IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters

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IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters: Enol−Keto Tautomers and Ion→Solvent Proton Transfer Published as part of The Journal of Physical Chemistry virtual special issue “Veronica Vaida Festschrift”. Daniel T. Mauney, Jonathon A. Maner, and Michael A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: Protonated ions of acetylacetone, H+(Hacac), and their argon-tagged analogues are produced via a pulsed discharge and cooled in a supersonic expansion. These ions are mass analyzed, selected in a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy using the method of rare-gas atom tagging. Computational studies at the DFT/B3LYP level are employed to elucidate the structures and spectra of these ions, which are expected to exist as either enol- or keto-based tautomers. The protonated acetylacetone ion is found to form a single enol-based isomer. Adding one or two water molecules to this ion, for example, H+(Hacac)(H2O)1,2, produces primarily enol-based structures, although a small concentration of keto structures also contribute to the spectra. The vibrational patterns resulting from hydrogen bonding in these systems are not well-described by theory. Addition of a third water molecule to form the H+(Hacac)(H2O)3 ion causes a significant change in the spectroscopy, attributed to proton transfer from the H+(Hacac) ion into the water solvent.

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INTRODUCTION Proton transfer is a key process in many areas of chemistry and biology including acid−base reactions, electrochemistry, and photosynthesis.1−8 It is also the basis for chemical ionization mass spectrometry,9,10 plays a major role in hydrogen fuel cells,11,12 and has been proposed in mechanisms for atmospheric and interstellar chemistry.13−18 In 1805, Grotthuss described the process of proton transfer in liquid water,19−21 and solvent assisted proton transfer has been implicated in a number of mechanisms across chemistry and biology.5−7,22,23 Therefore, the study of protonated systems and their solvation is an area of great interest. A number of recent studies have provided insight into the structure and solvation in hydrogenbonded systems using infrared spectroscopy of size-selected protonated water clusters in the gas phase, along with high-level computational chemistry.24−51 These studies allow the careful selection of specific ion−molecule complexes with known composition. In the current work, we use these methods to investigate protonated acetylacetone, hereafter denoted H+(Hacac), and its mixed H+(Hacac)(H2O)n clusters (n = 1−3). Proton-bound dimers represent intermediates in proton transfer. These systems have been studied extensively using mass spectrometry to determine binding energies and reactivities.52−55 A number of proton-bound dimers containing various molecular partners have been studied using infrared photodissociation spectroscopy.56−69 Johnson and co-workers showed that the frequency of the characteristic vibration arising from the motion of the shared proton is related to the © XXXX American Chemical Society

difference between the proton affinities of the neutral monomers involved.61 An interesting variation on this idea is intramolecular proton sharing. Intramolecular hydrogen bonds in neutral molecules are well-known noncovalent interactions contributing to the structures of many biological systems.70−76 Numerous examples of these systems have been documented in crystallography and investigated computationally. Johnson and co-workers investigated an ionized example of this kind of bonding in the infrared spectroscopy of protonated 1,8 disubstituted naphthalenes.77 Another class of well-studied compounds that exhibit OH···OC intramolecular hydrogen bonds are the β-diketones.73−76 Acetylacetone (Hacac, also known as 2,4-pentanedione) is the simplest β-diketone. It exists in the two tautomeric forms shown in Scheme 1. The enol form features an intramolecular Scheme 1

Received: July 20, 2017 Published: August 30, 2017 A

DOI: 10.1021/acs.jpca.7b07180 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A hydrogen bond and resonance stabilization through a conjugated π-system, whereas the diketo form contains two carbonyl groups with an ∼140° dihedral angle between the oxygens. These tautomers are distinguished easily using 1H NMR spectroscopy.78,79 Measurements using temperaturedependent photoelectron and UV spectroscopies have shown that the enol tautomer is lower in energy by ∼4 kcal/mol.80−83 The equilibrium has also been shown to shift depending on the environment, with the enol form dominating in gas phase measurements, while the keto form dominates in polar hydrogen-bonding solvents.84 The protonated form of Hacac has tautomers similar to the neutral, which should also be sensitive to solvation. H+(Hacac) may form an enol-like species with two equivalent carbonyl protonation sites or a keto-like species with a single proton bridging the carbonyl groups. The water solvation in the H+(Hacac)(H2O)2,3 complexes has been investigated previously by Chang and co-workers with infrared spectroscopy in the 2800−3800 cm−1 region.85 Here, we expand the investigation of this system, employing argon tagging methods and additional cluster sizes, while extending the spectra into the fingerprint region.

