Characterization of a Solvent-Separated Ion-Radical Pair in

Article pubs.acs.org/JPCA

Characterization of a Solvent-Separated Ion-Radical Pair in Cationized Water Networks: Infrared Photodissociation and ArAttachment Experiments for Water Cluster Radical Cations (H2O)n+ (n = 3−8) Kenta Mizuse† and Asuka Fujii* Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan S Supporting Information *

ABSTRACT: We present infrared spectra of nominal water cluster radical cations (H2O)n+ (n = 3−8), or to be precise, ion-radical complexes H+(H2O)n−1(OH), with and without an Ar tag. These clusters are closely related to the ionizing radiation-induced processes in water and are a good model to characterize solvation structures of the ion-radical pair. The spectra of Ar-tagged species show narrower bandwidths relative to those of the bare clusters due to the reduced internal energy via an Ar-attachment. The observed spectra are analyzed by comparing with those of the similar system, H+(H2O)n, and calculated ones. We find that the observed spectra are attributable to ionradical-separated motifs in n ≥ 5, as reported in the previous study (Mizuse et al. Chem. Sci. 2011, 2, 868−876). Beyond the structural trends found in the previous study, we characterize isomeric structures and determine the number of water molecules between the charged site and the OH radical in each cluster size. In all of the characterized cluster structures (n = 5−8), the most favorable position of OH radical is the next neighbor of the charged site (H3O+ or H5O2+). The positions and cluster structures are governed by the balance among the hydrogen-bonding abilities of the charged site, H2O, and OH radical. These findings on the ionized water networks lead to understanding of the detailed processes of ionizing radiation-initiated reactions in liquid water and aqueous solutions. anions (H2O)n− have been extensively studied as microscopic models for hydrated electrons in eq 2a. Photoelectron and optical (infrared (IR) and visible) spectroscopies as well as mass spectrometry have been applied to a broad size range (n ≤ ∼200) of (H2O)n−.16−29,84,89 In these studies, H-bond network structures, electron-binding motifs, and reactivities have been discussed in detail. For the microscopic models of the cationic moiety (eq 2b), namely, water cluster radical cations (H2O)n+, structural information has been rare, as mentioned in the following.62,87 The present study focuses on this problem. Previous studies on (H 2 O) n + have relied on mass spectrometry and photoionization/photoelectron spectroscopy.30−35,57,59,88,90 Because protonated water clusters H+(H2O)n are the major product of a typical cationic water cluster source, the production of relatively unstable (H2O)n+ itself has been of interest.30,31 After the first observation of (H2O) n +,30 dissociation patterns and reactivity have been investigated by a few groups.31−33 These studies have suggested three motifs of cluster structures. One of them is the dimer cation core type (hemibonded type) in which the positive charge is delocalized to the two water molecules and the other molecules solvate the (H2O)2+ core.33 The other is an ion radical pair type, denoted

1. INTRODUCTION Ionizing radiation-induced processes in aqueous systems are of great interest because of their importance in various chemical and biochemical processes such as radiation-induced biological damage and radiotherapy.1−4 The elementary steps of these processes are described as follows.1−8 H 2O → H 2O+ + e−

(1)

e− → e−aq

(2a)

H 2O+ + H 2O → H3O+ + OH

(2b)

Equation 1 is ionization of a water molecule; Equations 2a and 2b are the hydration of the ejected electron and the production of the OH radical via the H2O+−H2O ion− molecule reaction, respectively. These reactions occur in the liquid phase; therefore, it is important to understand their mechanisms, including the roles of the surrounding water molecules. In the condensed phase, microscopic details (especially about hydrogen (H)-bond network structures), however, tend to be veiled by the complexity of a great number of solvent molecules, competing processes, and thermal fluctuation. For microscopic understanding of structures and dynamics of H-bonded water networks, a hydrated cluster study in the gas phase is a powerful approach.9−90 For example, water cluster © 2013 American Chemical Society

