locations in each spectrum, from the representative structures of (H2O)nH . (Dotted curves mark the locations in each spectrum, from the low frequencylow frequency side, side, of aof symmetric a symmetric stretch stretch of of 1-coordinated1-coordinated water, water, a free a free OH OH stretch stretch of 3-coordinated of 3-coordinated water,water, a free a free OH OH stretch stretch of 2-coordinatedof 2-coordinated water water and an an antisymmetric antisymmetric stretch stretch of 1-coordinated of 1-coordinated water, water, respectively). Figure adapted from [15] with permission. Copyright 2011 The Royal Society of respectively). Figure adapted from [15] with permission. Copyright 2011 The Royal Society of Chemistry. Chemistry. + As discussed above, the OH radical water cluster cations (H2O)n • (n 3) should be regarded • + ≥ as “protonated water clusters with an OH radical”, (H2O)n 1H ( OH), as has been reported for + • − • + (H O) [99]. Mizuse et al. further reported the OH radical in (H O)n to be separated from the 2 2 • • 2 • protonated site by one water molecule in the n 5 clusters and attributed this separation to the OH ≥ • radical being a weaker hydrogen bond acceptor than is a water molecule, with the consequence being that the ﬁrst solvation shell of the protonated site is preferentially ﬁlled with water [110]. Figure 29 + presents experimentally-characterized structures of (H2O)n •. As mentioned above, these clusters + are easily constructed from the (H2O)nH structures by substituting one of the water molecules, which is the next neighbor of the charged site for OH radical [110]. The reactivity of water exposed to • ionizing radiation would be expected to depend on hydrogen bond network structures around the created radicals. Molecules 2020, 25, x FOR PEER REVIEW 25 of 38

However, the spectra of (H2O)n+• were noted to not be exactly identical to those of (H2O)nH+ in the 3500–3600 cm−1 region: an additional band was observed for (H2O)3–6+• and it is indicated by the arrow in Figure 3.8. The frequency of this band was too low to be attributed to any free OH stretch of the H2O moieties, as the gas-phase frequency of the stretch vibration of the •OH radical has been observed to be 3570 cm−1. Mizuse et al. assigned this band to the free stretch of an •OH radical moiety in the clusters. This assignment was consistent with the •OH radical stretch frequencies in (H2O)2+•• Ar 1,2 of 3511 and 3499 cm−1, respectively [99]. As discussed above, the •OH radical water cluster cations (H2O)n+• (n ≥ 3) should be regarded as “protonated water clusters with an •OH radical”, (H2O)n−1H+(•OH), as has been reported for (H2O)2+• [99]. Mizuse et al. further reported the •OH radical in (H2O)n+• to be separated from the protonated site by one water molecule in the n≥5 clusters and attributed this separation to the •OH radical being a weaker hydrogen bond acceptor than is a water molecule, with the consequence being that the first solvation shell of the protonated site is preferentially filled with water [110]. Figure 29 presents experimentally-characterized structures of (H2O)n+•. As mentioned above, these clusters are easily constructed from the (H2O)nH+ structures by substituting one of the water molecules, which is the next neighbor of the charged site for •OH radical [110]. The reactivity of water exposed to ionizing Moleculesradiation2020 would, 25, 3490 be expected to depend on hydrogen bond network structures around the created24 of 37 radicals.

++• Figure 29. Experimentally characterized clustercluster structuresstructures of of (H (H2O)2O)nn •..Figure Figure adapted adapted from from [50] [50] with permission. Copyright 2013 Springer Nature.

+ + Note the increasingincreasing similaritysimilarity ofof thethe (H (H2O)2O)nn+•• and (H22O)nHH+ spectraspectra with with increasing increasing size size of the cluster, a feature that can be attributed the decrease in the relative weight of the OH radical and the cluster, a feature that can be attributed the decrease in the relative weight of the •OH• radical and the “bridging”“bridging” waterwater site.site. In this regard, it is particularly difficult difficult to find find any marked difference difference between + + the spectraspectra ofof (H(H22O)O)1111+•• andand (H (H22O)O)1111HH+. However,. However, the the clear clear appearance appearance of of the the marker marker band band (as (as shown in Figure 28) and the clearly observed smooth changes between spectra with increasing n for n 10 in Figure 28) and the clearly observed smooth changes between spectra with increasing n for n ≤≤ 10 strongly suggest that the trendstrends continuecontinue inin clustersclusters largerlarger thanthan nn == 10.

