Tuning of the Internal Energy and Isomer Distribution in Small

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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 Kenta Mizuse† and Asuka Fujii* Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan S Supporting Information *

ABSTRACT: Infrared spectroscopy of gas-phase hydrated clusters provides us much information on structures and dynamics of water networks. However, interpretation of spectra is often difficult because of high internal energy (vibrational temperature) of clusters and coexistence of many isomers. Here we report an approach to vary these factors by using the inert gas (so-called “messenger”)-mediated cooling technique. Protonated water clusters with a messenger (M), H+(H2O)4−8·M (M = Ne, Ar, (H2)2), are formed in a molecular beam and probed with infrared photodissociation spectroscopy in the OH stretch region. Observed spectra are compared with each other and with bare H+(H2O)n. They show clear messenger dependence in their bandwidths and relative band intensities, reflecting different internal energy and isomer distribution, respectively. It is shown that the internal energy follows the order H+(H2O)n ≫ H+(H2O)n·(H2)2 > H+(H2O)n·Ar > H+(H2O)n·Ne, while the isomer-selectivity, which changes the isomer distribution in the bare system, follows the order H+(H2O)n·Ar > H+(H2O)n·(H2)2 > H+(H2O)n·Ne ∼ (H+(H2O)n). Although the origin of the isomer-selectivity is unclear, comparison among spectra measured with different messengers is very powerful in spectral analyses and makes it possible to easily assign spectral features of each isomer.

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difficulty of complexity in spectral interpretation.10 “Molecularlevel” understanding is, then, not necessarily achieved. One of the major sources of the complexity is the finite internal energy (vibrational temperature). Because typical ionic clusters formed in the gas phase have the internal energy corresponding to ∼100 K or higher temperature,10,19,21,27,28 low-frequency modes of the cluster are excited, affecting the spectral structure, that is, the bandwidths and hot band structures. Furthermore, the excitation of low-frequency modes, that is, large-amplitude motions, sometimes stimulates large anharmonic couplings, which would cause disappearance of a vibrational band.25,29 Another source of the complexity is coexistence of structural isomers, which makes it difficult to identify cluster structures on the basis of spectral patterns even when exactly size-selective spectra were measured.10 Such complexity is seen even in small clusters, in which the number of isomers is not so large.10 Conversely, these results suggest if the internal energy and isomer distribution are variable, more information on the cluster structure and H-bonded network should be available. Recently, IR-IR hole-burning spectroscopy has been introduced as a powerful tool to obtain isomer-specific spectra.30−32 However, tuning of internal energy and isomer distribution

INTRODUCTION Hydrated clusters in the gas phase, X·(H2O)n (X = solute), are the microscopic model for hydration, and studies of them provide insight for the hydrogen-bonded (H-bonded) water networks at the molecular level.1 Because H-bonding environments are well reflected in vibrational spectra, infrared (IR) spectroscopy has been a powerful tool to probe cluster structures and dynamics.1,2 For hydrated ion systems, IR photodissociation spectroscopy combined with a multistep mass spectrometer has been extensively used to measure sizeselective IR spectra since the pioneering studies by Lee and coworkers.3−6 Among various hydrated clusters investigated with this technique, protonated water clusters H+(H2O)n are one of the most extensively studied systems because of the fundamental importance of the hydrated proton.3,6−26 The broad applicability of this spectroscopy allows the exploration of the clusters in the broad cluster size (n) range from 1 to more than 100.3,6−26 In these studies, microhydration structures of H3O+ (“Eigen”) and H5O2+ (“Zundel”) type ion cores have been characterized at least for the clusters of n ≤ ∼10.3,6,7,9,10,17,22,24 In the same size range, chain-like, treelike, and net-like structures have been identified.10,18 Larger clusters (n > ∼20) show the signature of three-dimensional closed cage water networks, and they approach the bulk picture with increasing n.13,14,19,21,23,26 Despite these successive investigations of the structural trends, IR spectroscopy of hydrated clusters often faces the © 2012 American Chemical Society