Received: December 4, 2012 Revised: January 18, 2013 Published: January 18, 2013 929

dx.doi.org/10.1021/jp311909h | J. Phys. Chem. A 2013, 117, 929−938

The Journal of Physical Chemistry A

Article

by H+(H2O)n−1(OH).31,32 The latter type is further categorized into two types, depending on whether the radical is tightly bound to the protonated (charged) site or not. This is because reactivity of ionized water networks would be affected by the mobility of the OH radical (reactor) and the mobility would depend on the binding with the protonated site. Because these three motifs are different from each other,31−33 direct structural information has been expected. Furthermore, although there have been many theoretical studies on (H2O)n+,37−40,46,47,61,91 previous theoretical studies often suffered from symmetrybreaking, spin contamination, or self-interaction error in such open-shell doublet systems.58,61 Unique consensus for cluster structures has therefore not yet been achieved. Because H-bonding environments in a water network are sensitively reflected to its vibrational spectra, IR spectroscopy provides detailed structural information of hydrated clusters.15,92,93 With regard to (H2O)n+ systems, Gardenier et al. presented the first IR spectra of (H2O)2+ measured with the Armediated photodissociation technique, and they have shown that only the proton-transferred H 3 O+ −OH type was observable.62 Recent fine theoretical studies, one of which was combined with photoelectron spectroscopy, agree with this result.41,46,47,51−53,58,61,91 For larger systems, very recently, we have reported IR spectra of (H2O)3−11+ and found important structural trends:87 Nominal (H2O)n+ forms proton-transferred H+(H2O)n−1(OH) type structures. Frameworks of their Hbond networks are essentially the same as those of H+(H2O)n, which have been well-characterized in the previous studies.65,68,72,74,80 The first solvation shell of the protonated site is preferentially filled with water. As a result, the OH radical is separated from the protonated site by at least one water molecule in the n ≥ 5 clusters. Here we note that we often use the notation (H2O)n+ rather than H+(H2O)n−1(OH) for simplicity in the present study. In the following, we frequently refer to these trends as the previously reported structural trends. Our previous study has characterized the solvent-separated ion-radical pair, as suggested by Jongma et al.;31 however, the isomer-based analysis has been limited to n ≤ 5.87 Furthermore, we have not determined the exact number of water molecules (distance) between the protonated site and the OH radical in the systems larger than n = 5. In other words, this study focused only on the first and second solvation shells of the protonated site. In the condensed phase, much larger solvation shells should be taken into account to consider the reactivity of an OH radical produced by ionizing radiation. Here we extend our previous work to investigate detailed structures of (H2O)n+ or H+(H2O)n−1(OH), in which the second or higher solvation shells exist. In addition to the previously reported spectra of (H2O)3−11+, IR spectra of (H2O)3−7+·Ar are reported. Because Ar-tagged clusters have lower internal energy than bare clusters, simpler spectral structures are expected. For these experimental spectra, systematic analyses based on the comparison with the similar system, H+(H2O)n, and theoretical simulations are carried out. We characterize cluster structures of n ≤ 8 at the molecular level. Furthermore, in the presence of the third solvation shell, the most favorable position of the OH radical is determined experimentally.

