Near-Infrared Spectroscopy and Anharmonic Theory of Protonated

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Spectroscopy and Photochemistry; General Theory

Near-Infrared Spectroscopy and Anharmonic Theory of Protonated Water Clusters: Higher Elevations in the Hydrogen Bonding Landscape David C. McDonald, J. Philipp Wagner, Anne B. McCoy, and Michael A. Duncan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02499 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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The Journal of Physical Chemistry Letters

J. Phys. Chem. Lett.

Near-Infrared Spectroscopy and Anharmonic Theory of Protonated Water Clusters: Higher Elevations in the Hydrogen Bonding Landscape D. C. McDonald II,1 J. P. Wagner,1 A. B. McCoy,2* M. A. Duncan1* 1

2

Department of Chemistry, University of Georgia, Athens, GA 30602, U.S.A. Department of Chemistry, University of Washington, Seattle, WA 98195, U.S.A.

Abstract Near-infrared spectroscopy measurements are presented for protonated water clusters, H+(H2O)n, in the size range of n = 1−8. Clusters are produced in a pulsed-discharge supersonic expansion, mass selected, and studied with infrared laser photodissociation spectroscopy in the regions of 3600−4550 and 4850−7350 cm-1. Although there is some variation with cluster size, the main features of these spectra are a broad absorption near 5300 cm-1, a sharp doublet near 7200 cm-1, as well as a structured absorption near 4100 cm-1 for n ≥ 2. The vibrational patterns measured for the hydronium, Zundel and Eigen ions are compared to those predicted by different forms of anharmonic theory. Second order vibrational perturbation theory (VPT2) and a local mode treatment of the OH stretches both capture key aspects of the spectra, but suffer understandable deficiencies in the quantitative description of band positions and intensities.

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ToC Graphic:

Small protonated water clusters, H+(H2O)n, have been studied extensively with new experiments and theory as models for molecular proton accommodation, proton transfer intermediates, hydrogen bonding networks, and solvation.1-42 Infrared spectroscopy of sizeselected clusters has become possible with improved methods for ion cooling and manipulation with mass spectrometers, while various computational approaches have been employed to identify isomeric structures and their spectra for each cluster size. In studies to date, vibrational spectroscopy has been employed primarily in the mid-IR, where the fundamentals of free O−H stretches, hydrogen-bonded O−H stretches, and shared-proton vibrations occur. In the present report, we extend these measurements into the near-IR region, where combinations and overtones occur. These spectra reveal new vibrational patterns and test the capabilities of anharmonic theory for these prototype systems. Since the earliest spectroscopic studies, it has been recognized that protonated water clusters can form competing isomeric structures differing in their charge site and/or hydrogen bonding connectivity.1-7 The combined efforts of experiments and theory have been able to identify the key spectral features for these structures. Hydronium ion forms the core of many of these clusters, but the shared-proton Zundel moiety also plays a role at certain cluster sizes.14,21,29 Special consideration has also been given to the Eigen ion, H+(H2O)4, which has a symmetrically 2 ACS Paragon Plus Environment