Received: March 1, 2012 Revised: April 16, 2012 Published: May 4, 2012 4868

dx.doi.org/10.1021/jp302030d | J. Phys. Chem. A 2012, 116, 4868−4877

The Journal of Physical Chemistry A

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protonated water clusters, which have been one of the most extensively studied clusters with IR spectroscopy. In the previous studies on small protonated water clusters with messengers, H2, Ne, and Ar have been mainly used. Okumura et al. have reported pioneering messenger spectroscopy for H+(H2O)n·(H2)m (n ≤ 4) and H+(H2O)n·Ne (n ≤ 3) and have shown both the cooling effect and some perturbations of messengers.3,6,7 Headrick et al. have reported IR spectra of H+(H2O)n·Ar (n = 2−11) to cover the region of 1000− 3800 cm−1.17 Douberly et al. have carried out a systematic spectroscopic study of H+(H2O)2−5 and D+(H2O)2−5 with and without Ar attachment.22 More recently, we have reported IR spectroscopy of H+(H2O)6·Mm (M = Ne, Ar, Kr, Xe, H2, N2, and CH4) and have shown the messenger-dependent isomer selectivity; however, the aspect of the internal energy control has not necessarily been clear.24 Furthermore, only the balance between the chain type, simplest H-bonded network motif, isomers has been probed. As the next stage, isomer distribution including higher-order networks such as ring motifs should be probed to study much complicated network systems. In this study, to fill the gaps in messenger experiments of H+(H2O)n and to extract more information from experimental spectra, we report IR photodissociation spectroscopy of H+(H2O)4−8·M (M = Ne, Ar, (H2)2). We show the present approach is quite powerful to obtain internal energy- and isomer distribution-dependent spectra of hydrated clusters and to analyze spectra in more detail.

should be of much help also in hole-burning and related spectroscopy. For the control of the internal energy and isomer distribu tion of ionic hydrated clusters, several approaches have been reported. The most direct and active approach would be the use of temperature-variable ion trap because temperature is the controlling factor of both the internal energy and the isomer distribution under the thermal equilibrium.12,33−36 For example, Wang et al. have reported IR spectroscopy of H+(H2O)6 at the temperatures of 77 K to ∼200 K.12 The observed spectra show decrease of the bandwidth with lowering temperature, validating the cooling effect. On the other hand, no drastic change of the isomer distribution was observed in this temperature range. Further cooling would simplify the spectrum by suppressing hot bands and dynamic effect and it might lead to the isomer distribution change. In low temperature ( H+(H2O)5·Ar > H+(H2O)5·Ne, as indicated in their bandwidths. This order implies that the interaction between H2 and H+(H2O)5 is stronger and the magnitude of the perturbation is larger. Figures 2f−i show the calculated interaction (binding) energy and maximum temperature with each optimized lowest-energy structure. In the present calculation, we found that the most favorable position of H2 is in front of a free OH bond while those of Ar and Ne are under the H3O+ umbrella. It might be possible that the tiny splitting of the bands in H+(H2O)5·(H2)2 is a signature of such positions of the messenger. An alternative interpretation is that

IR spectra of bare, H2-tagged, Ar-tagged, and Ne-tagged H+(H2O)6, respectively. Table 1 summarizes frequencies. Bandwidths follow the order: H+(H2O)6 > H+(H2O)6·(H2)2 > H+(H2O)6·Ar ∼ H+(H2O)6·Ne, reflecting the internal energy as in the case of H+(H2O)4,5. On the basis of the simulated spectra (Figures 3e and 3f) and previously reported assignments (Table 1),10,24 the bands in blue at around 2980, 3320, and 3710 cm−1 can be regarded as markers of the “Eigen” type isomer (Figure 3h). On the other hand, the somewhat broad band in red at around 3160 cm−1 is the signature of the “Zundel” type (Figure 3g). We note that expected bands in the 4872