descriptions are given here. IR spectra of water cluster cations (H2O)n+ (n = 3−11) were measured as photodissociation spectra by using a tandem quadrupole mass spectrometer and a coherent IR source. IR spectra of (H2O)n+·Ar (n = 3−7) were also measured using essentially the same technique. Because intensities of H+(H2O)n and H+(H2O)n·Ar are much larger than those of (H2O)n+ and (H2O)n+·Ar, mass spectrometric separation of protonated and radical cationic species is difficult, especially in large systems. This fact limited the size region investigated in this study. Water cluster cations (H2O)n+ and (H2O)n+·Ar were generated in a supersonic jet expansion. The gaseous mixture of H2O (trace;17,18O-depleted H216O, 99.99% 16 O, ISOTEC) and Ar (5 MPa) was expanded into a vacuum chamber through a high-pressure pulsed valve (Even−Lavie valve).94 The gas pulse was crossed by the electron beam of 200 eV from an electron gun (Omegatron) in the collisional region of the jet. Cluster ions grew and were cooled following the collisions. We checked carrier gas dependences on (H2O)n+ yields. When we used Ne as a carrier gas, relative yields of (H2O)n+ to H+(H2O)n became smaller. In the case of He, only a much a smaller amount of (H2O)n+ was produced. Such carrier gas dependence was also in agreement with previous mass spectrometric studies.31 According to these results, we used only Ar as a carrier gas. Therefore, inert gas attachment technique is also limited to Ar-tagging, although variations of inert gases should provide more spectral information, as demonstrated in the previous studies on, for example, H+(H2O)n·M.11−13 The cluster ion of interest was selected by the first mass spectrometer (the mass resolution (Δm/z) was ∼1) and was irradiated by coherent IR light (Laser Vision OPO/OPA). IR spectra were measured by monitoring the photofragment intensity as functions of the IR wavelength. The second mass spectrometer was tuned to select the fragments of both the H2O-loss and the OH-loss channels (Δm/z ≈ 3) for spectroscopy of bare (H2O)n+. For the measurements for (H2O)n+·Ar, Ar-loss as well as (Ar + H2O)-loss and the (Ar + OH)-loss channels were independently measured and summed up. Additional details of fragment dependence are given in the following. The wavelength of the IR light was calibrated in vacuum wavenumbers (±2 cm−1) by recording spectra of ambient H2O and CO2 vapor. All of the presented IR spectra were normalized by the IR power. In the spectroscopy of (H2O)n+·Ar, we observed fragment dependence of the spectrum. In the case of small clusters (n = 3, 4), the Ar-loss channel was a unique path upon IR irradiation of the present wavelength range. In contrast, for n ≥ 5, lowfrequency (∼3000 cm−1) excitation led to the (Ar + H2O/OH) loss channel. We could not resolve the (Ar + H2O)-loss and (Ar + OH)-loss channels in fragment mass spectrometry due to the weak signal of the Ar-tagged species. In the intermediate frequency (2700−2900 cm−1), branching of the dissociation paths was observed. Figure S1 (in the Supporting Information) shows the fragment dependence of the measured spectrum of (H2O)5+·Ar. Because the IR spectrum should be the summation of all of the fragment channels, we took a weighted average of both channels. The resulting spectrum in Figure S1c in the Supporting Information was used as the IR spectrum in the following sections. Figure S2 in the Supporting Information shows similar spectra for the n = 6 system. We note that fragment dependence can be explained by simple energetics. Interaction energy of Ar is estimated to be ∼500 cm−1,11,12 and excitation energy of the IR

2. EXPERIMENTAL SECTION IR spectra of (H2O)n+ and (H2O)n+·Ar were measured with photodissiciation spectroscopy. Details of experimental setup have been described elsewhere.11,87 Therefore, only brief 930



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Figure 1. IR spectra of (a) (H2O)3−8+ and (b) (H2O)3−7+·Ar in the 2100−4000 cm−1 region. The red and blue bands are the H-bonded OH band of the H2O moiety toward the OH radical and the free OH radical stretch band, respectively.

detailed analyses. As shown in the previous study, the red and blue bands in Figure 1a,b are a H-bonded OH band of the H2O moiety toward the OH radical and the free OH stretch band of the radical moiety, respectively.87 The spectra of the bare and Ar-tagged clusters show similar spectral features in the n = 4−6 except for narrower bandwidths in the Ar-mediated spectra, whereas the spectra of n = 3 and 7 show remarkable differences. Table 1 collects frequencies and assignments of selected bands. On the basis of these experimental results, detailed cluster structures are discussed in the following.

