Infrared Spectroscopy of Small Protonated Water Clusters, H+(H2O

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Infrared Spectroscopy of Small Protonated Water Clusters, H+(H2O)n (n ) 2-5): Isomers, Argon Tagging, and Deuteration G. E. Douberly,† R. S. Walters,† J. Cui,‡ K. D. Jordan,‡ and M. A. Duncan*,† Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602-2556, and Department of Chemistry, UniVersity of Pittsburgh, 219 Parkman AVenue, Pittsburgh, PennsylVania 15260 ReceiVed: January 26, 2010; ReVised Manuscript ReceiVed: March 1, 2010

Infrared photodissociation spectroscopy is reported for mass-selected H+(H2O)n complexes and their deuterated analogues with and without argon “tagging.” H+(H2O)nArm and D+(D2O)nArm complexes are studied in the O-H (O-D) stretching region for clusters in the small size range (n ) 2-5). Upon infrared excitation, these clusters fragment by the loss of either argon atoms or one or more intact water molecules. Their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water and in the hydrogen bonding region. Experimental studies are complemented by computational work that explores the isomeric structures, their energetics and vibrational spectra. The addition of an argon atom is essential to obtain photodissociation for the n ) 2-3 complexes, and specific inclusion of the argon in calculations is necessary to reproduce the measured spectra. For n ) 3-5, spectra are obtained both with and without argon. The added argon atom allows selection of a subset of colder clusters and it increases the photodissociation yield. Although most of these clusters have more than one possible isomeric structure, the spectra measured correspond to a single isomer that is computed to be the most stable. Deuteration in these small cluster sizes leads to expected lowering of frequencies, but the spectra indicate the presence of the same single moststable isomer for each cluster size. 1. Introduction Proton transfer (PT) is ubiquitous in chemistry and biology, occurring in acid-base reactions,1,2 electrochemistry,1,2 biological processes such as photosynthesis,2,3 the operation of hydrogen fuels cells,2,4 and atmospheric5,6 and interstellar chemistry.7,8 The mechanism of proton transfer has been investigated extensively for many years.9-22 In liquid water, for example, the Grotthuss mechanism has been proposed to explain the unexpected high mobility of protons.9 Intermediates in PT processes are believed to have protons bonded to or shared between donor and acceptor structures, although recent experiments have found evidence for solvent-mediated processes with protons transferred through water.16-19 Therefore the accommodation of protons on small molecular systems has been an active area of investigation. Small clusters of protonated water are important model systems representing many of the complex issues in proton binding to small molecules, the structures they induce, and the dynamics of how they move from place to place. We describe here a new infrared spectroscopy study of these systems and their deuterated analogues in the O-H (O-D) stretching region using mass-selected photodissociation and the method of rare gas tagging. Protonated water clusters were observed in some of the earliest mass spectrometry experiments, and they have been studied extensively since the early 1970s.23-36 Indeed, the protonated water dimer is a standard reagent for chemical ionization.37 The isomeric structures of protonated water clusters have been investigated with a wide variety of theoretical methods.38-59 Because of the similarity of the energetics for different hydrogen bonding configurations, isomers having similar energies may coexist, depending on * Corresponding author. E-mail: [email protected]. † University of Georgia. ‡ University of Pittsburgh.

experimental conditions. Hydronium-based networks based on the “Eigen” motif, which has H3O+ symmetrically solvated forming the H3O+(H2O)3 core ion, have long been recognized. However, the unusual mobility of protons in solution gave rise to other structural concepts, including the shared proton H2O · · · H+ · · · OH2 “Zundel” structure. Discussions of proton transfer mechanisms and/ or the structures of protonated water now usually include both of these configurations. The first spectroscopic studies of protonated water clusters began with the pioneering high-resolution infrared absorption work of Saykally and co-workers60 and that of Oka and coworkers61 on hydronium produced in electrical discharges. Similar high-resolution absorption measurements have been described in pulsed-discharge molecular beams.62 The first IR measurements on protonated water clusters (mixed sizes) were reported by Schwarz.63 Using cluster ion beams and infrared photodissociation in the O-H stretching region, Lee and coworkers reported the first mass-selected spectroscopy studies of small protonated water clusters, H+(H2O)n (n ) 2-8).64 These investigators also employed ab initio calculations of the structures and spectra of different isomeric species to identify the important low-energy configurations for the small cluster sizes. Evidence was found throughout these initial studies for the presence of more than one isomer at many cluster sizes. Using the new technology of the FELIX free-electron laser (FEL), Asmis and co-workers first used multiphoton photodissociation to investigate the protonated water dimer in the lowfrequency region where proton bending and stretching modes occur.65 These studies, which were soon repeated at the CLIO FEL,66 established that the proton-bound dimer has the symmetric Zundel structure rather than that of hydronium solvated by a water molecule. Our groups, in collaboration with Johnson and co-workers, applied the expanded tuning range and increased brightness of pulsed optical parametric oscillator (OPO)