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Figure 1. Infrared photodissociation spectra for the H+(Hacac)(H2O)n complexes for n = 0−3. Each spectrum was measured by selecting one of the various H+(Hacac)(H2O)nAr ion masses and recording the fragment channel corresponding to the elimination of argon.

EXPERIMENTAL SECTION

but only in the 2800−3800 cm−1 region.85 Our spectra here have somewhat sharper bands with additional structure, but they are otherwise consistent with the spectra of Chang, which were measured via the elimination of water molecules. The water-free ion has three vibrational bands in the high-frequency region, where O−H stretches are expected, and the larger complexes have more bands in this region at slightly higher frequencies, where the O−H stretches of water might be expected. All of these ions also have signal in the low-frequency region near 1600 cm−1, where carbonyl stretches are expected. Each of the ions having attached water exhibits one or more broad resonances in the 2700−3500 cm−1 region of the spectrum, indicative of hydrogen-bonding vibrations. The n = 3 spectrum has less signal in the low-frequency range, possibly from a combination of low parent ion intensity and low photodissociation yield. To investigate the structures giving rise to these spectra, we performed computational studies on each of these ions with and without attached argon atoms. Multiple isomeric structures were found lying close in energy for each complex. These included the expected enol−keto structures as well as those with different attachment sites for water in the hydrated species. The full details of these computations are provided in the Supporting Information for this paper. We number these isomers such that the lowest enol structure for the cluster with one water is “EW1,” whereas the second-lowest keto isomer with two waters is “K2W2.” The lowest few isomers for each cluster size and their relative energies are shown as insets in Figures 2−5, and the relative energies are presented in Table 1. The argon atoms were found to bind weakly to each of these ions and, with the exception of the n = 0 complex, had little effect on the positions of vibrational bands or relative energies of isomers (see binding energies in Table 1 and Supporting Information). The infrared absorption spectra for each of these ions are compared to the respective infrared photodissociation spectra in Figures 2−5. A. H+(Hacac). Figure 2 shows the experimental spectrum obtained for H+(Hacac) (black) versus the spectra predicted by theory for the two lowest enol structures (E1, blue and E2, green) and the lowest keto structure (K1, red). The

H+(Hacac) and mixed H+(Hacac)(H2O)n ions are produced in a pulsed discharge/supersonic expansion of 10% H2 in Ar seeded with both Hacac (ReagentPlus, ≥99%, Sigma-Aldrich) and water at room temperature and ambient vapor pressure. The ions are mass selected using a reflectron time-of-flight spectrometer and interrogated using infrared photodissociation spectroscopy.41,42,62−69 The energy absorbed in a single photon at these frequencies is not enough to break the strong bonds present in the smaller clusters, so rare-gas tagging is employed.32,37−51 For this experiment, H+(Hacac)(H2O)nAr (n = 0−3) ions are produced and mass selected, and absorption of an IR photon causes the elimination of the argon tag atom. The spectrum is recorded as the fragment ion yield versus the laser frequency. The IR laser system used is an optical parametric oscillator/amplifier system (OPO/OPA; LaserVision) equipped with an external AgGaSe2 crystal, pumped by a Nd:YAG laser (Spectra Physics Pro-230). The spectra were recorded from 1000 to 4000 cm−1. Computational studies were performed at the DFT/B3LYP/ 6-311+G** level of theory using the Gaussian09 program package.86 These computations investigated each of the H+(Hacac)(H2O)n ions with and without attached argon. The energetics presented are corrected for the zero-point energies. Vibrational spectra were scaled based on a comparison between the calculated and known vibrational frequencies for acetone.87 We derived a factor of 0.965 for frequencies above 3000 cm−1, a factor of 0.973 for frequencies between 1750 and 3000 cm−1, and frequencies below 1750 cm−1 were not scaled.

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RESULTS AND DISCUSSION Figure 1 shows a comparison of the infrared spectra obtained for the different sized H+(Hacac)(H2O)nAr complexes (n = 0− 3), each measured in the mass channel corresponding to the elimination of argon. The top trace in black is the spectrum for H+(Hacac)Ar, whereas the spectra for the H+(Hacac)(H2O)1−3Ar ions are in order from top to bottom in blue, red, and green, respectively. The spectra for the n = 0 and 1 complexes have not been reported previously. The spectra for the n = 2 and 3 ions were reported by Chang and co-workers, B


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Figure 2. Infrared photodissociation spectra for the H+(Hacac)Ar ion compared to the spectra predicted by theory for several low-energy isomeric structures.