light is used for the evaporation of Ar. If residual energy (IR photon energy − ∼500 cm−1) is higher than evaporation energy OH or H2O, OH/H2O is removed as well as Ar. The present observation indicated the OH or H2O evaporation energy is ∼2400 cm−1 when we assume that the Ar evaporation energy is ∼500 cm−1. Furthermore, because H2O evaporation energy in cationic clusters is generally higher than 3000 cm−1, the present observations might be attributed to the OH and Ar loss channel. For detailed analyses of IR spectra and cluster structures, we carried out density functional theory (DFT) calculations at the MPW1K/6-311++G(3df,2p) level.95 For the (H2O)2+ system, Lee and Kim have reported that this functional gives similar results to the high-level calculations at CCSD(T)/CBS.54 Furthermore, it has been pointed out that the standard B3LYP functional overestimates the stabilization energy in dimer cation core types.54 Initial geometries were systematically constructed by replacing one of the H2O molecules in the known H+(H2O)n structure with the OH radical, according to the previously reported structural trends of (H2O)n+. These geometries were fully optimized and harmonic frequency calculations were performed. Calculated frequencies were scaled by a factor of 0.9185. To simulate IR spectra, we used Lorentzian functions with 10 and 50 cm−1 full widths at halfmaximum for free OH and H-bonded OH stretch modes, respectively. All calculations were carried out using the Gaussian 09 program.96 Cluster structures were visualized by the Molekel 4.3 program.97

4. DISCUSSIONS 4.1. (H2O)3−5+. In the previous study, we have already reported systematic structural and spectral analyses for n = 3− 5.87 Therefore, only brief descriptions on the cluster structures are given. The Ar-mediated spectra, reported for the first time, are compared with the spectra of the bare clusters. Table 1 shows the essential results of the spectral analyses. Figure 2a shows the IR spectrum of (H2 O) 3 +. As demonstrated in the previous study, the band at ∼3530 cm−1 has been assigned to the free OH radical stretch band.87 Because of the existence of the OH radical, (H2O)3+ has been characterized as H+(H2O)2(OH) as in Figure 2a. Figure 2b shows the IR spectrum of (H2O)3+·Ar in the same spectral range. The observed features seem to be different from the spectrum of the bare cluster, although the band at ∼3530 cm−1 implies that the spectral carrier also has the H+(H2O)2(OH) moiety. The observed differences can be attributed to the different internal energy and perturbation of Ar. The former one accounts for the narrower bandwidths of all bands. To estimate the effect of Ar attachment, we constructed (H2O)3+·Ar structures. The inset in Figure 2b shows the most plausible structure of (H2O)3+·Ar obtained with the

3. RESULTS Figure 1a shows IR spectra of (H2O)3−8+. Figure 1b shows IR spectra of (H2O)3−7+·Ar. The former ones are the same as those previously reported87 and shown for comparison and 931



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Table 1. Frequencies (in cm−1) and Assignments of Selected Bands in IR Spectra of (H2O)3−8+.and (H2O)3−7+·Ara freq. (cm−1)

assignment

(H2O)3+

freq. (cm−1)

assignment

+

3536(4)

OH radical

3624(1) 3697(3)

one-coordinated (νsym)/H3O+ one-coordinated (νantisym)

(H2O)4+ 3103(3)

HB to ·OH

3543(1) 3639(1) 3725(1)

OH radical one-coordinated (νsym) one-coordinated (νantisym)

(H2O)5 3458(1)

HB to ·OH

3553(1) 3647(1) 3707(1) 3734(1) (H2O)6+ 3496(1)

OH radical one-coordinated (νsym) two-coordinated H2O one-coordinated (νantisym) HB to ·OH

3557(1) 3649(1) 3711(1) 3737(1)

OH radical one-coordinated (νsym) two-coordinated H2O one-coordinated (νantisym)

(H2O)3+·Ar 3533(4) 3553(2) 3630(1)

OH radical/H3O+ to Ar

(H2O)5+·Ar 3458(1)

HB to ·OH

one-coordinated (νsym)

3549(1)

OH radical

3715(1)

one-coordinated (νantisym)

one-coordinated (νsym) two-coordinated H2O one-coordinated (νantisym)

(H2O)4+·Ar 3094(2)