10.1021/jp100778s  2010 American Chemical Society Published on Web 03/16/2010

IR of Small Protonated Water Clusters lasers to the study of protonated water systems, also using photodissociation.67 In a joint experimental/theoretical study, we explored the clathrate cage structures that may be found in the larger clusters and reported the spectrum of the H+(H2O)21 magic number species.67 Similar work done independently by Mikami and co-workers was reported at the same time.68 In a second study, we used OPO lasers operating in the lower frequency range to explore the proton stretching and bending modes for a number of cluster sizes.69 This work was able to identify both the Eigen and Zundel structural motifs for cluster sizes in the n ) 2-10 size range. As part of that work, we reported the O-H stretch vibrations measured with argon tagging that we investigate in more detail here. Since those studies, the group of Johnson has provided several in-depth investigations of the spectrum of the protonated water dimer measured with both argon and neon tagging,70 and several theory groups have attempted to elucidate the complex patterns in the proton stretching and bending vibrations at low frequency.53,56,58 Other groups have measured spectra for much larger protonated water clusters beyond the initial clathrate cage structure at n ) 21.71 In the present study, we examine the O-H stretching and hydrogen bonding regions in detail for the smaller clusters in the n ) 2-5 size range. We measure spectra both with and without argon tagging, and extend the study to perdeuterated species. Computational studies provide predicted spectra for several low-lying isomeric structures for comparison to the experimental studies. 2. Experimental Section Protonated water clusters, H+(H2O)n, are produced by a focused laser (Nd:YAG; 355 nm) initiated spark at the surface of a metal rod in a pulsed-nozzle supersonic expansion of helium seeded with water vapor at ambient temperature. The source employed is usually used for metal cluster generation;72 conditions of laser timing, laser intensity, and backing gas pressure are varied to make protonated water clusters rather than metal cation-water clusters. Independent experiments were also conducted in our lab on selected cluster sizes using a pulsed electrical discharge source. Spectra obtained by this method were essentially the same as those produced with the laser spark. Cluster ions are size-selected in a reflectron time-of-flight mass spectrometer72 and excited with a pulsed Nd:YAG-pumped infrared OPO laser system (LaserVision). Fragment ions are separated from selected parents in this instrument, and their yield is recorded with a digital oscilloscope (LeCroy model LT342) as a function of the wavelength to obtain a spectrum. IR wavelengths for the OPO system are calibrated by measuring the photoacoustic spectrum of methane, which has well-known rotationally resolved bands in the 2800-3200 cm-1 region. Mixed complexes with added argon are employed to enhance the photodissociation yields for these systems via the method of “messenger atom addition,” also known as “tagging”.72-75 These are produced using similar conditions in an argon expansion. Computational studies of these clusters were carried out at the MP2(fc)/aug-cc-pVDZ level of theory using the Gaussian 03W program package.76 Computed frequencies were scaled by a factor of 0.96 and plotted with a Lorentzian function of 10 cm-1 FWHM for comparison to the experimental spectra. Complete details of the computational work are provided in the Supporting Information file for this article. 3. Results and Discussion For comparison to the IR spectroscopy measurements, we have conducted computational studies on each of the H+(H2O)n

J. Phys. Chem. A, Vol. 114, No. 13, 2010 4571 TABLE 1: Energies of Different Isomers of the H+(H2O)n Clusters from Computational Studies (kcal/mol, Except As Noted)a isomer E(elec) 4I 4II 4III 4IV 4DI 4DII 4DIII 4DIV 5I 5II 5III 5IV 5V 5VI 5DI 5DII 5DIII 5DIV 5DV 5DVI

0 3.74 3.91 3.92

0 1.18 2.48 2.92 3.78 4.46

E(elec+ ZPE) G(50K) G(100K) G(150K) G(200K) 0 4.64 2.33 2.62 0 4.42 2.81 3.01 0.073 0 1.91 3.54 2.76 4.33 0 0.253 2.01 3.32 3.03 4.35

0 4.71 2.28 2.58 0 4.51 2.76 2.97 0.202 0 2.00 3.72 2.89 4.44 0 0.107 1.98 3.38 3.04 4.33

0 4.92 2.21 2.52 0 4.79 2.71 2.93 0.483 0 2.21 4.10 3.20 4.68 0.217 0 2.11 3.73 3.31 4.53

0 5.24 2.16 2.49 0 5.18 2.68 2.92 0.842 0 2.47 4.61 3.62 5.05 0.620 0 2.42 4.31 3.80 4.96

0 5.64 2.14 2.48 0 5.64 2.67 2.92 1.25 0 2.79 5.20 4.12 5.49 1.06 0 2.76 4.94 4.36 5.47

a Isomer numbering refers to structures in Figures 1, 3, and 5. Only one low-lying isomer was found for the n ) 2 and 3 clusters.

TABLE 2: Binding Energies (BE) for H+(H2O)n and H+(H2O)nAr Complexesa

n

BE H2O (kcal/mol)

experimentala (cm-1)

BE H2O computed (kcal/mol)

BE Ar computed (cm-1)

2 3 4

31.6 19.5 17.9

(11050) (6820) (6260)

34.0 23.7 20.1

(4440)

13.7

959 873 508 (isomer 1) 622 (isomer 2) 610

5

12.7 a

27 a

reference. These energetics were not corrected for BSSE or for zero-point energies.

and H+(H2O)nAr species studied. These allow us to identify the structural isomers expected and their corresponding predicted IR spectra. Tables 1 and 2 provide a summary of the results of these computational studies. Figure 1 shows the qualitative structures for the n ) 2-4 clusters; detailed structures are given in the Supporting Information. H+(H2O)2. The protonated water dimer is now generally recognized to have the symmetric Zundel structure (2I in Figure 1).38,45,47,53,55,56,58,64-66,69,70 The recent experimental studies of H5O2+ have motivated an intense theoretical effort to understand the observed vibrational structure. The proton stretching region was first measured by FEL experiments and found to have complex multiplet structure in the 1000-1200 cm-1 region.65,66 When multiphoton effects from the FEL experiments were eliminated by lower laser powers and rare gas tagging, this unusual structure was simplified but still contained more bands than were predicted by harmonic theory.69,70 Subsequently, anharmonic calculations have been carried out on this system that now reproduce most of the complexity of the measured vibrational bands.58 In more recent experiments, the vibrational spectrum for this complex has been explored including its dependence on selected isotopic substitutions.70 However, most of the theoretical studies have examined the isolated dimer complex, which ignores the impact of the tagging atom present in the recent experiments. Because of the low density of massselected ion beams, photodissociation experiments have been

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Douberly et al.