cm−1 region of the spectrum that is predicted to be an intense feature for all keto species. Therefore, we are confident in assigning the infrared spectrum for the H+(Hacac)Ar species to the lowest-energy E1 isomer. This isomer has two OH groups, with one forming an intramolecular COH···OC hydrogen bond. The argon binds on the bridging hydrogen. This and the hydrogen-bonding interaction shifts the O−H stretch to lower frequency (the 3326 cm−1 band), whereas the free O−H stretch has a higher frequency (3520 cm−1). A second enol-type isomer has two OH groups facing away from each other, and the two O−H stretches have similar intensities. Even though E1 and E2 lie close in energy, we apparently do not have appreciable amounts of E2, as it would produce additional O−H stretching bands. Note that the agreement between theory and experiment for this ion is not quantitative regarding either band positions or their relative intensities. The discrepancy in band positions likely reflects the limitations of theory for this system. The relative band intensities predicted for linear absorption spectra may not match the experiment, because we measure photodissociation yields. Interestingly, the infrared spectrum of the neutral acetylacetone spectrum was recently studied in solution and modeled computationally.82 Its shared proton vibration was much broader in that environment due to both thermal and dynamical effects that are not expected for these gas-phase systems. The observation of the enol structure for this cation is not too surprising. Neutral Hacac has the enol structure in the gas phase, and this configuration is stabilized by an intramolecular shared proton between the two oxygen atoms. This neutral enol structure is therefore likely to be already present in the vapor that mixes with our expansion gases. We add both hydrogen and water vapor to the discharge mix, and therefore protonation of Hacac most likely occurs by proton transfer from either H3+ or H3O+ ions in the discharge. The most probable protonation site is one of the lone pairs of an oxygen. Protonation here could occur without disrupting the shared proton. The second-lowest-energy enol isomer of the protonated ion (E2) does not have a shared proton, and

Table 1. Isomers of H+(Hacac)(H2O)n=0−3Ar relative energya (kcal/mol) isomer

bare

w/Ar

Ar binding energy (cm−1)

E1 E2 K1 E3 K2 K3 EW1 EW2 EW3 KW1 KW2 E2W1 E2W2 K2W1 K2W2 K2W3 E3W1 E3W2 K3W1 K3W2

0.0 2.8 5.6 12.2 15.6 17.1 0.0 7.1 9.8 11.5 13.0 0.0 4.6 7.3 8.9 11.3 0.0 1.7 5.9 6.5

0.0 2.8 5.6 12.2 15.6 17.1 0.0 6.5 9.3 11.1 12.6 0.0 4.4 7.2 8.7 11.4 0.0 1.6 5.8 6.6

63 206 4 254 390 390 14 177 168 159 140 42 100 81 120 30 22 10 39 23

a

Calculated at the B3LYP level of theory using Gaussian09 with the 6311+G** basis set. Relative energies are zero-point energy (ZPE) corrected.

experimental spectrum has two sharp resonances in the O−H stretching region at 3326 and 3520 cm−1, along with a peak at 2933 cm−1, where C−H stretches are typically found. The fingerprint region has three resolved peaks at 1253, 1523, and 1625 cm−1 and weak unresolved features between 1300 and 1500 cm−1. The band positions and their relative intensities in both the OH and fingerprint regions are in reasonably good agreement with those predicted for the lowest-energy enol isomer. There is no carbonyl stretch band in the 1900−2100 C


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Figure 3. Infrared photodissociation spectra for the H+(Hacac)(H2O)3Ar ion compared to the spectra predicted by theory for several low-energy isomeric structures. The intensities in predicted spectra above 3550 cm−1 were multiplied by 10× to make the weaker bands visible.

molecule (3663 and 3750 cm−1), the O−H stretch of the intramolecular hydrogen bond (3437 cm−1), and the much more intense O−H stretch of the hydrogen bond to water (2902 cm−1). The O−H stretches of water are both predicted within ∼20 cm−1 of the highest-frequency bands observed, but the hydrogen-bonded stretch is predicted much lower than the next-lowest feature. This is not too surprising, as the frequencies of shared-proton and hydrogen-bonding bands are often difficult to reproduce with theory.30−39,42,69,89 However, the number of bands here accounts for the three high-frequency bands and the broad structure in the hydrogenbonding region, although the measured intensities are mostly higher than those predicted. It is also not unusual for our photodissociation action spectra to have high-frequency bands more intense than those predicted for absorption spectra, as seen here, and hydrogen-bonding bands with water are often broadened significantly.42,69 Additionally, multiple C−H stretch vibrations are also predicted to fall in the same region as the hydrogen-bonding vibration, and couplings here may account for some of the width of the experimental feature. The bands predicted for this isomer in the fingerprint region also match reasonably well with the pattern detected between 1300 and 1700 cm−1. However, while the lowest-energy isomer accounts for most of the bands detected, it does not reproduce the features at 1928 or 3246 cm−1. These bands suggest that other isomers might be present. The second lowest-energy EW2 isomer also has the enol configuration, but without the intramolecular hydrogen bond. It is analogous to the second-most stable isomer of the solventfree ion in Figure 2 but now with water attached to one of the OH groups. Its spectrum is shown in the third trace in Figure 3 (red). This isomer has bands predicted for the symmetric and asymmetric stretches of the water (3648 and 3734 cm−1), the O−H stretch attached to argon (3583 cm−1), and the O−H stretch in the hydrogen bond to water (3215 cm−1). The band predicted at 3583 cm−1 falls near some of the extra structure seen at high frequency in the experimental spectrum, and the 3215 cm−1 band matches reasonably well with the experimental