HB to ·OH

3542(1)

OH radical

3630(1) 3642(1) 3717(1) 3727(1)

one-coordinated (νsym)

3646(1) 3704(1) 3737(1) (H2O)6+·Ar 3487(2) 3499(2) 3543(1) 3556(1) 3650(1)


3711(1)

two-coordinated H2O

3737(1)


freq. (cm−1) (H2O)7+ 3520(10) 3612(1) 3618(1) 3543(3) 3651(1) 3714(1) 3739(1) (H2O)8+ 3497(1) 3543(2) 3576(1) 3652(1) 3684(1) 3715(1) 3739(1) (H2O)7+·Ar 3612(1) 3618(1) 3542(1) 3552(1) 3652(1) 3716(1) 3739(1)

assignment HB to ·OH

OH radical one-coordinated (νsym) two-coordinated H2O one-coordinated (νantisym) HB to ·OH

one-coordinated (νsym) three-coordinated H2O two-coordinated H2O one-coordinated (νantisym) HB to ·OH OH radical one-coordinated (νsym) two-coordinated H2O one-coordinated (νantisym)

HB to ·OH OH radical one-coordinated (νsym)

“HB to ·OH” means hydrogen-bonded OH stretch of water towards an OH radical. All other bands shown here are attributed to free OH stretches of assigned moieties. νsym and νantisym are the symmetric and antisymmetric stretches, respectively. Band frequencies were determined by fitting each band with a single Lorentzian function. Errors in the fitting are shown in parentheses. Absolute accuracy is ±2 cm−1 due to the bandwidth of our IR source (10: Structural Strains in Hydrogen Bond Networks of Neutral Water Clusters. J. Phys. Chem. A 2009, 113, 12134−12141. (10) Mizuse, K.; Mikami, N.; Fujii, A. Infrared Spectra and Hydrogen-Bonded Network Structures of Large Protonated Water Clusters H+(H2O)n (n = 20−200). Angew. Chem., Int. Ed. 2010, 49, 10119−10122. (11) Mizuse, K.; Fujii, A. Infrared Photodissociation Spectroscopy of H+(H2O)6·Mm (M = Ne, Ar, Kr, Xe, H2, N2, and CH4): MessengerDependent Balance between H3O+ and H5O2+ Core Isomers. Phys. Chem. Chem. Phys. 2011, 13, 7129−7135. (12) Mizuse, K.; Fujii, A. Tuning of the Internal Energy and Isomer Distribution in Small Protonated Water Clusters H+(H2O)4−8: An Application of the Inert Gas Messenger Technique. J. Phys. Chem. A 2012, 116, 4868−4877. (13) Mizuse, K.; Fujii, A. Infrared Spectroscopy of Large Protonated Water Clusters H+(H2O)20−50 Cooled by Inert Gas Attachment. Chem. Phys. 2013, DOI: http://dx.doi.org/10.1016/j.chemphys.2012.07.012. (14) Buch, V.; Bauerecker, S.; Devlin, J. P.; Buck, U.; Kazimirski, J. K. Solid Water Clusters in the Size Range of Tens-Thousands of H2O: A Combined Computational/Spectroscopic Outlook. Int. Rev. Phys. Chem. 2004, 23, 375−433. (15) Buck, U.; Huisken, F. Infrared Spectroscopy of Size-Selected Water and Methanol Clusters. Chem. Rev. 2000, 100, 3863−3890. (16) Coe, J. V.; Lee, G. H.; Eaton, J. G.; Arnold, S. T.; Sarkas, H. W.; Bowen, K. H.; Ludewigt, C.; Haberland, H.; Worsnop, D. R. Photoelectron Spectroscopy of Hydrated Electron Cluster Anions, (H2O)n=2‑‑69−. J. Chem. Phys. 1990, 92, 3980−3982. (17) Coe, J. V. Fundamental Properties of Bulk Water from Cluster Ion Data. Int. Rev. Phys. Chem. 2001, 20, 33−58. (18) Bragg, A. E.; Verlet, J. R. R.; Kammrath, A.; Cheshnovsky, O.; Neumark, D. M. Hydrated Electron Dynamics: From Clusters to Bulk. Science 2004, 306, 669−671. (19) Verlet, J. R. R.; Bragg, A. E.; Kammrath, A.; Cheshnovsky, O.; Neumark, D. M. Observation of Large Water-Cluster Anions with Surface-Bound Excess Electrons. Science 2005, 307, 93−96. (20) Bragg, A. E.; Verlet, J. R. R.; Kammrath, A.; Cheshnovsky, O.; Neumark, D. M. Electronic Relaxation Dynamics of Water Cluster Anions. J. Am. Chem. Soc. 2005, 127, 15283−15295. (21) Neumark, D. M. Spectroscopy and Dynamics of Excess Electrons in Clusters. Mol. Phys. 2008, 106, 2183−2197. (22) Ehrler, O. T.; Neumark, D. M. Dynamics of Electron Solvation in Molecular Clusters. Acc. Chem. Res. 2009, 42, 769−777. (23) Maeyama, T.; Tsumura, T.; Fujii, A.; Mikami, N. Photodetachment of Small Water Cluster Anions in the Near-Infrared through the Visible Region. Chem. Phys. Lett. 1997, 264, 292−296. (24) Ayotte, P.; Johnson, M. A. Electronic Absorption Spectra of Size-Selected Hydrated Electron Clusters: (H2O)n−, n = 6−50. J. Chem. Phys. 1997, 106, 811−814. (25) Hammer, N. I.; Shin, J. W.; Headrick, J. M.; Diken, E. G.; Roscioli, J. R.; Weddle, G. H.; Johnson, M. A. How Do Small Water Clusters Bind an Excess Electron? Science 2004, 306, 675−679. (26) Hammer, N. I.; Roscioli, J. R.; Bopp, J. C.; Headrick, J. M.; Johnson, M. A. Vibrational Predissociation Spectroscopy of the (H2O)6−21− Clusters in the OH Stretching Region: Evolution of the Excess Electron-Binding Signature into the Intermediate Cluster Size Regime. J. Chem. Phys. 2005, 123, 244311. (27) Asmis, K. R.; Santambrogio, G.; Zhou, J.; Garand, E.; Headrick, J.; Goebbert, D.; Johnson, M. A.; Neumark, D. M. Vibrational