Figure 2. IR photodissociation spectrum of H+(H2O)2Ar compared to the computed MP2 harmonic spectra. The red and blue spectra are the theoretical results with and without argon, respectively.

Figure 1. Structures resulting from computational studies on the n ) 2-4 isomers, with and without argon.

used to study these systems rather than absorption spectroscopy. Because the binding energy of the dimer with respect to elimination of water is over 30 kcal/mol (10 500 cm-1),27,32 single-photon photodissociation of the bare dimer with infrared photons is not possible. Instead, this system must necessarily be studied with the method of tagging.64,69,70,72-75 Lee and coworkers employed hydrogen as the tag species,64 while recent work by our lab and by Johnson and co-workers have employed argon or neon.69,70 We compute the binding energy of argon to H5O2+ to be 2.74 kcal/mol (959 cm-1), which indicates that the tag atom can be eliminated by photodissociation in the mid-IR. Figure 2 shows the spectrum of H+(H2O)2Ar (black, upper trace) in the high-frequency OH stretching region compared to the predictions of theory (harmonic) for the tagged (red, middle trace, 2IA) versus untagged (blue, lower trace, 2I) species. As shown, the untagged species has higher symmetry, and only two high-frequency vibrations are expected corresponding to the symmetric and asymmetric O-H stretches. Argon is predicted to bind to the hydrogen of one water molecule, breaking the symmetry of the dimer and splitting the O-H stretches into four separate bands, as shown in spectrum 2IA in Figure 2. Indeed, the experimental spectrum has four main bands (3522, 3616, 3657, 3696 cm-1) in the free OH stretch region whose positions and intensities match well with those predicted. The bands at 3616 and 3696 cm-1 are slightly shifted from the bands computed for the isolated dimer, and these are then assigned to the water molecule remote from the argon atom binding site. These bands are shifted to the red from the corresponding vibrations of the free water molecule at 3657

(symmetric stretch) and 3756 cm-1 (asymmetric stretch).77 The bands at 3522 and 3657 cm-1 are more intense and each is shifted further to the red from the respective pair of bands in the isolated system, and these are assigned to the symmetric and asymmetric stretch of the water molecule bound to the argon atom. The red shift of these bands and their greater intensity are understandable if the Ar · · · HO system is viewed as a weak hydrogen bond. The red shift of this vibration and the intensity enhancement are consistent with the behavior seen for other hydrogen bonds. The band positions reported here for H5O2+Ar are slightly different ((1-2 cm-1) from those we reported in our earlier communication69 due to improved cooling and sharper line widths and improved wavelength calibration. These bands may also be compared to previous measurements in other laboratories. Using H2 tagging, Lee and co-workers reported bands at 3528, 3617, 3662, and 3693 cm-1, all of which are within a few wavenumbers of the present band positions.64a Using an IR F-center laser + CO2 laser (multiphoton) in a two-color experiment, Lee was able to measure the spectrum without tagging, finding bands at 3608.8 and 3684.4 cm-1.64a Johnson and co-workers70 measured the neon tagged spectrum and found two bands at almost exactly the positions of the bands in the untagged spectrum of Lee and co-workers. This confirms the general aspect of the symmetry reduction from the more strongly bound tag atom/molecule. Neon tagging induces a smaller perturbation than H2 or Ar tagging and produces a spectrum most like that of the free molecule. In addition to the main bands indicated here, two weak features at 3748 and 3825 cm-1 are measured. These bands are not predicted by theory for H5O2+ or H5O2+Ar at the harmonic level, and they were also not predicted by anharmonic calculations on the H+(H2O)2 ion.58 Similar high-frequency bands lying above the expected OH stretches have also been seen for several metal ion-water complexes.78,79 In a recent investigation of this effect for the Cu+(H2O)Ar2 complex, Carnegie et al. used a reduced dimensional anharmonic approach and showed that these additional bands come from combinations of the asymmetric O-H stretch with the H-O-H torsional vibration.79

IR of Small Protonated Water Clusters

Figure 3. IR photodissociation spectra of (a) H+(H2O)3 and (b) H+(H2O)3Ar compared to the computed MP2 harmonic spectra.

According to this analysis, these bands only occur when the system is tagged with argon, so that the torsional potential has a great enough barrier to hinder the internal rotation significantly. In the present system, this torsional vibration is predicted to have a harmonic frequency of 103 cm-1 (unscaled), whereas the first weak band lies 91 cm-1 above the asymmetric stretch not involving the argon. This same assignment of a combination between the asymmetric stretch and the torsional motion therefore seems to fit here. H+(H2O)3. The only isomer of H+(H2O)3 with high stability has a central hydronium ion, with “external” water molecules hydrogen bonded to two of its hydrogens (structure 3I in Figure 1). The third hydrogen atom is free in the untagged complex. As shown by our calculations and the work of others, this third hydronium hydrogen provides the binding site for tagging. The resulting argon complex structure is 3IA in Figure 1. The experimental binding energy of water in the neat complex is about 20 kcal/mol (7000 cm-1),27 and therefore one-photon photodissociation is not possible for this system in its ground vibrational state. Consistent with this, photodissociation of the neat complex is extremely inefficient. The top trace of Figure 3 shows the spectrum measured for H+(H2O)3 without Ar tagging. Unlike the n ) 2 complex, this system yields some photodissociation, but the signal is weak and gives rise to only broad ill-defined resonances. This behavior is consistent with selective photodissociation for the “hot fraction” of ions that were not cooled effectively in the supersonic expansion plasma. The majority of complexes this size are cold and do not contribute to this signal, but those clusters having residual internal energy from the plasma can use this together with the photoexcitation to accomplish photodissociation. The absolute size of this signal is quite small; its intensity is expanded in the figure. The two bands detected in this hot spectrum fall close to the frequencies predicted by theory for the symmetric and