therefore its formation from the stable neutral would involve breaking this hydrogen bond. As an interesting note, the lowest keto isomer (K1) also has an intramolecular shared proton. However, the formation of this isomer by protonation of the corresponding neutral keto form is less likely. The neutral has the two carbonyl groups misaligned from each other by ∼140° (Scheme 1), and therefore both internal rotation and protonation would be required to form the keto ion from the keto neutral. There is also much less of the neutral keto species in the gas phase. B. H+(Hacac)(H2O). Figure 3 shows the experimental spectrum measured for the H+(Hacac)(H2 O) complex compared to the spectra predicted for its different isomers. Adding water to the H+(Hacac) ion produces several new features. The experimental spectrum exhibits three sharp peaks in the O−H stretching region at 3563, 3645, and 3729 cm−1 along with a small resonance at 3246 cm−1, a broad feature spanning from 2500 to 3100 cm−1, four bands in the fingerprint region at 1335, 1425, 1573, and 1652 cm−1 similar to those seen for the H+(Hacac) ion, and a small new peak at 1928 cm−1. The broad signal centered near 2900 cm−1 is in the region typical of hydrogen bonding, as seen in protonated water clusters.25−51 New bands in the O−H stretching region are likely from the attached water. The most interesting new feature is the weak band at 1928 cm−1, which is in the region typically attributed to CO stretches. This kind of vibration should only be seen if a keto isomer is present. The spectra predicted for the lowest-energy isomers identified by theory are presented in the lower traces of Figure 3. The most stable structure (EW1) has the enol configuration seen for the solvent-free H+(Hacac), with a water molecule hydrogen bonding to one OH group, and the other OH forming the intramolecular hydrogen bond. This structure with the proton on Hacac makes sense, because its proton affinity (894 kJ/mol) is much greater than that of water (697 kJ/ mol).88 The spectrum predicted for this isomer is shown in the second trace of the figure in blue. In the high-frequency region it includes the symmetric and asymmetric stretches of the water D


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band at 3246 cm−1. Likewise, the structure predicted in the lowfrequency region is quite similar to that in the experimental spectrum and also might account for the extra bands here. Therefore, it seems likely that there is a small admixture of this isomer present in the experiment in addition to the one predicted to be most stable. However, neither of the lowest two isomers has a band in the region of the experimental peak at 1928 cm−1. Likewise, no other higher-energy enol-based isomers are predicted to have bands in this region (see Supporting Information). The only ion structure with a band here is the KW1 species, whose spectrum is shown in the lower trace of Figure 3 (green). The strong band predicted in this spectrum at 2143 cm−1 is the OH−OH2 stretch involved in the hydrogen bond with water, but now it is for the protonated carbonyl group. The position predicted for this vibration is significantly higher than that of the 1928 cm−1 band in the experiment, but again we note that shared-proton vibrations like this are notoriously difficult to handle with harmonic theory,30−39 and no bands for other isomers occur in this region. We therefore conclude that there is also an admixture of a keto-type isomer present in the experiment in addition to the more abundant enol species. The experimental spectrum therefore represents primarily the EW1 structure but with minor concentrations of the less stable EW2 and KW1 isomers. It is not too surprising that these minor isomers could be present. The discharge conditions in the ion source are energetic enough to sample different ion structures, but then the cooling in the supersonic expansion is rapid enough to freeze in these structures and inhibit rearrangements that might equilibrate to the lowest-energy structures. An interesting aspect of this spectrum is that it apparently contains vibrations associated with two different types of shared-proton moieties. The 2900 cm−1 structure is assigned to the OH+−OH2 configuration of the enol species, whereas the 1928 cm−1 band is assigned to the same kind of vibration for the keto species. Not too surprisingly, the local chemical environment has a significant effect on the proton stretch