+ 3.3. (H2O)n+• • Isomers in a Global Potential Energy Surface 3.3. (H2O)n Isomers in a Global Potential Energy Surface In addition to the simulationssimulations of the observed structuresstructures discussed above in detail, it was also valuable to characterize other stable structures onon thethe potentialpotential energyenergy surface.surface. Mizuse etet al.al. carried out systematic explorations of stable isomers of (H O) + on the HF/6-31G(d) and B3LYP/6-31+G(d) out systematic explorations of stable isomers of (H22O)3–83–8+•• on the HF/6-31G(d) and B3LYP/6-31+G(d) potential energy energy surfaces surfaces by by using using the the Global Global Reaction Reaction Route Route Map(GRRM) Map(GRRM) program program [111–116]. [111 –They116]. They reported the B3LYP/6-31+G(d) calculations yielding 3, 15 and 82 stable isomers for (H O) + , reported the B3LYP/6-31+G(d) calculations yielding 3, 15 and 82 stable isomers for (H2 2O)33+••, + + (H 2O)4 •and (H2O)5 •, respectively. Re-optimization at the MPW1K/6-311++G(3df, 2p) level yielded isomers at each size with global minimum energies. Calculations at the HF/6-31G(d) level yielded + 2, 9, 40, 62, 170 and 224 isomers for (H2O)3–8 •, respectively. The theoretical calculations indicated + the structures with the H3O cation separated from the OH radical to be lower in energy by + • 3–10 kJ/mol than those with H3O in contact with •OH, even when adopting the same network shapes (i.e., chains and rings). This result suggests stabilization aﬀorded by separating an ion from a radical to be a general phenomenon.

+ 4. Chemical Properties of (H2O)n •: Simulations and Experimental Studies

4.1. Proton Transfer to Form Hydroxyl Radicals Ionization of water clusters (liquid, molecular beam or vapor) has been investigated using several techniques, and the reaction was indicated according to the results to follow the equations Molecules 2020, 25, 3490 25 of 37

+ (H O)n + Ip [(H O)n •]ver + e− (Ionization) (6) 2 → 2 + + [(H2O)n •]ver (H3O )( OH)(H2O)n 2 (Complex Formation) → • − + H (H2O)n 1 + OH ( OH Dissociation) (7) → − • • + H (H2O)n k + (k 1)H2O + OH ( OH Dissociation + H2O Evaporation) → − − • • where Ip and ver denote the ionization potential and vertical ionized state from the parent neutral cluster, respectively. Based on a previous study in which the ionizations of water clusters (H2O)n (n = 2–6) were investigated using the direct ab initio molecular dynamics (AIMD) method at the Hartree–Fock (HF) level [117], Tachikawa et al. performed the direct AIMD calculation on a more accurate potential energy surface (MP2/6-311++G(d,p) level) to estimate the rate of proton transfer along the hydrogen bond in the water cluster cation [118]. The rate of the ﬁrst proton transfer was found to be strongly dependent on the cluster size: average time scales of proton transfer for n = 2, 3 and 4 were 28, 15 and 10 fs, respectively (from the MP2/6-311++G(d,p) level calculation), suggesting the proton transfer reactions to be very rapid processes in the three clusters. The clusters with n = 3 and Molecules 20204 also, 25 each, x FOR showed PEER a REVIEW second proton transfer (with average time scales of 120 fs and 40 fs, respectively, 27 of 38 + after the ionization), as shown in Figures 30 and 31 for the case of the water tetramer (H2O)4 •.

+ Figure 30. Snapshots of water tetramer cation (H2O)4 • after vertical ionization from optimized Figure 30. Snapshots of water tetramer cation (H2O)4+• after vertical ionization from optimized structure. The values are bond distance in Å. Figure adapted from [118] with permission. Copyright structure.2015 The The values Royal Societyare bond of Chemistry. distance in Å. Figure adapted from [118] with permission. Copyright 2015 The Royal Society of Chemistry.