J. Phys. Chem. A, Vol. 114, No. 13, 2010 4573 asymmetric stretches of the external water molecules. Another possible explanation for dissociation below the one-photon threshold is two (or more) photon absorption. The laser power used here is 5-10 mJ/pulse and some multiple photon absorption is conceivable, although it is not likely to be efficient. The lower experimental trace of Figure 3 shows the spectrum measured with argon tagging. The absolute signal levels in this case are much greater because of the efficient elimination of argon. This is expected because the argon binding energy for this complex is computed to be about 870 cm-1, which is well below the one-photon infrared energy in this region of the spectrum. The spectrum contains three main bands at 3577, 3638, and 3722 cm-1. The 3638 and 3722 cm-1 bands fall quite close to the two bands predicted for the external water O-H stretches in the untagged complex, and about the same positions are calculated for these bands in the tagged complex. These bands are only 20-30 cm-1 shifted to the red from the symmetric and asymmetric stretches in the free water molecule.77 The 3577 cm-1 band is more red-shifted than these from the vibrations in the isolated water molecule and is at the position predicted for the single hydronium O-H stretch bound to the argon. These three main vibrations were reported previously by Lee and co-workers with hydrogen tagging (3587, 3642, 3726 cm-1), with neon tagging (3640, 3658, 3722 cm-1) and with the two-color IR experiment without tagging (3637.4, 3667.0, 3721.6 cm-1).64 These previous experiments and ours here show that the external water vibrations remote from the tag atom are virtually unaffected by the tagging. These frequencies change by only 1-2 cm-1 with different tag species. However, as might be expected, the hydronium O-H attached to the tag has a frequency that changes significantly with different tags. As seen for the n ) 2 complex, the argon tagged band position is again quite close to that tagged with hydrogen, while the neon-tagged band position is closest to the untagged spectrum. Two other aspects of this n ) 3 spectrum are worth mentioning. Weaker higher frequency bands are detected at 3811 and 3825 cm-1. These are 89 and 103 cm-1 above the asymmetric stretch band, whereas two torsional vibrations are predicted by our harmonic theory at 87 and 131 cm-1. We therefore assign these as combination bands, tentatively associated with the asymmetric stretch-torsional modes. Another aspect of this spectrum is that two strong vibrations are predicted corresponding to the symmetric and asymmetric O-H stretches of the core hydronium ion in the 2400-2500 cm-1 region, but no bands are detected here. As we discussed previously,69 a strong band is instead detected at 1880 cm-1, where no band is predicted by harmonic calculations. The large anharmonic shifts of these vibrations are confirmed by vibrational SCF calculations. Apparently, there is unusually strong anharmonicity associated with this vibration because the core hydronium ion is partially sharing its protons with the attached water molecules. The associated vibrations have character mixed between that of the core ion and that of a proton-shared Zundel configuration. This mixed character and/or anharmonicity is not described adequately by harmonic calculations. A final aspect of this spectrum is a small but reproducible feature at 3098 cm-1, where no fundamental vibrations are predicted. Based on its frequency, this could be the overtone of the water scissors motion predicted to have a fundamental at 1599/1623 cm-1. However, the bend fundamentals are predicted to be quite weak and, as we reported previously, are also detected only as weak bands near 1500 and 1600 cm-1.69 It is not clear why the overtone of such weak vibrations would be detectable. Another possible assignment is the overtone of the hydronium stretch reported previously at

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Figure 4. IR photodissociation spectra of (a) H+(H2O)4 and (b) H+(H2O)4Ar compared to the computed MP2 harmonic spectra. The MP2 structures (indicated in Table 1 as 4I and 4IA) are shown as insets.

1880 cm-1.69 Strong anharmonicity could cause this overtone to fall in this region. H+(H2O)4. The n ) 4 complex is expected to be the classic “Eigen” ion, having a central hydronium surrounded symmetrically by three water molecules each hydrogen bonded to a single OH. This structure is confirmed by calculations to be the most stable for this ion (4I in Figure 1). The infrared spectra measured for this ion are shown in Figure 4. Surprisingly, even though the binding energy of water is measured and predicted to be 18-20 kcal/mol (6000-7000 cm-1),27 this ion photodissociates rather efficiently in the 3600-3800 region without tagging, producing the spectrum shown in the top trace of the figure. It is not clear whether this signal arises from singlephoton absorption plus added internal energy from warm ions or from multiple photon absorption. However, sharp bands are observed in the OH stretching region at 3643 and 3731 cm-1, unlike the broad structure seen for the untagged smaller clusters, and the signal here is quite large, suggesting that it does not represent a minor fraction of the ions. Because the majority of the ions are expected to be cooled efficiently, this signal is not likely to come from warm ions. The signal is much smaller in the hydrogen bonding region near 2800 cm-1. These overall characteristics are consistent with photodissociation without tagging being a two-photon process. The bands in the free-OH region have energies that exceed the two-photon threshold, but those in the hydrogen bonding region would be below this. The spectrum for the n ) 4 complex tagged with argon is shown in the middle trace of the figure. There are two sharp bands in the free-OH region at 3646 and 3732 cm-1 at essentially the same positions as those in the neat spectrum. At lower frequencies in the hydrogen bonding region, however, the broad signal seen for the neat complex is resolved into two stronger, somewhat broad, but discernible bands at 2670 and 3053 cm-1. The binding energy of argon to this complex is computed to be about 500 cm-1, and so the signal in this