vibration. This idea has been explored in recent studies of the stretch vibration for a number of A−H+−B proton-bound dimers. Johnson and co-workers showed that the frequency of the shared-proton stretch in these systems is correlated to the difference in proton affinity (ΔPA) of the two species involved.61 With the accepted proton affinity of Hacac in its gas-phase enol structure (874 kJ/mol),88 and that of water (697 kJ/mol), the ΔPA value for the enol−H+−OH2 is ∼177 kJ/ mol. According to the empirical plot in ref 61, the proton stretch vibration for this system should be ∼2800 cm−1, in good agreement with our experimental finding. A lower proton stretch vibrational frequency, such as the value of 1928 cm−1 for the keto−H+−OH2 moiety, indicates a lower proton affinity for the keto binding site. The proton affinity here has not been measured, but if we apply the plot of ref 61 in reverse, the frequency measured here would indicate a ΔPA of ∼95−100 kJ/mol. This suggests that the proton affinity of the keto tautomer is ∼800 kJ/mol, which is 74 kJ/mol less than the accepted gas-phase value for the enol species. The proton affinity at this keto site should be more relevant for work done in polar solvents, where the keto form is favored. C. H+(Hacac)(H2O)2. The experimental spectrum for the tagged H+(Hacac)(H2O)2 complex compared to the spectra predicted by theory is presented in Figure 4. In contrast to the spectrum for the single-water complex, there are now two broad, intense features in the hydrogen-bonding region. The single-water complex had a broad but much weaker feature here. This could indicate that there are multiple hydrogenbonding environments present, each with differing vibrational frequencies. This spectrum is also different in the O−H stretching region, with a more tightly grouped multiplet of three bands instead of the more widely spaced bands in the single-water complex. The multiplet of bands in the 1300−1600 cm−1 region is more similar to that in the single-water spectrum, as is the band at 1915 cm−1, which is shifted only 13 cm−1 from the position of the 1928 cm−1 band in the singleE


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the two OH−OH2 proton stretches. These two bands line up with the two broad hydrogen-bonding features at 2988 and 3208 cm−1, but the intensity ratios do not match those observed. This isomer is therefore difficult to rule out completely. It has several weak features in the 1300−1650 cm−1 region, which might also contribute to the unresolved structure in the experiment. Therefore, the E2W2 isomer may be present in some concentration, but by itself it cannot explain the most obvious structure. Its predicted spectrum also fails to account for the peak at 1915 cm−1. Because no enol-based isomers have any resonances near the 1915 cm−1 band, we investigated keto-based structures. Two such isomers are shown in Figure 4. The green trace shows the spectrum for the K2W1 isomer with hydronium bridging the carbonyl groups and an external water. This spectrum has two bands in the O−H stretching region, corresponding to the symmetric and asymmetric stretches of the water at 3674 and 3765 cm−1. A band at 3007 cm−1 comes from the symmetric stretch of the hydronium moiety, the one at 2835 cm−1 comes from the asymmetric stretch of two hydronium OH groups against the carbonyl groups, and the one at 2780 cm−1 is the OH−OH2 hydronium−water hydrogen-bond vibration. The 1714 cm−1 band is the scissors bend of the hydronium against the two carbonyl groups. The high-frequency O−H stretches for this structure fall in the same region as those for the enol isomers, and so these bands could also conceivably contribute to the structure measured in this region. The more intense bands at lower frequency do not match anything in the experiment, but each of these vibrations has a component of shared-proton motion, and their frequencies may not be described well by theory. Therefore, it does not appear that this isomer makes a contribution to the spectrum, but it is also difficult to rule it out completely. The purple trace corresponds to the K2W2 isomer with a bridging Zundel-type water dimer spanning across the two carbonyl groups. The three highfrequency bands at 3566, 3671, and 3729 cm−1 are all O−H stretches on the water. The 2528 cm−1 band is a hydroniumlike hydrogen-bonding stretch, whereas the 2139 cm−1 band is a Zundel-like shared-proton stretch. Both of these lowerfrequency vibrations involve shared-proton motions that are difficult to handle with harmonic theory. The shared-proton stretch predicted at 2139 cm−1 may well be lower in frequency, perhaps explaining the 1915 cm−1 experimental band. This is probably the best explanation for this band; the carbonyl stretch is also predicted in this same frequency region, but its IR intensity is much lower than that of the shared-proton motion. The infrared spectrum of this particular ion was studied previously by Chang and co-workers.85 Their experiment did not employ tagging but rather used the elimination of water molecules to detect photodissociation, and their infrared laser only covered the 2800−3800 cm−1 region. In the limited range of their experiment, our spectrum is consistent with theirs, although they did not detect the hydrogen-bonding feature we see at 2988 cm−1. Their O−H stretch bands are also broader than ours, presumably because of higher ion temperatures. With the limited structure available in their spectrum, Chang assigned it to a keto isomer with an open-chain water dimer attached to one carbonyl group, even though this isomer was computed to be less stable than others. However, as shown in Figure S37 in the Supporting Information, we find that the full IR spectrum of this isomer does not agree with the experimental spectrum.