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solvated hydronium structure. The smallest ions have well-established structures, but co-existing isomers may be present as cluster size increases, and their distribution may vary with the temperature. Therefore, ion preparation and cooling are critical concerns.24,29 In some cases, the spectral patterns predicted by theory are adequate to identify isomers. In other cases, new experiments have employed double-resonance methods to disentangle these spectra.26,27,29-32,41 Even when structures are established, as in the small hydronium, Zundel, and Eigen ions, protonsharing and hydrogen bonding interactions result in strong mode couplings and anharmonicity, with unanticipated vibrational patterns not described well by harmonic theory. Recent work has employed high level anharmonic methods to attack these issues.23,33-42 The effects of anharmonicity should have a strong influence on the spectroscopy in higher quantum levels, but unfortunately there is little information available beyond the fundamental region of the mid-IR. The near-IR spectrum of neutral water is well known, and its overtone and combination band absorptions are documented.43-49 Similar information is available for the neutral water dimer.50-56 Overtones of water and its complexes with small molecules drive important atmospheric chemistry,57,58 and near-IR spectroscopy is employed to investigate planetary or interstellar water ices.59,60 The prominent features in the near-IR include stretch-bend combination bands near 5300 cm-1 and stretch-stretch combination/overtones near 7200 cm-1. We expect similar bands for protonated water clusters. In the only experiments to date on these systems, Chang and coworkers investigated the H+(H2O)n (n = 3−5) clusters in the 7200 cm-1 region using photodissociation by elimination of water molecules.8 Smaller ions could not be studied because their dissociation energies are higher than the photon energy in this region.61 Band intensities were estimated to be 1−2 orders of magnitude weaker than the O−H stretch fundamentals. Here, we extend these studies to more cluster sizes and an expanded spectral range using argon tagging for the smaller clusters. 3 ACS Paragon Plus Environment


Figure 1 shows the infrared photodissociation spectra obtained for the H+(H2O)n (n = 1−8) clusters in the frequency range of 3600−4550 and 4850−7350 cm-1; the spectrum for hydronium includes frequencies down to below 3000 cm-1. In the left frame, the spectra are shown for the n = 1−4 ions measured with argon tagging, whereas the right frame contains the spectra for the n = 4−8 ions measured via the elimination of water molecules. Consistent with known binding energies,61 the n < 3 clusters do not dissociate by eliminating water upon IR excitation in these regions. The n = 4 cluster is the only one for which we were able to obtain spectra both with and without tagging. Spectra in the fundamental region of the IR for these ions were reported previously by our lab and others; we present a portion of those spectra here for comparison to the intensities in the new spectra. As shown in Figure 1, each of these spectra have signals in the regions of 5200−5300 and 7200 cm-1. Additionally, bands not documented previously near 4000−4100 cm-1 are seen for many of these clusters. The intensities of the bands in the 5000−7500 cm-1 region are estimated to be a factor of 100−150 weaker than the O−H stretch bands in the fundamental region, based on the size of the signals detected and the laser pulse energies employed. It should be noted that the intensities detected here result from both the absorption probability and the yield of photofragments. If the dissociation yield varies with the excess energy in the cluster, bands at lower energies may have intensities less than their computed absorption strengths. This is conceivable because the experiment only detects fragmentation events that occur on the timescale of 1−2 microseconds.62 Related to this, the photodissociation signal levels diminished considerably for the larger H+(H2O)n clusters, even though their intensities in the mass spectrum were strong. This is particularly noticeable for the n = 8 cluster, whose spectrum is shown at the lower right in Figure 1. This may be the result of faster IVR in the larger clusters, resulting in a 4 ACS Paragon Plus Environment

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dilution of the deposited energy and a lower fragmentation yield on the timescale of the experiment. The fact that we and others have detected photodissociation spectra for much larger H+(H2O)n clusters in the lower energy region of fundamental vibrations suggests that at least some multiphoton absorption may have contributed to those spectra. Multiphoton absorption is unlikely here for the much lower absorption cross sections of combinations and overtones. The H3O+Ar complex has relatively sharp bands in its spectrum throughout the region studied, including additional bands at 6014 and 6262 cm-1 where there is little or no signal for the other cluster sizes. All other clusters have broader bands in the 5300 cm-1 region, with traces of a doublet structure here for some cluster sizes. Although additional smaller bands are observed for some of the clusters, the most prominent feature at higher frequency is a doublet near 7200 cm-1. This doublet has sharper lines with a spacing of about 40 cm-1. Based on the known spectroscopy for neutral water, the 5300 cm-1 region is associated with a combination between the O−H stretch fundamentals (ν1 symmetric and ν3 antisymmetric at 3657 and 3756 cm-1, respectively) and the intramolecular H−O−H bend (ν2 fundamental at 1595 cm-1).43-49 The 7200 cm-1 doublet is in the region where overtones of the symmetric and antisymmetric stretches (2ν1, 2ν3) and the symmetric/antisymmetric stretch combination (ν1 + ν3) are expected.43-49 Chang and coworkers measured similar doublet bands in the 7200 cm-1 region for the H+(H2O)3-5 clusters.8 From consideration of the known fundamental frequencies, it is clear that not all of the possible combination and overtone bands are present in these spectra, and that there are significant anharmonic effects on the frequencies. Some insight can be obtained from the near-IR spectra of the isolated water molecule.43-49 Although a local mode description is more appropriate for higher quanta overtones, the bands in the present study can be discussed with a normal mode notation. The positions and intensities of the stretch-bend combination and stretch overtone/combinations for water have been documented 5 ACS Paragon Plus Environment