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2400−2600 cm−1 region of the “Eigen” type were not observed. This can be due to the large anharmonicity of those H3O+ bands, as in the case of H+(H2O)5. According to the spectral assignments (see also Table 1), the spectra of the bare, H2tagged, and Ne-tagged clusters are attributed to the coexistence of the “Eigen” and “Zundel” isomers while the Ar-mediated spectrum is accounted for only by the “Zundel” type. These results are (at least partial) achievement of control of the isomer-distribution. In addition to the previously reported discussion for the messenger-dependent isomer-selectivity, we also emphasize that the present control method enables us to deduce isomer specific spectral features. A typical example is found in the 2800−3400 cm−1 region. In the bare H+(H2O)6 spectrum, the red band is not clear while the spectra of H2 and Ne-tagged clusters shows three clear bands by virtue of the cooling effect. Because of the single band feature of the Ar-mediated spectrum and agreement of the band position, the central band in H2- and Ne- mediated spectra can be immediately assigned to the same isomer as that found in H+(H2O)6·Ar (even without calculations). Such analysis procedure is similar to that in hole-burning spectroscopy, in which isomer-specific bands can be deduced. We will show this approach is useful to deal with more complicated spectra in the next section. Here, we also shortly comment on the origin of the isomerselectivity. If the thermal equilibrium is really achieved at low temperature, only an energetically favored isomer should be observed. In the “coldest” system, H+(H2O)6·Ne, however, two isomers coexist. This means that there is no clear correlation between the internal energy and isomer distribution. That is, the isomer distribution does not necessarily follow the Boltzmann distribution expected by the vibrational temperature. Recently, theoretical studies have suggested non-negligible effects of messengers themselves in energetics of some systems.53,58,79−81 These results suggest that the inert gas attachment would affect potential energy surfaces (relative energies of isomers and barrier heights of interconversion). For example, the Ar-tagging of the V3O6−8 clusters causes clear reordering of the relative energies of isomers.80 As for the energetics consideration, we carried out DFT calculates for H+(H2O)6·M as in the case of H+(H2O)4,5. The most favorable positions of the messengers are similar to those in H+(H2O)4,5·M: H2 favors front of an OH bond of H2O; Ar/Ne lies near the H3O+ or shared proton (see Supporting Information, Figure S1). However, we did not find a remarkable messenger dependence in the relative energies. Another plausible origin is the carrier gas dependence of the jetcooling processes. It has been shown that more efficient rotational-cooling in a supersonic jet expansion is achieved by using a heavier carrier gas.82 If this effect occurs also for the temperature of the isomer distribution, this is in agreement with the fact that Ar (heavier atom)-tagged cluster achieves the global minimum formation. However, for bare clusters, both isomers are formed even when Ar is used as a carrier gas. There are other controlling factors, that is, restricted internal energy and potential barrier in the isomerization. Because the inert gas attachment limits the internal energy of the system, interconversions between isomers can be sometimes restricted. In this context, if the system once goes into a local minimum and its energy is reduced below isomerization barriers, the system is trapped in this minimum. According to these discussions, we think the observed isomer-selectivity can be accounted for by the balance between the cooling to the global minimum and the trapping effect at the local minimum. If the

cooling effect is superior, the system gets to the global minimum. On the other hand, if the trapping effect comes first, some clusters are trapped at the local minima, and then the internal (vibrational) energy is reduced. The former case agrees well with Ar-tagged case while the latter agrees with the H2- and Ne-tagged cases. For bare clusters, the situation is explainable if both the cooling and the trapping effects are too small and the internal energy exceeds the isomerization barrier. We note that the present discussion provides one of the plausible explanations. Unfortunately, it is difficult to treat the interplay between the messenger-mediated energetics and the cooling dynamics at the present stage. For the deeper understanding for the origin of the messenger effect, systematic experimental and theoretical investigations for various systems would be a valuable help. Though the physical origin is still unclear, the present messenger experiments imply the empirical trend of the isomerselectivity for jet-cooled clusters, that is, Ar reduces isomers observed while H2 and Ne are less affective to the isomerdistribution in the bare system. We should note that the cooling process in cryogenic traps would be different from that in a supersonic jet and the isomer-selectivity by the messenger might be different. H+(H2O)7. Figures 4a−d show IR spectra of bare, H2-tagged, Ar-tagged, and Ne-tagged H+(H2O)7, respectively. H+(H2O)7 is also a system in which mainly two isomers coexist, as demonstrated by Jiang et al.10 Figures 4g and 4h show structures of the major isomers and Figures 4e and 4f are simulated spectra of these isomers, respectively. Table 1 collects frequencies and assignments of selected bands. This system is also known as the smallest size in which the ring network arises (Figure 4g).10 We can probe the balance between chain and ring network motifs. The clear signature of the “Ring” isomer is the bands at around 3550 cm−1 (Figure 4e).10 In the case of bare H+(H2O)7, these marker bands are quite weak, indicating the main part of the spectrum can be accounted for by the “Chain” type isomer (Figure 4a). Other isomers can also populate as indicated in the band at ∼3680 cm−1 because this band is assigned to 3-coord H2O and both of the two major isomers do not include such a H2O molecule.10 However, their contribution should be small, and we focus the discussion only on the “Ring” isomer (with the Zundel core) and “Chain” isomer (Eigen core). As shown in the previous Ar-tagging experiment by Headrick et al., the features of the “Ring” isomer become dominant also in the present spectrum of H+(H2O)7·Ar and their bandwidth becomes narrower than the bare cluster.17 These results show the isomer-selectivity and lowering of the internal energy upon the Ar-tagging. On the other hand, the H+(H2O)7·Ne spectrum shows similar spectral signatures to those of the bare cluster except for the narrower bandwidths. As for the isomer distribution, the spectrum of H+(H2O)7·(H2)2 seems to be an intermediate case between the bare/Ne-tagged and Ar-tagged clusters because the marker bands of the “Ring” isomer are quite weaker than in the Ar-tagged cluster but stronger than in the bare/Ne-tagged clusters. In this system we achieved broader and finer tuning of the isomer distribution in hydrated clusters than in the case of H+(H2O)6. These results also suggest that the surprising isomer-selectivity of Ar is the strongest and H2 comes second. Ne is the weakest perturber to the isomer-distribution of the bare clusters. These results are quite similar to those in the H+(H2O)6 system. Here we comment on the entropic effect, which would be important in the balance between the “Ring” and “Chain” 4873