Douberly et al. spectrum should correspond to a one-photon process throughout the energy region shown. Our computational studies for H+(H2O)4 find three lowlying isomers, consistent with the results from previous work.38-41,45,47,49,50,53,56 The Eigen structure mentioned above (isomer 4I) has the lowest energy, but two other isomers based on the Zundel motif lie only 3.5-4.0 kcal/mol above this (see Supporting Information). Isomer 4II has the Zundel ion in a four-water ring, while isomer 4III has an open structure with a Zundel core and two external diagonally attached waters. As shown in Table 1, if we include vibrational zero-point energy, isomer 4I remains the lowest energy species, but the order of isomers 4II and 4III are reversed. With the inclusion of entropy contributions at T ) 50-200 K, the ordering of the isomers remains unchanged but the free-energy difference relative to isomer 4I increases for isomer 4II and decreases for isomer 4III. Both isomers 4II and 4III have bands in the hydrogen bonding region like the structure seen here. However, because of the reduced symmetry, isomers 4II and 4III have multiple bands in the free-OH stretching region (see Supporting Information). The sharp two-band pattern measured experimentally is found only for the Eigen structure because of its high symmetry. Theoretical spectra with and without argon based on this structure are shown in Figure 4. Therefore, this signature confirms that only the Eigen structure is present in our experiment. This same conclusion was reached in our previous study based on the spectrum in the shared-proton stretching region. Both isomers 4II and 4III contain the Zundel core, and both are predicted to have strong shared proton vibrations in the 1000-1200 cm-1 region, but no strong bands were measured there.69 Our computations also find two isomers (4IA1 and 4IA2) associated with different binding sites for the argon atom that differ energetically by only about 100 cm-1 (0.3 kcal/mol; see Table 1 and Figure 1). Isomer 4IA2 has the argon atom on a terminal water OH group, while the lowest energy configuration (isomer 4IA1) has the argon atom bound symmetrically over the center of the complex interacting simultaneously with three OH groups. The predicted spectra for these two isomers are shown in the lower two traces of Figure 4. The asymmetric binding of argon leads to multiplet structure for the two free-OH stretches and two equally intense hydrogen bonding bands, neither of which are seen. Therefore, even though these two isomers are extremely close in energy, apparently only the lower energy structure that has the higher symmetry argon binding site is observed experimentally. The IR spectrum of the H+(H2O)4 ion was measured previously with hydrogen molecule tagging and with the two-color IR method without tagging by Lee and co-workers.64 The hydrogen tagging experiment produced four vibrational bands (3636, 3648, 3723, 3733 cm-1), which were assigned to the Eigen structure with the hydrogen molecule bound to a terminal water molecule (as in argon isomer 4IA2 above). As discussed above for 4IA2, the lower symmetry leads to more bands in the OH stretching region. In this case then, because of the different binding site, argon tagging is less intrusive than hydrogen tagging. The neat spectrum of Lee and co-workers has two OH stretching bands, as expected for the high-symmetry Eigen ion at 3644.9 and 3730.4 cm-1. These band positions are with 1-2 cm-1 of those observed here with argon tagging, and in similar agreement with the bands in our neat spectrum. Argon tagging is therefore particularly nonintrusive in this system. It is worth mentioning the line widths seen in different parts of these spectra. The free-OH stretches here give rise to sharp

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Figure 5. The IR photodissociation spectra of a) H+(H2O)5 and b) H+(H2O)5Ar compared to the computed MP2 harmonic spectra.

bands in the 3600-3700 cm-1 region, whereas the bands in the hydrogen bonding region (2670 and 3053 cm-1) are much broader. This same pattern is seen below for larger clusters studied here. Similar broad bands in the hydrogen bonding region have been seen and discussed previously for many other water-containing clusters, including protonated water species64,69 and metal cation-water clusters.78 However, the mechanism of the broadening of bands associated with hydrogen bonding has not always been completely understood. Inhomogeneous effects from different isomeric species, temperature effects, and predissociation lifetimes have been invoked in these discussions. In the present system, we have demonstrated that this broadening is present for a system in which only one isomer is present, eliminating inhomogeneous effects. The widths of the free-OH bands provide an upper limit to the rotational contours expected for this system at the (apparently) cold temperature of the experiment. The higher energy free-OH bands are sharper than the lower energy hydrogen bonding bands, and therefore predissociation lifetimes, which should be longer at lower energy, cannot be the source of this broadening. The remaining effect is the relative rates of intramolecular vibrational relaxation (IVR), which can be quite different for different vibrational modes with different molecular coupling strengths, particularly in clusters such as these.80 Apparently, the free-OH vibrations here are less well-coupled to the molecular framework than the hydrogen bonding modes, and the rate of IVR for the latter is much greater, giving rise to their broader lines. This makes sense because of the direct connectivity of the hydrogen bonds to other molecules in the cluster, which is not the case for the free-OH vibrations. H+(H2O)5. The spectra measured for the n ) 5 complex with and without argon tagging are presented in Figure 5. The binding energy of water to the neat complex is about 13 kcal/mol (4500 cm-1),27 and thus one-photon photodissociation should not be possible for cold molecules. Nevertheless, a spectrum with reasonably high signal levels is obtained, which has essentially all the structural features found in the argon-tagged spectrum. The argon binding in this system is computed to be about 600 cm-1, which makes one-photon photodissociation possible throughout the frequency range shown. Consistent with this,

Figure 6. Low-energy isomers of the n ) 5 complex found in the MP2 optimizations.