Figure 31. (a) Superposition of snapshots for the first proton transfer process. (b) Potential energy curve for the proton transfer process as derived from a static ab initio calculation (MP2/6-311++G (d, p) level). The values obtained from the direct ab initio molecular dynamics (AIMD) calculation are given as filled circles. (c) Potential energy and (d) geometrical parameters of the water tetramer cation

Molecules 2020, 25, x FOR PEER REVIEW 27 of 38

Figure 30. Snapshots of water tetramer cation (H2O)4+• after vertical ionization from optimized structure. The values are bond distance in Å. Figure adapted from [118] with permission. Copyright

2015 TheMolecules Royal2020 Society, 25, 3490 of Chemistry. 26 of 37

Figure 31. (a) Superposition of snapshots for the ﬁrst proton transfer process. (b) Potential energy Figure 31. (curvea) Superposition for the proton transfer of snapshots process as derivedfor the from first a static proton ab initio transfer calculation process. (MP2/6-311 (b++) PotentialG energy curve for the(d, proton p) level). transfer The values process obtained from as derived the direct ab fr initioom moleculara static dynamicsab initio (AIMD) calculation calculation (MP2/6-311++G are (d, given as ﬁlled circles. (c) Potential energy and (d) geometrical parameters of the water tetramer cation p) level). Thefollowing values the obtained vertical ionization from each the as direct a function ab of initio time. Figure molecular adapted fromdynamics [118] with (AIMD) permission. calculation are given as filledCopyright circles. 2015 (c The) Potential Royal Society energy of Chemistry. and (d) geometrical parameters of the water tetramer cation

As illustrated above, on the basis of the presented calculations, a reaction model of ionizations of the water clusters is proposed. Upon ionization of water, a hole would become localized on one of

the water molecules of (H2O)n, and a proton would rapidly transfer—within a time scale of about + + 10 fs—from H2O • (W1 •) to H2O (W2) along the hydrogen bond. Next, the second proton transfer + would occur from W2 (H ) to W3 (H2O) within a time scale of 50–100 fs. The second proton transfer + + would result in the separation of the OH radical from H3O to form H3O -H2O-•OH, as discussed in • + detail above in Section3. Upon completion of this separation, any attraction between H 3O and the OH radical would be masked by the intervening H O, immediately resulting in dissociation of the • 2 OH radical from the system. • + 4.2. Initial Ultrarapid Charge Migration in the Chemistry of H2O • Generally, the OH radical dissociated from H O+ is considered to be the main reactive species • 2 • inducing one electron oxidation, OH adduct formation and H atom abstraction reactions. In theory, + • H2O • should be considered as an ultra-oxidizing species. In the gas phase, oxidation of small molecules such as O2 has been reported, as discussed above [29,30]. In the condensed phase, a prerequisite for the direct oxidation process to be competitive is that the target molecule should be in close proximity to + + H2O •; thus, the oxidation process induced by H2O • can be mimicked in highly concentrated solutions, Molecules 2020, 25, x FOR PEER REVIEW 28 of 38