and examined thoroughly with various anharmonic theory approaches.43-49 The stretch-bend ν1 + ν2 and ν2 + ν3 combinations occur at 5235 and 5331 cm-1, respectively, with the latter more intense (about 22×) than the former.47 The 2ν1, ν1 + ν3, and 2ν3 O−H stretch overtones and combination occur at 7202, 7250 and 7445 cm-1.47 The approximate intensity ratio of these is 16:150:1.47 On the basis of these band positions and intensities for the water molecule, Chang and coworkers assigned the doublet bands they detected for the protonated water clusters near 7200 cm-1 to the 2ν1 and ν1 + ν3 transitions seen for water itself, but without any theory to confirm these assignments.8 To further investigate these spectra, we employed computations using two anharmonic treatments for the vibrational frequencies and intensities (details given below and in the Supporting Information). Figures 2−4 show the spectra for the H+(H2O)n clusters for n = 1, 2 and 4 compared to the spectra predicted by anharmonic theory using the VPT2 and local mode methods. A similar spectrum for the n = 3 cluster is presented in the Supporting Information (Figure S13). These computations employ the hydronium, Zundel and Eigen structures that are well-known for these ions. Although the structure of the n = 4 ion has been questioned, recent anharmonic theory confirms that it has the symmetrically-solvated hydronium structure.39,40,42 We computed the near-IR vibrational pattern for the Zundel-based n = 4, and found no compelling evidence for this structure (see Figure S14). The experimental and predicted band positions and their assignments are given in Tables S1−S4 in the Supporting Information. The spectrum of the H3O+Ar ion (Figure 2) in the fundamental region includes an intense lower frequency band at 3135 cm-1 assigned to the O–HAr stretch, and bands at 3498 and 3562 cm-1 assigned to the symmetric and antisymmetric stretches of the two "free" hydrogens. The latter band has partially resolved K-type rotational structure (see rotational band simulation in Figure S2). Both the VPT2 and local mode methods reproduce these bands with reasonable 6 ACS Paragon Plus Environment

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accuracy. Because the local mode treatment focuses on the OH stretching vibrations, only the VPT2 calculations find additional bands that can explain the peaks observed at 3208, 3243, 3314, and 3769 cm-1. These weak features are assigned respectively to the overtone of the freeHOH bend (ߥb ), the overtone of the argon-bound OH bend (ߥOHAr-b ), the combination of the O−HAr stretch (ߥOHAr ) with the OH−Ar argon stretch (ߥAr ), and the combination of the s asymmetric free O−H stretch (ߥOH ) with the argon-bound ArH−O−H bend. In the region of the