water complex, although there is additional weaker structure in this region. Theory suggests that there are several isomeric enol and keto structures lying close in energy. The most stable E2W1 structure is again an enol species but with two water molecules attached in a sequential arrangement at an OH protonation site, with a separate OH−O intramolecular hydrogen bond. A second-most stable enol structure (E2W2) has no intramolecular hydrogen bond, with a single water attached at each of the two roughly equivalent OH sites. Two low-lying keto isomers each have protonated water structures bridging their carbonyl groups. In K2W1, a hydronium moiety forms this bridge, with a water attached to it on the back side via a hydrogen bond. In K2W2, a Zundel-like protonated water dimer forms the bridge. It is shown in the figure in its equilibrium hydronium-water configuration, but symmetry suggests that vibrational averaging of the proton position may occur between equivalent hydroniums as it does in the Zundel ion. As with the single-water complex, comparison with theory indicates that this spectrum is not completely consistent with any single isomer. However, the lowest-energy structure (E2W1, blue trace) accounts for many of the experimental bands. It is analogous to the lowest-energy structure for the single-water complex (EW1, Figure 3, blue), with the second water connected to the first via a hydrogen bond. In the O−H stretching region, the predicted spectrum has a triplet of peaks that match the pattern in the experiment reasonably well, although each member of the triplet is ∼20 cm−1 higher than the frequencies in the experiment. This same kind of offset was noted above for the spectrum of the single-water complex. The intensities of the predicted bands in this region are multiplied by a factor of 10 to allow them to be seen in the figure. The predicted spectrum also has two hydrogen-bond stretches at 3284 and 3407 cm−1 representing the shared proton between the two water molecules and the intramolecular OH−O vibration in the Hacac structure, respectively. These overlap with the broad structure in the higher-energy part of the hydrogen-bonding region. In the fingerprint region, the spectrum for this isomer has bands at 1375 and 1637 cm−1 that match reasonably well with the more intense bands here. However, the experiment again has a higher-energy band at 1915 cm−1 that is not present in the theory. The most prominent feature in this region in the predicted spectrum is the H-bond stretch at 2432 cm−1 coming from the shared proton between water and Hacac. This band is well above the experimental bands in the fingerprint region but well below those in the hydrogen stretching region. Although it does not match the experimental spectrum, harmonic theory can have difficulty predicting these shared proton stretches, as noted earlier. If we assume that this band is either very broad or higher than predicted, it could explain the broad, weak hydrogen-bonding structure below 2900 cm−1 or even perhaps the more intense structure in the 2988 cm−1 region. The next-lowest E2W2 structure (Figure 4, red) corresponds to the water molecules each attaching to a separate OH from an enol-based structure. Bands predicted at 3669 and 3756 cm−1 correspond to the symmetric and asymmetric stretches of the water molecules. The doublet predicted here does not match the experiment, but some contribution from these bands might explain the unresolved doublet structure of the 3651 and 3747 cm−1 bands. The spectrum predicted also contains peaks at 3030 and 3309 cm−1 in the H-bonding region corresponding to F


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Overall, as shown from this discussion, we find that the most stable isomer (E2W1, Figure 4, blue trace) reproduces most of the measured vibrational bands. Small admixtures of the other isomers, particularly the K2W2 species, are necessary to account for other features detected. It is clear that the exact hydrogen-bonding frequencies for these various isomers are quite important, but unfortunately these are not described well enough by theory to give us more confidence in these assignments. D. H+(Hacac)(H2O)3. The experimental spectrum obtained for the H+(Hacac)(H2O)3 ion is shown in Figure 5, where it is compared to the spectra predicted by theory for different isomeric structures of this ion. It is immediately clear that this spectrum is significantly different from those of the smaller complexes, with only a single main sharp band at high frequency (3750 cm−1), a single feature at low frequency (1617 cm−1), and two broad bands in the hydrogen-bonding region (3162 and 3363 cm−1). There are three much weaker peaks in the high-frequency region. The hydrogen-bonding bands are somewhat sharper than those for the other complexes, and shifted to higher frequencies. Just as for the smaller complexes, theory finds a number of both enol and keto isomers lying close together in energy. The structural patterns are similar to those for the H+(Hacac)(H2O)2 ion but with the additional water bound in different hydrogen-bonding positions. As seen for the other complexes, the enol-type isomers lie lower in energy than the keto forms. In the higher-frequency free O−H stretching region, none of the predicted spectra reproduce the experiment perfectly, but the doublet for isomer E3W1 has the correct spacing between two of the measured features and has a strong asymmetric stretch band that matches the 3750 cm−1 reasonably well. In the lower-frequency region, all of the enol or keto forms of this