spectroscopy with and without Ar-attachment was carried out. The observed IR spectra show the characteristic signatures of the ion-radical complexes H+(H2O)n−1(OH). Because an OH radical is a weaker H-bond-acceptor than a water molecule, the first solvation shell of the protonated site is preferentially filled with water. As a result, the OH radical is separated from the protonated site by one water molecule in the n ≥ 5 clusters. In larger clusters including up to the third solvation shell, we found that the OH radical tends to act as a H-bond donor in the second solvation shell. According to these findings, structures of (H2O)n+ in the size range of n ≤ 8 can be constructed from the structures of H+(H2O)n by substituting one of the next neighbor molecules from the protonated site with an OH radical. Reactivity of water exposed to ionizing radiation should depend on H-bond network structures around the created radicals. The present findings on the ionized water networks would lead to understanding of the detailed processes of ionizing radiation-initiated reactions in liquid water and aqueous solutions.

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ASSOCIATED CONTENT

S Supporting Information *

Fragment-dependent IR photodissociation spectra of (H2O)5,6+·Ar. Complete author lists of refs 2, 57, and 96. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Institute for Molecular Science, Nishi-Gonaka 38, Myodaiji, Okazaki 444−8585, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. J.-L. Kuo in Institute of Atomic and Molecular Sciences, Academia Sinica for stimulating discussions. This study was supported by the Grant-in-Aid for Scientific Research (project no. 19056001 from MEXT Japan, and nos. 23850018, and 2235001 from JSPS). Part of the calculations was performed at the Research Center for Computational Science, Okazaki (Japan).

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REFERENCES

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