the tagged spectrum has much improved signal levels and sharper bands compared to the neat spectrum. Our computational studies are consistent with previous work, locating six isomers within the first 5 kcal/mol of energy. These isomers are labeled 5I-5VI and are shown in Figure 6. The relative electronic energies of these isomers and the changes caused by the inclusion of zero-point energy and entropy at different temperatures are summarized in Table 1. Isomer 5I has a hydronium core ion with three waters bound to it in essentially the Eigen structure. The fifth water molecule is bound in a bifurcated double-acceptor hydrogen bonding configuration with two of these molecules, completing a four-water ring. Isomer 5II has a similar Eigen-based structure with one additional molecule having only a single hydrogen bond connection in a single-acceptor configuration. Isomer 5III has connectivity similar to that of isomer 5II but differs primarily in the conformation of the external water molecule. Isomer 5IV has connectivity similar to isomer 5I, but with a different conformation. Isomer 5V is the only one of the low-lying isomers with a Zundel core, and this resides in a five-water ring. Isomer 5VI returns to the hydronium motif, with this core ion in a four-water ring, and the fifth water having a single hydrogen bond in a single-acceptor configuration. The detailed structures of these isomers and their computed spectra are presented in the Supporting Information. As might be expected, the relative stabilities of these isomers are sensitive to both zero-point corrections and the inclusion of entropy. In particular, the inclusion of zero-point energy reverses the order of isomers 5I and 5II. Isomer 5II remains slightly lower in free energy (0.1-0.5 kcal/mol) as entropy is included for temperatures up to 100 K. Isomers 5III-5VI are consistently 2-5 kcal/mol

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higher in free energy under all conditions studied. The same trend in relative stabilities is seen for the argon tagged species. Based on the relative energies, more than one isomeric structure could be present for H+(H2O)5. As shown in the Supporting Information, all of these isomers have distinctive IR patterns, allowing an evaluation of the structures actually present. In particular, isomers 5I and 5II have quite different computed spectra. Isomer 5I is predicted to have four strong bands (2 strong, two weak) and two much weaker ones in the free-OH stretching region, and a tight multiplet of three hydrogen bonding bands, while isomer 5II has a triplet of bands in the free-OH region and three much more widely spaced hydrogen bonding bands. As shown in Figure 5, the spectra of both the neat and tagged clusters match nicely with the spectral pattern predicted for isomer 5II. The only major disagreement is that a strong hydrogen bonding band is predicted near 2400 cm-1, where none is observed. This band is the same kind of hydronium-based partially shared-proton vibration predicted but also not observed in this same region for the n ) 3 complex. Like the corresponding vibration for the n ) 3 complex, this hydronium vibration was reported in our previous study to occur at lower frequency near 1880 cm-1, with the large shift from the calculated harmonic position being due to vibrational anharmonicity. With this one caveat then, the spectrum for the n ) 5 complex is in excellent agreement with that predicted for the isomer 5II structure. Significantly, no other major bands are observed that cannot be assigned to this isomer. A weak combination band is observed at 3187 cm-1 and a very weak broad band is detected in the hydrogen bonding region at 3430 cm-1. This latter feature is not expected for isomer 5I, and only isomers 5V and 5VI have predicted bands near this frequency. Therefore, this spectrum is consistent with the presence of primarily one isomer, with the possible very minor concentration of one or more other isomers. This is quite remarkable considering the close energies of these isomers. Moreover, the prominent isomer seen is not the one identified to have the lowest energy unless zero-point energy is included. Zero-point energy gives this isomer only the slimmest margin of energy preference (0.1-0.5 kcal/mol), but this is apparently enough to cause preferential formation of only this structure. The spectrum presented here can be compared to that reported previously by Lee and co-workers, which was measured without tagging.64 In their work, a similar pattern of bands was reported and assigned to the same structural isomer (VII) that we selected. Free-OH bands were detected at 3647, 3709, and 3736 cm-1, which fall between the band positions that we measure for the tagged versus neat species. Two broad hydrogen bonding bands were reported at 2879 and 2967 cm-1, for which the agreement is not as close. In particular, the higher frequency hydrogen bonding band from the previous study is over 100 cm-1 to the red of the band that we observe. All the bands in our spectra are narrower than those in this previous work. It is therefore likely that our clusters are colder than those in the Lee work. Apparently, this difference does not affect the isomer preference (consistent with our free energy analysis), but it does have a significant affect on this higher frequency hydrogen bond. This hydrogen bond frequency is associated with the central water molecule in this structure, which is the most highly connected. It is possible that the frequency of this vibration might be higher in a colder, more rigid structure and lower in a hotter, less rigid structure. Deuterated Complexes. Deuterated analogues of these protonated water clusters will have lower vibrational frequencies, as expected for the O-D stretching vibrations of D2O (sym-

Douberly et al.

Figure 7. IR photodissociation spectra of D+(D2O)3Ar and D+(D2O)4Ar compared to the computed MP2 harmonic spectra for the most stable isomers of the corresponding all-H clusters.

metric and asymmetric stretches at 2671 and 2788 cm-1).77 As a result of these lower frequencies, the zero-point energies of these clusters are lower, resulting in higher effective dissociation energies and activation barriers toward rearrangements. As we discussed recently in a study of the D+(D2O)n clusters in the size range of 18-24,81 if they contain the same amount of internal energy as a corresponding all-H species, the deuterated water clusters have more rigid structures, due to zero-point energy differences. Because zero-point energies shift by different amounts for free-OH f free OD versus the corresponding hydrogen bonded species, isotopic substitutions may have quite different energetic effects on different structural isomers. In protonated clusters having multiple isomers with close energies, it is conceivable that such a differential zero-point lowering resulting from deuteration might shift the isomer preference. Johnson and co-workers have studied several isotopic substitutions of the n ) 2 species,70d,e and our group has reported a study of the larger perdeuterated clusters,81 but there is no previous infrared spectroscopy of deuterated protonated water clusters in the n ) 3-5 range. The spectrum of the perdeuterated n ) 2 species tagged with argon has a four-band pattern much like that seen for H+(H2O)2Ar, except that it is shifted to a frequency range near that of the D2O O-D stretches. This spectrum is presented in the Supporting Information. Our four bands at 2563, 2633, 2703, and 2742 cm-1 agree with the positions of bands shown in the published spectrum of Johnson and co-workers,70d,e although band positions were not tabulated in that report. As discussed above, this four-band pattern results from the binding of the argon atom to one OD group, which breaks the approximate degeneracies of the pairs of symmetric and asymmetric stretch vibrations. Figure 7 shows the spectra measured for the n ) 3 and 4 perdeuterated complexes tagged with argon together with theoretical spectra associated with the most stable isomeric structures calculated for the all-H complexes. The observed spectrum of the D+(D2O)3Ar complex, indicated as 3DA (for n ) 3, D for deuterated, A for argon tagged, etc.) has strong bands at 2627, 2652, and 2762 cm-1, with a pattern similar to that of the corresponding all-H complex. We therefore assign these