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For spectroscopic purposes, as in the case of H+(H2O)6, we found that the use of various messengers is a quite powerful tool to assign isomer (H-bonded network)-specific spectral features. Figure 5 shows the expanded spectra of Figure 4. We

Figure 5. (a−d) Expanded IR spectra of bare, H2-, Ar-, and Ne-tagged H+(H2O)7, respectively. The bands in red show those of the “Ring” isomer. The bands in blue show those of the “Chain” isomer. Dashed lines are guides for the eyes.

assigned the red and blue bands in Figures 4 and 5 to those of the “Ring” and “Chain” isomers, respectively. These spectral assignments can be done according to the following simple analyses. In the H+(H2O)7·Ne spectrum, the 3550 cm−1 band is weaker, indicating the main component is the “Chain” isomer. The main band in this spectrum (3350 cm−1) is, then, assigned to that of the “Chain” isomer (blue). In the H2- and Armediated spectra, the bands at essentially the same frequency are also attributed to the “Chain” isomer (blue bands). In the Ar-mediated spectrum, the bands which are weak in the Nemediated spectrum get the larger intensities. These bands can be assigned to the “Ring” isomer (red bands). This is because they are accompanied with the emergence of the 3550 cm−1 bands, which are the marker of the “Ring” isomer. Following a similar consideration, the bands in the H2-mediated spectrum can be categorized as shown in Figures 4b and 5b. Such experimental efforts make it easier to compare the experimental and simulated spectra and assign each band. In the 3300− 3400 cm−1 region, several bands of both the isomers are seen, and the previously reported assignments were not isomerresolved.10 By virtue of the isomer distribution control, each band can be assigned as follows. The bands at ∼3180, ∼3310, and ∼3330 cm−1 are attributed to the H-bonded OH stretches of the H5O2+ moiety of the “Ring”; ∼3350 cm−1 band is due to the H-bonded OH stretch of the 2-coord H2O moiety in the “Chain”. We emphasize that these spectroscopic assignments are very difficult without the isomer separation by using multiple messengers even if simulated spectra are available. H+(H2O)8. Figures 6a−c show IR spectra of bare, H2-, and Ar-tagged H+(H2O)8, respectively. Table 1 collects frequencies and assignments of selected bands. Unfortunately, significant spontaneous dissociation of H+(H2O)8·Ne made the measurement of its spectrum difficult. In the previous studies of H+(H2O)8, it has been suggested that the branched ring

Figure 4. (a−d) IR spectra of bare, H2-, Ar-, and Ne-tagged H+(H2O)7, respectively. The bands in red show those of the “Ring” isomer. The bands in blue show those of the “Chain” isomer. (e,f) DFT-simulated IR spectra of the structures (g,h) obtained with the B3LYP/6-31+G(d) level, respectively. (g,h) Optimized cluster structures of H+(H2O)7. For the calculations including the messenger, see Supporting Information,Figure S2 .

isomers. The “Ring” isomer has more (seven) H-bonds than “Chain” (six) and therefore it is enthalpically favored while its relatively rigid ring framework lowers entropy relative to the flexible chain.10,78,83 These considerations suggest that the “Ring” isomer is dominant at lower temperature while the “Chain” becomes preferred at higher temperature. Recent theoretical study indicates that the transition temperature of this switch is ∼120 K.78 Such an entropic effect once succeeded in the explanation of the difference between the previously reported spectra of H+(H2O)7 and its Ar-tagged cluster, because the Ar-tagged cluster is “colder” than the bare cluster and then the “Ring” isomer is expected to be dominant.17,78 However, the present findings on the Ne- and (H2)2-tagged clusters conflict with such a simple interpretation by the entropic effect on the isomer distribution. In the “coldest” system, H+(H2O)7·Ne, the signature of “Ring” is very weak, meaning vibrational cooling upon the inert gas tagging is not necessarily accompanied by dominant population of enthalpically favored isomers. This conclusion is essentially the same as in H+(H2O)6 although the entropic effect in this system is expected to be important. 4874



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of each messenger. The magnitude of the perturbation to the isomer distribution in the bare system follows the order H+(H2O) n·Ar ≫ H+(H 2O)n·(H 2)2 > H +(H2O) n·Ne ∼ (H+(H2O)n). While the origin of the isomer-selectivity is not necessarily clear, the present study, at least partly, achieves the control of both the internal energy and isomer distribution. It is hoped that the present approach opens the way for detailed analyses of IR spectra and H-bonded network structures of larger and more complex systems.