stretch-bend combination, prominent bands are detected at 5105, 5142 and 5173 cm-1, with weaker features at 4845 and 4926 cm-1. The VPT2 calculations predict two bands here with intensities comparable to those of bands in the higher frequency region (6000−7000 cm-1). The two bands are assigned to the symmetric and antisymmetric free-OH stretches, each in combination with the HOH bend. The relative intensities of these bands predicted by VPT2 match reasonably well with those in the experiment. In the higher frequency region, an intense sharp band at 6014 cm-1 is reproduced well by both theory methods and assigned to the overtone of the O–HAr stretch. Likewise, both methods have a similar pattern at higher frequency, predicting weak combinations of the O–HAr stretch with each of the symmetric and asymmetric free O–H stretches, and more intense transitions at higher frequencies. Both methods predict a strong symmetric stretch overtone, consistent with the 6875 cm-1 band, and the symmetric + antisymmetric free O−H stretch combination, consistent with the 6901 cm-1 band. The experiment confirms this latter assignment. The 6901 cm-1 band has K-type rotational structure, which is only expected for a band with perpendicular band character like the symmetric + antisymmetric O−H stretch combination. An expanded view and simulation of this is presented in Figure S3. Neither theoretical method accounts for the reasonably intense doublet measured at 6238 and 6262 cm-1. However, on the basis of the computed fundamental frequencies, these 7 ACS Paragon Plus Environment


bands could be assigned to a 2:1 Fermi resonance involving one quantum in the free OH stretch and two quanta in the bend, along with contributions from argon stretching in combination with the O–HAr overtone. The present anharmonic theory methods do not include such transitions, explaining why these bands are missed. While not reporting frequencies and intensities for such three quanta transitions, the VPT2 calculation provides anharmonic constants, and these parameters can be used to approximate the frequencies of these transitions. The results of this analysis for possible Fermi resonances involving OH stretch overtones are provided in Tables S14−S15. In addition to the peaks near 6250 cm-1, a Fermi resonance built off of the Ar-bound OH stretch could explain the smaller features at 4845 and 4926 cm-1, which were also not captured in the anharmonic calculations. The spectrum for the H+(H2O)2Ar ion in Figure 3 has less structure than that for hydronium. There are four known bands in the fundamental O–H stretching region (3522, 3616, 3657, and 3696 cm-1),21 which arise from the symmetric and antisymmetric stretches on the two water molecules, with one having an attached argon. Both theory methods agree on the assignment of these intense fundamentals. In addition to this, there is a new band at 4157 cm-1, where no strong feature is predicted by either theory method. Based on the frequency, this could be a combination of one of the O−H stretch fundamentals with the in-plane wag of the outer water molecules in the 500−550 cm-1 range. This intensity arises from the transition moment for the OH stretches having a component that lies perpendicular to the plane of the outer water molecules, leading to excitation of hindered rotation of the water molecules in their molecular planes. The local mode method did not include bending modes and could not capture this, while the VPT2 calculation fails to capture the intensity of these transitions. This may reflect the large anharmonicity of the in-plane wags. In the near-IR, we detect a broad feature with a sharp peak near its center at 5267 cm-1. VPT2 predicts stretch-bend combinations here, with additional 8 ACS Paragon Plus Environment

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structure from other vibrations, perhaps consistent with the broad band. Because the local mode treatment includes only O−H stretches, it does not produce any signal here. The higher frequency region has an experimental pattern with four bands, consisting of combinations and overtones of the O−H stretches. Because of the argon tagging in a position on an OH group, the modes are distinguished as the O−HAr stretch and the free O−Hf stretch adjacent to this, whereas there are separate symmetric and asymmetric stretches on the opposite water molecule (νOHs,a). A red-shifted band at 6841 cm-1 is assigned to the overtone of the O–HAr stretch by both theory methods, and both methods find a weaker asymmetric stretch overtone at higher frequency (2νOHa), which could account for the 7215 cm-1 feature. However, the two methods disagree on the source of the two more intense bands at 7121 and 7160 cm-1. The VPT2 method suggests that there is overlapping intensity from the overtone of the symmetric O−H stretch (2νOHs), the symmetric plus asymmetric stretch combination (νOHs + νOHa) and the combination of the two O−H stretches on the argon-bound water (νOHf + νOHAr) to produce the lower frequency band, whereas the higher frequency band is from the overtone of the argon-side free O−H stretch (2νOHf). The local mode treatment finds intensity from the same three vibrations that overlap in the lower frequency VPT2 band, but predicts a triplet pattern, while finding insignificant intensity for the 2νOHf overtone. The appearance of the experimental spectrum here matches more closely with that predicted by VPT2, whereas the local mode spectrum has more bands spread out over a wider frequency range. The spectrum of the Zundel ion is remarkable in that the main structure can be accounted for by anharmonic approaches, even though neither method is able to capture contributions from the shared-proton motion. The local mode treatment does not include the shared proton motion, whereas the VPT2 methods seems to have serious trouble in handling it. In combinations 9 ACS Paragon Plus Environment