complex have spectra dominated by the intense bands associated with hydrogen-bonding vibrations on the water molecules. Isomer E3W1 has a water attached to the enol OH group, with two flanking neutral waters. It has a 1779 cm−1 band predicted for the OH−OH2 scissors vibration and a doublet near 3354 cm−1 predicted for the symmetric and asymmetric stretches of the water molecule involved in this proton sharing. Isomer E3W2 has its three waters in a chain, with intense OH−OH2 (2118 cm−1) and OH2−OH2 (3030 cm−1) hydrogen-bonding vibrations along the length of this chain. Isomer K3W1 has a water dimer structure bridging the carbonyl groups much like isomer K2W2, with strong hydrogen-bonding vibrations (2429 and 2833 cm−1) associated with these bridging waters. Isomer K3W2 has a hydronium-like structure bridging the carbonyl groups, with a strong hydronium stretch vibration at 2204 cm−1. Unfortunately, as shown in Figure 5, none of the predicted spectra in the mid-IR frequency range match the experiment. The same is true for the low-frequency region, where all isomers have predicted spectra with multiple bands, but the experiment has only a single peak at 1617 cm−1. Chang and co-workers assigned their spectrum of this ion in only the higher-frequency region to be that of a keto isomer,85 but our data in a wider range shows that the spectrum does not match those predicted for either enol or keto forms of H+(Hacac)(H2O)3. We mentioned earlier how harmonic theory has severe difficulties in the treatment of hydrogen bonding and shared-proton vibrations. This ion is apparently another example where such problems arise and apparently become even more significant. We examined the present H+(Hacac)(H2O)3 ion with dispersion-corrected density functional theory (DFT) and with MP2 calculations using comparable basis sets and found no significant improvement in the results. G


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dynamics of each system. Each cluster also has free−OH stretches at higher frequencies. The top trace shows the spectrum for the H+(H2O)4 ion, which has three water molecules surrounding a central hydronium, and which is known as the “Eigen” ion.42,44,49 The second trace shows the spectrum when this Eigen ion binds to a neutral benzene molecule via an OH−π hydrogen bond (its vibration is the 3473 cm−1 band).69 The inductive forces from binding to benzene result in stiffening of the hydrogen-bonding network, and the vibrations shift to higher frequencies while maintaining the same basic intensity pattern. A similar effect occurs when hydronium is surrounded by three heavier nitrogen molecules rather than water molecules (third trace).68 Here, the hydrogen-bonding frequencies are even higher, while maintaining a similar intensity ratio between the two hydronium bands. As shown, the spectrum of the H+(Hacac)(H2O)3 ion fits nicely into this same sequence, with hydrogen-bonding vibrations that are even higher in frequency than the others in the series. We therefore suggest that the best assignment for the spectrum of the H+(Hacac)(H2O)3 ion is to a structure like the lowest-energy E3W1 species, but one in which the proton involved in the OH−OH2 bond has transferred over to produce a CO−OH3+(H2O)2 (solvated hydronium) configuration in the water. This makes sense for several reasons. The spectrum for the H+(Hacac)(H2O)3 ion changes significantly from those of the smaller clusters, indicating the presence of a new infrared chromophore. The E3W1 structure is otherwise computed to be stable and differs from the proposed structure only in the position of the bridging proton. The hydrogen-bonding pattern fits the trend shown in Figure 6 for a solvated hydronium species. It makes sense that a hydronium tethered to a carbonyl group would be in a strong binding interaction, pushing the hydrogen-bonding modes to higher frequencies. The E3W1 structure has a higher symmetry arrangement for the flanking water molecules, which is consistent with the simple pattern in the O−H stretching region. If the charge is on water, then it makes sense that the water-based vibrations would be more intense than those of the organic framework. This works for the hydrogen-bonding vibrations but also for the hydronium bend at 1617 cm−1. This scenario also makes sense energetically. The proton affinity of the enol form of Hacac (874 kJ/mol) is greater than that for a single water molecule (697 kJ/mol), and consistent with this the smaller complexes here that have the proton on the Hacac moiety. However, the proton affinity of water clusters increases with their size, and that for the n = 3 cluster has been estimated to be 893 kJ/mol, favoring the proton transfer into the water.92 Although the E3W1 isomer works for this assignment, it is clear that we have lost spectral information about the organic moiety and cannot distinguish clearly between enol versus keto forms there. Proton transfer processes are an appealing aspect of molecular cluster science, and several examples of these processes have been suggested to occur previously. For example, in the case of ROH-H+−water mixtures, changes in collisional fragmentation channels or infrared spectral features were noted by several groups.28,93−95 However, the present system provides more highly resolved spectroscopic features, with clear evidence for changes in the infrared spectrum at a specific cluster size. The present system also demonstrates the complexity of vibrational patterns for intermediate-sized ions undergoing solvation. Harmonic theory at its present level cannot be expected to provide a clear picture of the vibrational patterns in such systems, and full-dimensional anharmonic