IR of Small Protonated Water Clusters

Figure 8. IR photodissociation spectrum of D+(D2O)5Ar compared to the computed MP2 harmonic spectra for the two lowest energy isomers of the corresponding all-H cluster.

bands to isomer 3A, with the higher frequency bands corresponding to the symmetric and asymmetric stretches, and the 2627 band to the O-D stretch of the deuterium to which the argon binds. The computed spectrum is in good agreement with the experimental one but underestimates the splitting between the two lower frequency bands. A combination band, as discussed above, is seen at 2826 cm-1, and another weak feature is seen at 2421 cm-1. This latter band could be from the bend overtone, but it could also come from a small amount of the mass-coincident D3O+-Ar2 cluster, which should also have a band at this position. The hydrogen bonding region is also expected to have bands at lower frequencies below the region of these measurements. The D+(D2O)4 complex has a simple spectrum containing only two bands in the O-D stretching region at 2657 and 2768 cm-1. This simple pattern agrees well with that computed for the Eigen structure with the symmetric argon tag, 4DIA1. Again, the expected hydrogen bonding bands would lie below the region of the experiment. The agreement between these experimental spectra and the ones computed shows that these two perdeuterated species have the same most stable structures as the corresponding all-H clusters. Figure 8 shows the experimental spectrum for the perdeuterated n ) 5 cluster, together with the computed spectra for two of the lower energy isomers, 5DIA and 5DIIA. As shown in the figure, the spectrum matches nicely with that predicted for isomer 5DIIA, which has the same structure determined above for the all-H isomer. However, unlike isomer 5I, isomer 5DI is computed to lie lowest in energy even when zero-point energy is included (Table 1). Only when entropy contributions for temperatures greater than 50 K are included is isomer 5DII calculated to be lower in free energy than isomer 5DI. It is not clear whether this is a real example of an entropic isomer switch because of the elevated temperature of our ions, or just an artifact due to approximations in the calculations. The free energy differences involved are quite small, and indeed argon atom binding (not considered in the free energy calculations) might reverse the trend again. However, considering the close energetics, it is remarkable that only one isomer is seen in the experiment. Again, the observed deuterated isomer is the same as that detected for the corresponding all-H cluster. 4. Conclusions In the experiments reported here, infrared photodissociation is employed to measure vibrational spectra for small protonated

J. Phys. Chem. A, Vol. 114, No. 13, 2010 4577 water clusters, H+(H2O)n, in neat form, tagged with argon or in perdeuterated form. Because dissociation energies are wellknown for these systems from both theory and experiment, the effects of tagging can be evaluated carefully. Tagging is essential for obtaining the spectra for the n ) 2 and 3 clusters, consistent with their known high dissociation energies. Surprisingly, spectra can be measured without tagging for the n ) 3 and 4 clusters, even though the photoexcitation energies are below the one-photon dissociation limits for these species. The appearance of the spectra in these cases is much the same as those measured with tagging. A small amount of photodissociation at energies below threshold is possible for a minor population of clusters with unquenched internal energy. However, the efficient photodissociation seen here indicates that twophoton absorption leads is also likely contributing to these signals. This unexpected observation should provide a cautionary point to those using spectra such as these to derive unknown dissociation energies. These studies provide some of the best examples yet for the effects of tagging, as these same clusters have now been studied in neat form and tagged with argon, neon, or hydrogen. The present data show that when spectra are measured both with and without tagging for a particular cluster size, the main band structure seen is essentially the same. Tagging is most important for the smaller clusters with greater dissociation energies, but it improves the dissociation yields and provides narrower bands for all cluster sizes. Comparison with previous work shows that hydrogen and argon tagging both induce somewhat similar small shifts in vibrational band positions. However, neon tagging provides spectra with virtually the same band positions as those measured for the isolated cold molecule. Even though small shifts are present in the spectra measured with argon tagging, the structural isomer detected for each of these small clusters remains the same with or without tagging. The computational results presented here are in essential agreement with previous work. The n ) 2 and 3 clusters each have only one low-lying isomer, and these are responsible for the experimental spectra. The n ) 2 species has the expected Zundel structure, and the n ) 3 species has a hydronium-based structure. Although the n ) 4 species has other low-lying isomers, the most stable isomer, the symmetric Eigen ion, is found to be formed exclusively in the experiment. Two isomers differing in the binding position of argon are predicted to lie close in energy, but only that with argon in the high-symmetry position is detected. The n ) 5 cluster has six isomers computed to lie low in energy. The experimental spectrum matches that computed for the most stable isomer, but this species is only the lowest in energy when zero-point energy is included. The structure is Eigen based, with one water hydrogen bonding in the second sphere. Remarkably, even though computed energy differences are quite small, the experimental spectra for each cluster size can be interpreted with the presence of only a single isomer. This work provides the first report for IR spectroscopy of the perdeuterated protonated n ) 3-5 water clusters. In each case, the experiment finds the same single isomer for the deuterated clusters that was found for the corresponding all-H species. Because of the lower zero-point energies and consequently higher dissociation energies, argon tagging is essential to measure these spectra. Computational studies find the same most-stable isomer for the deuterated species as found for the all-H species, with one exception. The observed n ) 5 isomer does not have the lowest energy at zero Kelvin, but it has the lowest free energy at temperatures above 50 K. This temperature