containing this mode, VPT2 predicts an unrealistic frequency interval for its component of about 200 cm-1, compared to the harmonic fundamental of 1250 cm-1. While this motion could be responsible for some of the broadening near 4157 and 5267 cm-1, there does not appear to be strong bands in the near-IR spectrum associated with the shared-proton motion, even though this vibration is by far the most intense of the fundamentals. Figure 4 shows the spectrum for the H+(H2O)4 complex, again compared to the predictions of the two forms of anharmonic theory. Similar data for the n = 3 complex are provided in the Supporting Information (Figure S13). The spectra for the n = 3 and 4 complexes vary in the fundamental region, but are essentially identical in the higher frequency region. The spectrum for H+(H2O)4 contains the known fundamentals of the O−H stretches at 3643 and 3731 cm-1 and broader bands at 3797 and 4095 cm-1 just above this. A broad band at 4095 cm-1, analogous to the 4157 cm-1 band seen for the Zundel ion, is associated with the free O−H stretches in combination with the in-plane wagging motion of the water molecule, but the predicted intensity is still much weaker than in the experiment. In the higher frequency region, there is a broad noisy band near 5300 cm-1, and a much sharper doublet near 7200 cm-1. The n = 3 and 4 clusters are the smallest to have hydrogen bonds, and therefore the hydrogen bonding OH−O vibrations, which have fundamentals in the 2700−3100 cm-1 range21 can also participate in combinations and overtones. The Eigen structure for n = 4 has a symmetric hydrogen bonding stretch of the hydronium core ion and a corresponding degenerate asymmetric stretch. These hydrogen bonding vibrations are predicted to participate in overtones and combinations by both theory methods. However, it has been shown in previous work that a local mode treatment has trouble with both the frequencies and intensities of hydrogen bonding vibrations when low frequency intermolecular modes are not included.55,56


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The 5300 cm-1 region is associated with the free O−H stretch plus HOH bend combination in the isolated water molecule, like in the smaller protonated species. Similar signal is predicted here for these combinations by both anharmonic theory methods for the n = 3 and 4 clusters. However, the greater intensity bands here are predicted by both methods to arise from the combinations and overtones of the hydrogen bonding stretch vibrations. On the other hand, the validity of this prediction is questionable, since both methods also predict free O−H plus hydrogen bonded OH−O stretch combinations near 6200 cm-1, where no signal is detected in H+(H2O)4 and at best a weak feature is detected in H+(H2O)3. It is well known that VPT2 calculations cannot adequately describe the observed width or shape of hydrogen bonding bands measured experimentally in the fundamental region, which is attributed to the dynamical effects of spectral diffusion.29 In the fundamental region, the band associated with the shared proton stretch is quite broad, and shows evidence of strong couplings between these vibrations and overtones and combination bands involving low-frequency modes. This kind of strong coupling has been attributed to proton transfer being driven by vibrational excitation.63 The same kind of strong coupling should be enhanced in the overtone region, perhaps leading to the additional broadening here. This would explain the predicted, but weak or missing, signal in the 6200 cm-1 range. It is also conceivable that the signal near 5300 cm-1 has less of the predicted hydrogen bonding character and more character from free-OH stretch-bend combinations, as seen in the isolated water molecule. Local mode calculations on the neutral water dimer including the low frequency intermolecular modes found very weak hydrogen bonding overtones.55,56 Both theory methods agree on the nature of the vibrations producing the sharp doublet at 7198/7239 cm-1. The intensity here is derived from the overtone of the symmetric O−H stretch and the symmetric-plus-asymmetric O−H stretch of the external water molecules. This free-OH