4.2. Initial Ultrarapid Charge Migration in the Chemistry of H2O+•

Generally, the •OH radical dissociated from H2O+• is considered to be the main reactive species inducing one electron oxidation, •OH adduct formation and H atom abstraction reactions. In theory, H2O+• should be considered as an ultra-oxidizing species. In the gas phase, oxidation of small moleculesMolecules 2020 such, 25, 3490 as O2 has been reported, as discussed above [29,30]. In the condensed phase,27 of 37a prerequisite for the direct oxidation process to be competitive is that the target molecule should be in close proximity to H2O+•; thus, the oxidation process induced by H2O+• can be mimicked in highly + concentratedwhere the nearest solutions, neighbors where of the H2 Onearest• may neighbors be molecules of H other2O+• may than be water molecules [119,120 other]. By than effectively water [119,120].solving the By bottleneck, effectively itsolving was not the possible bottleneck, to exclude it was thenot possibilitypossible to of exclude an OH the radical possibility reaction of an in • •OHthe oxidation radical reaction process in by the using oxidation nanosecond process or by microsecond using nanosecond electron or pulses microsecond as well aselectron the problems pulses asof well the absenceas the problems of information of the absence on the of yield information of a direct on e theffect yield of radiation of a direct on effect the solute,of radiation picosecond on the + solute,pulse radiolysis picosecond clearly pulse showed radiolysis that clearly H2O •showedcan also that be consideredH2O+• can also as an be ultra-oxidizing considered as speciesan ultra- in oxidizingsolution and species presents in asolution reactivity and diff presentserent from a thatreactivity of the differentOH radical. from Furthermore, that of the several •OH attemptsradical. + • Furthermore,have been made several to experimentally attempts have investigatebeen made Hto2 experimentallyO • in different investigate ways based H2 onO+• femtosecond in different ways laser basedtechnology on femtosecond [23,25,26]. laser technology [23,25,26]. One of the femtosecond laser-driven accelerators established established at at ELYSE ELYSE (a facility named after Lysis (Greek for degradation) by Electrons, which achieves a high energy (7–8(7-8 Mev) electron beam with aa pulsepulse widthwidth of of 7 7 ps) ps) is is shown shown in Figurein Figure 32. 32 This. This accelerator accelerator can achievecan achieve a high-energy a high-energy (7–8 MeV (7–8) MeV)electron electron beam withbeam a pulsewith a width pulse of width 7 ps [ 121of 7]. ps Its detection[121]. Its detection system is basedsystem on is transient based on absorption transient absorptionspectroscopy spectroscopy with probe with light, probe with wavelengthslight, with wavelengths ranging from ranging 380 nm from to 1500380 nm nm to [122 1500]. Therefore,nm [122]. Therefore,the unique the time-resolved unique time-resolved technique based technique on a high-energy based on a high-energy electron pulse electron enabled pulse various enabled investigators various + investigatorsto explore ultra-rapid to explore chemical ultra-rapid reactions chemical of Hreactions2O • through of H2O the+• through scavenging the scavenging method in method a variety in of a varietyhighly concentratedof highly concentrated aqueous solutions, aqueous i.e., solutions, by observing i.e., theby observing formation ofthe secondary formation radicals of secondary such as radicalsNO3•, SO such4•− ,Xas 2NO•− (X3•, =SOCl,4•−, Br) X2• or− (X H =2 POCl, 4Br)•. Byor H further2PO4•. using By further diffusion-kinetic using diffus simulationsion-kinetic simulations of the spur + ofreaction the spur induced reaction by theinduced incident by electrons,the incident the electr first semi-quantitativeons, the first semi-quantitative estimation of the estimation H2O • radicals of the Hscavenging2O+• radicals fractions scavenging for a widefractions range for of a soluteswide range could of be solutes established. could be established.

Figure 32. (a) Photograph of the pulse radiolysis facility (the only one in Europe) based on the ELYSE (a facility named after Lysis (Greek for degradation) by Electrons, which achieves a high energy (7–8 Mev) electron beam with a pulse width of 7 ps) picosecond pulsed electron accelerator from the Physical Chemistry Laboratory in Orsay, France. (b) Schematic description of the synchronization of the electron beam for ionization with a laser beam to probe the species created by the electron pulse. Figure adapted from [123] with permission. Copyright 2018 Multidisciplinary Digital Publishing Institute. Molecules 2020, 25, x FOR PEER REVIEW 29 of 38

Figure 32. (a) Photograph of the pulse radiolysis facility (the only one in Europe) based on the ELYSE (a facility named after Lysis (Greek for degradation) by Electrons, which achieves a high energy (7-8 Mev) electron beam with a pulse width of 7 ps)picosecond pulsed electron accelerator from the Physical Chemistry Laboratory in Orsay, France. (b) Schematic description of the synchronization of the electron beam for ionization with a laser beam to probe the species created by the electron pulse. Figure adapted from [123] with permission. Copyright 2018 Multidisciplinary Digital Publishing Molecules 2020, 25, 3490 28 of 37 Institute.