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is not unreasonable for this experiment, but it should also be noted that these isomer energy differences are quite small. Because harmonic calculations are employed, these small energy differences may not be physically significant. The change in zero-point energy has the effect of compressing energy differences between isomers. Although the isomer energies are even closer than those for the all-H species, again only a single isomer is observed for each perdeuterated cluster size. The number of possible low-energy isomers grows rapidly with the number of water molecules in H+(H2O)n clusters. As a result, the influence of deuteration on the relative stabilities, and of Ar-tagging on the spectra can be even more important in larger clusters. We will report results on the n ) 6-9 clusters (all-H and perdeuterated) in a future article.82 Acknowledgment. We gratefully acknowledge support for this work from the National Science Foundation, M.A.D. grant no. CHE-0551202 and K.D.J. grant no. CHE-0518253. Supporting Information Available: Full citation for ref 76 and the complete details of the DFT computations done in support of the spectroscopy here, including the spectra, structures, energetics, and vibrational frequencies for each of the structures considered. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bell, R. P. The Proton in Chemistry; Chapman & Hall: London, 1973. (2) Hynes, J. T., Klinman, J. P., Limbach, H.-H., Schowen, R. L., Eds. Hydrogen-Transfer Reactions; Wiley-VCH Publishers,: Weinheim, 2006. (3) Bertini, I.; Gray, H. B.; Stiefel, E. I.; Valentine, J. S. Biological Inorganic Chemistry; University Science Books: Sausalito, CA, 2007. (4) Sorensen, B. Hydrogen and Fuel Cells; Elsevier Academic Press: Burlington, MA, 2005. (5) Ferguson, E. E.; Fehsenfield, F. C.; Albritton, D. L. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 1, p 45. (6) D’Auria, R.; Turco, R. P. Geophys. Res. Lett. 2001, 28, 3871. (7) Duley, W. W. Astrophys. J. 1996, 471, L57. (8) Tielens, A. G. G. M. The Physics and Chemistry of the Interstellar Medium; Cambridge University Press: Cambridge, U.K., 2005. (9) (a) de Grotthuss, C. J. T. Ann. Chim. (Paris) 1805, LVIII, 54. (b) de Grotthuss, C. J. T. Biochim. Biophys. Acta 2006, 1757, 871 (Engl. transl. (a)). (10) (a) Eigen, M. Z. Phys. Chem. (N.F. Frankfurt) 1954, 1, 176. (b) Eigen, M. Angew. Chem. 1963, 75, 489. (c) Eigen, M. Angew. Chem., Int. Ed. Engl. 1964, 3, 1. (d) Eigen, M.; Kruse, W.; Maass, G.; DeMaeyer, L. Prog. React. Kinet. 1964, 2, 285. (11) Weller, A. Prog. React. Kinet. 1961, 1, 187. (12) (a) Zundel, G.; Metzer, H. Z. Z. Phys. Chem. (N.F. Frankfurt) 1968, 58, 225. (b) Zundel, G. AdV. Chem. Phys. 2000, 111, 1. (13) (a) Borgis, D.; Hynes, J. T. J. Phys. Chem. 1996, 100, 1118. (b) Kiefer, P.; Hynes, J. T. J. Phys. Chem. A 2002, 106, 1834. (c) Kiefer, P.; Hynes, J. T. J. Phys. Chem. A 2004, 108, 11793. (14) Agmon, N. Chem. Phys. Lett. 1995, 244, 456. (15) (a) Lobaugh, J.; Voth, G. A. J. Chem. Phys. 1996, 104, 2056. (b) Schmitt, U. W.; Voth, G. A. J. Chem. Phys. 1999, 111, 9361. (c) Day, T. J. F.; Soudackov, A. V.; Cuma, M.; Schmitt, U. W.; Voth, G. A. J. Chem. Phys. 2002, 117, 5839. (d) Voth, G. A. Acc. Chem. Res. 2006, 39, 143. (e) Swanson, J. M. J.; Maupin, C. M.; Chen, H.; Petersen, M. K.; Xu, J.; Wu, Y.; Voth, G. A. J. Phys. Chem. B 2007, 111, 4300. (f) Morkovitch, O.; Chen, H.; Izvekov, S.; Paesani, F.; Voth, G. A.; Agman, N. J. Phys. Chem. B 2008, 112, 9456. (g) Paesani, F.; Voth, G. A. J. Phys. Chem. B 2009, 113, 5702. (16) Mohammed, O. F.; Pines, D.; Dreyer, J.; Pines, E.; Nibbering, E. T. Science 2005, 310, 83. (17) Garczarek, F.; Gerwert, K. Nature 2006, 439, 109. (18) Wraight, C. A. Biochim. Biophys. Acta 2006, 1757, 886. (19) Siwick, B. J.; Bakker, H. J. J. Am. Chem. Soc. 2007, 129, 13412. (20) Marx, D. ChemPhysChem 2007, 7, 1848. (21) Hammes-Schiffer, S.; Soudackov, A. V. J. Phys. Chem. B 2008, 112, 14108. (22) Peters, K. S. Acc. Chem. Res. 2009, 42, 89. (23) Lin, S. S. ReV. Sci. Instrum. 1973, 44, 516.

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