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ASSOCIATED CONTENT

S Supporting Information *

The calculated most favorable positions of the messengers in the H+(H2O)6−8 systems. Complete references 56 and 73. 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, Myodaiji, Okazaki 444-8585, Japan.

Figure 6. (a−d) IR spectra of bare, H2-, and Ar-tagged H+(H2O)8, respectively. The red band is a marker band of the structure in (e) (3-coordinated H2O). (d) DFT-simulated IR spectrum of the structure in (e) obtained with the B3LYP/6-31+G(d) level. Dashed lines show a plausible correspondence between the experiment and theory. (e) Optimized cluster structure of H+(H2O)8. The OH bond of 3-coordinated H2O is highlighted in red. For the calculations including the messenger, see Supporting Information, Figure S3.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.M. thanks Dr. M. Hayashi and Prof. Y. Ohshima of the Institute for Molecular Science for the valuable discussion on various H2-tagged clusters. 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). K.M. was supported by JSPS Research Fellowships for Young Scientists.

structure (Figure 6e) reproduces well the experimental spectrum of the bare cluster (Figure 6d).10,84 This structure is evidenced with the band of the 3-coordinated water molecule at around 3680 cm−1 as highlighted in red in Figure 6. Coexistence of many isomers has been also implied, though further discussion has been difficult because of a large number of possible isomers and limited information from the broadened spectrum.10,84,85 In Figure 6, the Ar-mediated spectrum is reproduced well by the ring isomer as reported previously. Interestingly, the H2- mediated spectrum shows a very similar trend to that of the Ar-tagged cluster even though Ar and H2 show the different isomer-selectivities in H+(H2O)6,7. These results imply the isomer in Figure 6e is dominant in the H+(H2O)8 system and then it is always dominant even when the cluster is tagged by Ar/H2. An alternative interpretation is that the effect of the Ar and H2 attachment becomes similar to each other in this size. Although the messenger technique successfully eliminates the broadening of the spectra, further discussion needs more experimental efforts such as IR-IR hole-burning spectroscopy, which enables us to measure isomer specific IR spectra using isomer-specific IR transitions.30

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REFERENCES

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CONCLUSION We probed the messenger effect on both the internal (vibrational) energy and isomer distribution in the IR spectroscopy of H+(H2O)4−8·M (M = Ne, Ar, (H2)2). All messengers can lower the internal energy, which generally causes for example, spectral broadening. It was shown that the internal energy follows the order: H+(H2O)n ≫ H+(H2O)n·(H2)2 > H+(H2O)n·Ar > H+(H2O)n·Ne, reflecting the interaction energy 4875



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overestimated since the peak positions of the bands of the bare cluster should be affected by hot bands. In Table 1, it is seen that the effective shifts by the tagging are actually smaller in larger clusters (for example, a few cm−1 for the free OH stretches). Suppression of hot bands in bare clusters (because of the lowering the dissociation energy) would be a reason though lowering the interaction strength is an alternative interpretation. (78) Luo, Y.; Maeda, S.; Ohno, K. J. Comput. Chem. 2009, 30, 952− 961. (79) Morita, M.; Takahashi, K. Phys. Chem. Chem. Phys. 2012, 14, 2797−2808. (80) Asmis, K.; Wende, T.; Brummer, M.; Gause, O.; Santambrogio, G.; Stanca-Kaposta, E. C.; Dobler, J.; Niedziela, A.; Sauer, J. Phys. Chem. Chem. Phys. 2012, DOI: 10.1039/c2cp40245a. (81) Kuo, J.-L. J. Phys, Conf. Ser. 2006, 28, 87−90. (82) Amirav, A.; Even, U.; Jortner, J. Chem. Phys. 1980, 51, 31−42. (83) Shin, I.; Park, M.; Min, S. K.; Lee, E. C.; Suh, S. B.; Kim, K. S. J. Chem. Phys. 2006, 125, 234305. (84) Karthikeyan, S.; Park, M.; Shin, I.; Kim, K. S. J. Phys. Chem. A 2008, 112, 10120−10124. (85) Luo, Y.; Maeda, S.; Ohno, K. J. Phys. Chem. A 2007, 111, 10732−10737.

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Tuning of the Internal Energy and Isomer Distribution in Small

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