The problems with harmonic theory for hydrogen-bonding and shared-proton vibrations are now well-established for many examples of protonated water clusters or their mixtures with other hydrogen-bonding partners. Even for small protonated water clusters such as the H+(H2O)n (n = 3,4) ions, strong hydrogen-bonding vibrational bands are predicted by theory in regions where no signal is observed.26−51 These small protonated water clusters have now been examined with anharmonic theory in different forms to investigate these issues.90,91 The anharmonic studies demonstrate convincingly how unreliable the harmonic calculations can be for such systems. In larger systems like the ions described here, anharmonic theory is simply not feasible. We encountered a similar problem in the clusters of protonated water−benzene mixtures.69 In that system, computations were insufficient to explain the measured vibrational patterns, but we used comparisons between the spectra for protonated water− benzene mixtures and those for pure protonated water clusters to analyze the patterns. A similar approach could be useful here, because the infrared pattern for H+(Hacac)(H2O)3 looks very much like those of other protonated water clusters we studied previously.42,44,49,68,69 In particular, the free OH stretch, two kinds of hydrogen-bonding vibrations, and the single band where water bending is expected are all consistent with the vibrations being localized on a protonated water center. To illustrate this further, we compare the vibrational spectra of other related protonated water clusters to that measured here for the H+(Hacac)(H2O)3 ion in Figure 6. In each complex, the

Figure 6. Infrared photodissociation spectra for selected ions containing hydronium in different environments compared to the spectrum of the H+(Hacac)(H2O)3 ion.

cluster is interpreted to contain hydronium in slightly different hydrogen-bonding environments, and its vibrations dominate the spectrum. The hydrogen-bonding region has two broad bands, with the lower-frequency asymmetric stretch of hydronium more intense than its higher-frequency symmetric stretch, although line widths vary because of the individual H


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theory cannot handle systems of this size. As demonstrated by other groups, double-resonance methods provide one approach toward disentangling such spectra,46−51 but these systems will remain challenging for both experiment and theory.

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CONCLUSION Complexes of protonated acetylacetone and water were produced with a pulsed electrical discharge, and their infrared spectra were obtained between 1000 and 4000 cm−1 using mass-selected infrared photodissociation spectroscopy. The experimental spectrum for H+(Hacac) shows that a single enolbased isomer can account for most of the spectrum, although small concentrations of other isomers must also be present, including keto-based structures. As this H+(Hacac) ion is solvated with water molecules, the spectrum becomes more complex, including different forms of hydrogen-bonding vibrations that are not well-described by theory. Although many spectral features are consistent with those predicted for low-lying enol isomers, multiple isomers, including contributions from keto forms, are identified for the single- and doublewater complexes. The spectrum for the H+(Hacac)(H2O)3 complex changes significantly from those of the smaller clusters, with virtually no active vibrations from the organic framework of the molecule. Instead, new features are characteristic of a protonated water cluster attached to the organic scaffold. This pattern makes sense if the protonated H+(Hacac) ion has transferred its proton into the solvating water.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b07180. Full details of the DFT computations done in support of the spectroscopy presented here, including the structures, energetics, vibrational frequencies, and predicted spectra for each of the complexes considered. The full citation for ref 86 is also given (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael A. Duncan: 0000-0003-4836-106X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to gratefully acknowledge funding for this research by the National Science Foundation (Grant No. CHE-1464708).

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REFERENCES

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IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters

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