+•+ H22O • hashas been been suggested suggested to to be be a stronger oxidative radical than other oxidants in aqueousaqueous + solutionssolutions basedbased on on an estimationan estimation of its of redox its potentialredox potential value: the value: standard the redoxstandard potential redox of Hpotential2O •/H 2ofO coupleH2O+•/H has2O beencouple recently has been estimated recently to estimated be higher to than be 3hi Vgher vs. normalthan 3 V hydrogen vs. normal electrode hydrogen (NHE) electrode [123]. + +• A(NHE) 7 ps pulse[123]. radiolysisA 7 ps pulse set-up radiolys wasis used set-up to demonstratewas used to demonstr the oxidationate the reaction oxidation of H reaction2O • in of various H2O + +• solutions.in various solutions. As the lifetime As theof lifetime H2O •ofis H too2O shortis too forshort it tofor be it to observed be observed directly, directly, the product the product of the of ++• oxidationthe oxidation reaction reaction was was observed observed just just after after the the electron electron pulse. pulse. The The oxidation oxidation of of M M by by H H2O2O• andand by ++• direct eeffectﬀect givesgives MM •,, which can bebe observed.observed. Other oxidation reactions, such as oxidationoxidation byby thethe ++• •OHOH radical,radical, dodo notnot occuroccur soso quickly.quickly. Therefore,Therefore, thethe yieldyield ofof MM • measured precisely at 7 psps couldcould • ++• provideprovide informationinformation aboutabout itsits oxidationoxidation byby HH22O •(as(as shown shown in in Figure Figure 33).33).

Figure 33. Schematic description of the reactions occurring in solutions containing a solute M at high Figure 33. Schematic description of the reactions occurring in solutions containing a solute M at high concentration. Figure adapted from [123] with permission. Copyright 2018 Multidisciplinary Digital concentration. Figure adapted from [123] with permission. Copyright 2018 Multidisciplinary Digital Publishing Institute. Publishing Institute. Several studies in highly concentrated solutions were performed with the purpose of scavenging Several studies in highly concentrated+ solutions were performed with the purpose of scavenging the radical cation of water, i.e., H2O • [124–128]. Highly concentrated sulfuric acid solutions were the radical cation of water, i.e., H2O+• [124–128]. Highly concentrated sulfuric acid solutions were shown to be appropriate probe systems to elucidate the diﬀerent mechanisms [129–134] and showed shown+ to be appropriate probe systems to elucidate the different mechanisms [129–134] and showed H2O • acting as a strong oxidant towards weak electron donors such as HSO4− or H2PO4− [135]. +• − − H2O acting as a strong+ oxidant towards+ weak electron donors such as HSO4 or H2PO4 [135]. The The reactivities of H2O • and D2O • were probed in hydrogenated and deuterated sulfuric acid reactivities of H2O+• and D2O+• were probed in hydrogenated and deuterated sulfuric acid solutions solutions of various concentrations, combined with theoretical simulations by Wang et al. [136] of various concentrations, combined with theoretical simulations by Wang et al. [136] As illustrated in Figures 34 and 35, electron transfer was indicated, based on the kinetics observed As illustrated in Figures 34 and 35, electron transfer was indicated, based on the kinetics by Wang et al. [136], to be favored over proton transfer in competition reactions of the radical cation observed by Wang et al. [136], to be favored over proton transfer in competition reactions of the of water in deuterated solutions. These latest studies thus indicated the radical cation of water to radical cation of water in deuterated solutions. These latest studies thus indicated the radical+ cation+ be engaged not only in proton transfer reactions but also in redox reactions when H2O • (D2O •) is of water to be engaged not only in proton transfer reactions but also in redox reactions when H2O+• formed in the vicinity of molecules diﬀerent from water—with charge migration propelled by the (D2O+•) is formed in the vicinity of molecules different from water—with charge migration propelled excess energy present in the electron cloud just after ionization and not by nuclear motions as in standard chemical theories of electron transfer [136,137].