pattern has the same character seen for the isolated water molecule in this frequency region.43-49 Whereas the VPT2 method finds multiple bands here with close frequencies, the local mode method finds a clearer doublet matching the experiment quite nicely, except that the doublet spacing predicted (60 cm-1) is larger than that observed (41 cm-1). Apparently, in this larger cluster size, the local mode treatment produces a more accurate portrayal of the vibrations than the normal mode picture inherent to the VPT2 method. Neither method predicts the weak band at 6875 cm-1, which is likely a Fermi resonance involving one quantum of OH stretch and two quanta of excitation in the HOH bend. These bands are analyzed in Tables S16 and S17. The experimental patterns identified here for the smaller clusters continue in the larger species which have not yet been investigated with anharmonic theory. However, we expect that the essential vibrational character identified here will persist in the larger species. According to the discussion so far, 4000−4100 cm-1 bands seen for all clusters larger than hydronium likely arise from O−H stretch plus wag-type bend combinations. The 5300 and 7200 cm-1 signals are derived from stretch-HOH bend combinations and stretch-stretch overtones/combinations of the free-OH groups associated with the external water molecules. Internal vibrations such as the shared-proton vibration of the n = 2 Zundel ion or the hydrogen-bonding OH−O stretches of the Eigen ion are not predicted to have strong overtones or combinations. At least some vibrations predicted for the hydrogen bonding vibrations are not observed. The hydrogen bonding combinations like those predicted for the Eigen ion near 6200 cm-1 may begin to contribute signal in the larger n = 5 and 6 clusters, where some weak, broad signal is seen. Another possibility for this signal for the n = 6 cluster is activity in shared-proton stretch combinations, since a Zundel configuration is predicted to be the global minimum for this species. The n = 6 cluster is the only one with a widely spaced doublet in the 5300−5400 cm-1 region, which may also be related in some way to its Zundel structure. In any event, there are no strong vibrations 12 ACS Paragon Plus Environment

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predicted or observed from "internal" vibrations. It therefore seems that the vibrational spectra in the near-IR are dominated by the free-OH oscillators of the peripheral waters molecules in these clusters. It is surprising that the sharp doublet structure seen near 7200 cm-1 is essentially invariant with cluster size up to the largest species (n = 8) studied here. According to previous studies in our lab under similar conditions, the n = 7−8 clusters should have co-existing isomeric structures. Other groups have pointed out the presence of minor isomers for the n = 5 and 6 cluster.26,31 If there were co-existing isomers, we might expect to see additional multiplet members here or broadening of the doublet peaks. Instead, the 7200 cm-1 region contains only a clear sharp doublet up to the n = 7 cluster (the signal is too noisy to be sure for the n = 8 species). Other spectral features also seem to be described reasonably well by a single isomer at each cluster size. However, if other isomers are present, then this data implies that the overtone/combination bands here fall at essentially the same frequency and are indistinguishable for different isomers. If this is true, then near-IR spectra such as these will have limited value in distinguishing different isomeric structures. These near-IR spectra for small protonated water clusters provide the first opportunity to test the capabilities of anharmonic theory on the higher vibrational levels of these prototype hydrogen bonding systems. Because of their large amplitude motions involving the charge centers, and their hydrogen bond connections, these systems exhibit a complex mixture of electrical and mechanical anharmonicity affecting line positions and intensities. As shown here, even the smallest of these systems challenge the capability of anharmonic theory. Whereas both VPT2 and the local mode method capture many of the qualitative aspects of the spectroscopy, both have difficulties in predicting quantitative frequencies and intensities. Further development



of these or other methods appears to be required for the more refined treatment of near-IR spectra for these kinds of systems.