Molecules 2020, 25, x FOR PEER REVIEW 30 of 38

Moleculesby the excess2020, 25 ,energy x FOR PEER present REVIEW in the electron cloud just after ionization and not by nuclear motions30 of 38as in standard chemical theories of electron transfer [136,137]. by the excess energy present in the electron cloud just after ionization and not by nuclear motions as Moleculesin standard2020, 25chemical, 3490 theories of electron transfer [136,137]. 29 of 37

Figure 34. Electronic dynamics simulations. (a) Evolution of the charge of the water molecule Figurehydrogen 34. Electronic bonded to dynamics HSO4- as simulations. a function of (a time) Evolution after ionization of the charge took of place, the water defined molecule to be hydrogent = 0. Each

bondedFigureplot (1, 34.to2, 2 HSO′Electronic and4 −3)as corresponds a dynamics function to ofsimulations. a timedifferent after electronic ionization(a) Evolution dynamics took of place, thegenerated definedcharge by of to depopulating bethe t water= 0. Eachmolecule different plot −3/2 (hydrogen1,valence 2, 20 and MOs, bonded 3) corresponds the torepresentations HSO4- toas aa difunctionff oferent which of electronic time are shownafter dynamics ionization in panel generated took(b) for place, isosurfaces by defined depopulating ofto ±be 0.08 t di= ff0.bohrerent Each . 3/2 valenceplotThe (1, two 2, MOs, 2green′ and the curves3)representations corresponds (2 and 2 ′to) correspond a of different which are electronicto ionization shown dynamics in panelfrom the ( bgenerated) sa forme isosurfaces MO by but depopulating with of different0.08 bohrdifferent values− . − ± Thevalenceof the two hydrogen greenMOs, curvesthe bond representations (2 length and 20 )between correspond of which H2O to andare ionization shownHSO4 , in1.8 from panel and the 2.4 (b same )Å, for respectively, MOisosurfaces but with andof di ±ff (erent0.08c) isosurfaces bohr values−3/2. ofTheof the the two hydrogen difference green curves bond charge length(2 and density between2′) correspond with H respect2O and to toionization HSO the 4initial−, 1.8 from andtime the 2.4 for sa Å, dynamicsme respectively, MO but 2 and with and for different (cdifferent) isosurfaces values times ofofspanning thethe dihydrogenfference the first chargebond half length densityperiod between of with charge respect H2 Omigration. and to theHSO initial Blue4−, 1.8 and time and green for2.4 dynamicsÅ, isosurfaces respectively, 2 and indicate forand di ( cffaccumulation)erent isosurfaces times spanningofand the depletion difference the first of charge electron half period density density, of chargewith respectively. respect migration. to thFiguree Blue initial adapted and time green for from isosurfacesdynamics [136] with 2 indicateand permission. for different accumulation Copyright times andspanning2017 depletion The theRoyal first of electronSociety half period of density, Chemistry. of charge respectively. migration. Figure Blue adapted and green from isosurfaces [136] with permission. indicate accumulation Copyright 2017and depletion The Royal of Society electron of density, Chemistry. respectively. Figure adapted from [136] with permission. Copyright 2017 The Royal Society of Chemistry.

+ Figure 35. Schematic of the probing of the competition reactions of the radical cation D2O •. A time resolution of 7 ps was used in the setup for probing the sulfate radical in deuterated sulfuric acid

Molecules 2020, 25, 3490 30 of 37

5. Summary and Outlook

5.1. Summary

+ 1. The currently known methods and mechanisms of formation of (H2O)n • can be summarized as follows:

A. Method: wet nitrogen ionized as a result of being subjected to electron bombardment or corona discharge. Mechanism: a series of charge transfer reactions started by the formation + of N2 •. B. Method: photoionization of a water molecular beam or vapor, even including + helium nanodroplets. Mechanism: (i) weakly bonded ions, e.g., (Ar)k(H2O)n •, + were cooled and then (H2O)n • were formed after further evaporating the carrier gas; (ii) the Franck–Condon factor could be enlarged due to the presence of carrier gas atoms in + complex ions, (Ar)k(H2O)n •, preventing the contraction of the O-O bond upon ionization. + (H2O)n • were generated after the complex ions’ dissociation; (iii) rapid evaporation of + the carrier gas could remove the excess energy in the (H2O)n •; (iv) the excess energies + remained in (H2O)n • after exposure to high-energy photos or particles would be removed by the exiting electron. C. Method: photoionization by high-energy photons or 70 eV electrons impactions. + Mechanism: the inner valence electrons could be excited and ﬁnally two H2O • ion radicals could be formed through an ICD process. + 2. The current knowledge about the structure of (H2O)n • can be summarized as follows:

+ A. For (H2O)2 •, a theoretical simulation led to minimum-energy arrangements of both + the structure resulting from proton transfer (H3O -•OH) and the dimer cation structure + [H2O ... OH2] •, but with the former indicated to be more stable than the latter by 8.8 kcal/mol with an interconversion barrier of 15.1 kcal/mol. Experimental results based on infrared spectroscopy and collision-induced dissociation MS spectroscopy also tended to support the formation of structures resulting from proton transfer. B. A recently reported simulation of the dynamical process following water dimer ionization suggests that both the structure resulting from proton transfer and the dimer cation structures as well as other dissociated structures (H O+ + OH, H + OH + H O+ ) could 3 • • • 2 • be generated. The relative yields of these species are controlled by the populated electronic state of the radical cation. Proton transfer resulting from HOMO electron ionization is an ultrafast process, taking less than 100 fs; in the case of higher-energy ionization, the dynamical processes occur on longer timescales (200–300 fs). + C. For the larger (H O)n clusters (i.e., n 3), most of the simulations and experimental results 2 • ≥ indicated the presence of the structures resulting from proton transfer. At ﬁrst, researchers expected OH to be bonded tightly with H O+. Later, however, they proposed that the • 3 OH group would be separated from the protonated site by one water molecule since an • OH is a weaker hydrogen bond acceptor than is a water molecule. Then, researchers • found a preference for the OH to stay on the terminal site of the H-bond network—with, • in the case of the clusters becoming bigger (n > 10), the OH groups beginning to serve as • H-bond donors (instead of acceptors) and in this way form H-bonds with water molecules. + 3. It was recently found that, besides the ultra-fast proton transfer reaction between (H2O)n • and its neighboring water molecules to generate protonated water clusters and highly oxidative + hydroxyl radicals, (H2O)n • should also be regarded as the strongest oxidant in the solution and + that ultrafast charge migration could occur from a solute M to (H2O)n • in highly concentrated solutions. The reaction rate of charge migration was indicated by both the simulation and experimental results to be competitive with the rate of proton transfer. Molecules 2020, 25, 3490 31 of 37

5.2. Outlook

+ 1. According to the electric ﬁeld ionization theory, (H2O)n • may be expected to be produced under ambient conditions by the exposure of water vapor to the low-energy corona discharge in open air. 2. More accurate methods are needed to reliably predict the structures and chemical properties of + (H2O)n •. + 3. Experimental investigation of chemical properties of (H2O)n • is greatly hindered due to the + low amount of the generated (H2O)n • in current methods. New methods are needed to allow a + higher abundance of (H2O)n •. + 4. Attosecond photoelectron spectroscopy based on high harmonic generation resolves H2O • decay + in pure water and therefore shows high potential for the studies of (H2O)n •. + 5. Future studies of (H2O)n • reactivity need to take into account the competition of alternative reactions such as H O+ addition or H abstraction with proton transfer and charge 2 • • migration reactions. + 6. More research on the reactivity of (H2O)n • in the condensed phase is essential in radio-biochemistry, nuclear industry and many other disciplines.

Author Contributions: D.M. and K.C. conceived the idea of current manuscript. D.M. wrote the manuscript and K.C. supervised writing. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the National Natural Science Foundation of China (grant number 21520102007) and the work of K.C. was funded by the Russian Science Foundation (agreement number 20-65-46014). Acknowledgments: Here, I extend special thanks to Bingqing Wei, Yixuan Mao and Junqiang Xu for their help with organizing the figures, charts and references. Conflicts of Interest: The authors declare no conflict of interest.

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