Experimental and Computational Details Protonated water ions were produced in a pulsed-nozzle/pulsed-discharge supersonic beam source as described previously.21 A representative mass spectrum is presented in the Supporting Information (Figure S1). These ions were mass selected in a reflectron time-of-flight spectrometer62 and studied with infrared laser photodissociation spectroscopy. Small ions were tagged with argon and the spectra recorded in the argon elimination mass channel, while the spectra of larger ions were studied in the water-elimination mass channel. The infrared and nearinfrared radiation was provided by a Nd:YAG-pumped OPO/OPA laser system (Laser Vision). In the near-IR, this laser provides 10−15 mJ/pulse of radiation. Its diverging beam was focused with a 20 cm lens to improve the overlap with the ion beam and thus optimize sensitivity. Computational studies were performed with two different approaches to handle the anharmonic effects necessary to describe overtone and combination bands. In the first, we employed second order vibrational perturbation theory (VPT2) as implemented in the Gaussian program package,64,65 at the MP2/aug-cc-pVTZ level. In the second method, a local mode approach was employed at the same level of theory. Here we followed the work of Sibert and co-workers66 and generated linear combinations of the normal coordinates that correspond to an orthogonal set of local OH stretches. With these coordinates defined, we generated quartic expansions of the potential along each coordinate along with the off-diagonal quadratic force constants and a quadratic expansion of the dipole surface. Second order perturbation theory was applied to this Hamiltonian, omitting the quadratic force constants. The anharmonic frequencies of the fundamentals, overtones and combination bands in the OH stretches were evaluated along 14 ACS Paragon Plus Environment

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with the transition moments to each of these excited states. Since the off-diagonal quadratic force constants were not included in the perturbation theory treatment, the OH oscillators are decoupled at this stage. These off-diagonal terms are then introduced into a variational calculation of the coupled energies and intensities. The resulting spectrum is identified as local mode in the main text. It should be noted that this treatment considers only the OH stretches and will not identify intensity in transitions that involve the HOH bends or any of the low frequency vibrations. A more detailed description of these calculations can be found elsewhere.67

Associated Content

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. This includes a mass spectrum, optimized geometries, unscaled harmonic frequencies, anharmonic frequencies, additional spectra, and full citations for references 47, 64 and 65.

Author Information Corresponding Authors *E-mail: [email protected]

ORCID J. Philipp Wagner: 0000-0002-1433-0292 Michael A. Duncan: 0000-0003-4836-106X Anne B. McCoy: 0000-0001-6851-6634



Acknowledgments We gratefully acknowledge support for this work by the National Science Foundation (MAD grant CHE-1464708; ABM grant CHE-1663636). JPW acknowledges the Alexander von Humboldt Foundation for a Feodor Lynen Postdoctoral Fellowship. Parts of this work were performed using the Ilahie cluster at the University of Washington, which was purchased using funds from a MRI grant from the National Science Foundation (CHE-1624430).

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Figure Captions

Figure 1. The near-IR spectra obtained for H+(H2O)nAr (left) and H+(H2O)n (right) clusters, using photodissociation and the elimination of argon (left) or water molecules (right), respectively.

Figure 2. The IR and near-IR photodissociation spectrum of H3O+Ar compared to the predictions of theory using the VPT2 (middle, blue) and local mode methods (lower, red).

Figure 3. The IR and near-IR photodissociation spectrum of H+(H2O)2Ar compared to the predictions of theory using the VPT2 (middle, blue) and local mode methods (lower, red).

Figure 4. The IR and near-IR photodissociation spectrum of H+(H2O)4Ar compared to the predictions of theory using the VPT2 (middle, blue) and local mode methods (lower, red).



Figure 1.


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Figure 2. 27 ACS Paragon Plus Environment


Figure 3. 28 ACS Paragon Plus Environment

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Figure 4.


Near-Infrared Spectroscopy and Anharmonic Theory of Protonated

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